Java示例中的死鎖

教學
Java

在Java中，死锁是一种编程情况，其中两个或更多线程被永久地阻塞。Java死锁情况至少涉及两个线程和两个或多个资源。这里我编写了一个简单的程序，会导致Java死锁场景，然后我们将看到如何分析它。

Java中的死锁

让我们看一个简单的程序，我将在Java线程中创建死锁。

package com.journaldev.threads;

public class ThreadDeadlock {

    public static void main(String[] args) throws InterruptedException {
        Object obj1 = new Object();
        Object obj2 = new Object();
        Object obj3 = new Object();
    
        Thread t1 = new Thread(new SyncThread(obj1, obj2), "t1");
        Thread t2 = new Thread(new SyncThread(obj2, obj3), "t2");
        Thread t3 = new Thread(new SyncThread(obj3, obj1), "t3");
        
        t1.start();
        Thread.sleep(5000);
        t2.start();
        Thread.sleep(5000);
        t3.start();
        
    }

}

class SyncThread implements Runnable{
    private Object obj1;
    private Object obj2;

    public SyncThread(Object o1, Object o2){
        this.obj1=o1;
        this.obj2=o2;
    }
    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        System.out.println(name + " acquiring lock on "+obj1);
        synchronized (obj1) {
         System.out.println(name + " acquired lock on "+obj1);
         work();
         System.out.println(name + " acquiring lock on "+obj2);
         synchronized (obj2) {
            System.out.println(name + " acquired lock on "+obj2);
            work();
        }
         System.out.println(name + " released lock on "+obj2);
        }
        System.out.println(name + " released lock on "+obj1);
        System.out.println(name + " finished execution.");
    }
    private void work() {
        try {
            Thread.sleep(30000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

在上面的程序中，SyncThread实现了Runnable接口，并通过在每个对象上一个接一个地使用同步块来运行。在main方法中，我有三个线程运行SyncThread，并且每个线程之间有一个共享资源。这些线程以一种方式运行，以便能够在第一个对象上获取锁，但当它尝试在第二个对象上获取锁时，它会进入等待状态，因为它已被另一个线程锁定。这导致了线程之间资源的循环依赖，从而导致死锁。当我执行上述程序时，会生成以下输出，但程序永远不会终止，因为Java线程中存在死锁。

t1 acquiring lock on java.lang.Object@6d9dd520
t1 acquired lock on java.lang.Object@6d9dd520
t2 acquiring lock on java.lang.Object@22aed3a5
t2 acquired lock on java.lang.Object@22aed3a5
t3 acquiring lock on java.lang.Object@218c2661
t3 acquired lock on java.lang.Object@218c2661
t1 acquiring lock on java.lang.Object@22aed3a5
t2 acquiring lock on java.lang.Object@218c2661
t3 acquiring lock on java.lang.Object@6d9dd520

在这里，我们可以清楚地从输出中识别出死锁情况，但在实际应用程序中，要找到死锁情况并调试它们是非常困难的。

如何在Java中檢測死鎖

要在java中檢測死鎖，我們需要查看應用程序的java線程傾印，在上一篇文章中，我解釋了如何使用VisualVM分析器或使用jstack實用程序生成線程傾印。以下是上述程序的線程傾印。

2012-12-27 19:08:34
Full thread dump Java HotSpot(TM) 64-Bit Server VM (23.5-b02 mixed mode):

"Attach Listener" daemon prio=5 tid=0x00007fb0a2814000 nid=0x4007 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"DestroyJavaVM" prio=5 tid=0x00007fb0a2801000 nid=0x1703 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"t3" prio=5 tid=0x00007fb0a204b000 nid=0x4d07 waiting for monitor entry [0x000000015d971000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f658> (a java.lang.Object)
	- locked <0x000000013df2f678> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)

"t2" prio=5 tid=0x00007fb0a1073000 nid=0x4207 waiting for monitor entry [0x000000015d209000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f678> (a java.lang.Object)
	- locked <0x000000013df2f668> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)

"t1" prio=5 tid=0x00007fb0a1072000 nid=0x5503 waiting for monitor entry [0x000000015d86e000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f668> (a java.lang.Object)
	- locked <0x000000013df2f658> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)

"Service Thread" daemon prio=5 tid=0x00007fb0a1038000 nid=0x5303 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" daemon prio=5 tid=0x00007fb0a1037000 nid=0x5203 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" daemon prio=5 tid=0x00007fb0a1016000 nid=0x5103 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=5 tid=0x00007fb0a4003000 nid=0x5003 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=5 tid=0x00007fb0a4800000 nid=0x3f03 in Object.wait() [0x000000015d0c0000]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x000000013de75798> (a java.lang.ref.ReferenceQueue$Lock)
	at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
	- locked <0x000000013de75798> (a java.lang.ref.ReferenceQueue$Lock)
	at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
	at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:177)

