Monitors the AWT event dispatch thread for events that take longer than a certain time to be dispatched
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* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*/
import java.awt.*;
import java.awt.event.*;
import java.lang.management.*;
import java.util.*;
import java.util.Timer;
import javax.swing.*;
/**
* Monitors the AWT event dispatch thread for events that take longer than
* a certain time to be dispatched.
* <p/>
* The principle is to record the time at which we start processing an event,
* and have another thread check frequently to see if we're still processing.
* If the other thread notices that we've been processing a single event for
* too long, it prints a stack trace showing what the event dispatch thread
* is doing, and continues to time it until it finally finishes.
* <p/>
* This is useful in determining what code is causing your Java application's
* GUI to be unresponsive.
*
* <p>The original blog can be found here<br>
* <a href="http://elliotth.blogspot.com/2005/05/automatically-detecting-awt-event.html">
* Automatically detecting AWT event dispatch thread hangs</a>
* </p>
*
* @author Elliott Hughes <enh@jessies.org>
*
* Advice, bug fixes, and test cases from
* Alexander Potochkin and Oleg Sukhodolsky.
*
* https://swinghelper.dev.java.net/
*/
public final class EventDispatchThreadHangMonitor extends EventQueue {
private static final EventDispatchThreadHangMonitor INSTANCE = new EventDispatchThreadHangMonitor();
// Time to wait between checks that the event dispatch thread isn't hung.
private static final long CHECK_INTERVAL_MS = 100;
// Maximum time we won't warn about. This used to be 500 ms, but 1.5 on
// late-2004 hardware isn't really up to it; there are too many parts of
// the JDK that can go away for that long (often code that has to be
// called on the event dispatch thread, like font loading).
private static final long UNREASONABLE_DISPATCH_DURATION_MS = 1000;
// Help distinguish multiple hangs in the log, and match start and end too.
// Only access this via getNewHangNumber.
private static int hangCount = 0;
// Prevents us complaining about hangs during start-up, which are probably
// the JVM vendor's fault.
private boolean haveShownSomeComponent = false;
// The currently outstanding event dispatches. The implementation of
// modal dialogs is a common cause for multiple outstanding dispatches.
private final LinkedList<DispatchInfo> dispatches = new LinkedList<DispatchInfo>();
private static class DispatchInfo {
// The last-dumped hung stack trace for this dispatch.
private StackTraceElement[] lastReportedStack;
// If so; what was the identifying hang number?
private int hangNumber;
// The EDT for this dispatch (for the purpose of getting stack traces).
// I don't know of any API for getting the event dispatch thread,
// but we can assume that it's the current thread if we're in the
// middle of dispatching an AWT event...
// We can't cache this because the EDT can die and be replaced by a
// new EDT if there's an uncaught exception.
private final Thread eventDispatchThread = Thread.currentThread();
// The last time in milliseconds at which we saw a dispatch on the above thread.
private long lastDispatchTimeMillis = System.currentTimeMillis();
public DispatchInfo() {
// All initialization is done by the field initializers.
}
public void checkForHang() {
if (timeSoFar() > UNREASONABLE_DISPATCH_DURATION_MS) {
examineHang();
}
}
// We can't use StackTraceElement.equals because that insists on checking the filename and line number.
// That would be version-specific.
private static boolean stackTraceElementIs(StackTraceElement e, String className, String methodName, boolean isNative) {
return e.getClassName().equals(className) && e.getMethodName().equals(methodName) && e.isNativeMethod() == isNative;
}
// Checks whether the given stack looks like it's waiting for another event.
// This relies on JDK implementation details.
private boolean isWaitingForNextEvent(StackTraceElement[] currentStack) {
return stackTraceElementIs(currentStack[0], "java.lang.Object", "wait", true) && stackTraceElementIs(currentStack[1], "java.lang.Object", "wait", false) && stackTraceElementIs(currentStack[2], "java.awt.EventQueue", "getNextEvent", false);
}
private void examineHang() {
StackTraceElement[] currentStack = eventDispatchThread.getStackTrace();
if (isWaitingForNextEvent(currentStack)) {
// Don't be fooled by a modal dialog if it's waiting for its next event.
