Java精通并发-透过字节码理解synchronized关键字

在上一次https://www.cnblogs.com/webor2006/p/11428408.html中对于synchronized关键字的作用做了一个实例详解，下面再来看一下这个程序：

请问下，如果一个线程访问了同一个对象的method1()方法之后，另外一个线程能否访问同一个对角的method4的静态方法呢？答案是肯定的，因为method1的锁是锁的对象，而method4锁的是MyClass的类对象，锁是不一样的，当然就可以并行的进行访问啦，

关于synchronized其实从使用角度来说是比较容易理解的，但是要想充分理解它的底层其实并不是那么简单的，“偏向锁”、“轻量级锁”、“重量级锁”、“自旋锁”，可能提到这些东东顺间就懵逼了，这些其实都是在底层所能隐射出来的知识点，所以接下来会多底层来审视这个synchronized，下面一点点来，先看一下新的例子：

对于这个代码其实在实际代码中是非常之常见的，那提个疑问：

其实，改成任何对象其效果都是一模一样的，比如改一下：

而为啥都用Object可能是成了一种标准了，所以实际代码中同步块中都会去锁一个Object对象，这里要注意：其实声明任何类型的对象其效果都是一模一样的，至于为啥在未来的底层探索中会来揭示的。

下面对这个简单的程序进行一下字节码的反编译来看一下synchronized在底层的表现：

xiongweideMacBook-Pro:main xiongwei$ javap -c com/javacurrency/test3/MyTest1.class

Compiled from "MyTest1.java"

public class com.javacurrency.test3.MyTest1 {

  public com.javacurrency.test3.MyTest1();

    Code:

       0: aload_0

       1: invokespecial #1                  // Method java/lang/Object."<init>":()V

       4: aload_0

       5: new           #2                  // class java/util/Date

       8: dup

       9: invokespecial #3                  // Method java/util/Date."<init>":()V

      12: putfield      #4                  // Field object:Ljava/util/Date;

      15: return

  public void method();

    Code:

       0: aload_0

       1: getfield      #4                  // Field object:Ljava/util/Date;

       4: dup

       5: astore_1

       6: monitorenter

       7: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;

      10: ldc           #6                  // String hello world

      12: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V

      15: aload_1

      16: monitorexit

      17: goto          25

      20: astore_2

      21: aload_1

      22: monitorexit

      23: aload_2

      24: athrow

      25: return

    Exception table:

       from    to  target type

           7    17    20   any

          20    23    20   any

}

其中如果想要看到更多的信息，比如常量池【这个在当时的JVM学习中已经详细学过了，就不过多解释了】，则可以用javap -v，如下：

xiongweideMacBook-Pro:main xiongwei$ javap -v com/javacurrency/test3/MyTest1.class

Classfile /Users/xiongwei/Documents/workspace/IntelliJSpace/java_javacurrency/build/classes/java/main/com/javacurrency/test3/MyTest1.class

  Last modified 2019-8-30; size 719 bytes

  MD5 checksum 44fd4afb0a37765559faa8ec0abb158f

  Compiled from "MyTest1.java"

public class com.javacurrency.test3.MyTest1

  minor version: 0

  major version: 52

  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:

   #1 = Methodref          #9.#26         // java/lang/Object."<init>":()V

   #2 = Class              #27            // java/util/Date

   #3 = Methodref          #2.#26         // java/util/Date."<init>":()V

   #4 = Fieldref           #8.#28         // com/javacurrency/test3/MyTest1.object:Ljava/util/Date;

   #5 = Fieldref           #29.#30        // java/lang/System.out:Ljava/io/PrintStream;

   #6 = String             #31            // hello world

   #7 = Methodref          #32.#33        // java/io/PrintStream.println:(Ljava/lang/String;)V

   #8 = Class              #34            // com/javacurrency/test3/MyTest1

   #9 = Class              #35            // java/lang/Object

  #10 = Utf8               object

  #11 = Utf8               Ljava/util/Date;

