Thread.java

package java.lang;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.security.AccessController;
import java.security.AccessControlContext;
import java.security.PrivilegedAction;
import java.util.Map;
import java.util.HashMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

import sun.nio.ch.Interruptible;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
import sun.security.util.SecurityConstants;


/**
 * A <i>thread</i> is a thread of execution in a program. The Java
 * Virtual Machine allows an application to have multiple threads of
 * execution running concurrently.
 * <p>
 * Every thread has a priority. Threads with higher priority are
 * executed in preference to threads with lower priority. Each thread
 * may or may not also be marked as a daemon. When code running in
 * some thread creates a new <code>Thread</code> object, the new
 * thread has its priority initially set equal to the priority of the
 * creating thread, and is a daemon thread if and only if the
 * creating thread is a daemon.
 * <p>
 * When a Java Virtual Machine starts up, there is usually a single
 * non-daemon thread (which typically calls the method named
 * <code>main</code> of some designated class). The Java Virtual
 * Machine continues to execute threads until either of the following
 * occurs:
 * <ul>
 * <li>The <code>exit</code> method of class <code>Runtime</code> has been
 *     called and the security manager has permitted the exit operation
 *     to take place.
 * <li>All threads that are not daemon threads have died, either by
 *     returning from the call to the <code>run</code> method or by
 *     throwing an exception that propagates beyond the <code>run</code>
 *     method.
 * </ul>
 * <p>
 * There are two ways to create a new thread of execution. One is to
 * declare a class to be a subclass of <code>Thread</code>. This
 * subclass should override the <code>run</code> method of class
 * <code>Thread</code>. An instance of the subclass can then be
 * allocated and started. For example, a thread that computes primes
 * larger than a stated value could be written as follows:
 * <hr><blockquote><pre>
 *     class PrimeThread extends Thread {
 *         long minPrime;
 *         PrimeThread(long minPrime) {
 *             this.minPrime = minPrime;
 *         }
 *
 *         public void run() {
 *             // compute primes larger than minPrime
 *             &nbsp;.&nbsp;.&nbsp;.
 *         }
 *     }
 * </pre></blockquote><hr>
 * <p>
 * The following code would then create a thread and start it running:
 * <blockquote><pre>
 *     PrimeThread p = new PrimeThread(143);
 *     p.start();
 * </pre></blockquote>
 * <p>
 * The other way to create a thread is to declare a class that
 * implements the <code>Runnable</code> interface. That class then
 * implements the <code>run</code> method. An instance of the class can
 * then be allocated, passed as an argument when creating
 * <code>Thread</code>, and started. The same example in this other
 * style looks like the following:
 * <hr><blockquote><pre>
 *     class PrimeRun implements Runnable {
 *         long minPrime;
 *         PrimeRun(long minPrime) {
 *             this.minPrime = minPrime;
 *         }
 *
 *         public void run() {
 *             // compute primes larger than minPrime
 *             &nbsp;.&nbsp;.&nbsp;.
 *         }
 *     }
 * </pre></blockquote><hr>
 * <p>
 * The following code would then create a thread and start it running:
 * <blockquote><pre>
 *     PrimeRun p = new PrimeRun(143);
 *     new Thread(p).start();
 * </pre></blockquote>
 * <p>
 * Every thread has a name for identification purposes. More than
 * one thread may have the same name. If a name is not specified when
 * a thread is created, a new name is generated for it.
 * <p>
 * Unless otherwise noted, passing a {@code null} argument to a constructor
 * or method in this class will cause a {@link NullPointerException} to be
 * thrown.
 *
 * @see     Runnable
 * @see     Runtime#exit(int)
 * @see     #run()
 * @see     #stop()
 */
public class Thread implements Runnable {
    /**
     * 类加载的时候，调用本地的注册本地静态方法
     */
    private static native void registerNatives();

    static {
        registerNatives();
    }

    /**
     * 线程名称
     */
    private volatile String name;

    /**
     * 优先级
     */
    private int priority;

    /**
     * 是否是守护线程，true 是守护线程。如果是守护线程的话，是不能够阻止 JVM 的退出的，级别很低
     */
    private boolean daemon = false;

    /**
     * JVM 状态
     */
    private boolean stillborn = false;

    /**
     * 实际被执行的对象
     */
    private Runnable target;

    /**
     * 该线程的线程组
     */
    private ThreadGroup group;

    /**
     * 该线程的上下文类加载器
     */
    private ClassLoader contextClassLoader;

    /* The inherited AccessControlContext of this thread */
    private AccessControlContext inheritedAccessControlContext;

    /**
     * 给线程自动生成名称所用
     */
    private static int threadInitNumber;

    /**
     * 同一时刻只会有一个线程修改 threadInitNumber 的值，线程安全
     *
     * @return
     */
    private static synchronized int nextThreadNum() {
        return threadInitNumber++;
    }

    /**
     * ThreadLocal 的 ThreadLocalMap 是线程的一个属性，所以在多线程环境下 threadLocals 是线程安全的
     */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /**
     * 当创建子线程时，子线程可以得到父线程的 inheritableThreadLocals
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

    /**
     * 此线程请求的堆栈大小
     */
    private long stackSize;

    /*
     * JVM-private state that persists after native thread termination.
     */
    private long nativeParkEventPointer;

    /**
     * 线程 ID
     */
    private long tid;

    /**
     * 用于生成线程 ID
     */
    private static long threadSeqNumber;

    /**
     * 线程状态
     */
    private volatile int threadStatus = 0;

    /**
     * 同一时刻只会有一个线程修改 threadSeqNumber 的值，线程安全
     *
     * @return
     */
    private static synchronized long nextThreadID() {
        return ++threadSeqNumber;
    }

    /**
     * 此对象不为null时说明线程进入了park（阻塞）状态，参见 LockSupport
     *
     * @see java.util.concurrent.locks.LockSupport
     */
    volatile Object parkBlocker;

    /**
     * 线程中断回调标记，设置此标记后，可在线程被中断时调用标记对象的回调方法
     */
    private volatile Interruptible blocker;

    /**
     * 临时使用的锁，在设置/获取线程中断回调标记时使用
     */
    private final Object blockerLock = new Object();

    /**
     * 为当前线程设置一个线程中断回调标记，以便在线程被中断时调用该标记的回调方法
     */
    void blockedOn(Interruptible b) {
        synchronized (blockerLock) {
            blocker = b;
        }
    }

    /**
     * 最低优先级
     */
    public final static int MIN_PRIORITY = 1;

   /**
     * 普通优先级，也是默认的
     */
    public final static int NORM_PRIORITY = 5;

    /**
     * 最高优先级
     */
    public final static int MAX_PRIORITY = 10;

