死锁分析 | Knight Dawn

极客时间链接
Q: 什么情况下 Java 程序会产生死锁？如何定位、修复？

死锁

多线程环境下，多个线程之间，相互持有对方所需的锁，永久处于等待中

写一个死锁例子

public class DeadLockDemo extends Thread {
    private Object first;
    private Object second;
    public DeadLockDemo(String name, Object first, Object second) {
        super(name);
        this.first = first;
        this.second = second;
    }
    @Override
    public void run() {
        synchronized (first) {
            System.out.println(getName() + " get lock " + first.toString());
            synchronized (second) {
                System.out.println(getName() + " get lock " + second.toString());
            }
        }
    }
    public static void main(String[] args) throws InterruptedException {
        String a = "lockA";
        String b = "lockB";
        Thread thread1 = new DeadLockDemo("thread---a", a, b);
        Thread thread2 = new DeadLockDemo("thread---b", b, a);
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
    }
}
// 
output:
thread---b get lock lockB
thread---a get lock lockA

定位死锁

jstack 分析线程堆栈

定位pid

1
2
3

ps -ef|grep DeadLockDemo
501 34056   288   0 12:12下午 ??         0:00.22 .../target/classes net.sudo.deadlock.DeadLockDemo
501 34062 32674   0 12:12下午 ttys008    0:00.00 grep --color=auto --exclude-dir=.bzr --exclude-dir=CVS --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn DeadLock

jstack命令

cd ${JAVA_HOME}/bin
jstack 34056

Found one Java-level deadlock:
=============================
"thread---b":
  waiting to lock monitor 0x00007fd7e5080608 (object 0x000000076ac7b178, a java.lang.String),
  which is held by "thread---a"
"thread---a":
  waiting to lock monitor 0x00007fd7e60079f8 (object 0x000000076ac7b1b0, a java.lang.String),
  which is held by "thread---b"

ThreadMXBean

新起一个线程定时扫描检查

ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
ThreadMXBean mbean = ManagementFactory.getThreadMXBean();
scheduler.scheduleAtFixedRate(() -> {
    long[] threadIds = mbean.findDeadlockedThreads();
    if (threadIds != null) {
        ThreadInfo[] threadInfos = mbean.getThreadInfo(threadIds);
        System.out.println("Detected deadlock threads:");
        for (ThreadInfo threadInfo : threadInfos) {
            System.out.println("ThreadName: " + threadInfo.getThreadName());
            System.out.println("ThreadState: " + threadInfo.getThreadState());
        }
    }
}, 5L, 10L, TimeUnit.SECONDS);

output:
thread---a get lock lockA
thread---b get lock lockB
Detected deadlock threads:
ThreadName: thread---b
ThreadState: BLOCKED
ThreadName: thread---a
ThreadState: BLOCKED

ThreadMXBean扩展，打印所有线程

public class ThreadMxBeanDemo {
    public static void main(String[] args) {
        System.out.println("hellWorld");
        ThreadMXBean mbean = ManagementFactory.getThreadMXBean();
        long[] threadIds = mbean.getAllThreadIds();
        if (threadIds != null) {
            ThreadInfo[] threadInfos = mbean.getThreadInfo(threadIds);
            for (ThreadInfo threadInfo : threadInfos) {
                System.out.println("ThreadName: " + threadInfo.getThreadName());
                System.out.println("ThreadState: " + threadInfo.getThreadState());
            }
        }
    }
}
output:
hellWorld
ThreadName: Monitor Ctrl-Break
ThreadState: RUNNABLE
ThreadName: Signal Dispatcher   -- jvm接收系统命令的线程
ThreadState: RUNNABLE
ThreadName: Finalizer    -- 处理Finalizer方法线程
ThreadState: WAITING
ThreadName: Reference Handler   --  处理引用对象的垃圾回收
ThreadState: WAITING
ThreadName: main       -- 主线程
ThreadState: RUNNABLE