synchronized关键字所生成的字节码详细分析

在之前已经将如下这样的源文件对应的字节码文件完整的分析完了，如下：

这次再来写一个内容稍丰富一点的类，准备再来从头至尾的来分析一下，对其字节码的理解进一步巩固，如下：

然后用javap -verbose来查看一下反编译信息：

xiongweideMacBook-Pro:jvm_lectue xiongwei$ cd out/production/classes/

xiongweideMacBook-Pro:classes xiongwei$ javap -verbose com/jvm/bytecode/MyTest2.class

Classfile /Users/xiongwei/Documents/workspace/IntelliJSpace/jvm_lectue/out/production/classes/com/jvm/bytecode/MyTest2.class

  Last modified Sep 11, 2018; size 831 bytes

  MD5 checksum 22c52151c374d39068e39938d1b3e72c

  Compiled from "MyTest2.java"

public class com.jvm.bytecode.MyTest2

  minor version: 0

  major version: 52

  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:

   #1 = Methodref          #10.#34        // java/lang/Object."<init>":()V

   #2 = String             #35            // Welcome

   #3 = Fieldref           #5.#36         // com/jvm/bytecode/MyTest2.str:Ljava/lang/String;

   #4 = Fieldref           #5.#37         // com/jvm/bytecode/MyTest2.x:I

   #5 = Class              #38            // com/jvm/bytecode/MyTest2

   #6 = Methodref          #5.#34         // com/jvm/bytecode/MyTest2."<init>":()V

   #7 = Methodref          #5.#39         // com/jvm/bytecode/MyTest2.setX:(I)V

   #8 = Methodref          #40.#41        // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;

   #9 = Fieldref           #5.#42         // com/jvm/bytecode/MyTest2.in:Ljava/lang/Integer;

  #10 = Class              #43            // java/lang/Object

  #11 = Utf8               str

  #12 = Utf8               Ljava/lang/String;

  #13 = Utf8               x

  #14 = Utf8               I

  #15 = Utf8               in

  #16 = Utf8               Ljava/lang/Integer;

  #17 = Utf8               <init>

  #18 = Utf8               ()V

  #19 = Utf8               Code

  #20 = Utf8               LineNumberTable

  #21 = Utf8               LocalVariableTable

  #22 = Utf8               this

  #23 = Utf8               Lcom/jvm/bytecode/MyTest2;

  #24 = Utf8               main

  #25 = Utf8               ([Ljava/lang/String;)V

  #26 = Utf8               args

  #27 = Utf8               [Ljava/lang/String;

  #28 = Utf8               myTest2

  #29 = Utf8               setX

  #30 = Utf8               (I)V

  #31 = Utf8               <clinit>

  #32 = Utf8               SourceFile

  #33 = Utf8               MyTest2.java

  #34 = NameAndType        #17:#18        // "<init>":()V

  #35 = Utf8               Welcome

  #36 = NameAndType        #11:#12        // str:Ljava/lang/String;

  #37 = NameAndType        #13:#14        // x:I

  #38 = Utf8               com/jvm/bytecode/MyTest2

  #39 = NameAndType        #29:#30        // setX:(I)V

  #40 = Class              #44            // java/lang/Integer

  #41 = NameAndType        #45:#46        // valueOf:(I)Ljava/lang/Integer;

  #42 = NameAndType        #15:#16        // in:Ljava/lang/Integer;

  #43 = Utf8               java/lang/Object

  #44 = Utf8               java/lang/Integer

  #45 = Utf8               valueOf

  #46 = Utf8               (I)Ljava/lang/Integer;

{

  java.lang.String str;

    descriptor: Ljava/lang/String;

    flags:

  public static java.lang.Integer in;

    descriptor: Ljava/lang/Integer;

    flags: ACC_PUBLIC, ACC_STATIC

  public com.jvm.bytecode.MyTest2();

    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: ldc           #2                  // String Welcome

         7: putfield      #3                  // Field str:Ljava/lang/String;

        10: aload_0

        11: iconst_5

        12: putfield      #4                  // Field x:I

        15: return

      LineNumberTable:

        line 3: 0

        line 4: 4

        line 6: 10

      LocalVariableTable:

        Start  Length  Slot  Name   Signature

            0      16     0  this   Lcom/jvm/bytecode/MyTest2;

  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           #5                  // class com/jvm/bytecode/MyTest2

         3: dup

         4: invokespecial #6                  // Method "<init>":()V

         7: astore_1

         8: aload_1

         9: bipush        8

        11: invokevirtual #7                  // Method setX:(I)V

        14: bipush        20

        16: invokestatic  #8                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;

        19: putstatic     #9                  // Field in:Ljava/lang/Integer;

