1 files changed, 153 insertions, 2 deletions
diff --git a/src/main/java/org/junit/internal/runners/statements/FailOnTimeout.java b/src/main/java/org/junit/internal/runners/statements/FailOnTimeout.java
index 9fad35b..9362cc1 100644
--- a/src/main/java/org/junit/internal/runners/statements/FailOnTimeout.java
+++ b/src/main/java/org/junit/internal/runners/statements/FailOnTimeout.java
@@ -1,5 +1,8 @@
 package org.junit.internal.runners.statements;
 
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
 import java.util.concurrent.Callable;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutionException;
@@ -7,6 +10,9 @@ import java.util.concurrent.FutureTask;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 
+import org.junit.internal.management.ManagementFactory;
+import org.junit.internal.management.ThreadMXBean;
+import org.junit.runners.model.MultipleFailureException;
 import org.junit.runners.model.Statement;
 import org.junit.runners.model.TestTimedOutException;
 
@@ -14,6 +20,7 @@ public class FailOnTimeout extends Statement {
     private final Statement originalStatement;
     private final TimeUnit timeUnit;
     private final long timeout;
+    private final boolean lookForStuckThread;
 
     /**
      * Returns a new builder for building an instance.
@@ -40,6 +47,7 @@ public class FailOnTimeout extends Statement {
         originalStatement = statement;
         timeout = builder.timeout;
         timeUnit = builder.unit;
+        lookForStuckThread = builder.lookForStuckThread;
     }
 
     /**
@@ -48,6 +56,7 @@ public class FailOnTimeout extends Statement {
      * @since 4.12
      */
     public static class Builder {
+        private boolean lookForStuckThread = false;
         private long timeout = 0;
         private TimeUnit unit = TimeUnit.SECONDS;
 
@@ -80,6 +89,20 @@ public class FailOnTimeout extends Statement {
         }
 
         /**
+         * Specifies whether to look for a stuck thread.  If a timeout occurs and this
+         * feature is enabled, the test will look for a thread that appears to be stuck
+         * and dump its backtrace.  This feature is experimental.  Behavior may change
+         * after the 4.12 release in response to feedback.
+         *
+         * @param enable {@code true} to enable the feature
+         * @return {@code this} for method chaining.
+         */
+        public Builder withLookingForStuckThread(boolean enable) {
+            this.lookForStuckThread = enable;
+            return this;
+        }
+
+        /**
          * Builds a {@link FailOnTimeout} instance using the values in this builder,
          * wrapping the given statement.
          *
@@ -97,7 +120,8 @@ public class FailOnTimeout extends Statement {
     public void evaluate() throws Throwable {
         CallableStatement callable = new CallableStatement();
         FutureTask<Throwable> task = new FutureTask<Throwable>(callable);
-        Thread thread = new Thread(task, "Time-limited test");
+        ThreadGroup threadGroup = threadGroupForNewThread();
+        Thread thread = new Thread(threadGroup, task, "Time-limited test");
         thread.setDaemon(true);
         thread.start();
         callable.awaitStarted();
@@ -107,6 +131,31 @@ public class FailOnTimeout extends Statement {
         }
     }
 
+    private ThreadGroup threadGroupForNewThread() {
+        if (!lookForStuckThread) {
+            // Use the default ThreadGroup (usually the one from the current
+            // thread).
+            return null;
+        }
+
+        // Create the thread in a new ThreadGroup, so if the time-limited thread
+        // becomes stuck, getStuckThread() can find the thread likely to be the
+        // culprit.
+        ThreadGroup threadGroup = new ThreadGroup("FailOnTimeoutGroup");
+        if (!threadGroup.isDaemon()) {
+            // Mark the new ThreadGroup as a daemon thread group, so it will be
+            // destroyed after the time-limited thread completes. By ensuring the
+            // ThreadGroup is destroyed, any data associated with the ThreadGroup
+            // (ex: via java.beans.ThreadGroupContext) is destroyed.
+            try {
+                threadGroup.setDaemon(true);
+            } catch (SecurityException e) {
+                // Swallow the exception to keep the same behavior as in JUnit 4.12.
+            }
+        }
+        return threadGroup;
+    }
+
     /**
      * Wait for the test task, returning the exception thrown by the test if the
      * test failed, an exception indicating a timeout if the test timed out, or
@@ -131,12 +180,114 @@ public class FailOnTimeout extends Statement {
 