// As long as the modal dialog's event pump doesn't get stuck, it's okay for the outer pump to be suspended.
return;
}
if (stacksEqual(lastReportedStack, currentStack)) {
// Don't keep reporting the same hang every time the timer goes off.
return;
}
hangNumber = getNewHangNumber();
String stackTrace = stackTraceToString(currentStack);
lastReportedStack = currentStack;
Log.warn("(hang #" + hangNumber + ") event dispatch thread stuck processing event for " + timeSoFar() + " ms:" + stackTrace);
checkForDeadlock();
}
private static boolean stacksEqual(StackTraceElement[] a, StackTraceElement[] b) {
if (a == null) {
return false;
}
if (a.length != b.length) {
return false;
}
for (int i = 0; i < a.length; ++i) {
if (a[i].equals(b[i]) == false) {
return false;
}
}
return true;
}
/**
* Returns how long this dispatch has been going on (in milliseconds).
*/
private long timeSoFar() {
return (System.currentTimeMillis() - lastDispatchTimeMillis);
}
public void dispose() {
if (lastReportedStack != null) {
Log.warn("(hang #" + hangNumber + ") event dispatch thread unstuck after " + timeSoFar() + " ms.");
}
}
}
private EventDispatchThreadHangMonitor() {
initTimer();
}
/**
* Sets up a timer to check for hangs frequently.
*/
private void initTimer() {
final long initialDelayMs = 0;
final boolean isDaemon = true;
Timer timer = new Timer("EventDispatchThreadHangMonitor", isDaemon);
timer.schedule(new HangChecker(), initialDelayMs, CHECK_INTERVAL_MS);
}
private class HangChecker extends TimerTask {
@Override
public void run() {
synchronized (dispatches) {
if (dispatches.isEmpty() || !haveShownSomeComponent) {
// Nothing to do.
// We don't destroy the timer when there's nothing happening
// because it would mean a lot more work on every single AWT
// event that gets dispatched.
return;
}
// Only the most recent dispatch can be hung; nested dispatches
// by their nature cause the outer dispatch pump to be suspended.
dispatches.getLast().checkForHang();
}
}
}
/**
* Sets up hang detection for the event dispatch thread.
*/
public static void initMonitoring() {
Toolkit.getDefaultToolkit().getSystemEventQueue().push(INSTANCE);
}
/**
* Overrides EventQueue.dispatchEvent to call our pre and post hooks either
* side of the system's event dispatch code.
*/
@Override
protected void dispatchEvent(AWTEvent event) {
try {
preDispatchEvent();
super.dispatchEvent(event);
} finally {
postDispatchEvent();
if (!haveShownSomeComponent &&
event instanceof WindowEvent && event.getID() == WindowEvent.WINDOW_OPENED) {
haveShownSomeComponent = true;
}
}
}
private void debug(String which) {
if (false) {
for (int i = dispatches.size(); i >= 0; --i) {
System.out.print(' ');
}
System.out.println(which);
}
}
/**
* Starts tracking a dispatch.
*/
private synchronized void preDispatchEvent() {
debug("pre");
synchronized (dispatches) {
dispatches.addLast(new DispatchInfo());
}
}
/**
* Stops tracking a dispatch.
*/
private synchronized void postDispatchEvent() {
synchronized (dispatches) {
// We've finished the most nested dispatch, and don't need it any longer.
DispatchInfo justFinishedDispatch = dispatches.removeLast();
justFinishedDispatch.dispose();
// The other dispatches, which have been waiting, need to be credited extra time.
// We do this rather simplistically by pretending they've just been redispatched.
Thread currentEventDispatchThread = Thread.currentThread();
for (DispatchInfo dispatchInfo : dispatches) {
if (dispatchInfo.eventDispatchThread == currentEventDispatchThread) {
dispatchInfo.lastDispatchTimeMillis = System.currentTimeMillis();
}
}
}
debug("post");
}
private static void checkForDeadlock() {
ThreadMXBean threadBean = ManagementFactory.getThreadMXBean();
long[] threadIds = threadBean.findMonitorDeadlockedThreads();
if (threadIds == null) {
return;
}
Log.warn("deadlock detected involving the following threads:");
ThreadInfo[] threadInfos = threadBean.getThreadInfo(threadIds, Integer.MAX_VALUE);
for (ThreadInfo info : threadInfos) {
Log.warn("Thread #" + info.getThreadId() + " " + info.getThreadName() +
" (" + info.getThreadState() + ") waiting on " + info.getLockName() +
" held by " + info.getLockOwnerName() + stackTraceToString(info.getStackTrace()));
}
}
private static String stackTraceToString(StackTraceElement[] stackTrace) {
StringBuilder result = new StringBuilder();
// We used to avoid showing any code above where this class gets
// involved in event dispatch, but that hides potentially useful
// information when dealing with modal dialogs. Maybe we should
// reinstate that, but search from the other end of the stack?
