公共代码

  1. public static void sleep() {
  2.     try {
  3.         Thread.sleep(100000000L);
  4.     } catch (InterruptedException interruptedException) {
  5.         interruptedException.printStackTrace();
  6.     }
  7. }

锁实例方法

  1. public synchronized void method1() {
  2.     sleep();
  3. }
  5. public void method0() {
  6.     synchronized (this) {
  7.         sleep();
  8.     }
  9. }

字节码

  1. // access flags 0x21 method1()
  2. public synchronized method1()V
  3.  L0
  4.   LINENUMBER 34 L0
  5.   INVOKESTATIC com/raycloud/api/common/core/http/ThreadTest.sleep ()V
  6.  L1
  7.   LINENUMBER 35 L1
  8.   RETURN
  9.  L2
  10.   LOCALVARIABLE this Lcom/raycloud/api/common/core/http/ThreadTest; L0 L2 0
  11.   MAXSTACK = 0
  12.   MAXLOCALS = 1
  13. ===================================================================================
  14. public method0()V
  15.   TRYCATCHBLOCK L0 L1 L2 null
  16.   TRYCATCHBLOCK L2 L3 L2 null
  17.  L4
  18.   LINENUMBER 47 L4
  19.   ALOAD 0
  20.   DUP
  21.  ASTORE 1
  22.  MONITORENTER
  23. L0
  24.  LINENUMBER 48 L0
  25.  INVOKESTATIC com/raycloud/api/common/core/http/ThreadTest.sleep ()V
  26. L5
  27.  LINENUMBER 49 L5
  28.  ALOAD 1
  29.  MONITOREXIT
  30. L1
  31.  GOTO L6
  32. L2
  33. FRAME FULL [com/raycloud/api/common/core/http/ThreadTest java/lang/Object] [java/lang/Throwable]
  34.  ASTORE 2
  35.  ALOAD 1
  36.  MONITOREXIT
  37. L3
  38.  ALOAD 2
  39.  ATHROW
  40. L6
  41.  LINENUMBER 50 L6
  42. FRAME CHOP 1
  43.  RETURN
  44. L7
  45.  LOCALVARIABLE this Lcom/raycloud/api/common/core/http/ThreadTest; L4 L7 0
  46.  MAXSTACK = 2
  47.  MAXLOCALS = 3

其中的区别在于 synchronized 放方法上就是一个简单的方法访问标志，而放方法内部，同步this的处理就有 MONITORENTER 和 MONITOREXIT 等数据了。

Q: 锁实例方法和锁this代码块，是不是同一个monitor lock呢?
查看堆栈

  1. public class ThreadTest {
  2.     public static void main(String[] args) throws Exception {
  3.         ThreadTest threadTest = new ThreadTest();
  4.         new Thread(threadTest::method1).start();
  5.         Thread.sleep(100L
  6.         );
  7.         threadTest.method0();
  8.     }
  9. }
  1. "main" #1 prio=5 os_prio=31 tid=0x00007fe61780e800 nid=0xd03 waiting for monitor entry [0x000070000644d000]
  2.    java.lang.Thread.State: BLOCKED (on object monitor)
  3.     at com.raycloud.api.common.core.http.ThreadTest.method0(ThreadTest.java:32)
  4.     - waiting to lock <0x000000076ae40ef8> (a com.raycloud.api.common.core.http.ThreadTest)
  5.     at com.raycloud.api.common.core.http.ThreadTest.main(ThreadTest.java:14)
  7. "Thread-0" #11 prio=5 os_prio=31 tid=0x00007fe6198f7000 nid=0x5a03 waiting on condition [0x0000700007a12000]
  8.    java.lang.Thread.State: TIMED_WAITING (sleeping)
  9.     at java.lang.Thread.sleep(Native Method)
  10.     at com.raycloud.api.common.core.http.ThreadTest.sleep(ThreadTest.java:24)
  11.     at com.raycloud.api.common.core.http.ThreadTest.method1(ThreadTest.java:18)
  12.     - locked <0x000000076ae40ef8> (a com.raycloud.api.common.core.http.ThreadTest)
  13.     at com.raycloud.api.common.core.http.ThreadTest$$Lambda$1/1940447180.run(Unknown Source)
  14.     at java.lang.Thread.run(Thread.java:748)

