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Synchronized全解读-01

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Synchronized的使用方式

Synchronized作用于普通方法、静态方法、同步代码块。通过javap -v命令深入查看。

代码示例：

public class SyncUsingWay {

    /**
     * 普通方法
     * 锁对象：实例对象，谁调用这个方法，锁就作用于谁身上
     */
    public synchronized void syncMethod() {
        System.out.println("Synchronized Method");
    }

    /**
     * 静态方法
     * 锁对象：对象的class，全局唯一
     * 类型 包括 类的class文件的二进制文件最终都加载到了运行时数据区的方法区
     */
    public synchronized static void staticSyncMethod() {
        System.out.println("Static Synchronized Method");
    }

    public void method() {
        // 静态代码块
        synchronized (this) {
            System.out.println("Method");
        }
    }
}

编译之后，使用javap -v SyncUsingWay.class查看信息：

Classfile /Users/codejuzi/Documents/CodeWorkSpace/Practice/juc-demo/target/classes/com/juzi/juc/sync/SyncUsingWay.class
  Last modified 2023-12-1; size 839 bytes
  MD5 checksum 594bba00a6d14034bd77c93025a9dd13
  Compiled from "SyncUsingWay.java"
public class com.juzi.juc.sync.SyncUsingWay
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#25         // java/lang/Object."<init>":()V
   #2 = Fieldref           #26.#27        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = String             #28            // Synchronized Method
   #4 = Methodref          #29.#30        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = String             #31            // Static Synchronized Method
   #6 = String             #32            // Method
   #7 = Class              #33            // com/juzi/juc/sync/SyncUsingWay
   #8 = Class              #34            // java/lang/Object
   #9 = Utf8               <init>
  #10 = Utf8               ()V
  #11 = Utf8               Code
  #12 = Utf8               LineNumberTable
  #13 = Utf8               LocalVariableTable
  #14 = Utf8               this
  #15 = Utf8               Lcom/juzi/juc/sync/SyncUsingWay;
  #16 = Utf8               syncMethod
  #17 = Utf8               staticSyncMethod
  #18 = Utf8               method
  #19 = Utf8               StackMapTable
  #20 = Class              #33            // com/juzi/juc/sync/SyncUsingWay
  #21 = Class              #34            // java/lang/Object
  #22 = Class              #35            // java/lang/Throwable
  #23 = Utf8               SourceFile
  #24 = Utf8               SyncUsingWay.java
  #25 = NameAndType        #9:#10         // "<init>":()V
  #26 = Class              #36            // java/lang/System
  #27 = NameAndType        #37:#38        // out:Ljava/io/PrintStream;
  #28 = Utf8               Synchronized Method
  #29 = Class              #39            // java/io/PrintStream
  #30 = NameAndType        #40:#41        // println:(Ljava/lang/String;)V
  #31 = Utf8               Static Synchronized Method
  #32 = Utf8               Method
  #33 = Utf8               com/juzi/juc/sync/SyncUsingWay
  #34 = Utf8               java/lang/Object
  #35 = Utf8               java/lang/Throwable
  #36 = Utf8               java/lang/System
  #37 = Utf8               out
  #38 = Utf8               Ljava/io/PrintStream;
  #39 = Utf8               java/io/PrintStream
  #40 = Utf8               println
  #41 = Utf8               (Ljava/lang/String;)V
{
  public com.juzi.juc.sync.SyncUsingWay();
    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 6: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/juzi/juc/sync/SyncUsingWay;

  public synchronized void syncMethod();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_SYNCHRONIZED
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String Synchronized Method
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 13: 0
        line 14: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       9     0  this   Lcom/juzi/juc/sync/SyncUsingWay;

  public static synchronized void staticSyncMethod();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED
    Code:
      stack=2, locals=0, args_size=0
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #5                  // String Static Synchronized Method
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 22: 0
        line 23: 8

  public void method();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=1
         0: aload_0
         1: dup
         2: astore_1
         3: monitorenter
         4: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         7: ldc           #6                  // String Method
         9: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        12: aload_1
        13: monitorexit
        14: goto          22
        17: astore_2
        18: aload_1
        19: monitorexit
        20: aload_2
        21: athrow
        22: return
      Exception table:
         from    to  target type
             4    14    17   any
            17    20    17   any
      LineNumberTable:
        line 27: 0
        line 28: 4
        line 29: 12
        line 30: 22
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      23     0  this   Lcom/juzi/juc/sync/SyncUsingWay;
      StackMapTable: number_of_entries = 2
        frame_type = 255 /* full_frame */
          offset_delta = 17
          locals = [ class com/juzi/juc/sync/SyncUsingWay, class java/lang/Object ]
          stack = [ class java/lang/Throwable ]
        frame_type = 250 /* chop */
          offset_delta = 4
}
SourceFile: "SyncUsingWay.java"

SyncMethod普通同步方法

public synchronized void syncMethod();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_SYNCHRONIZED
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String Synchronized Method
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 13: 0
        line 14: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       9     0  this   Lcom/juzi/juc/sync/SyncUsingWay;

flag中ACC_SYNCHRONIZED是加锁的标志，编译期生成。

StaticSyncMethod静态同步方法中flag部分相比于普通同步方法增加了ACC_STATIC。

method同步代码块的字节码信息的flag信息中并没有任何与synchronized有关的标志。但是在Code区域中，存在monitorenter和monitorexit。

3: monitorenter
...
13: monitorexit
...
19: monitorexit
...
Exception table:
...
  • monitorenter：标志进入同步代码块（临界范围内，锁的原子内部）
  • 第一个monitorexit：正常退出同步代码块
  • 第二个monitorexit：异常情况下的退出同步代码块。在异常情况下，JVM依然可以正常退出锁。
  • Exception table：在异常情况下配合JVM退出锁

资料：https://www.bilibili.com/video/BV1dv411c79B/ 强推 河北王校长

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