for (StackTraceElement stackTraceElement : stackTrace) {
String indentation = " ";
result.append("\n" + indentation + stackTraceElement);
}
return result.toString();
}
private synchronized static int getNewHangNumber() {
return ++hangCount;
}
public static void main(String[] args) {
initMonitoring();
//special case for deadlock test
if (args.length > 0 && "deadlock".equals(args[0])) {
EventDispatchThreadHangMonitor.INSTANCE.haveShownSomeComponent = true;
Tests.runDeadlockTest();
return;
}
Tests.main(args);
}
private static class Tests {
public static void main(final String[] args) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
for (String arg : args) {
final JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLocationRelativeTo(null);
if (arg.equals("exception")) {
runExceptionTest(frame);
} else if (arg.equals("focus")) {
runFocusTest(frame);
} else if (arg.equals("modal-hang")) {
runModalTest(frame, true);
} else if (arg.equals("modal-no-hang")) {
runModalTest(frame, false);
} else {
System.err.println("unknown regression test '" + arg + "'");
System.exit(1);
}
frame.pack();
frame.setVisible(true);
}
}
});
}
private static void runDeadlockTest() {
class Locker {
private Locker locker;
public void setLocker(Locker locker) {
this.locker = locker;
}
public synchronized void tryToDeadlock() {
locker.toString();
}
public synchronized String toString() {
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
return super.toString();
}
}
final Locker one = new Locker();
final Locker two = new Locker();
one.setLocker(two);
two.setLocker(one);
//Deadlock expected here:
for (int i = 0; i < 100; i++) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
one.tryToDeadlock();
}
});
two.tryToDeadlock();
}
}
// If we don't do our post-dispatch activity in a finally block, we'll
// report bogus hangs.
private static void runExceptionTest(final JFrame frame) {
JButton button = new JButton("Throw Exception");
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
// This shouldn't cause us to report a hang.
throw new RuntimeException("Nobody expects the Spanish Inquisition!");
}
});
frame.add(button);
}
// A demonstration of nested calls to dispatchEvent caused by SequencedEvent.
private static void runFocusTest(final JFrame frame) {
final JDialog dialog = new JDialog(frame, "Non-Modal Dialog");
dialog.add(new JLabel("Close me!"));
dialog.pack();
dialog.setLocationRelativeTo(frame);
dialog.addWindowFocusListener(new WindowAdapter() {
public void windowGainedFocus(WindowEvent e) {
System.out.println("FocusTest.windowGainedFocus");
// If you don't cope with nested calls to dispatchEvent, you won't detect this.
// See java.awt.SequencedEvent for an example.
sleep(2500);
}
});
JButton button = new JButton("Show Non-Modal Dialog");
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
dialog.setVisible(true);
}
});
frame.add(button);
}
// A demonstration of the problems of dealing with modal dialogs.
private static void runModalTest(final JFrame frame, final boolean shouldSleep) {
System.out.println(shouldSleep ? "Expect hangs!" : "There should be no hangs...");
JButton button = new JButton("Show Modal Dialog");
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
if (shouldSleep) {
sleep(2500); // This is easy.
}
JDialog dialog = new JDialog(frame, "Modal dialog", true);
dialog.setLayout(new FlowLayout());
dialog.add(new JLabel("Close this dialog!"));
final JLabel label = new JLabel(" ");
dialog.add(label);
dialog.pack();
dialog.setLocation(frame.getX() - 100, frame.getY());
// Make sure the new event pump has some work to do, each unit of which is insufficient to cause a hang.
new Thread(new Runnable() {
public void run() {
for (int i = 0; i <= 100000; ++i) {
final int value = i;
EventQueue.invokeLater(new Runnable() {
public void run() {
label.setText(Integer.toString(value));
}
});
}
}
}).start();
dialog.setVisible(true);
if (shouldSleep) {
sleep(2500); // If you don't distinguish different stack traces, you won't report this.
}
}
});
frame.add(button);
}
private static void sleep(long ms) {
try {
System.out.println("Sleeping for " + ms + " ms on " + Thread.currentThread() + "...");
Thread.sleep(ms);
System.out.println("Finished sleeping...");
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
private static class Log {
public static void warn(String str) {
System.out.println(str);
}
}
}
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