    /**
     * 返回当前正在执行的线程对象的引用
     *
     * @return 当前正在执行的线程
     */
    public static native Thread currentThread();

    /**
     * 当前线程做出让步，放弃当前 cpu，让线程重新选择 cpu，避免线程过度使用 cpu
     * 让步不是不执行，也有可能重新选中自己
     * 当前线程让出 cpu 时间片，重新抢占执行权
     */
    public static native void yield();

    /**
     * 沉睡，锁不会释放，在 millis 毫秒之后会自己醒来
     * 使线程进入 TIMED_WAITING 状态，millis毫秒后自己醒来（不释放锁）
     *
     * @param millis 睡眠时间
     * @throws IllegalArgumentException 若 millis 为负数
     * @throws InterruptedException     if any thread has interrupted the current thread. The
     *                                  <i>interrupted status</i> of the current thread is
     *                                  cleared when this exception is thrown.
     */
    public static native void sleep(long millis) throws InterruptedException;

    /**
     * 使线程进入TIMED_WAITING状态
     * 至少等待millis毫秒，nanos是一个纳秒级的附加时间，用来微调millis参数（不释放锁）
     *
     * @param millis 睡眠时间
     * @param nanos  {@code 0-999999} 额外的纳秒睡眠时间
     * @throws IllegalArgumentException if the value of {@code millis} is negative, or the value of
     *                                  {@code nanos} is not in the range {@code 0-999999}
     * @throws InterruptedException     if any thread has interrupted the current thread. The
     *                                  <i>interrupted status</i> of the current thread is
     *                                  cleared when this exception is thrown.
     */
    public static void sleep(long millis, int nanos) throws InterruptedException {
        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (nanos < 0 || nanos > 999999) {
            throw new IllegalArgumentException(
                                "nanosecond timeout value out of range");
        }
        // 纳秒大于 0.5 毫秒，算一个毫秒
        if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
            millis++;
        }

        sleep(millis);
    }

    /**
     * Initializes a Thread with the current AccessControlContext.
     * @see #init(ThreadGroup,Runnable,String,long,AccessControlContext,boolean)
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
        init(g, target, name, stackSize, null, true);
    }

    /**
     * 初始化一个线程
     *
     * @param g 代表线程组，线程组可以对组内的线程进行批量的操作，比如批量的打断 interrupt
     * @param target run（）方法被调用的对象，也就是我们要运行的对象
     * @param name 新线程的名称
     * @param stackSize 可以设置堆栈的大小
     * @param acc the AccessControlContext to inherit, or
     *            AccessController.getContext() if null
     * @param inheritThreadLocals if {@code true}, inherit initial values for
     *            inheritable thread-locals from the constructing thread
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }

        this.name = name;

        // 当前线程作为父线程
        Thread parent = currentThread();
        // 如果安全管理器不为空，从安全管理器中拿线程组
        // 否则继承父线程的线程组
        SecurityManager security = System.getSecurityManager();
        if (g == null) {
            /* Determine if it's an applet or not */

            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }

            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            if (g == null) {
                g = parent.getThreadGroup();
            }
        }

        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
        g.checkAccess();

        /*
         * Do we have the required permissions?
         */
        if (security != null) {
            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }

        g.addUnstarted();

        this.group = g;
        // 子线程会继承父线程的守护属性
        this.daemon = parent.isDaemon();
        // 子线程继承父线程的优先级属性
        this.priority = parent.getPriority();
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        this.target = target;
        setPriority(priority);
        // 当父线程的 inheritableThreadLocals 的值不为空时
        // 会把 inheritableThreadLocals 里面的值全部传递给子线程
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        // 线程 id 自增
        tid = nextThreadID();
    }

    /**
     * Throws CloneNotSupportedException as a Thread can not be meaningfully
     * cloned. Construct a new Thread instead.
     *
     * @throws  CloneNotSupportedException
     *          always
     */
    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }

    /**
     * Allocates a new {@code Thread} object. This constructor has the same
     * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
     * {@code (null, null, gname)}, where {@code gname} is a newly generated
     * name. Automatically generated names are of the form
     * {@code "Thread-"+}<i>n</i>, where <i>n</i> is an integer.
     */
    public Thread() {
        init(null, null, "Thread-" + nextThreadNum(), 0);
    }

    /**
     * Allocates a new {@code Thread} object. This constructor has the same
     * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
     * {@code (null, target, gname)}, where {@code gname} is a newly generated
     * name. Automatically generated names are of the form
     * {@code "Thread-"+}<i>n</i>, where <i>n</i> is an integer.
     *
     * @param  target
     *         the object whose {@code run} method is invoked when this thread
     *         is started. If {@code null}, this classes {@code run} method does
     *         nothing.
     */
    // 名称的初始化
    public Thread(Runnable target) {
        init(null, target, "Thread-" + nextThreadNum(), 0);
    }

    /**
     * Creates a new Thread that inherits the given AccessControlContext.
     * This is not a public constructor.
     */
    Thread(Runnable target, AccessControlContext acc) {
        init(null, target, "Thread-" + nextThreadNum(), 0, acc, false);
    }

    /**
     * Allocates a new {@code Thread} object. This constructor has the same
     * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
     * {@code (group, target, gname)} ,where {@code gname} is a newly generated
     * name. Automatically generated names are of the form
     * {@code "Thread-"+}<i>n</i>, where <i>n</i> is an integer.
     *
     * @param  group
     *         the thread group. If {@code null} and there is a security
     *         manager, the group is determined by {@linkplain
     *         SecurityManager#getThreadGroup SecurityManager.getThreadGroup()}.
     *         If there is not a security manager or {@code
     *         SecurityManager.getThreadGroup()} returns {@code null}, the group
     *         is set to the current thread's thread group.
     *
     * @param  target
     *         the object whose {@code run} method is invoked when this thread
     *         is started. If {@code null}, this thread's run method is invoked.
     *
     * @throws  SecurityException
     *          if the current thread cannot create a thread in the specified
     *          thread group
     */
    public Thread(ThreadGroup group, Runnable target) {
        init(group, target, "Thread-" + nextThreadNum(), 0);
    }

    /**
     * Allocates a new {@code Thread} object. This constructor has the same
     * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
     * {@code (null, null, name)}.
     *
     * @param   name
     *          the name of the new thread
     */
    public Thread(String name) {
        init(null, null, name, 0);
    }

    /**
     * Allocates a new {@code Thread} object. This constructor has the same
     * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
     * {@code (group, null, name)}.
     *
     * @param  group
     *         the thread group. If {@code null} and there is a security
     *         manager, the group is determined by {@linkplain
     *         SecurityManager#getThreadGroup SecurityManager.getThreadGroup()}.
     *         If there is not a security manager or {@code
     *         SecurityManager.getThreadGroup()} returns {@code null}, the group
     *         is set to the current thread's thread group.
     *
     * @param  name
     *         the name of the new thread
     *
     * @throws  SecurityException
     *          if the current thread cannot create a thread in the specified
     *          thread group
     */
    public Thread(ThreadGroup group, String name) {
        init(group, null, name, 0);
    }

