併發編程之死鎖解析

時間 2019-12-05

標籤併發編程死鎖解析简体版

原文原文鏈接

前言

在 Java 的併發編程中，有一個問題須要特別注意，那就是死鎖，若是發生了死鎖，基本就是重啓，而重啓將會丟失運行中的數據。因此，瞭解死鎖的造成並排查死鎖到預防死鎖成了一個重要的問題。java

咱們瞭解任何一個事情的步驟是：what，how，why，why not。編程

1. 什麼是死鎖？

咱們仍是直接寫一段代碼來看看：windows

package hello;

public class DeadLock {

  public static void main(String[] args) {

    new Thread(() -> {
      try {
         new DeadLock().resource1();
      } catch (InterruptedException e) {
      }
    }
    ).start();

    new Thread(() -> {
      try {
         new DeadLock().resource2();
      } catch (InterruptedException e) {
      }
    }
    ).start();
  }

  void resource1() throws InterruptedException {

    synchronized ("resource1") {
      System.out.println("獲取資源1");
       // 等待 1 秒讓另外一個線程拿到鎖
      Thread.sleep(1000);
      resource2();
    }


  }

  void resource2() throws InterruptedException {
    synchronized ("resource2") {
      System.out.println("獲取資源2");
      // 等待 1 秒讓另外一個線程拿到鎖
      Thread.sleep(1000);
      resource1();
    }
  }
}

上面的代碼中，咱們啓用了兩個線程，分別搶佔2個資源，但這兩個資源又分別被不一樣的對象（字符串）鎖住了。當第一個線程調用 resource1 方法，進入同步塊，拿到鎖，並等待 1 秒鐘讓另外一個線程進入 resource2 同步塊，當第二個線程進入同步塊後，注意：此時，拿着 resourec1 鎖的線程企圖拿到 resource2 的鎖，但這個時候，拿着 resource2 的線程也想去拿 resource1 的鎖。因而就出現了互相僵持的狀況，誰也沒法拿到對方的鎖，整個系統就卡死了。併發

這種狀況就是死鎖。app

像咱們如今寫的代碼是本身故意造出來的死鎖，咱們可以發現，那若是是線上環境怎麼辦，假如咱們的系統卡死了，咱們怎麼知道究竟是哪一段代碼出現了問題，有沒有可能使死鎖的問題。也就是如何檢測死鎖。socket

2. 如何檢測死鎖？

因爲死鎖極難經過人工的方式查出來，所以JDK 提供了命令來檢測某個java進程中心線程的狀況，並排查有沒有死鎖。上面命令呢？ jps , 用來查看java 程序的進程號，固然在 Linux 中也能夠經過別的方式獲取， jstack 進程號命令則能夠答應對應進程的棧信息，並找到死鎖。spa

咱們就剛剛的程序，在 windows 上使用該命令。.net

C:\Users\stateis0>jps
11060
2084 Launcher
10712 RemoteMavenServer
18040 Jps
11820 DeadLock





C:\Users\stateis0>jstack 11820
2017-12-29 18:52:38
Full thread dump Java HotSpot(TM) Client VM (25.131-b11 mixed mode):

"DestroyJavaVM" #11 prio=5 os_prio=0 tid=0x051fe800 nid=0x1e0c waiting on condition [0x00000000]
   java.lang.Thread.State: RUNNABLE

"Thread-1" #10 prio=5 os_prio=0 tid=0x18777800 nid=0x5664 waiting for monitor entry [0x18e0f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at hello.DeadLock.resource1(DeadLock.java:31)
        - waiting to lock <0x07415a50> (a java.lang.String)
        at hello.DeadLock.resource2(DeadLock.java:43)
        - locked <0x0742bd18> (a java.lang.String)
        at hello.DeadLock.lambda$main$1(DeadLock.java:20)
        at hello.DeadLock$$Lambda$2/4983748.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)

"Thread-0" #9 prio=5 os_prio=0 tid=0x18776c00 nid=0x4dc4 waiting for monitor entry [0x18d7f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at hello.DeadLock.resource2(DeadLock.java:41)
        - waiting to lock <0x0742bd18> (a java.lang.String)
        at hello.DeadLock.resource1(DeadLock.java:33)
        - locked <0x07415a50> (a java.lang.String)
        at hello.DeadLock.lambda$main$0(DeadLock.java:11)
        at hello.DeadLock$$Lambda$1/5592464.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)

