Java 多线程中的死锁概述

死锁

死锁的定义

发生在并发中

当两个线程（或更多）线程（或线程）相互持有对方所需要的资源，又不主动释放，导致所有线程都无法继续执行，是程序陷入无尽的阻塞，这就是死锁。

如果多个线程之间的依赖关系是环形，存在环形的锁的依赖关系，那么也可能会发生死锁。

死锁的影响

死锁的影响在不同的系统中是不一样的，这取决于系统对死锁的处理能力。

• 数据库中：检测并放弃事务；
• JVM中：无法自动处理，但是提供了工具可以帮助我们取检测；

程序中的死锁

• 一旦发生，多是高并发场景，影响用户多；
• 整个系统崩溃，子系统崩溃，性能降低；
• 压力测试无法找出所有潜在的死锁；

例子

例一

public class MustDeadLock extends Thread {
    int flag = 1;
    static Object o1 = new Object();
    static Object o2 = new Object();

    public static void main(String[] args) {
        MustDeadLock run1 = new MustDeadLock();
        MustDeadLock run2 = new MustDeadLock();
        run1.flag = 1;
        run2.flag = 0;
        run1.start();
        run2.start();
    }

    @Override
    public void run() {
        System.out.println("flag = " + flag);
        if (flag == 1) {
            synchronized (o1) {
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (o2) {
                    System.out.println("线程1成功拿到两把锁!");
                }
            }
        }
        if (flag == 0) {
            synchronized (o2) {
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (o1) {
                    System.out.println("线程2成功拿到两把锁!");
                }
            }
        }
    }
}

案例分析

• 当类的对象 flag=1 时（T1），先锁定 O1，睡眠 500 毫秒，然后锁定 O2；
• T1 在睡眠的过程中，另一个 flag=0（T2）线程启动，先锁定 O2，睡眠 500 毫秒，等待 T1 释放 O1;
• T1 睡眠结束后需要锁定 O2 才能继续执行，而此时 O2 已被 T2 锁定；
• T2 睡眠结束后需要锁定 O1 才能继续执行，而此时 O1 已被 T1 锁定；
• 此时 T1，T2 相互等待，都需要对方锁定的资源才能继续执行，于是便发生死锁了。

例二（转账操作）

public class TransferMoney implements Runnable {

    int flag = 1;
    static Account a = new Account(500);
    static Account b = new Account(500);

    public static void main(String[] args) throws InterruptedException {
        TransferMoney r1 = new TransferMoney();
        TransferMoney r2 = new TransferMoney();
        r1.flag = 1;
        r2.flag = 0;
        Thread t1 = new Thread(r1);
        Thread t2 = new Thread(r2);
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println("a.balance = " + a.balance);
        System.out.println("b.balance = " + b.balance);
    }

    @Override
    public void run() {
        if (flag == 1) {
            transferMoney(a, b, 200);
        }
        if (flag == 0) {
            transferMoney(b, a, 200);
        }
    }

    private void transferMoney(Account from, Account to, int amount) {
        synchronized (from) {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (to) {
                if (from.balance - to.balance < 0) {
                    System.out.println("余额不足，转账失败！");
                }
                from.balance -= to.balance;
                to.balance += from.balance;
                System.out.println("转账成功，转账共：" + amount);
            }
        }
    }

    static class Account {
        int balance;

        public Account(int balance) {
            this.balance = balance;
        }
    }
}

死锁产生的必要条件

产生死锁必须同时满足以下四个条件，只要其中一条不成立，死锁就不会发生。

① 互斥条件

进程要求对所分配的资源（如打印机）进行排他性控制，即在一段时间内某资源仅为一个进程所占有。此时若有其他进程请求该资源，则请求进程只能等待。

② 请求与保持条件

进程已经保持了至少一个资源，但又提出了新的资源请求，而该资源已被其他进程占有，此时请求进程被阻塞，但对自己已获得的资源保持不放。

③ 不剥夺条件

进程已经保持了至少一个资源，但又提出了新的资源请求，而该资源已被其他进程占有，此时请求进程被阻塞，但对自己已获得的资源保持不放。

④ 循环等待条件

存在一种进程资源的循环等待链，链中每一个进程已获得的资源同时被链中下一个进程所请求。即存在一个处于等待状态的进程集合{Pl, P2, ..., pn}，其中Pi等 待的资源被P(i+1)占有（i=0, 1, ..., n-1)，Pn等待的资源被P0占有，如例一所示。

