ArrayComparisonFailure serialization incompatibility fix:

Override getCause() to allow fallback to the deprecated fCause field.
Run tests around possible forward incompatibility of the class from r4.11, 4.12.

src/main/java/org/junit/internal/ArrayComparisonFailure.java

@@ -42,6 +42,11 @@ public void addDimension(int index) {
         fIndices.add(0, index);
     }
 
+    @Override
+    public synchronized Throwable getCause() {
+        return super.getCause() == null ? fCause : super.getCause();
+    }
+
     @Override
     public String getMessage() {
         StringBuilder sb = new StringBuilder();

src/test/java/org/junit/internal/AllInternalTests.java

@@ -16,7 +16,8 @@
         FailOnTimeoutTest.class,
         MethodSorterTest.class,
         StacktracePrintingMatcherTest.class,
-        ThrowableCauseMatcherTest.class
+        ThrowableCauseMatcherTest.class,
+        ArrayComparisonFailureTest.class
 })
 public class AllInternalTests {
 }

src/test/java/org/junit/internal/ArrayComparisonFailureTest.java

@@ -0,0 +1,68 @@
+package org.junit.internal;
+
+import org.junit.Test;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.ObjectInputStream;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.fail;
+
+public class ArrayComparisonFailureTest {
+
+    private static final String ARRAY_COMPARISON_FAILURE_411 = "arrayComparisonFailure_411";
+    private static final String ARRAY_COMPARISON_FAILURE_412 = "arrayComparisonFailure_412";
+
+    /*
+        Test compatibility of older versions of ArrayComparisonFailure
+        Setup:
+        - checkout prior versions of the codebase (r4.11, r4.12 in this case)
+        - catch the exception resulting from:
+            assertArrayEquals(new int[]{0, 1}, new int[]{0, 5});
+        - serialize the resulting exception to a file, moving into the test/resources path
+            Ex., for v4.11's resulting exception {@link org/junit/internal/arrayComparisonFailure_411}
+        Current unit test:
+        - deserialize the above files casting it to the current version of the class
+            (catches any forward incompatibility with missing fields)
+        - assert the results from existing methods: getCause(), toString() -> getMessage()
+            (catches incompatible usages of fields)
+
+        This does not test if an instance of the current version of the class is able to deserialize to a previous ver.
+     */
+
+    @Test
+    public void classShouldAccept411Version() throws Exception {
+        assertFailureSerializableFromOthers(ARRAY_COMPARISON_FAILURE_411);
+    }
+
+    @Test
+    public void classShouldAccept412Version() throws Exception {
+        assertFailureSerializableFromOthers(ARRAY_COMPARISON_FAILURE_412);
+    }
+
+    private void assertFailureSerializableFromOthers(String failureFileName) throws IOException,
+            ClassNotFoundException {
+        try {
+            assertArrayEquals(new int[]{0, 1}, new int[]{0, 5});
+            fail();
+        } catch (ArrayComparisonFailure e) {
+            ArrayComparisonFailure arrayComparisonFailureFromFile = deserializeFailureFromFile(failureFileName);
+            assertNotNull("ArrayComparisonFailure.getCause() should fallback to the deprecated fCause field"
+                            + " for compatibility with older versions of junit4 that didn't use Throwable.initCause().",
+                    arrayComparisonFailureFromFile.getCause());
+            assertEquals(e.getCause().toString(), arrayComparisonFailureFromFile.getCause().toString());
+            assertEquals(e.toString(), arrayComparisonFailureFromFile.toString());
+        }
+    }
+
+    private ArrayComparisonFailure deserializeFailureFromFile(String fileName) throws IOException,
+            ClassNotFoundException {
+        InputStream resource = getClass().getResourceAsStream(fileName);
+        ObjectInputStream objectInputStream = new ObjectInputStream(resource);
+        return (ArrayComparisonFailure) objectInputStream.readObject();
+    }
+
+}