Java命令javap

javap命令主要用于反编译java的class文件，查看java编译器生成的字节码。

看一段java代码:

public class SynchronizedUse {
    public static void main(String[] args) {
        synchronized (SynchronizedUse.class){
            
        }
        
        display();
    }

    public static synchronized void display() {
        System.out.println(1);
    }
}

对编译后的class文件执行 javap -v SynchronizedUse.class>output.txt:

Classfile /E:/ideaProjects2017/java/out/production/classes/com/gsonkeno/java/concurrent/SynchronizedUse.class
  Last modified 2017-8-31; size 750 bytes
  MD5 checksum f647c188b699918adeede598aff4d432
  Compiled from "SynchronizedUse.java"
public class com.gsonkeno.java.concurrent.SynchronizedUse
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#25         // java/lang/Object."<init>":()V
   #2 = Class              #26            // com/gsonkeno/java/concurrent/SynchronizedUse
   #3 = Methodref          #2.#27         // com/gsonkeno/java/concurrent/SynchronizedUse.display:()V
   #4 = Fieldref           #28.#29        // java/lang/System.out:Ljava/io/PrintStream;
   #5 = Methodref          #30.#31        // java/io/PrintStream.println:(I)V
   #6 = Class              #32            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               LocalVariableTable
  #12 = Utf8               this
  #13 = Utf8               Lcom/gsonkeno/java/concurrent/SynchronizedUse;
  #14 = Utf8               main
  #15 = Utf8               ([Ljava/lang/String;)V
  #16 = Utf8               args
  #17 = Utf8               [Ljava/lang/String;
  #18 = Utf8               StackMapTable
  #19 = Class              #17            // "[Ljava/lang/String;"
  #20 = Class              #32            // java/lang/Object
  #21 = Class              #33            // java/lang/Throwable
  #22 = Utf8               display
  #23 = Utf8               SourceFile
  #24 = Utf8               SynchronizedUse.java
  #25 = NameAndType        #7:#8          // "<init>":()V
  #26 = Utf8               com/gsonkeno/java/concurrent/SynchronizedUse
  #27 = NameAndType        #22:#8         // display:()V
  #28 = Class              #34            // java/lang/System
  #29 = NameAndType        #35:#36        // out:Ljava/io/PrintStream;
  #30 = Class              #37            // java/io/PrintStream
  #31 = NameAndType        #38:#39        // println:(I)V
  #32 = Utf8               java/lang/Object
  #33 = Utf8               java/lang/Throwable
  #34 = Utf8               java/lang/System
  #35 = Utf8               out
  #36 = Utf8               Ljava/io/PrintStream;
  #37 = Utf8               java/io/PrintStream
  #38 = Utf8               println
  #39 = Utf8               (I)V
{
  public com.gsonkeno.java.concurrent.SynchronizedUse();
    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/gsonkeno/java/concurrent/SynchronizedUse;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: ldc           #2                  // class com/gsonkeno/java/concurrent/SynchronizedUse
         2: dup
         3: astore_1
         4: monitorenter
         5: aload_1
         6: monitorexit
         7: goto          15
        10: astore_2
        11: aload_1
        12: monitorexit
        13: aload_2
        14: athrow
        15: invokestatic  #3                  // Method display:()V
        18: return
      Exception table:
         from    to  target type
             5     7    10   any
            10    13    10   any
      LineNumberTable:
        line 8: 0
        line 10: 5
        line 12: 15
        line 13: 18
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      19     0  args   [Ljava/lang/String;
      StackMapTable: number_of_entries = 2
        frame_type = 255 /* full_frame */
          offset_delta = 10
          locals = [ class "[Ljava/lang/String;", class java/lang/Object ]
          stack = [ class java/lang/Throwable ]
        frame_type = 250 /* chop */
          offset_delta = 4

  public static synchronized void display();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED
    Code:
      stack=2, locals=0, args_size=0
         0: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: iconst_1
         4: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V
         7: return
      LineNumberTable:
        line 16: 0
        line 17: 7
}
SourceFile: "SynchronizedUse.java"

参考

• 1. 说说javap命令