locked <0x000000076ae40ef8> waiting to lock <0x000000076ae40ef8>

⚠️ 注意

锁实例，针对的是每一个实例对象，所以每一个实例对象都持有一个lock，因此不能实例之间的 synchronized 是相互不影响的。

  1. public class ThreadTest {
  2.     public static void main(String[] args) throws Exception {
  3.         ThreadTest first = new ThreadTest();
  4.         ThreadTest second = new ThreadTest();
  5.         new Thread(first::method1).start();
  6.         new Thread(second::method1).start();
  7.     }
  8. }
  1. "Thread-1" #12 prio=5 os_prio=31 tid=0x00007f9ade156000 nid=0x5a03 waiting on condition [0x0000700001b1a000]
  2.    java.lang.Thread.State: TIMED_WAITING (sleeping)
  3.     at java.lang.Thread.sleep(Native Method)
  4.     at com.raycloud.api.common.core.http.ThreadTest.sleep(ThreadTest.java:25)
  5.     at com.raycloud.api.common.core.http.ThreadTest.method1(ThreadTest.java:19)
  6.     - locked <0x000000076ae40f30> (a com.raycloud.api.common.core.http.ThreadTest)
  7.     at com.raycloud.api.common.core.http.ThreadTest$$Lambda$2/2121744517.run(Unknown Source)
  8.     at java.lang.Thread.run(Thread.java:748)
  10. "Thread-0" #11 prio=5 os_prio=31 tid=0x00007f9adf89c000 nid=0xa803 waiting on condition [0x0000700001a17000]
  11.    java.lang.Thread.State: TIMED_WAITING (sleeping)
  12.     at java.lang.Thread.sleep(Native Method)
  13.     at com.raycloud.api.common.core.http.ThreadTest.sleep(ThreadTest.java:25)
  14.     at com.raycloud.api.common.core.http.ThreadTest.method1(ThreadTest.java:19)
  15.     - locked <0x000000076ae40f20> (a com.raycloud.api.common.core.http.ThreadTest)
  16.     at com.raycloud.api.common.core.http.ThreadTest$$Lambda$1/1940447180.run(Unknown Source)
  17.     at java.lang.Thread.run(Thread.java:748)

锁静态方法

  1. public synchronized static void method2() {
  2.     sleep();
  3. }
  5. public static void method3() {
  6.     synchronized (ThreadTest.class) {
  7.         sleep();
  8.     }
  9. }

字节码

  1.   // access flags 0x29
  2.   public static synchronized method2()V
  3.    L0
  4.     LINENUMBER 37 L0
  5.     INVOKESTATIC com/raycloud/api/common/core/http/ThreadTest.sleep ()V
  6.    L1
  7.     LINENUMBER 38 L1
  8.     RETURN
  9.     MAXSTACK = 0
  10.     MAXLOCALS = 0
  11. ===================================================================================
  12.       // access flags 0x9
  13.   public static method3()V
  14.     TRYCATCHBLOCK L0 L1 L2 null
  15.     TRYCATCHBLOCK L2 L3 L2 null
  16.    L4
  17.     LINENUMBER 41 L4
  18.     LDC Lcom/raycloud/api/common/core/http/ThreadTest;.class
  19.     DUP
  20.     ASTORE 0
  21.     MONITORENTER
  22.    L0
  23.     LINENUMBER 42 L0
  24.     INVOKESTATIC com/raycloud/api/common/core/http/ThreadTest.sleep ()V
  25.    L5
  26.     LINENUMBER 43 L5
  27.     ALOAD 0
  28.     MONITOREXIT
  29.    L1
  30.     GOTO L6
  31.    L2
  32.    FRAME FULL [java/lang/Object] [java/lang/Throwable]
  33.     ASTORE 1
  34.     ALOAD 0
  35.     MONITOREXIT
  36.    L3
  37.     ALOAD 1
  38.     ATHROW
  39.    L6
  40.     LINENUMBER 44 L6
  41.    FRAME CHOP 1
  42.     RETURN
  43.     MAXSTACK = 2
  44.     MAXLOCALS = 2

和锁实例方法一样的配置，不过这次aload，和store的是 ThreadTest.class， 所谓锁的对象发生了变化.
静态方法和 xxx.class是不是同一把锁呢

  1. public class ThreadTest {
  2.     public static void main(String[] args) throws Exception {
  3.         new Thread(ThreadTest::method2).start();
  4.         Thread.sleep(100L
  5.         );
  6.         ThreadTest.method3();
  7.     }
  8. }
  1. "main" #1 prio=5 os_prio=31 tid=0x00007fd11881a000 nid=0xd03 waiting for monitor entry [0x00007000054b8000]
  2.    java.lang.Thread.State: BLOCKED (on object monitor)
  3.     at com.raycloud.api.common.core.http.ThreadTest.method3(ThreadTest.java:41)
  4.     - waiting to lock <0x000000076ae3e398> (a java.lang.Class for com.raycloud.api.common.core.http.ThreadTest)
  5.     at com.raycloud.api.common.core.http.ThreadTest.main(ThreadTest.java:13)
  7. "Thread-0" #11 prio=5 os_prio=31 tid=0x00007fd11694e000 nid=0xa703 waiting on condition [0x00007000069fa000]
  8.    java.lang.Thread.State: TIMED_WAITING (sleeping)
  9.     at java.lang.Thread.sleep(Native Method)
  10.     at com.raycloud.api.common.core.http.ThreadTest.sleep(ThreadTest.java:23)
  11.     at com.raycloud.api.common.core.http.ThreadTest.method2(ThreadTest.java:36)
  12.     - locked <0x000000076ae3e398> (a java.lang.Class for com.raycloud.api.common.core.http.ThreadTest)
  13.     at com.raycloud.api.common.core.http.ThreadTest$$Lambda$1/1940447180.run(Unknown Source)
  14.     at java.lang.Thread.run(Thread.java:748)

  • locked <0x000000076ae3e398> (a java.lang.Class for com.raycloud.api.common.core.http.ThreadTest)

  • waiting to lock <0x000000076ae3e398> (a java.lang.Class for com.raycloud.api.common.core.http.ThreadTest)

可重入性

当同步锁住一个实例方法进行调用时，是否可以多次调用同一个方法。

答案是可以，反证法如下:

  • 如果不能多次同步调用，就会出现死锁了，我要向下执行就需要再一次拥有锁，但是又不能获得锁
  • 所以，可重入性是必然的