    /**
     * Allocates a new {@code Thread} object. This constructor has the same
     * effect as {@linkplain #Thread(ThreadGroup,Runnable,String) Thread}
     * {@code (null, target, name)}.
     *
     * @param  target
     *         the object whose {@code run} method is invoked when this thread
     *         is started. If {@code null}, this thread's run method is invoked.
     *
     * @param  name
     *         the name of the new thread
     */
    public Thread(Runnable target, String name) {
        init(null, target, name, 0);
    }

    /**
     * Allocates a new {@code Thread} object so that it has {@code target}
     * as its run object, has the specified {@code name} as its name,
     * and belongs to the thread group referred to by {@code group}.
     *
     * <p>If there is a security manager, its
     * {@link SecurityManager#checkAccess(ThreadGroup) checkAccess}
     * method is invoked with the ThreadGroup as its argument.
     *
     * <p>In addition, its {@code checkPermission} method is invoked with
     * the {@code RuntimePermission("enableContextClassLoaderOverride")}
     * permission when invoked directly or indirectly by the constructor
     * of a subclass which overrides the {@code getContextClassLoader}
     * or {@code setContextClassLoader} methods.
     *
     * <p>The priority of the newly created thread is set equal to the
     * priority of the thread creating it, that is, the currently running
     * thread. The method {@linkplain #setPriority setPriority} may be
     * used to change the priority to a new value.
     *
     * <p>The newly created thread is initially marked as being a daemon
     * thread if and only if the thread creating it is currently marked
     * as a daemon thread. The method {@linkplain #setDaemon setDaemon}
     * may be used to change whether or not a thread is a daemon.
     *
     * @param  group
     *         the thread group. If {@code null} and there is a security
     *         manager, the group is determined by {@linkplain
     *         SecurityManager#getThreadGroup SecurityManager.getThreadGroup()}.
     *         If there is not a security manager or {@code
     *         SecurityManager.getThreadGroup()} returns {@code null}, the group
     *         is set to the current thread's thread group.
     *
     * @param  target
     *         the object whose {@code run} method is invoked when this thread
     *         is started. If {@code null}, this thread's run method is invoked.
     *
     * @param  name
     *         the name of the new thread
     *
     * @throws  SecurityException
     *          if the current thread cannot create a thread in the specified
     *          thread group or cannot override the context class loader methods.
     */
    public Thread(ThreadGroup group, Runnable target, String name) {
        init(group, target, name, 0);
    }

    /**
     * Allocates a new {@code Thread} object so that it has {@code target}
     * as its run object, has the specified {@code name} as its name,
     * and belongs to the thread group referred to by {@code group}, and has
     * the specified <i>stack size</i>.
     *
     * <p>This constructor is identical to {@link
     * #Thread(ThreadGroup,Runnable,String)} with the exception of the fact
     * that it allows the thread stack size to be specified.  The stack size
     * is the approximate number of bytes of address space that the virtual
     * machine is to allocate for this thread's stack.  <b>The effect of the
     * {@code stackSize} parameter, if any, is highly platform dependent.</b>
     *
     * <p>On some platforms, specifying a higher value for the
     * {@code stackSize} parameter may allow a thread to achieve greater
     * recursion depth before throwing a {@link StackOverflowError}.
     * Similarly, specifying a lower value may allow a greater number of
     * threads to exist concurrently without throwing an {@link
     * OutOfMemoryError} (or other internal error).  The details of
     * the relationship between the value of the <tt>stackSize</tt> parameter
     * and the maximum recursion depth and concurrency level are
     * platform-dependent.  <b>On some platforms, the value of the
     * {@code stackSize} parameter may have no effect whatsoever.</b>
     *
     * <p>The virtual machine is free to treat the {@code stackSize}
     * parameter as a suggestion.  If the specified value is unreasonably low
     * for the platform, the virtual machine may instead use some
     * platform-specific minimum value; if the specified value is unreasonably
     * high, the virtual machine may instead use some platform-specific
     * maximum.  Likewise, the virtual machine is free to round the specified
     * value up or down as it sees fit (or to ignore it completely).
     *
     * <p>Specifying a value of zero for the {@code stackSize} parameter will
     * cause this constructor to behave exactly like the
     * {@code Thread(ThreadGroup, Runnable, String)} constructor.
     *
     * <p><i>Due to the platform-dependent nature of the behavior of this
     * constructor, extreme care should be exercised in its use.
     * The thread stack size necessary to perform a given computation will
     * likely vary from one JRE implementation to another.  In light of this
     * variation, careful tuning of the stack size parameter may be required,
     * and the tuning may need to be repeated for each JRE implementation on
     * which an application is to run.</i>
     *
     * <p>Implementation note: Java platform implementers are encouraged to
     * document their implementation's behavior with respect to the
     * {@code stackSize} parameter.
     *
     *
     * @param  group
     *         the thread group. If {@code null} and there is a security
     *         manager, the group is determined by {@linkplain
     *         SecurityManager#getThreadGroup SecurityManager.getThreadGroup()}.
     *         If there is not a security manager or {@code
     *         SecurityManager.getThreadGroup()} returns {@code null}, the group
     *         is set to the current thread's thread group.
     *
     * @param  target
     *         the object whose {@code run} method is invoked when this thread
     *         is started. If {@code null}, this thread's run method is invoked.
     *
     * @param  name
     *         the name of the new thread
     *
     * @param  stackSize
     *         the desired stack size for the new thread, or zero to indicate
     *         that this parameter is to be ignored.
     *
     * @throws  SecurityException
     *          if the current thread cannot create a thread in the specified
     *          thread group
     *
     * @since 1.4
     */
    public Thread(ThreadGroup group, Runnable target, String name,
                  long stackSize) {
        init(group, target, name, stackSize);
    }

    /**
     * 该方法可以创建一个新的线程出来，返回的仍然是主线程
     * 启动线程，线程状态从 NEW 进入 RUNNABLE
     *
     * @throws IllegalThreadStateException 若线程已经启动
     * @see #run()
     * @see #stop()
     */
    public synchronized void start() {
        // 如果没有初始化，抛异常
        if (threadStatus != 0)
            throw new IllegalThreadStateException();

        // 将当前线程加入到所在的线程组，记录为活跃线程
        group.add(this);