     private Exception createTimeoutException(Thread thread) {
         StackTraceElement[] stackTrace = thread.getStackTrace();
+        final Thread stuckThread = lookForStuckThread ? getStuckThread(thread) : null;
         Exception currThreadException = new TestTimedOutException(timeout, timeUnit);
         if (stackTrace != null) {
             currThreadException.setStackTrace(stackTrace);
             thread.interrupt();
         }
-        return currThreadException;
+        if (stuckThread != null) {
+            Exception stuckThreadException = 
+                new Exception("Appears to be stuck in thread " +
+                               stuckThread.getName());
+            stuckThreadException.setStackTrace(getStackTrace(stuckThread));
+            return new MultipleFailureException(
+                Arrays.<Throwable>asList(currThreadException, stuckThreadException));
+        } else {
+            return currThreadException;
+        }
+    }
+
+    /**
+     * Retrieves the stack trace for a given thread.
+     * @param thread The thread whose stack is to be retrieved.
+     * @return The stack trace; returns a zero-length array if the thread has 
+     * terminated or the stack cannot be retrieved for some other reason.
+     */
+    private StackTraceElement[] getStackTrace(Thread thread) {
+        try {
+            return thread.getStackTrace();
+        } catch (SecurityException e) {
+            return new StackTraceElement[0];
+        }
+    }
+
+    /**
+     * Determines whether the test appears to be stuck in some thread other than
+     * the "main thread" (the one created to run the test).  This feature is experimental.
+     * Behavior may change after the 4.12 release in response to feedback.
+     * @param mainThread The main thread created by {@code evaluate()}
+     * @return The thread which appears to be causing the problem, if different from
+     * {@code mainThread}, or {@code null} if the main thread appears to be the
+     * problem or if the thread cannot be determined.  The return value is never equal 
+     * to {@code mainThread}.
+     */
+    private Thread getStuckThread(Thread mainThread) {
+        List<Thread> threadsInGroup = getThreadsInGroup(mainThread.getThreadGroup());
+        if (threadsInGroup.isEmpty()) {
+            return null;
+        }
+
+        // Now that we have all the threads in the test's thread group: Assume that
+        // any thread we're "stuck" in is RUNNABLE.  Look for all RUNNABLE threads. 
+        // If just one, we return that (unless it equals threadMain).  If there's more
+        // than one, pick the one that's using the most CPU time, if this feature is
+        // supported.
+        Thread stuckThread = null;
+        long maxCpuTime = 0;
+        for (Thread thread : threadsInGroup) {
+            if (thread.getState() == Thread.State.RUNNABLE) {
+                long threadCpuTime = cpuTime(thread);
+                if (stuckThread == null || threadCpuTime > maxCpuTime) {
+                    stuckThread = thread;
+                    maxCpuTime = threadCpuTime;
+                }
+            }               
+        }
+        return (stuckThread == mainThread) ? null : stuckThread;
+    }
+
+    /**
+     * Returns all active threads belonging to a thread group.  
+     * @param group The thread group.
+     * @return The active threads in the thread group.  The result should be a
+     * complete list of the active threads at some point in time.  Returns an empty list
+     * if this cannot be determined, e.g. because new threads are being created at an
+     * extremely fast rate.
+     */
+    private List<Thread> getThreadsInGroup(ThreadGroup group) {
+        final int activeThreadCount = group.activeCount(); // this is just an estimate
+        int threadArraySize = Math.max(activeThreadCount * 2, 100);
+        for (int loopCount = 0; loopCount < 5; loopCount++) {
+            Thread[] threads = new Thread[threadArraySize];
+            int enumCount = group.enumerate(threads);
+            if (enumCount < threadArraySize) {
+                return Arrays.asList(threads).subList(0, enumCount);
+            }
+            // if there are too many threads to fit into the array, enumerate's result
+            // is >= the array's length; therefore we can't trust that it returned all
+            // the threads.  Try again.
+            threadArraySize += 100;
+        }
+        // threads are proliferating too fast for us.  Bail before we get into 
+        // trouble.
+        return Collections.emptyList();
+    }
+
+    /**
+     * Returns the CPU time used by a thread, if possible.
+     * @param thr The thread to query.
+     * @return The CPU time used by {@code thr}, or 0 if it cannot be determined.
+     */
+    private long cpuTime(Thread thr) {
+        ThreadMXBean mxBean = ManagementFactory.getThreadMXBean();
+        if (mxBean.isThreadCpuTimeSupported()) {
+            try {
+                return mxBean.getThreadCpuTime(thr.getId());
+            } catch (UnsupportedOperationException e) {
+            }
+        }
+        return 0;
     }
 
     private class CallableStatement implements Callable<Throwable> {