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Java 多线程中的死锁概述

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；

  3. image-20220306170329091

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:developmentjdk1.8jdkin>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:developmentjdk1.8jdkin>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；
hmoban主题是根据ripro二开的主题，极致后台体验，无插件，集成会员系统
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