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Java线程之活跃性危险

1. 活跃性危险

在安全性和活跃性之间存在某种制衡，我们使用加锁机制来确保线程安全，但是过度使用加锁，或使用不当，可能导致活跃性故障，最严重的情况下导致死锁。

1.1 死锁

在线程A持有锁L的同时，又想继续持有锁R；而线程B持有锁R，又想继续持有锁L。这两个线程将永远等待下去。

第一种解决办法是，让多组状态变量合并，从而变成只需要一把锁，这样多线程之间就不会发生死锁了。第二种办法是，如果合并不了，那就要控制锁的顺序，比如让所有线程都先拿锁L，再拿锁R。第三种办法是，尽量避免在一个加了锁的代码块里面再去获取别的锁。

1.2 饥饿

如果对线程的优先级设置不当，或者在持有锁的线程无限循环或无限等待某个资源，就会导致有的线程永远得不到锁，或很少机会得到锁，这就是饥饿。

应该尽量不要使用线程优先级，因为每个平台实现不一样，会产生不可预测的结果。

1.3 长时间不响应

如果某些计算密集型的线程一直在运行，就会导致其它线程很难得到CPU的时钟周期，导致其它线程响应慢。比如前台线程把任务交给后台线程后，后台线程执行计算密集型任务，会影响前台线程给客户响应的速度。

这种情况可以适当给计算密集型任务添加一些休眠时间，让其他线程有机会得到CPU的时钟周期。

1.4 活锁

有一种活锁是指线程不停重复执行相同操作，而且总是失败，屡败屡战。比如，当一个线程从一个任务队列里面取一个任务执行，失败后，把任务重新放回队列，稍后又执行，如此往复……这种活锁通常是由过度的错误恢复代码造成的，将不可修复的错误作为可修复的错误。

还有一种活锁出现在多个相互协作的线程场景下，假设有两个过于礼貌的人在路上面对面相遇，两个人先同时向右让，然后又同时向左让，如此反复……

要避免活锁问题，通常需要在重试机制中引入随机性，比如不要固定时间重试。

2. 监控线程

既然锁容易导致活跃性危险，就应该能够通过技术手段诊断出这些危险。下面介绍通过jstack来跟踪诊断问题。

2.1 死锁的示例代码

首先通过一段死锁代码开始：

public class DeadLock {
    public static void main(String[] args) {
        LeftThread lt = new LeftThread();
        RightThread rt = new RightThread();

        lt.setRightThread(rt);
        rt.setLeftThread(lt);


        new Thread(lt).start();
        new Thread(rt).start();

    }
}


class LeftThread implements Runnable {

    private RightThread rightThread;

    public void setRightThread(RightThread rightThread) {
        this.rightThread = rightThread;
    }

    @Override
    public void run() {
        this.letf();
    }

    public synchronized void letf() {
        System.out.println(Thread.currentThread().getId()+":I get left");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        rightThread.right();
    }

}

class RightThread implements Runnable {

    private LeftThread leftThread;

    public void setLeftThread(LeftThread leftThread) {
        this.leftThread = leftThread;
    }

    @Override
    public void run() {
        this.right();

    }

    public synchronized void right() {
        System.out.println(Thread.currentThread().getId()+":I get right");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        leftThread.letf();
    }

}

2.2 找出Java代码的进程PID

Windows系统可以通过资源管理器得到，Linux系统可以通过ps命令。

2.3 使用jstack命令查看进程内的所有线程状态

$ jstack pid

C:\Users\lenovo>jstack 4804
2018-06-10 10:15:19
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.51-b03 mixed mode):

"DestroyJavaVM" #12 prio=5 os_prio=0 tid=0x0000000003184800 nid=0x1354 waiting o
n condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

"Thread-1" #11 prio=5 os_prio=0 tid=0x000000001908d800 nid=0x107c waiting for mo
nitor entry [0x0000000019c6f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at LeftThread.letf(DeadLock.java:32)
- waiting to lock <0x00000000d6f4a378> (a LeftThread)
at RightThread.right(DeadLock.java:64)
- locked <0x00000000d6f4bba0> (a RightThread)
at RightThread.run(DeadLock.java:53)
at java.lang.Thread.run(Thread.java:745)

"Thread-0" #10 prio=5 os_prio=0 tid=0x000000001908b000 nid=0x4b0 waiting for mon
itor entry [0x0000000019b6f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at RightThread.right(DeadLock.java:58)
- waiting to lock <0x00000000d6f4bba0> (a RightThread)
at LeftThread.letf(DeadLock.java:38)
- locked <0x00000000d6f4a378> (a LeftThread)
at LeftThread.run(DeadLock.java:28)
at java.lang.Thread.run(Thread.java:745)

"Service Thread" #9 daemon prio=9 os_prio=0 tid=0x0000000018ff0800 nid=0x15c run
nable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

"C1 CompilerThread2" #8 daemon prio=9 os_prio=2 tid=0x0000000017c48000 nid=0x13c
0 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #7 daemon prio=9 os_prio=2 tid=0x0000000017c45000 nid=0xaf4
waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #6 daemon prio=9 os_prio=2 tid=0x0000000017c3f800 nid=0x129
8 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x0000000018fb3800 nid=0x190 wa
iting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x0000000017c33000 nid=0xf60
runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x000000000327a800 nid=0x131c in Obje
ct.wait() [0x0000000018f6f000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000d6c06f58> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
- locked <0x00000000d6c06f58> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x0000000017be9800 nid=0xc1c
in Object.wait() [0x0000000018e6f000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000d6c06998> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:157)
- locked <0x00000000d6c06998> (a java.lang.ref.Reference$Lock)

"VM Thread" os_prio=2 tid=0x0000000017be7000 nid=0xb74 runnable

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x000000000319b000 nid=0xcb0 runna
ble

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x000000000319c800 nid=0x12f4 runn
able

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x000000000319e000 nid=0x480 runna
ble

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00000000031a0000 nid=0xda8 runna
ble

"VM Periodic Task Thread" os_prio=2 tid=0x0000000018ff1800 nid=0x874 waiting on
condition

JNI global references: 6

Found one Java-level deadlock:

"Thread-1":
waiting to lock monitor 0x0000000003277858 (object 0x00000000d6f4a378, a LeftT
hread),
which is held by "Thread-0"
"Thread-0":
waiting to lock monitor 0x000000000327a198 (object 0x00000000d6f4bba0, a Right
Thread),
which is held by "Thread-1"

Java stack information for the threads listed above:

"Thread-1":
at LeftThread.letf(DeadLock.java:32)
- waiting to lock <0x00000000d6f4a378> (a LeftThread)
at RightThread.right(DeadLock.java:64)
- locked <0x00000000d6f4bba0> (a RightThread)
at RightThread.run(DeadLock.java:53)
at java.lang.Thread.run(Thread.java:745)
"Thread-0":
at RightThread.right(DeadLock.java:58)
- waiting to lock <0x00000000d6f4bba0> (a RightThread)
at LeftThread.letf(DeadLock.java:38)
- locked <0x00000000d6f4a378> (a LeftThread)
at LeftThread.run(DeadLock.java:28)
at java.lang.Thread.run(Thread.java:745)

Found 1 deadlock.

分析结果是一个线程运行的快照，把执行jstack命令的那一刻的线程状态全部记录下来，可以看到每个线程的状态，可以看到被阻塞的线程持有的锁，希望获取的锁……等信息。如果有死锁，还会直接给予提示。

原文链接：https://blog.csdn.net/weixin_34160277/article/details/92469797

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