【并发编程】多把锁

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多把锁

多个工作用同一把锁，并发度是很低的

示例

如下代码，在一个大房子里，不同工作共用一把锁（睡觉，学习...）导致并发度很低。

class BigRoom {
    public void sleep() {
        synchronized (this) {
            log.debug("sleeping 2 小时");
            Sleeper.sleep(2);
        }
    }
    public void study() {
        synchronized (this) {
            log.debug("study 1 小时");
            Sleeper.sleep(1);
        }
    }
}

执行

BigRoom bigRoom = new BigRoom();
new Thread(() -> {
    bigRoom.compute();
},"小南").start();
new Thread(() -> {
    bigRoom.sleep();
},"小女").start();

某次结果

改进

不同的任务用不同的锁

class BigRoom {
    private final Object studyRoom = new Object();
    private final Object bedRoom = new Object();
    public void sleep() {
        synchronized (bedRoom) {
            log.debug("sleeping 2 小时");
            Sleeper.sleep(2);
        }
    }
    public void study() {
        synchronized (studyRoom) {
            log.debug("study 1 小时");
            Sleeper.sleep(1);
        }
    }
}    

某次执行结果

将锁的粒度细分：

好处：是可以增强并发度

坏处：如果一个线程需要同时获得多把锁，就容易发生死锁

活跃性

死锁

有这样的一种情况：一个线程需要同时获取多把锁，这时就容器发生死锁。

t1线程获得A对象锁，接下来想获得B对象的锁。

t2线程获得B对象锁，接下来想获得A对象的锁。

Object A=new Object();
        Object B=new Object();
        Thread t1=new Thread(()->{
        synchronized (A){
        log.debug("lock A");
        sleep(1);
        synchronized (B){
        log.debug("lock B");
        log.debug("操作...");
        }
       }
     },"t1");
        Thread t2=new Thread(()->{
        synchronized (B){
        log.debug("lock B");
        sleep(0.5);
        synchronized (A){
        log.debug("lock A");

        log.debug("操作...");
        }
      }
    },"t2");
      t1.start();
      t2.start();

运行结果?

定位死锁的工具

1.检测死锁可以使用jconsole工具，WIN+R窗口输入jconsole

2.使用jps定位进程id,再用jstack定位死锁

$ jps                                           
26400 RemoteMavenServer36
18708 Launcher
19668 Jps 
14568     
10124 Test


$ jstack 10124
2024-01-18 10:42:01
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.121-b13 mixed mode):

"DestroyJavaVM" #22 prio=5 os_prio=0 tid=0x0000000002d84000 nid=0x3dac waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"t2" #21 prio=5 os_prio=0 tid=0x00000000215c2000 nid=0x6acc waiting for monitor entry [0x0000000021d2f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at aaa.Test.lambda$main$1(Test.java:36)
        - waiting to lock <0x000000076c12dc10> (a java.lang.Object)
        - locked <0x000000076c12dc20> (a java.lang.Object)
        at aaa.Test$$Lambda$2/1031980531.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:745)

"t1" #20 prio=5 os_prio=0 tid=0x00000000215c1800 nid=0x64c0 waiting for monitor entry [0x0000000021c2f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at aaa.Test.lambda$main$0(Test.java:22)
        - waiting to lock <0x000000076c12dc20> (a java.lang.Object)
        - locked <0x000000076c12dc10> (a java.lang.Object)
        at aaa.Test$$Lambda$1/1793329556.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:745)