"Service Thread" #8 daemon prio=9 os_prio=0 tid=0x186e4c00 nid=0x172c runnable [0x00000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x186af000 nid=0x53f8 waiting on condition [0x00000000]
   java.lang.Thread.State: RUNNABLE

"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x1861e800 nid=0x3928 runnable [0x18b3f000]
   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 <0x07861da0> (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 <0x07861da0> (a java.io.InputStreamReader)
        at java.io.BufferedReader.readLine(BufferedReader.java:389)
        at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:64)

"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x179c0800 nid=0x40a0 waiting on condition [0x00000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x17985c00 nid=0x5004 runnable [0x00000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x17972400 nid=0x41a8 in Object.wait() [0x17cff000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x0ca1b830> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
        - locked <0x0ca1b830> (a java.lang.ref.ReferenceQueue$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=0x17960000 nid=0x4ef0 in Object.wait() [0x17c6f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x0ca1b9d0> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:502)
        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
        - locked <0x0ca1b9d0> (a java.lang.ref.Reference$Lock)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"VM Thread" os_prio=2 tid=0x1795a800 nid=0x3f54 runnable

"VM Periodic Task Thread" os_prio=2 tid=0x18739400 nid=0x4a14 waiting on condition

JNI global references: 229

// 找到一個死鎖
Found one Java-level deadlock:
=============================
"Thread-1":
  waiting to lock monitor 0x17978de4 (object 0x07415a50, a java.lang.String),
  which is held by "Thread-0"
"Thread-0":
  waiting to lock monitor 0x1797a974 (object 0x0742bd18, a java.lang.String),
  which is held by "Thread-1"

Java stack information for the threads listed above:
===================================================
"Thread-1":
        at hello.DeadLock.resource1(DeadLock.java:31)
         // 等待 0x07415a50 鎖
        - waiting to lock <0x07415a50> (a java.lang.String)
        at hello.DeadLock.resource2(DeadLock.java:43)
        // 持有 0x0742bd18
        - locked <0x0742bd18> (a java.lang.String)
        at hello.DeadLock.lambda$main$1(DeadLock.java:20)
        at hello.DeadLock$$Lambda$2/4983748.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)
"Thread-0":
        at hello.DeadLock.resource2(DeadLock.java:41)
        // 等待 0x0742bd18 鎖
        - waiting to lock <0x0742bd18> (a java.lang.String)
        at hello.DeadLock.resource1(DeadLock.java:33)
        // 持有 0x07415a50
        - locked <0x07415a50> (a java.lang.String)
        at hello.DeadLock.lambda$main$0(DeadLock.java:11)
        at hello.DeadLock$$Lambda$1/5592464.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)

// 發現了一個死鎖
Found 1 deadlock.


C:\Users\stateis0>

Thread-1 waiting to lock <0x07415a50> locked <0x0742bd18>
Thread-0 waiting to lock <0x0742bd18> locked <0x07415a50>

咱們首先使用 jps 命令找到 java 進程號，而後使用 jstack 進程號 打印進程棧的信息，其中，在最後的部分，jstack 告訴咱們，他找到了一個死鎖，其中又詳細的信息：Thread-1 線程（這裏咱們沒有給線程其合適的名字，若是在線上，給線程起一個合適的名字將更有利於排查）持有 String 類型的編號爲 0x07415a50 的鎖，等待編號爲 0x07415a50 的鎖，但這個鎖由 Thread-0 持有，於此同時，Thread-0 和 Thread-1 相反。Thread-0 線程持有 0x07415a50 的鎖，等待 0x07415a50 的鎖。咱們的註釋裏也寫上了。線程

那麼發生了死鎖，該怎麼辦呢？最簡單的辦法就是重啓，重啓以後，對 jstack 中打印的堆棧信息中的代碼進行修改。從新發布。固然還有一些高級策略，好比讓進程回滾到死鎖前的狀態，而後讓他們順序進入同步塊。code