如何定位死锁?

jstack

这里以案例一为基础进行展示。

第一步：先运行列一；

第二步：找到 Java 在系统中的进程 id；

方式一（直接通过任务管理器获取）

1. 我是在 Windows 环境下进行演示，我可以先打开任务管理，然后再运行例一；

2. 找到对应的进行 id；

方式二（通过 Java 提供的程序获取）

1. 找到 Java JDK 的安装路径下的 bin 目录；

2. 在此目录下打开 CMD 窗口；

3. 然后直接运行 jps 后就会打印出我们正在运行的进程 id；

4. D:\development\jdk\1.8jdk\bin>jps
   12160 Jps
   45224 Launcher
   23020 MustDeadLock
   

第三步：通过工具定位到死锁

还是在 Java JDK 的安装路径下的 bin 目录下打开 cmd 窗口，然后运行 jstack + 进程id

D:\development\jdk\1.8jdk\bin>jstack 23020
2022-03-06 17:13:11
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.281-b09 mixed mode):

"DestroyJavaVM" #14 prio=5 os_prio=0 tid=0x00000204548cb000 nid=0x12220 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Thread-1" #13 prio=5 os_prio=0 tid=0x00000204719b6000 nid=0xbd44 waiting for monitor entry [0x0000000c528ff000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at main.threaddemo.MustDeadLock.run(MustDeadLock.java:45)
        - waiting to lock <0x000000076c09a3a0> (a java.lang.Object)
        - locked <0x000000076c09a3b0> (a java.lang.Object)

"Thread-0" #12 prio=5 os_prio=0 tid=0x00000204719b3000 nid=0xd260 waiting for monitor entry [0x0000000c527ff000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at main.threaddemo.MustDeadLock.run(MustDeadLock.java:33)
        - waiting to lock <0x000000076c09a3b0> (a java.lang.Object)
        - locked <0x000000076c09a3a0> (a java.lang.Object)

"Service Thread" #11 daemon prio=9 os_prio=0 tid=0x000002047198c800 nid=0x48f4 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread3" #10 daemon prio=9 os_prio=2 tid=0x00000204718eb800 nid=0xa35c waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x00000204718e6800 nid=0x9288 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x00000204718e4000 nid=0x7dcc waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x00000204718e1000 nid=0x101a4 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x00000204718de000 nid=0xb3b4 runnable [0x0000000c520fe000]
   java.lang.Thread.State: RUNNABLE
        at java.net.SocketInputStream.socketRead0(Native Method)
        at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
        at java.net.SocketInputStream.read(SocketInputStream.java:171)
        at java.net.SocketInputStream.read(SocketInputStream.java:141)
        at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
        at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
        at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
        - locked <0x000000076bf8f8a0> (a java.io.InputStreamReader)
        at java.io.InputStreamReader.read(InputStreamReader.java:184)
        at java.io.BufferedReader.fill(BufferedReader.java:161)
        at java.io.BufferedReader.readLine(BufferedReader.java:324)
        - locked <0x000000076bf8f8a0> (a java.io.InputStreamReader)
        at java.io.BufferedReader.readLine(BufferedReader.java:389)
        at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:47)

"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x0000020471893000 nid=0x109e0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x0000020471892800 nid=0x6798 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x000002046f4cb000 nid=0x5848 in Object.wait() [0x0000000c51dfe000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000076be08ee0> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
        - locked <0x000000076be08ee0> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:216)

"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x000002046f4c4800 nid=0xee60 in Object.wait() [0x0000000c51cff000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000076be06c00> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:502)
        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
        - locked <0x000000076be06c00> (a java.lang.ref.Reference$Lock)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"VM Thread" os_prio=2 tid=0x000002046f499000 nid=0x7284 runnable

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00000204548e1800 nid=0xf530 runnable

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00000204548e3000 nid=0x12aa0 runnable

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00000204548e4000 nid=0x11d58 runnable

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00000204548e5800 nid=0x67d8 runnable

"GC task thread#4 (ParallelGC)" os_prio=0 tid=0x00000204548e7800 nid=0xed20 runnable

"GC task thread#5 (ParallelGC)" os_prio=0 tid=0x00000204548e8800 nid=0x2e10 runnable

"GC task thread#6 (ParallelGC)" os_prio=0 tid=0x00000204548eb800 nid=0xd504 runnable

