我如何在Java程序中以编程方式检测死锁的发生?
我如何在Java程序中以编程方式检测死锁的发生?
您可以使用随JDK一起提供的ThreadMXBean
来以编程方式实现此操作:
ThreadMXBean bean = ManagementFactory.getThreadMXBean();
long[] threadIds = bean.findDeadlockedThreads(); // Returns null if no threads are deadlocked.
if (threadIds != null) {
ThreadInfo[] infos = bean.getThreadInfo(threadIds);
for (ThreadInfo info : infos) {
StackTraceElement[] stack = info.getStackTrace();
// Log or store stack trace information.
}
}
显然,你应该尝试隔离执行死锁检查的线程,否则如果该线程发生死锁,它将无法运行检查!
顺便说一下,这就是 JConsole 在背后使用的方法。
一个有用的调查提示:
如果你能够在应用程序出现死锁的情况下及时捕捉到它,可以在java.exe控制台窗口上按下"Ctrl-Break"(或者在Solaris/Linux上按下"Ctrl-\"),JVM将会输出所有线程的当前状态和堆栈追踪信息,从而找出死锁并准确描述它们。
输出内容大致如下:
Full thread dump Java HotSpot(TM) Client VM (1.5.0_09-b03 mixed mode):
"[Test Timer] Request Queue" prio=6 tid=0x13d708d0 nid=0x1ec in Object.
wait() [0x1b00f000..0x1b00fb68]
at java.lang.Object.wait(Native Method)
at java.lang.Object.wait(Unknown Source)
at library.util.AsyncQueue.run(AsyncQueue.java:138)
- locked <0x02e70000> (a test.server.scheduler.SchedulerRequestQueue)
...
Found one Java-level deadlock:
=============================
"Corba service":
waiting to lock monitor 0x13c06684 (object 0x04697d90, a java.lang.Object),
which is held by "[Server Connection] Heartbeat Timer"
"[Server Connection] Heartbeat Timer":
waiting to lock monitor 0x13c065c4 (object 0x0467e728, a test.proxy.ServerProxy), which is held by "Corba service"
Java stack information for the threads listed above:
===================================================
"Corba service":
at test.proxy.ServerProxy.stopHBWatchDog(ServerProxy:695)
- waiting to lock <0x04697d90> (a java.lang.Object)
...
您可以使用ThreadMXBean类来编程地检测死锁线程。以下是代码,
ThreadMXBean bean = ManagementFactory.getThreadMXBean();
long ids[] = bean.findMonitorDeadlockedThreads();
if(ids != null)
{
ThreadInfo threadInfo[] = bean.getThreadInfo(ids);
for (ThreadInfo threadInfo1 : threadInfo)
{
System.out.println(threadInfo1.getThreadId()); //Prints the ID of deadlocked thread
System.out.println(threadInfo1.getThreadName()); //Prints the name of deadlocked thread
System.out.println(threadInfo1.getLockName()); //Prints the string representation of an object for which thread has entered into deadlock.
System.out.println(threadInfo1.getLockOwnerId()); //Prints the ID of thread which currently owns the object lock
System.out.println(threadInfo1.getLockOwnerName()); //Prints name of the thread which currently owns the object lock.
}
}
else
{
System.out.println("No Deadlocked Threads");
}
点击这里了解更多有关如何检测死锁线程的信息。
JArmus 是用于死锁检测和避免的库。它支持以下内容:Thread.join
、CyclicBarrier
、CountDownLatch
、Phaser
和ReentrantLock
。
要使用 JArmus,您需要对代码进行仪器化。可以通过其经过仪器化的类之一或使用 JArmus 仪器化程序jarmusc
自动进行。
java -jar jarmusc.jar yourprogram.jar checkedprogram.jar
输入yourprogram.jar
是您要检查的程序。
输出是具有检查以自动查找任何死锁的相同程序。
使用类CyclicBarrier
、CountDownLatch
、Phaser
验证死锁有点棘手---例如,JConsole无法检测到这些类型的死锁。JArmus 需要您的一点帮助:您必须指定哪些线程影响同步,我们将这些称为已注册线程。
尽快,线程必须标记自己已注册。标记已注册线程的好地方是在Runnable.run
方法的开始处。
JArmus.register(latch);
下面使程序发生死锁,可以被 JArmus 正确识别:
final CountDownLatch latch = new CountDownLatch(2);
final CyclicBarrier barrier = new CyclicBarrier(2);
final Queue<Exception> exceptions = new ArrayDeque<>();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
try {
JArmus.register(barrier); // do not forget to register!
JArmus.register(latch); // do not forget to register!
latch.countDown();
latch.await();
barrier.await();
} catch (Exception e) {
exceptions.add(e);
}
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
try {
JArmus.register(barrier); // do not forget to register!
JArmus.register(latch); // do not forget to register!
barrier.await();
latch.countDown();
latch.await();
} catch (Exception e) {
exceptions.add(e);
}
}
});
t1.start();
t2.start();
如果您不需要编程检测,可以通过 JConsole 完成;在线程选项卡中有一个“检测死锁”按钮。在JDK6中,此功能可检测内部监视器和 j.u.c
Lock
的锁。
通过使用$JAVA_HOME/bin/jconsole
命令启动 JConsole。
魔法发生在ThreadMonitor.findDeadlock()
中:
public boolean findDeadlock() {
long[] tids;
if (findDeadlocksMethodName.equals("findDeadlockedThreads")
&& tmbean.isSynchronizerUsageSupported()) {
tids = tmbean.findDeadlockedThreads();
if (tids == null) {
return false;
}
System.out.println("Deadlock found :-");
ThreadInfo[] infos = tmbean.getThreadInfo(tids, true, true);
for (ThreadInfo ti : infos) {
printThreadInfo(ti);
printLockInfo(ti.getLockedSynchronizers());
System.out.println();
}
} else {
tids = tmbean.findMonitorDeadlockedThreads();
if (tids == null) {
return false;
}
ThreadInfo[] infos = tmbean.getThreadInfo(tids, Integer.MAX_VALUE);
for (ThreadInfo ti : infos) {
// print thread information
printThreadInfo(ti);
}
}
return true;
}
这个调用ThreadMXBean
的API在Java 5和6中有不同的名称(因此有外部的if()
)。
代码示例还允许中断锁定,因此您甚至可以打破死锁。