"Reference Handler" daemon prio=5 tid=0x00007fb0a4002000 nid=0x3e03 in Object.wait() [0x000000015cfbd000]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x000000013de75320> (a java.lang.ref.Reference$Lock)
	at java.lang.Object.wait(Object.java:503)
	at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)
	- locked <0x000000013de75320> (a java.lang.ref.Reference$Lock)

"VM Thread" prio=5 tid=0x00007fb0a2049800 nid=0x3d03 runnable 

"GC task thread#0 (ParallelGC)" prio=5 tid=0x00007fb0a300d800 nid=0x3503 runnable 

"GC task thread#1 (ParallelGC)" prio=5 tid=0x00007fb0a2001800 nid=0x3603 runnable 

"GC task thread#2 (ParallelGC)" prio=5 tid=0x00007fb0a2003800 nid=0x3703 runnable 

"GC task thread#3 (ParallelGC)" prio=5 tid=0x00007fb0a2004000 nid=0x3803 runnable 

"GC task thread#4 (ParallelGC)" prio=5 tid=0x00007fb0a2005000 nid=0x3903 runnable 

"GC task thread#5 (ParallelGC)" prio=5 tid=0x00007fb0a2005800 nid=0x3a03 runnable 

"GC task thread#6 (ParallelGC)" prio=5 tid=0x00007fb0a2006000 nid=0x3b03 runnable 

"GC task thread#7 (ParallelGC)" prio=5 tid=0x00007fb0a2006800 nid=0x3c03 runnable 

"VM Periodic Task Thread" prio=5 tid=0x00007fb0a1015000 nid=0x5403 waiting on condition 

JNI global references: 114


Found one Java-level deadlock:
=============================
"t3":
  waiting to lock monitor 0x00007fb0a1074b08 (object 0x000000013df2f658, a java.lang.Object),
  which is held by "t1"
"t1":
  waiting to lock monitor 0x00007fb0a1010f08 (object 0x000000013df2f668, a java.lang.Object),
  which is held by "t2"
"t2":
  waiting to lock monitor 0x00007fb0a1012360 (object 0x000000013df2f678, a java.lang.Object),
  which is held by "t3"

Java stack information for the threads listed above:
===================================================
"t3":
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f658> (a java.lang.Object)
	- locked <0x000000013df2f678> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)
"t1":
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f668> (a java.lang.Object)
	- locked <0x000000013df2f658> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)
"t2":
	at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
	- waiting to lock <0x000000013df2f678> (a java.lang.Object)
	- locked <0x000000013df2f668> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:722)

Found 1 deadlock.

線程傾印輸出清楚地顯示了死鎖情況以及導致死鎖情況的線程和資源。為了分析死鎖，我們需要尋找狀態為BLOCKED的線程，然後查看它正在等待鎖定的資源。每個資源都有一個唯一的ID，我們可以使用該ID找出哪個線程已經在對象上持有鎖。例如，線程“t3”正在等待鎖定0x000000013df2f658，但它已經被線程“t1”鎖定。一旦我們分析了死鎖情況並找到了導致死鎖的線程，我們需要對代碼進行更改以避免死鎖情況。

如何在java中避免死鎖

這些是我們可以避免大多數死鎖情況的一些指南。

  • 避免嵌套鎖定：這是死鎖的最常見原因，如果已經持有一個鎖，則應避免鎖定另一個資源。如果只使用一個物件鎖，幾乎不可能發生死鎖情況。例如，這是另一個不使用嵌套鎖定的run()方法的實現，程式成功運行且不會發生死鎖情況。

        public void run() {
        String name = Thread.currentThread().getName();
        System.out.println(name + " acquiring lock on " + obj1);
        synchronized (obj1) {
            System.out.println(name + " acquired lock on " + obj1);
            work();
        }
        System.out.println(name + " released lock on " + obj1);
        System.out.println(name + " acquiring lock on " + obj2);
        synchronized (obj2) {
            System.out.println(name + " acquired lock on " + obj2);
            work();
        }
        System.out.println(name + " released lock on " + obj2);

        System.out.println(name + " finished execution.");
    }

  • 僅鎖定所需的資源: 您應該僅對您需要處理的資源進行鎖定，在上述程式中，例如我正在鎖定完整的物件資源，但如果我們只對其中一個欄位感興趣，那麼我們應該僅鎖定該特定欄位而不是完整的物件。

  • 避免無限期等待: 如果兩個執行緒互相等待對方無限期地完成，則可能會發生死結，使用執行緒 join。 如果您的執行緒必須等待另一個執行緒完成，最好始終使用 join，並設置您希望等待執行緒完成的最大時間。

這就是Java執行緒中的死結的所有內容。

Source:
https://www.digitalocean.com/community/tutorials/deadlock-in-java-example