"GC task thread#7 (ParallelGC)" os_prio=0 tid=0x00000204548ec800 nid=0x10cc0 runnable

"GC task thread#8 (ParallelGC)" os_prio=0 tid=0x00000204548ed800 nid=0x8548 runnable

"GC task thread#9 (ParallelGC)" os_prio=0 tid=0x00000204548ee800 nid=0xac70 runnable

"VM Periodic Task Thread" os_prio=2 tid=0x00000204719b1000 nid=0x4b64 waiting on condition

JNI global references: 12

Found one Java-level deadlock:
=============================
"Thread-1":
  waiting to lock monitor 0x000002046f4c8368 (object 0x000000076c09a3a0, a java.lang.Object),
  which is held by "Thread-0"
"Thread-0":
  waiting to lock monitor 0x000002046f4caca8 (object 0x000000076c09a3b0, a java.lang.Object),
  which is held by "Thread-1"

Java stack information for the threads listed above:
===================================================
"Thread-1":
        at main.threaddemo.MustDeadLock.run(MustDeadLock.java:45)
        - waiting to lock <0x000000076c09a3a0> (a java.lang.Object)
        - locked <0x000000076c09a3b0> (a java.lang.Object)
"Thread-0":
        at main.threaddemo.MustDeadLock.run(MustDeadLock.java:33)
        - waiting to lock <0x000000076c09a3b0> (a java.lang.Object)
        - locked <0x000000076c09a3a0> (a java.lang.Object)

Found 1 deadlock.

运行后主要观察

Java stack information for the threads listed above:
===================================================
"Thread-1":
        at main.threaddemo.MustDeadLock.run(MustDeadLock.java:45)
        - waiting to lock <0x000000076c09a3a0> (a java.lang.Object)
        - locked <0x000000076c09a3b0> (a java.lang.Object)
"Thread-0":
        at main.threaddemo.MustDeadLock.run(MustDeadLock.java:33)
        - waiting to lock <0x000000076c09a3b0> (a java.lang.Object)
        - locked <0x000000076c09a3a0> (a java.lang.Object)

Found 1 deadlock.

通过 ThreadMXBean 工具类去检测死锁

代码

public class ThreadMXBeanDetection implements Runnable {

    int flag = 1;

    static Object lock1 = new Object();
    static Object lock2 = new Object();

    public static void main(String[] args) throws InterruptedException {
        ThreadMXBeanDetection r1 = new ThreadMXBeanDetection();
        ThreadMXBeanDetection r2 = new ThreadMXBeanDetection();
        r1.flag = 1;
        r2.flag = 0;
        Thread t1 = new Thread(r1);
        Thread t2 = new Thread(r2);
        t1.start();
        t2.start();
        Thread.sleep(3000);
        ThreadMXBean threadMXBean = ManagementFactory.getThreadMXBean();
        long[] deadlockedThreads = threadMXBean.findDeadlockedThreads();
        if (deadlockedThreads != null && deadlockedThreads.length > 0) {
            for (int i = 0; i < deadlockedThreads.length; i++) {
                ThreadInfo threadInfo = threadMXBean.getThreadInfo(deadlockedThreads[i]);
                System.out.println("发现死锁" + threadInfo.getThreadName());
            }
        }
    }

    @Override
    public void run() {
        System.out.println("flag = " + flag);
        if (flag == 1) {
            synchronized (lock1) {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (lock2) {
                    System.out.println("线程1成功拿到两把锁!");
                }
            }
        }
        if (flag == 0) {
            synchronized (lock2) {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (lock1) {
                    System.out.println("线程2成功拿到两把锁!");
                }
            }
        }
    }
}

输出

flag = 1
flag = 0
发现死锁Thread-1
发现死锁Thread-0

常见的修复方式

①避免策略

避免相反的获取锁的顺序，也就是在编写程序的时候就规划好锁的获取，从而破坏死锁产生的四个必要条件的其中一个。

②检测与恢复策略

检测到锁的时候再将其恢复，不过这个时候已经产生了一定的影响了。

实际开发中如何避免死锁

• 设置超时时间；
• 多使用并发类而不是自己设计锁；
• 尽量降低锁的使用粒度：用不同的锁而不是一个锁；
• 如果能使用同步代码块，就不使用同步方法：自已指定所对象；
• 创建线程的时候命名尽量达到见名知义，方便后面排查问题；