  #12 = Utf8               <init>

  #13 = Utf8               ()V

  #14 = Utf8               Code

  #15 = Utf8               LineNumberTable

  #16 = Utf8               LocalVariableTable

  #17 = Utf8               this

  #18 = Utf8               Lcom/javacurrency/test3/MyTest1;

  #19 = Utf8               method

  #20 = Utf8               StackMapTable

  #21 = Class              #34            // com/javacurrency/test3/MyTest1

  #22 = Class              #35            // java/lang/Object

  #23 = Class              #36            // java/lang/Throwable

  #24 = Utf8               SourceFile

  #25 = Utf8               MyTest1.java

  #26 = NameAndType        #12:#13        // "<init>":()V

  #27 = Utf8               java/util/Date

  #28 = NameAndType        #10:#11        // object:Ljava/util/Date;

  #29 = Class              #37            // java/lang/System

  #30 = NameAndType        #38:#39        // out:Ljava/io/PrintStream;

  #31 = Utf8               hello world

  #32 = Class              #40            // java/io/PrintStream

  #33 = NameAndType        #41:#42        // println:(Ljava/lang/String;)V

  #34 = Utf8               com/javacurrency/test3/MyTest1

  #35 = Utf8               java/lang/Object

  #36 = Utf8               java/lang/Throwable

  #37 = Utf8               java/lang/System

  #38 = Utf8               out

  #39 = Utf8               Ljava/io/PrintStream;

  #40 = Utf8               java/io/PrintStream

  #41 = Utf8               println

  #42 = Utf8               (Ljava/lang/String;)V

{

  public com.javacurrency.test3.MyTest1();

    descriptor: ()V

    flags: ACC_PUBLIC

    Code:

      stack=3, locals=1, args_size=1

         0: aload_0

         1: invokespecial #1                  // Method java/lang/Object."<init>":()V

         4: aload_0

         5: new           #2                  // class java/util/Date

         8: dup

         9: invokespecial #3                  // Method java/util/Date."<init>":()V

        12: putfield      #4                  // Field object:Ljava/util/Date;

        15: return

      LineNumberTable:

        line 5: 0

        line 6: 4

      LocalVariableTable:

        Start  Length  Slot  Name   Signature

            0      16     0  this   Lcom/javacurrency/test3/MyTest1;

  public void method();

    descriptor: ()V

    flags: ACC_PUBLIC

    Code:

      stack=2, locals=3, args_size=1

         0: aload_0

         1: getfield      #4                  // Field object:Ljava/util/Date;

         4: dup

         5: astore_1

         6: monitorenter

         7: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;

        10: ldc           #6                  // String hello world

        12: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V

        15: aload_1

        16: monitorexit

        17: goto          25

        20: astore_2

        21: aload_1

        22: monitorexit

        23: aload_2

        24: athrow

        25: return

      Exception table:

         from    to  target type

             7    17    20   any

            20    23    20   any

      LineNumberTable:

        line 9: 0

        line 10: 7

        line 11: 15

        line 12: 25

      LocalVariableTable:

        Start  Length  Slot  Name   Signature

            0      26     0  this   Lcom/javacurrency/test3/MyTest1;

      StackMapTable: number_of_entries = 2

        frame_type = 255 /* full_frame */

          offset_delta = 20

          locals = [ class com/javacurrency/test3/MyTest1, class java/lang/Object ]

          stack = [ class java/lang/Throwable ]

        frame_type = 250 /* chop */

          offset_delta = 4

}

SourceFile: "MyTest1.java"

这里重点来看一下方法，首先是默认构造方法：

当然它不是我们要看的重点，要看的是method()方法：

关于上面的字节码的所有信息都在当时JVM的学习中学习过了，也不过多说，只看跟同步相关的东东:

因为要锁对象首先肯定得先获取对象才行，如下：

接着就执行同步块里面的方法了：

也就是：

当同步块执行完了，则会看到字节码中会退出锁：

但是！！！

下次再说~