目錄

一、什么是死鎖

二、代碼實現一個死鎖

三、如何判斷是不是死鎖

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一、什么是死鎖

? ? 死鎖是指兩個或兩個以上的進程在執行過程中，因爭奪資源而造成的一種互相等待的現象，若無外力干涉那它們都將無法推進下去，如果系統資源充足，進程的資源請求都能夠得到滿足，死鎖出現的可能性就很低，否則就會因爭奪有限的資源而陷入死鎖。

二、代碼實現一個死鎖

/** * 死鎖發生，兩個以上線程因爭奪資源互相等待 * 持有自己的鎖，還妄圖獲得別人的鎖 */ public class DeadLockDemo {public static void main(String[] args) {String LockA = "lockA";String LockB = "lockB";new Thread(new HoldLockThread(LockA, LockB), "t1").start();new Thread(new HoldLockThread(LockB, LockA), "t2").start();} }class HoldLockThread implements Runnable{private String lockA;private String lockB;public HoldLockThread(String lockA, String lockB){this.lockA = lockA;this.lockB = lockB;}@Overridepublic void run() {synchronized (lockA){System.out.println(Thread.currentThread().getName() + "得到"+lockA+"鎖嘗試獲得"+lockB+"鎖");try {Thread.sleep(200);} catch (InterruptedException e) {e.printStackTrace();}synchronized (lockB){System.out.println(Thread.currentThread().getName() + "得到" +lockB);}}} }

三、如何判斷是不是死鎖

1.jps -l命令定位進程號

2.jstack找到死鎖查看

E:\eclipseWorkspace\MySpringboot>jstack 9500 2021-04-23 16:07:15 Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.131-b11 mixed mode):"DestroyJavaVM" #13 prio=5 os_prio=0 tid=0x0000000002b93800 nid=0x32d8 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"t2" #12 prio=5 os_prio=0 tid=0x000000001a12f800 nid=0x4474 waiting for monitor entry [0x000000001b2ff000]java.lang.Thread.State: BLOCKED (on object monitor)at com.boot.security.server.myUtils.jiagou2021.locks.HoldLockThread.run(DeadLockDemo.java:45)- waiting to lock <0x00000000d6327610> (a java.lang.String)- locked <0x00000000d6327648> (a java.lang.String)at java.lang.Thread.run(Thread.java:748)"t1" #11 prio=5 os_prio=0 tid=0x000000001a290800 nid=0x43ec waiting for monitor entry [0x000000001b1ff000]java.lang.Thread.State: BLOCKED (on object monitor)at com.boot.security.server.myUtils.jiagou2021.locks.HoldLockThread.run(DeadLockDemo.java:45)- waiting to lock <0x00000000d6327648> (a java.lang.String)- locked <0x00000000d6327610> (a java.lang.String)at java.lang.Thread.run(Thread.java:748)"Service Thread" #10 daemon prio=9 os_prio=0 tid=0x00000000198ed000 nid=0xaa0 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C1 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x00000000198ec000 nid=0x7d4 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x000000001987f800 nid=0x255c waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x0000000019831000 nid=0x17dc waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x00000000198fa000 nid=0x219c runnable [0x0000000019c2e000]java.lang.Thread.State: RUNNABLEat 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 <0x00000000d629f910> (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 <0x00000000d629f910> (a java.io.InputStreamReader)at java.io.BufferedReader.readLine(BufferedReader.java:389)at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:64)"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x0000000018d73800 nid=0x41c4 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x000000001799a800 nid=0x2954 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x0000000017971800 nid=0x2d04 in Object.wait() [0x0000000018ccf000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on <0x00000000d5d08ec8> (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)- locked <0x00000000d5d08ec8> (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=0x0000000017949800 nid=0x14cc in Object.wait() [0x0000000018bcf000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on <0x00000000d5d06b68> (a java.lang.ref.Reference$Lock)at java.lang.Object.wait(Object.java:502)at java.lang.ref.Reference.tryHandlePending(Reference.java:191)- locked <0x00000000d5d06b68> (a java.lang.ref.Reference$Lock)at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)"VM Thread" os_prio=2 tid=0x0000000017947800 nid=0x2f70 runnable"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000002bb1000 nid=0x4a64 runnable"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x0000000002bb2800 nid=0x33cc runnable"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x0000000002bb4800 nid=0x3bd4 runnable"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x0000000002bb6000 nid=0x4e7c runnable"VM Periodic Task Thread" os_prio=2 tid=0x00000000198ee000 nid=0x20e8 waiting on conditionJNI global references: 33Found one Java-level deadlock: ============================= "t2":waiting to lock monitor 0x00000000179516b8 (object 0x00000000d6327610, a java.lang.String),which is held by "t1" "t1":waiting to lock monitor 0x0000000017950218 (object 0x00000000d6327648, a java.lang.String),which is held by "t2"Java stack information for the threads listed above: =================================================== "t2":at com.boot.security.server.myUtils.jiagou2021.locks.HoldLockThread.run(DeadLockDemo.java:45)- waiting to lock <0x00000000d6327610> (a java.lang.String)- locked <0x00000000d6327648> (a java.lang.String)at java.lang.Thread.run(Thread.java:748) "t1":at com.boot.security.server.myUtils.jiagou2021.locks.HoldLockThread.run(DeadLockDemo.java:45)- waiting to lock <0x00000000d6327648> (a java.lang.String)- locked <0x00000000d6327610> (a java.lang.String)at java.lang.Thread.run(Thread.java:748)Found 1 deadlock.

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