        // started 是个标识符，我们在初始化一些东西的时候，经常这么写
        boolean started = false;
        try {
            // 这里会创建一个新的线程，执行完成之后，新的线程已经在运行了，既 target 的内容已经在运行了
            start0();
            // 这里执行的还是主线程
            started = true;
        } finally {
            try {
                // 如果失败，把线程从线程组中删除
                if (!started) {
                    group.threadStartFailed(this);
                }
                // 这里的 catch 捕捉也是值得我们学习的，我们在工作中 catch 时也应该多用 Throwable，少用 Exception
                // 比如对于异步线程抛出来的异常，Exception 是捕捉不住的，Throwable 却可以
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }

    /**
     * 开启新线程使用的是 native 方法
     */
    private native void start0();

    /**
     * 简单的运行，不会新起线程
     *
     * @see #start()
     * @see #stop()
     * @see #Thread(ThreadGroup, Runnable, String)
     */
    @Override
    public void run() {
        if (target != null) {
            target.run();
        }
    }

    /**
     * This method is called by the system to give a Thread
     * a chance to clean up before it actually exits.
     */
    private void exit() {
        if (group != null) {
            group.threadTerminated(this);
            group = null;
        }
        /* Aggressively null out all reference fields: see bug 4006245 */
        target = null;
        /* Speed the release of some of these resources */
        threadLocals = null;
        inheritableThreadLocals = null;
        inheritedAccessControlContext = null;
        blocker = null;
        uncaughtExceptionHandler = null;
    }

    /**
     * Forces the thread to stop executing.
     * <p>
     * If there is a security manager installed, its <code>checkAccess</code>
     * method is called with <code>this</code>
     * as its argument. This may result in a
     * <code>SecurityException</code> being raised (in the current thread).
     * <p>
     * If this thread is different from the current thread (that is, the current
     * thread is trying to stop a thread other than itself), the
     * security manager's <code>checkPermission</code> method (with a
     * <code>RuntimePermission("stopThread")</code> argument) is called in
     * addition.
     * Again, this may result in throwing a
     * <code>SecurityException</code> (in the current thread).
     * <p>
     * The thread represented by this thread is forced to stop whatever
     * it is doing abnormally and to throw a newly created
     * <code>ThreadDeath</code> object as an exception.
     * <p>
     * It is permitted to stop a thread that has not yet been started.
     * If the thread is eventually started, it immediately terminates.
     * <p>
     * An application should not normally try to catch
     * <code>ThreadDeath</code> unless it must do some extraordinary
     * cleanup operation (note that the throwing of
     * <code>ThreadDeath</code> causes <code>finally</code> clauses of
     * <code>try</code> statements to be executed before the thread
     * officially dies).  If a <code>catch</code> clause catches a
     * <code>ThreadDeath</code> object, it is important to rethrow the
     * object so that the thread actually dies.
     * <p>
     * The top-level error handler that reacts to otherwise uncaught
     * exceptions does not print out a message or otherwise notify the
     * application if the uncaught exception is an instance of
     * <code>ThreadDeath</code>.
     *
     * @exception  SecurityException  if the current thread cannot
     *               modify this thread.
     * @see        #interrupt()
     * @see        #checkAccess()
     * @see        #run()
     * @see        #start()
     * @see        ThreadDeath
     * @see        ThreadGroup#uncaughtException(Thread,Throwable)
     * @see        SecurityManager#checkAccess(Thread)
     * @see        SecurityManager#checkPermission
     * @deprecated This method is inherently unsafe.  Stopping a thread with
     *       Thread.stop causes it to unlock all of the monitors that it
     *       has locked (as a natural consequence of the unchecked
     *       <code>ThreadDeath</code> exception propagating up the stack).  If
     *       any of the objects previously protected by these monitors were in
     *       an inconsistent state, the damaged objects become visible to
     *       other threads, potentially resulting in arbitrary behavior.  Many
     *       uses of <code>stop</code> should be replaced by code that simply
     *       modifies some variable to indicate that the target thread should
     *       stop running.  The target thread should check this variable
     *       regularly, and return from its run method in an orderly fashion
     *       if the variable indicates that it is to stop running.  If the
     *       target thread waits for long periods (on a condition variable,
     *       for example), the <code>interrupt</code> method should be used to
     *       interrupt the wait.
     *       For more information, see
     *       <a href="{@docRoot}/../technotes/guides/concurrency/threadPrimitiveDeprecation.html">Why
     *       are Thread.stop, Thread.suspend and Thread.resume Deprecated?</a>.
     */
    @Deprecated
    public final void stop() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            checkAccess();
            if (this != Thread.currentThread()) {
                security.checkPermission(SecurityConstants.STOP_THREAD_PERMISSION);
            }
        }
        // A zero status value corresponds to "NEW", it can't change to
        // not-NEW because we hold the lock.
        if (threadStatus != 0) {
            resume(); // Wake up thread if it was suspended; no-op otherwise
        }

        // The VM can handle all thread states
        stop0(new ThreadDeath());
    }

    /**
     * Throws {@code UnsupportedOperationException}.
     *
     * @param obj ignored
     *
     * @deprecated This method was originally designed to force a thread to stop
     *        and throw a given {@code Throwable} as an exception. It was
     *        inherently unsafe (see {@link #stop()} for details), and furthermore
     *        could be used to generate exceptions that the target thread was
     *        not prepared to handle.
     *        For more information, see
     *        <a href="{@docRoot}/../technotes/guides/concurrency/threadPrimitiveDeprecation.html">Why
     *        are Thread.stop, Thread.suspend and Thread.resume Deprecated?</a>.
     */
    @Deprecated
    public final synchronized void stop(Throwable obj) {
        throw new UnsupportedOperationException();
    }

    /**
     * 中断线程（只是给线程预设一个标记，不是立即让线程停下来）
     *
     * @throws SecurityException 若当前线程无法修改
     */
    public void interrupt() {
        // 如果由别的线程对当前线程发起中断
        if (this != Thread.currentThread())
            checkAccess();

        synchronized (blockerLock) {
            Interruptible b = blocker;
            // 如果存在线程中断回调标记
            if (b != null) {
                // 设置中断标记
                interrupt0();
                b.interrupt(this);
                return;
            }
        }
        interrupt0();
    }

    /**
     * Tests whether the current thread has been interrupted.  The
     * <i>interrupted status</i> of the thread is cleared by this method.  In
     * other words, if this method were to be called twice in succession, the
     * second call would return false (unless the current thread were
     * interrupted again, after the first call had cleared its interrupted
     * status and before the second call had examined it).
     *
     * <p>A thread interruption ignored because a thread was not alive
     * at the time of the interrupt will be reflected by this method
     * returning false.
     *
     * @return  <code>true</code> if the current thread has been interrupted;
     *          <code>false</code> otherwise.
     * @see #isInterrupted()
     * @revised 6.0
     */
    public static boolean interrupted() {
        return currentThread().isInterrupted(true);
    }