"Service Thread" #19 daemon prio=9 os_prio=0 tid=0x000000001eadb000 nid=0x6a64 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread11" #18 daemon prio=9 os_prio=2 tid=0x000000001ea7f000 nid=0x2338 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread10" #17 daemon prio=9 os_prio=2 tid=0x000000001ea7e800 nid=0x44c4 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread9" #16 daemon prio=9 os_prio=2 tid=0x000000001ea80800 nid=0x3168 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread8" #15 daemon prio=9 os_prio=2 tid=0x000000001ea7c000 nid=0x660c waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread7" #14 daemon prio=9 os_prio=2 tid=0x000000001ea57000 nid=0x208c waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread6" #13 daemon prio=9 os_prio=2 tid=0x000000001ea38800 nid=0x1d9c waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread5" #12 daemon prio=9 os_prio=2 tid=0x000000001ea0c800 nid=0x54d0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread4" #11 daemon prio=9 os_prio=2 tid=0x000000001ea04000 nid=0x5be0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread3" #10 daemon prio=9 os_prio=2 tid=0x000000001e9fb000 nid=0x5938 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x000000001e9f2800 nid=0x26d8 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x000000001e9ef800 nid=0x27f0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x000000001e9eb000 nid=0x6bb4 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x000000001e9a7000 nid=0x2078 runnable [0x000000002042e000]
   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 <0x000000076b84a128> (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 <0x000000076b84a128> (a java.io.InputStreamReader)
        at java.io.BufferedReader.readLine(BufferedReader.java:389)
        at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:56)

"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x000000001e7fb000 nid=0x5b64 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x000000001e858000 nid=0x473c runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x000000001cd78800 nid=0x55a8 in Object.wait() [0x000000002012e000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000076b588ec8> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
        - locked <0x000000076b588ec8> (a java.lang.ref.ReferenceQueue$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=0x000000001cd70000 nid=0x4d34 in Object.wait() [0x000000002002e000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000076b586b68> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:502)
        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
        - locked <0x000000076b586b68> (a java.lang.ref.Reference$Lock)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"VM Thread" os_prio=2 tid=0x000000001e7b5000 nid=0x4554 runnable

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000002d99800 nid=0x27bc runnable

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x0000000002d9b000 nid=0x2c3c runnable 

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x0000000002d9c800 nid=0x5770 runnable

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x0000000002d9e000 nid=0x3f2c runnable

"GC task thread#4 (ParallelGC)" os_prio=0 tid=0x0000000002da0000 nid=0x2284 runnable

"GC task thread#5 (ParallelGC)" os_prio=0 tid=0x0000000002da2800 nid=0xe4c runnable

"GC task thread#6 (ParallelGC)" os_prio=0 tid=0x0000000002da5800 nid=0x5914 runnable

"GC task thread#7 (ParallelGC)" os_prio=0 tid=0x0000000002da6800 nid=0x1044 runnable

"GC task thread#8 (ParallelGC)" os_prio=0 tid=0x0000000002da8000 nid=0x4c6c runnable

"GC task thread#9 (ParallelGC)" os_prio=0 tid=0x0000000002da9000 nid=0x467c runnable

"GC task thread#10 (ParallelGC)" os_prio=0 tid=0x0000000002daa000 nid=0x4c60 runnable

"GC task thread#11 (ParallelGC)" os_prio=0 tid=0x0000000002dad800 nid=0x4aac runnable

"GC task thread#12 (ParallelGC)" os_prio=0 tid=0x0000000002dae800 nid=0x57dc runnable

"VM Periodic Task Thread" os_prio=2 tid=0x0000000021320800 nid=0x65e8 waiting on condition

JNI global references: 343


Found one Java-level deadlock:
=============================
"t2":
  waiting to lock monitor 0x000000001cd74b18 (object 0x000000076c12dc10, a java.lang.Object),
  which is held by "t1"
"t1":
  waiting to lock monitor 0x000000001cd76f88 (object 0x000000076c12dc20, a java.lang.Object),
  which is held by "t2"

Java stack information for the threads listed above:
===================================================
"t2":
        at aaa.Test.lambda$main$1(Test.java:36)
        - waiting to lock <0x000000076c12dc10> (a java.lang.Object)
        - locked <0x000000076c12dc20> (a java.lang.Object)
        at aaa.Test$$Lambda$2/1031980531.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:745)
"t1":
        at aaa.Test.lambda$main$0(Test.java:22)
        - waiting to lock <0x000000076c12dc20> (a java.lang.Object)
        - locked <0x000000076c12dc10> (a java.lang.Object)
        at aaa.Test$$Lambda$1/1793329556.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:745)

Found 1 deadlock.

避免死锁要注意加锁顺序

另外如果由于某个线程进入了死循环，导致其他线程一直等待。对于这种情况Linux下可以通过top先定位CPU占用高的java进程，再利用top -Hp 进程id来定位是哪个线程，最后再用jstack排查