synchronized原理

//简单的小例子观察字节码文件的差异：

package com.ydlclass.lock;

public class Test {
    public static void main(String[] args) {
        synchronized (Test.class){
            int a = 1;
        }
    }
}

下面是字节码文件，发现多了Monitorenter,monitorexit关键字

Classfile /D:/code/MyReviewer/out/production/api/com/ydlclass/lock/Test.class
  Last modified 2022-4-13; size 507 bytes
  MD5 checksum 26d60269103a6ca368976d152fc706e0
  Compiled from "Test.java"
public class com.ydlclass.lock.Test
  minor version: 0
  major version: 55
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #3.#20         // java/lang/Object."<init>":()V
   #2 = Class              #21            // com/ydlclass/lock/Test
   #3 = Class              #22            // java/lang/Object
   #4 = Utf8               <init>
   #5 = Utf8               ()V
   #6 = Utf8               Code
   #7 = Utf8               LineNumberTable
   #8 = Utf8               LocalVariableTable
   #9 = Utf8               this
  #10 = Utf8               Lcom/ydlclass/lock/Test;
  #11 = Utf8               main
  #12 = Utf8               ([Ljava/lang/String;)V
  #13 = Utf8               args
  #14 = Utf8               [Ljava/lang/String;
  #15 = Utf8               StackMapTable
  #16 = Class              #14            // "[Ljava/lang/String;"
  #17 = Class              #23            // java/lang/Throwable
  #18 = Utf8               SourceFile
  #19 = Utf8               Test.java
  #20 = NameAndType        #4:#5          // "<init>":()V
  #21 = Utf8               com/ydlclass/lock/Test
  #22 = Utf8               java/lang/Object
  #23 = Utf8               java/lang/Throwable
{
  public com.ydlclass.lock.Test();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/ydlclass/lock/Test;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=4, args_size=1
         0: ldc           #2                  // class com/ydlclass/lock/Test
         2: dup
         3: astore_1
         4: monitorenter
         5: iconst_1
         6: istore_2
         7: aload_1
         8: monitorexit
         9: goto          17
        12: astore_3
        13: aload_1
        14: monitorexit
        15: aload_3
        16: athrow
        17: return
      Exception table:
         from    to  target type
             5     9    12   any
            12    15    12   any
      LineNumberTable:
        line 5: 0
        line 6: 5
        line 7: 7
        line 8: 17
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      18     0  args   [Ljava/lang/String;
      StackMapTable: number_of_entries = 2
        frame_type = 255 /* full_frame */
          offset_delta = 12
          locals = [ class "[Ljava/lang/String;", class java/lang/Object ]
          stack = [ class java/lang/Throwable ]
        frame_type = 250 /* chop */
          offset_delta = 4
}
SourceFile: "Test.java"
PS D:\code\MyReviewer\out\production\api\com\ydlclass\lock>

对象头：对象头中包括标记字段Mark Word(锁信息，分代年纪)和类型指针KClass pointer