    /**
     * Tests whether this thread has been interrupted.  The <i>interrupted
     * status</i> of the thread is unaffected by this method.
     *
     * <p>A thread interruption ignored because a thread was not alive
     * at the time of the interrupt will be reflected by this method
     * returning false.
     *
     * @return  <code>true</code> if this thread has been interrupted;
     *          <code>false</code> otherwise.
     * @see     #interrupted()
     * @revised 6.0
     */
    public boolean isInterrupted() {
        return isInterrupted(false);
    }

    /**
     * Tests if some Thread has been interrupted.  The interrupted state
     * is reset or not based on the value of ClearInterrupted that is
     * passed.
     */
    private native boolean isInterrupted(boolean ClearInterrupted);

    /**
     * Throws {@link NoSuchMethodError}.
     *
     * @deprecated This method was originally designed to destroy this
     *     thread without any cleanup. Any monitors it held would have
     *     remained locked. However, the method was never implemented.
     *     If if were to be implemented, it would be deadlock-prone in
     *     much the manner of {@link #suspend}. If the target thread held
     *     a lock protecting a critical system resource when it was
     *     destroyed, no thread could ever access this resource again.
     *     If another thread ever attempted to lock this resource, deadlock
     *     would result. Such deadlocks typically manifest themselves as
     *     "frozen" processes. For more information, see
     *     <a href="{@docRoot}/../technotes/guides/concurrency/threadPrimitiveDeprecation.html">
     *     Why are Thread.stop, Thread.suspend and Thread.resume Deprecated?</a>.
     * @throws NoSuchMethodError always
     */
    @Deprecated
    public void destroy() {
        throw new NoSuchMethodError();
    }

    /**
     * 判断当前线程是否仍然存活（没有到达 TERMINATED 状态）
     *
     * @return 若存活返回 true
     */
    public final native boolean isAlive();

    /**
     * Suspends this thread.
     * <p>
     * First, the <code>checkAccess</code> method of this thread is called
     * with no arguments. This may result in throwing a
     * <code>SecurityException </code>(in the current thread).
     * <p>
     * If the thread is alive, it is suspended and makes no further
     * progress unless and until it is resumed.
     *
     * @exception  SecurityException  if the current thread cannot modify
     *               this thread.
     * @see #checkAccess
     * @deprecated   This method has been deprecated, as it is
     *   inherently deadlock-prone.  If the target thread holds a lock on the
     *   monitor protecting a critical system resource when it is suspended, no
     *   thread can access this resource until the target thread is resumed. If
     *   the thread that would resume the target thread attempts to lock this
     *   monitor prior to calling <code>resume</code>, deadlock results.  Such
     *   deadlocks typically manifest themselves as "frozen" processes.
     *   For more information, see
     *   <a href="{@docRoot}/../technotes/guides/concurrency/threadPrimitiveDeprecation.html">Why
     *   are Thread.stop, Thread.suspend and Thread.resume Deprecated?</a>.
     */
    @Deprecated
    public final void suspend() {
        checkAccess();
        suspend0();
    }

    /**
     * Resumes a suspended thread.
     * <p>
     * First, the <code>checkAccess</code> method of this thread is called
     * with no arguments. This may result in throwing a
     * <code>SecurityException</code> (in the current thread).
     * <p>
     * If the thread is alive but suspended, it is resumed and is
     * permitted to make progress in its execution.
     *
     * @exception  SecurityException  if the current thread cannot modify this
     *               thread.
     * @see        #checkAccess
     * @see        #suspend()
     * @deprecated This method exists solely for use with {@link #suspend},
     *     which has been deprecated because it is deadlock-prone.
     *     For more information, see
     *     <a href="{@docRoot}/../technotes/guides/concurrency/threadPrimitiveDeprecation.html">Why
     *     are Thread.stop, Thread.suspend and Thread.resume Deprecated?</a>.
     */
    @Deprecated
    public final void resume() {
        checkAccess();
        resume0();
    }

    /**
     * 设置线程优先级
     *
     * @param newPriority priority to set this thread to
     * @throws IllegalArgumentException 如果优先级不在 MIN_PRIORITY 到 MAX_PRIORITY 范围内
     * @throws SecurityException 若不能修改当前线程
     * @see #getPriority
     * @see #checkAccess()
     * @see #getThreadGroup()
     * @see #MAX_PRIORITY
     * @see #MIN_PRIORITY
     * @see ThreadGroup#getMaxPriority()
     */
    public final void setPriority(int newPriority) {
        ThreadGroup g;
        checkAccess();
        if (newPriority > MAX_PRIORITY || newPriority < MIN_PRIORITY) {
            throw new IllegalArgumentException();
        }
        if((g = getThreadGroup()) != null) {
            if (newPriority > g.getMaxPriority()) {
                newPriority = g.getMaxPriority();
            }
            setPriority0(priority = newPriority);
        }
    }

    /**
     * 返回该线程的优先级
     *
     * @return  该线程的优先级
     * @see     #setPriority
     */
    public final int getPriority() {
        return priority;
    }

    /**
     * 设置线程名称
     *
     * @param name 线程名称
     * @throws SecurityException if the current thread cannot modify this
     *                           thread.
     * @see #getName
     * @see #checkAccess()
     */
    public final synchronized void setName(String name) {
        checkAccess();
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }

        this.name = name;
        if (threadStatus != 0) {
            setNativeName(name);
        }
    }

    /**
     * 返回该线程的名字
     *
     * @return 该线程的名字
     * @see #setName(String)
     */
    public final String getName() {
        return name;
    }

    /**
     * 返回此线程所属的线程组
     * 若该线程已经死亡（停止），则返回null
     *
     * @return 此线程所属的线程组
     */
    public final ThreadGroup getThreadGroup() {
        return group;
    }

    /**
     * 递归获取当前线程所在线程组的所有活跃线程数量（可能与实际数量有出入，因为线程数量动态变化），建议仅用作监视目的
     *
     * @return 当前线程所在线程组的所有活跃线程数量
     */
    public static int activeCount() {
        return currentThread().getThreadGroup().activeCount();
    }

    /**
     * 递归获取当前线程所在线程组的所有线程（可能与实际状态有出入，因为线程数量动态变化），建议仅用作监视目的
     *
     * @param tarray 当前线程的线程组
     * @return 线程组的所有数量数量
     * @throws SecurityException if {@link java.lang.ThreadGroup#checkAccess} determines that
     *                           the current thread cannot access its thread group
     */
    public static int enumerate(Thread tarray[]) {
        return currentThread().getThreadGroup().enumerate(tarray);
    }

