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1 对于普通同步方法，锁是当前实例对象

2 对于静态同步方法，锁是当前类Class对象

3 对于同步方法块，锁是Synchronized括号里配置的对象

4 JVM基于进入和退出Monitor对象来显示方法同步和代码块同步，但两者的实现细节不一样。代码块同步是使用monitorenter和monitorexit指令实现的，而方法同步是使用另外一种方式实现的。

monitorenter指令是在编译后插入到同步代码块的开始位置，而monitorexit是插入到方法的结束处和异常处，JVM保证每个monitorenter必须有对应的monitorexit与之配对。


<code winbatch>
C:\Users\morga>javap -c -verbose D:\gitee\projectx\ml-common\target\classes\com\xiaosq\common\MainTestSynchronized.class
Classfile /D:/gitee/projectx/ml-common/target/classes/com/xiaosq/common/MainTestSynchronized.class
  Last modified 2019-9-10; size 841 bytes
  MD5 checksum 3d25a1b989989b9b30f9e22feee77676
  Compiled from "MainTestSynchronized.java"
public class com.xiaosq.common.MainTestSynchronized
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#31         // java/lang/Object."<init>":()V
   #2 = Fieldref           #4.#32         // com/xiaosq/common/MainTestSynchronized.aa:I
   #3 = Fieldref           #4.#33         // com/xiaosq/common/MainTestSynchronized.bb:I
   #4 = Class              #34            // com/xiaosq/common/MainTestSynchronized
   #5 = Methodref          #4.#31         // com/xiaosq/common/MainTestSynchronized."<init>":()V
   #6 = Methodref          #4.#35         // com/xiaosq/common/MainTestSynchronized.first:()V
   #7 = Methodref          #4.#36         // com/xiaosq/common/MainTestSynchronized.second:()V
   #8 = Class              #37            // java/lang/Object
   #9 = Utf8               aa
  #10 = Utf8               I
  #11 = Utf8               bb
  #12 = Utf8               <init>
  #13 = Utf8               ()V
  #14 = Utf8               Code
  #15 = Utf8               LineNumberTable
  #16 = Utf8               LocalVariableTable
  #17 = Utf8               this
  #18 = Utf8               Lcom/xiaosq/common/MainTestSynchronized;
  #19 = Utf8               first
  #20 = Utf8               second
  #21 = Utf8               StackMapTable
  #22 = Class              #34            // com/xiaosq/common/MainTestSynchronized
  #23 = Class              #37            // java/lang/Object
  #24 = Class              #38            // java/lang/Throwable
  #25 = Utf8               main
  #26 = Utf8               ([Ljava/lang/String;)V
  #27 = Utf8               args
  #28 = Utf8               [Ljava/lang/String;
  #29 = Utf8               SourceFile
  #30 = Utf8               MainTestSynchronized.java
  #31 = NameAndType        #12:#13        // "<init>":()V
  #32 = NameAndType        #9:#10         // aa:I
  #33 = NameAndType        #11:#10        // bb:I
  #34 = Utf8               com/xiaosq/common/MainTestSynchronized
  #35 = NameAndType        #19:#13        // first:()V
  #36 = NameAndType        #20:#13        // second:()V
  #37 = Utf8               java/lang/Object
  #38 = Utf8               java/lang/Throwable
{
  public com.xiaosq.common.MainTestSynchronized();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: aload_0
         5: iconst_0
         6: putfield      #2                  // Field aa:I
         9: aload_0
        10: iconst_0
        11: putfield      #3                  // Field bb:I
        14: return
      LineNumberTable:
        line 8: 0
        line 10: 4
        line 12: 9
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      15     0  this   Lcom/xiaosq/common/MainTestSynchronized;

  public synchronized void first();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_SYNCHRONIZED
    Code:
      stack=3, locals=1, args_size=1
         0: aload_0
         1: aload_0
         2: getfield      #2                  // Field aa:I
         5: iconst_1
         6: iadd
         7: putfield      #2                  // Field aa:I
        10: return
      LineNumberTable:
        line 15: 0
        line 16: 10
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  this   Lcom/xiaosq/common/MainTestSynchronized;

  public void second();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=3, args_size=1
         0: aload_0
         1: dup
         2: astore_1
         3: monitorenter
         4: aload_0
         5: aload_0
         6: getfield      #3                  // Field bb:I
         9: iconst_1
        10: iadd
        11: putfield      #3                  // Field bb:I
        14: aload_1
        15: monitorexit
        16: goto          24
        19: astore_2
        20: aload_1
        21: monitorexit
        22: aload_2
        23: athrow
        24: return
      Exception table:
         from    to  target type
             4    16    19   any
            19    22    19   any
      LineNumberTable:
        line 19: 0
        line 20: 4
        line 21: 14
        line 22: 24
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      25     0  this   Lcom/xiaosq/common/MainTestSynchronized;
      StackMapTable: number_of_entries = 2
        frame_type = 255 /* full_frame */
          offset_delta = 19
          locals = [ class com/xiaosq/common/MainTestSynchronized, class java/lang/Object ]
          stack = [ class java/lang/Throwable ]
        frame_type = 250 /* chop */
          offset_delta = 4

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=2, args_size=1
         0: new           #4                  // class com/xiaosq/common/MainTestSynchronized
         3: dup
         4: invokespecial #5                  // Method "<init>":()V
         7: astore_1
         8: aload_1
         9: invokevirtual #6                  // Method first:()V
        12: aload_1
        13: invokevirtual #7                  // Method second:()V
        16: return
      LineNumberTable:
        line 26: 0
        line 27: 8
        line 29: 12
        line 30: 16
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      17     0  args   [Ljava/lang/String;
            8       9     1  main   Lcom/xiaosq/common/MainTestSynchronized;
}
SourceFile: "MainTestSynchronized.java"
</code>