死锁

死锁的定义

死锁的影响

• 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

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. 
   
2. 
   
3. 
   

方式二（通过 Java 提供的程序获取）

1. 
   
2. 
   
3. 
   
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

②检测与恢复策略

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；

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