    /**
     * Counts the number of stack frames in this thread. The thread must
     * be suspended.
     *
     * @return     the number of stack frames in this thread.
     * @exception  IllegalThreadStateException  if this thread is not
     *             suspended.
     * @deprecated The definition of this call depends on {@link #suspend},
     *             which is deprecated.  Further, the results of this call
     *             were never well-defined.
     */
    @Deprecated
    public native int countStackFrames();

    /**
     * 使该方法的调用者所在的线程进入 WAITING 或 TIMED_WAITING 状态，直到当前线程死亡，或者等待超时之后，再去执行上述调用者线程
     *
     * @param millis 等待时间
     * @throws IllegalArgumentException if the value of {@code millis} is negative
     * @throws InterruptedException     if any thread has interrupted the current thread. The
     *                                  <i>interrupted status</i> of the current thread is
     *                                  cleared when this exception is thrown.
     */
    public final synchronized void join(long millis) throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            // 其他线程好了之后，当前线程的状态是 TERMINATED,isAlive 返回 false
            // NEW false
            // RUNNABLE true
            while (isAlive()) {
                // 等待其他线程,一直等待
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                // 等待一定的时间，如果在 delay 时间内，等待的线程仍没有结束，放弃等待
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

    /**
     * 使该方法的调用者所在的线程进入 WAITING 或 TIMED_WAITING 状态，直到当前线程死亡，或者等待超时之后，再去执行上述调用者线程
     * 注：millis的单位是毫秒，且为非负数；nanos的单位是纳秒，其取值范围在1毫秒之内，与millis共同组成超时限制
     *
     * @param millis 等待时间
     * @param nanos  {@code 0-999999} 额外等待时间
     * @throws IllegalArgumentException if the value of {@code millis} is negative, or the value
     *                                  of {@code nanos} is not in the range {@code 0-999999}
     * @throws InterruptedException     if any thread has interrupted the current thread. The
     *                                  <i>interrupted status</i> of the current thread is
     *                                  cleared when this exception is thrown.
     */
    public final synchronized void join(long millis, int nanos) throws InterruptedException {

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        // 纳秒的取值在1毫秒之内
        if (nanos < 0 || nanos > 999999) {
            throw new IllegalArgumentException(
                                "nanosecond timeout value out of range");
        }

        if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
            millis++;
        }

        join(millis);
    }

    /**
     * 使该方法的调用者所在的线程进入 WAITING 状态，直到当前线程死亡之后，再去执行上述调用者线程
     *
     * @throws InterruptedException if any thread has interrupted the current thread. The
     *                              <i>interrupted status</i> of the current thread is
     *                              cleared when this exception is thrown.
     */
    public final void join() throws InterruptedException {
        join(0);
    }

    /**
     * 打印当前线程的异常栈信息，此方法仅用于调试
     *
     * @see Throwable#printStackTrace()
     */
    public static void dumpStack() {
        new Exception("Stack trace").printStackTrace();
    }

    /**
     * 设置当前线程为守护线程/非守护线程
     * 线程开始之前此方法必须被调用
     *
     * @param on 若为 true，则设置为守护线程
     * @throws IllegalThreadStateException if this thread is {@linkplain #isAlive alive}
     * @throws SecurityException           if {@link #checkAccess} determines that the current
     *                                     thread cannot modify this thread
     */
    public final void setDaemon(boolean on) {
        checkAccess();
        if (isAlive()) {
            throw new IllegalThreadStateException();
        }
        daemon = on;
    }

    /**
     * 判断该线程是否为守护线程
     *
     * @return 返回true代表当前线程是守护线程
     * @see #setDaemon(boolean)
     */
    public final boolean isDaemon() {
        return daemon;
    }

    /**
     * Determines if the currently running thread has permission to
     * modify this thread.
     * <p>
     * If there is a security manager, its <code>checkAccess</code> method
     * is called with this thread as its argument. This may result in
     * throwing a <code>SecurityException</code>.
     *
     * @exception  SecurityException  if the current thread is not allowed to
     *               access this thread.
     * @see        SecurityManager#checkAccess(Thread)
     */
    public final void checkAccess() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkAccess(this);
        }
    }

    /**
     * 返回该线程的字符串表示形式，包括线程名称，优先级和线程组
     *
     * @return 该线程的字符串表示
     */
    public String toString() {
        ThreadGroup group = getThreadGroup();
        if (group != null) {
            return "Thread[" + getName() + "," + getPriority() + "," +
                           group.getName() + "]";
        } else {
            return "Thread[" + getName() + "," + getPriority() + "," +
                            "" + "]";
        }
    }

    /**
     * 获取当前线程上下文类加载器
     *
     * @return the context ClassLoader for this Thread, or {@code null}
     * indicating the system class loader (or, failing that, the
     * bootstrap class loader)
     * @throws SecurityException 如果当前线程无法获取上下文类加载器
     */
    @CallerSensitive
    public ClassLoader getContextClassLoader() {
        if (contextClassLoader == null)
            return null;
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            ClassLoader.checkClassLoaderPermission(contextClassLoader,
                                                   Reflection.getCallerClass());
        }
        return contextClassLoader;
    }

    /**
     * 设置线程上下文类加载器
     *
     * @param cl the context ClassLoader for this Thread, or null  indicating the
     *           system class loader (or, failing that, the bootstrap class loader)
     * @throws SecurityException 如果当前线程无法设置上下文类加载器
     */
    public void setContextClassLoader(ClassLoader cl) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("setContextClassLoader"));
        }
        contextClassLoader = cl;
    }

    /**
     * 判断是否只有当前线程持有 obj 锁
     *
     * @param  obj the object on which to test lock ownership
     * @throws NullPointerException 若 obj 为 null
     * @return 若只有当前线程持有 obj 锁，则返回 true
     * @since 1.4
     */
    public static native boolean holdsLock(Object obj);

    /**
     * 空栈帧
     */
    private static final StackTraceElement[] EMPTY_STACK_TRACE = new StackTraceElement[0];

    /**
     * 获取当前线程中的栈帧
     *
     * @return StackTraceElement 数组，每一个元素代表一个栈帧
     * @throws SecurityException if a security manager exists and its
     *                           <tt>checkPermission</tt> method doesn't allow
     *                           getting the stack trace of thread.
     * @see SecurityManager#checkPermission
     * @see RuntimePermission
     * @see Throwable#getStackTrace
     */
    public StackTraceElement[] getStackTrace() {
        if (this != Thread.currentThread()) {
            // check for getStackTrace permission
            SecurityManager security = System.getSecurityManager();
            if (security != null) {
                security.checkPermission(
                    SecurityConstants.GET_STACK_TRACE_PERMISSION);
            }
            // optimization so we do not call into the vm for threads that
            // have not yet started or have terminated
            if (!isAlive()) {
                return EMPTY_STACK_TRACE;
            }
            StackTraceElement[][] stackTraceArray = dumpThreads(new Thread[] {this});
            StackTraceElement[] stackTrace = stackTraceArray[0];
            // a thread that was alive during the previous isAlive call may have
            // since terminated, therefore not having a stacktrace.
            if (stackTrace == null) {
                stackTrace = EMPTY_STACK_TRACE;
            }
            return stackTrace;
        } else {
            // Don't need JVM help for current thread
            return (new Exception()).getStackTrace();
        }
    }