        22: return

      LineNumberTable:

        line 11: 0

        line 13: 8

        line 15: 14

        line 16: 22

      LocalVariableTable:

        Start  Length  Slot  Name   Signature

            0      23     0  args   [Ljava/lang/String;

            8      15     1 myTest2   Lcom/jvm/bytecode/MyTest2;

  public void setX(int);

    descriptor: (I)V

    flags: ACC_PUBLIC

    Code:

      stack=2, locals=2, args_size=2

         0: aload_0

         1: iload_1

         2: putfield      #4                  // Field x:I

         5: return

      LineNumberTable:

        line 19: 0

        line 20: 5

      LocalVariableTable:

        Start  Length  Slot  Name   Signature

            0       6     0  this   Lcom/jvm/bytecode/MyTest2;

            0       6     1     x   I

  static {};

    descriptor: ()V

    flags: ACC_STATIC

    Code:

      stack=1, locals=0, args_size=0

         0: bipush        10

         2: invokestatic  #8                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;

         5: putstatic     #9                  // Field in:Ljava/lang/Integer;

         8: return

      LineNumberTable:

        line 8: 0

}

SourceFile: "MyTest2.java"

由于这次的类中声明有静态变量，所以标红处多了一个static块，下面将“-verbose”参数去掉可以看到一个更精简的信息，如下：

其中注意此时有一个setX()方法的，那如果将setX()方法访问修饰符由“public”改为"private"，看会发生啥：

为啥呢？其实并非私有的方法在字节码信息中就不存在了，而是用javap命令需要再加一个参数才能看到私有的方法，如下：

由于此篇讨论的话题是关于synchronized关键字在字节码中的含义，所以接下来咱们给setX()方法加一个同步锁然后再来分析其字节码，在加之前先来看下不带synchronized所对应字节码的信息，如下：

接下来给方法加上synchronized，如下：

然后看一下此时对应的字节码会发生啥变化：

貌似网上有很多资料看到对于同步方法在字节码中表现通常会有“moniterenter”和“moniterexit”，但是为啥咱们自己通过javap -verbose查看不到呢？因为修饰的是实例方法，也就是给当前对象上锁，表现则是在方法说明之上而非代码中，如果改为修饰方法中的代码的话则就会可以看到啦，如下：

当然上面的这个是对参数str进行上锁，实际没啥意义，因为str是一个可变的值，正常的做法应该是对一个不可变的对象进行上锁，这里只是为了说明问题，下面再来看一下字节码信息：

private void test(java.lang.String);

    descriptor: (Ljava/lang/String;)V

    flags: ACC_PRIVATE

    Code:

      stack=3, locals=4, args_size=2

         0: aload_1

         1: dup

         2: astore_2

         3: monitorenter

         4: getstatic     #10                 // Field java/lang/System.out:Ljava/io/PrintStream;

         7: ldc           #11                 // String hello world~

         9: iconst_0

        10: anewarray     #12                 // class java/lang/Object

        13: invokevirtual #13                 // Method java/io/PrintStream.printf:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;

        16: pop

        17: aload_2

        18: monitorexit

        19: goto          27

        22: astore_3

        23: aload_2

        24: monitorexit

        25: aload_3

        26: athrow

        27: return

      Exception table:

         from    to  target type

             4    19    22   any

            22    25    22   any

      LineNumberTable:

        line 23: 0

        line 24: 4

        line 25: 17

        line 26: 27

      LocalVariableTable:

        Start  Length  Slot  Name   Signature

            0      28     0  this   Lcom/jvm/bytecode/MyTest2;

            0      28     1   str   Ljava/lang/String;

      StackMapTable: number_of_entries = 2

        frame_type = 255 /* full_frame */

          offset_delta = 22

          locals = [ class com/jvm/bytecode/MyTest2, class java/lang/String, class java/lang/Object ]

          stack = [ class java/lang/Throwable ]

        frame_type = 250 /* chop */

          offset_delta = 4

确实是看到了monitorenter，monitorexit了，其加了同步锁在字节码的流程可以大致分析一下：

另外synchronized还可以修饰静态方法，下面写一个：

很显然它锁的是整个class对象，下面也来看一下它对应的字节码信息：

由于synchronized修饰的是方法，而非它里面的代码，所以在javap中只看到了方法签名上有同步信息。

关于同步锁需要了解一下如下概念：
重入锁：所谓重入锁就是说同一个线程可以访问多个同步方法，比如说：

访问一次同步方法其引用计数就加1，像上面的引用计数就会变为2，而当setB()执行完之后，则引用计数就会减为1，再setX()执行完则引用计数会减为0。

非重入锁：是指多个线程而言的，比如一个线程访问了一个同步方法，另一个线程再访问同一个对象的该方法则会阻塞。