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。