    /**
     * 获取当前 JVM 中所有活跃线程的栈帧
     *
     * @throws SecurityException if a security manager exists and its
     *                           <tt>checkPermission</tt> method doesn't allow
     *                           getting the stack trace of thread.
     * @see #getStackTrace
     * @see SecurityManager#checkPermission
     * @see RuntimePermission
     * @see Throwable#getStackTrace
     */
    public static Map<Thread, StackTraceElement[]> getAllStackTraces() {
        // check for getStackTrace permission
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkPermission(
                SecurityConstants.GET_STACK_TRACE_PERMISSION);
            security.checkPermission(
                SecurityConstants.MODIFY_THREADGROUP_PERMISSION);
        }

        // Get a snapshot of the list of all threads
        Thread[] threads = getThreads();
        StackTraceElement[][] traces = dumpThreads(threads);
        Map<Thread, StackTraceElement[]> m = new HashMap<>(threads.length);
        for (int i = 0; i < threads.length; i++) {
            StackTraceElement[] stackTrace = traces[i];
            if (stackTrace != null) {
                m.put(threads[i], stackTrace);
            }
            // else terminated so we don't put it in the map
        }
        return m;
    }


    private static final RuntimePermission SUBCLASS_IMPLEMENTATION_PERMISSION =
                    new RuntimePermission("enableContextClassLoaderOverride");

    /** cache of subclass security audit results */
    /* Replace with ConcurrentReferenceHashMap when/if it appears in a future
     * release */
    private static class Caches {
        /** cache of subclass security audit results */
        static final ConcurrentMap<WeakClassKey,Boolean> subclassAudits =
            new ConcurrentHashMap<>();

        /** queue for WeakReferences to audited subclasses */
        static final ReferenceQueue<Class<?>> subclassAuditsQueue =
            new ReferenceQueue<>();
    }

    /**
     * Verifies that this (possibly subclass) instance can be constructed
     * without violating security constraints: the subclass must not override
     * security-sensitive non-final methods, or else the
     * "enableContextClassLoaderOverride" RuntimePermission is checked.
     */
    private static boolean isCCLOverridden(Class<?> cl) {
        if (cl == Thread.class)
            return false;

        processQueue(Caches.subclassAuditsQueue, Caches.subclassAudits);
        WeakClassKey key = new WeakClassKey(cl, Caches.subclassAuditsQueue);
        Boolean result = Caches.subclassAudits.get(key);
        if (result == null) {
            result = Boolean.valueOf(auditSubclass(cl));
            Caches.subclassAudits.putIfAbsent(key, result);
        }

        return result.booleanValue();
    }

    /**
     * Performs reflective checks on given subclass to verify that it doesn't
     * override security-sensitive non-final methods.  Returns true if the
     * subclass overrides any of the methods, false otherwise.
     */
    private static boolean auditSubclass(final Class<?> subcl) {
        Boolean result = AccessController.doPrivileged(
            new PrivilegedAction<Boolean>() {
                public Boolean run() {
                    for (Class<?> cl = subcl;
                         cl != Thread.class;
                         cl = cl.getSuperclass())
                    {
                        try {
                            cl.getDeclaredMethod("getContextClassLoader", new Class<?>[0]);
                            return Boolean.TRUE;
                        } catch (NoSuchMethodException ex) {
                        }
                        try {
                            Class<?>[] params = {ClassLoader.class};
                            cl.getDeclaredMethod("setContextClassLoader", params);
                            return Boolean.TRUE;
                        } catch (NoSuchMethodException ex) {
                        }
                    }
                    return Boolean.FALSE;
                }
            }
        );
        return result.booleanValue();
    }

    private native static StackTraceElement[][] dumpThreads(Thread[] threads);

    /**
     * 获取当前JVM内所有线程
     *
     * @return
     */
    private native static Thread[] getThreads();

    /**
     * 返回该线程的 ID。线程 ID 是在创建该线程时产生正长数。线程 ID 是唯一的，它的生命周期内保持不变。当一个线程被终止时，该线程ID可以重复使用
     *
     * @return 该线程的 ID
     */
    public long getId() {
        return tid;
    }

    /**
     * 线程状态
     *
     * NEW 尚未启动的线程处于这种状态
     * RUNNABLE 在 JVM 上执行的线程处于这种状态
     * BLOCKED 被阻止等待监视器锁的线程处于这种状态
     * WAITING 即无限期地等待另一个线程来执行某一特定操作的线程处于这种状态
     * TIMED_WAITING 正在等待另一个线程来达到一个指定的等待时间执行动作的线程处于这种状态
     * TERMINATED 已退出的线程处于这种状态
     * 一个线程可以在给定时间点只能处于一种状态。这些状态是 JVM 的状态并没有反映任何操作系统线程状态
     *
     * @see #getState
     */
    public enum State {
        /**
         * 线程还没有启动
         */
        NEW,

        /**
         * 运行中的线程
         */
        RUNNABLE,

        /**
         * 阻塞的，可能是在等待进入同步块/方法时被阻塞的
         * WAITING 不同在于， BLOCKED 是还没有进入同步块/方法时被阻塞，WAITING 是已经进去到获取同步块的过程中了，但却获取不到锁
         */
        BLOCKED,

        /**
         * 等待，遇到 Object#wait()、Thread.join、LockSupport#park() 这些方法时，线程就会等待
         * 等待另外一个线程执行特定的操作
         * 一个线程 Object.wait() 后，需要等待另外一个线程执行同一个 Object 的 notify()
         * 或者线程执行 thread1.join()，等待 thread1 来打断
         */
        WAITING,

        /**
         * 等待一定的时间
         */
        TIMED_WAITING,

        /**
         * 终止线程
         */
        TERMINATED;
    }

    /**
     * 返回该线程的状态。此方法被设计以用于监测的系统状态，而不是同步控制
     *
     * @return 该线程的状态
     */
    public State getState() {
        // 获取当前线程状态
        return sun.misc.VM.toThreadState(threadStatus);
    }

    // Added in JSR-166

    /**
     * 未捕获异常处理接口
     *
     * @see #setDefaultUncaughtExceptionHandler
     * @see #setUncaughtExceptionHandler
     * @see ThreadGroup#uncaughtException
     * @since 1.5
     */
    @FunctionalInterface
    public interface UncaughtExceptionHandler {
        /**
         * JVM检测到未捕获异常时的回调方法
         *
         * @param t 线程
         * @param e 异常
         */
        void uncaughtException(Thread t, Throwable e);
    }

    // null unless explicitly set
    private volatile UncaughtExceptionHandler uncaughtExceptionHandler;

    // null unless explicitly set
    private static volatile UncaughtExceptionHandler defaultUncaughtExceptionHandler;

    /**
     * Set the default handler invoked when a thread abruptly terminates
     * due to an uncaught exception, and no other handler has been defined
     * for that thread.
     *
     * <p>Uncaught exception handling is controlled first by the thread, then
     * by the thread's {@link ThreadGroup} object and finally by the default
     * uncaught exception handler. If the thread does not have an explicit
     * uncaught exception handler set, and the thread's thread group
     * (including parent thread groups)  does not specialize its
     * <tt>uncaughtException</tt> method, then the default handler's
     * <tt>uncaughtException</tt> method will be invoked.
     * <p>By setting the default uncaught exception handler, an application
     * can change the way in which uncaught exceptions are handled (such as
     * logging to a specific device, or file) for those threads that would
     * already accept whatever &quot;default&quot; behavior the system
     * provided.
     *
     * <p>Note that the default uncaught exception handler should not usually
     * defer to the thread's <tt>ThreadGroup</tt> object, as that could cause
     * infinite recursion.
     *
     * @param eh the object to use as the default uncaught exception handler.
     * If <tt>null</tt> then there is no default handler.
     *
     * @throws SecurityException if a security manager is present and it
     *         denies <tt>{@link RuntimePermission}
     *         (&quot;setDefaultUncaughtExceptionHandler&quot;)</tt>
     *
     * @see #setUncaughtExceptionHandler
     * @see #getUncaughtExceptionHandler
     * @see ThreadGroup#uncaughtException
     * @since 1.5
     */
    public static void setDefaultUncaughtExceptionHandler(UncaughtExceptionHandler eh) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(
                new RuntimePermission("setDefaultUncaughtExceptionHandler")
                    );
        }

         defaultUncaughtExceptionHandler = eh;
     }

    /**
     * Returns the default handler invoked when a thread abruptly terminates
     * due to an uncaught exception. If the returned value is <tt>null</tt>,
     * there is no default.
     * @since 1.5
     * @see #setDefaultUncaughtExceptionHandler
     * @return the default uncaught exception handler for all threads
     */
    public static UncaughtExceptionHandler getDefaultUncaughtExceptionHandler(){
        return defaultUncaughtExceptionHandler;
    }

    /**
     * Returns the handler invoked when this thread abruptly terminates
     * due to an uncaught exception. If this thread has not had an
     * uncaught exception handler explicitly set then this thread's
     * <tt>ThreadGroup</tt> object is returned, unless this thread
     * has terminated, in which case <tt>null</tt> is returned.
     * @since 1.5
     * @return the uncaught exception handler for this thread
     */
    public UncaughtExceptionHandler getUncaughtExceptionHandler() {
        return uncaughtExceptionHandler != null ?
            uncaughtExceptionHandler : group;
    }

    /**
     * Set the handler invoked when this thread abruptly terminates
     * due to an uncaught exception.
     * <p>A thread can take full control of how it responds to uncaught
     * exceptions by having its uncaught exception handler explicitly set.
     * If no such handler is set then the thread's <tt>ThreadGroup</tt>
     * object acts as its handler.
     * @param eh the object to use as this thread's uncaught exception
     * handler. If <tt>null</tt> then this thread has no explicit handler.
     * @throws  SecurityException  if the current thread is not allowed to
     *          modify this thread.
     * @see #setDefaultUncaughtExceptionHandler
     * @see ThreadGroup#uncaughtException
     * @since 1.5
     */
    public void setUncaughtExceptionHandler(UncaughtExceptionHandler eh) {
        checkAccess();
        uncaughtExceptionHandler = eh;
    }

    /**
     * 当前线程内出现未捕获异常时，JVM 会调用此方法
     */
    private void dispatchUncaughtException(Throwable e) {
        getUncaughtExceptionHandler().uncaughtException(this, e);
    }

    /**
     * Removes from the specified map any keys that have been enqueued
     * on the specified reference queue.
     */
    static void processQueue(ReferenceQueue<Class<?>> queue,
                             ConcurrentMap<? extends
                             WeakReference<Class<?>>, ?> map)
    {
        Reference<? extends Class<?>> ref;
        while((ref = queue.poll()) != null) {
            map.remove(ref);
        }
    }

    /**
     *  Weak key for Class objects.
     **/
    static class WeakClassKey extends WeakReference<Class<?>> {
        /**
         * saved value of the referent's identity hash code, to maintain
         * a consistent hash code after the referent has been cleared
         */
        private final int hash;

        /**
         * Create a new WeakClassKey to the given object, registered
         * with a queue.
         */
        WeakClassKey(Class<?> cl, ReferenceQueue<Class<?>> refQueue) {
            super(cl, refQueue);
            hash = System.identityHashCode(cl);
        }

        /**
         * Returns the identity hash code of the original referent.
         */
        @Override
        public int hashCode() {
            return hash;
        }

        /**
         * Returns true if the given object is this identical
         * WeakClassKey instance, or, if this object's referent has not
         * been cleared, if the given object is another WeakClassKey
         * instance with the identical non-null referent as this one.
         */
        @Override
        public boolean equals(Object obj) {
            if (obj == this)
                return true;

            if (obj instanceof WeakClassKey) {
                Object referent = get();
                return (referent != null) &&
                       (referent == ((WeakClassKey) obj).get());
            } else {
                return false;
            }
        }
    }


    // The following three initially uninitialized fields are exclusively
    // managed by class java.util.concurrent.ThreadLocalRandom. These
    // fields are used to build the high-performance PRNGs in the
    // concurrent code, and we can not risk accidental false sharing.
    // Hence, the fields are isolated with @Contended.

    /** The current seed for a ThreadLocalRandom */
    @sun.misc.Contended("tlr")
    long threadLocalRandomSeed;

    /** Probe hash value; nonzero if threadLocalRandomSeed initialized */
    @sun.misc.Contended("tlr")
    int threadLocalRandomProbe;

    /** Secondary seed isolated from public ThreadLocalRandom sequence */
    @sun.misc.Contended("tlr")
    int threadLocalRandomSecondarySeed;

    /* Some private helper methods */
    private native void setPriority0(int newPriority);
    private native void stop0(Object o);
    private native void suspend0();
    private native void resume0();
    private native void interrupt0();
    private native void setNativeName(String name);
}