使用javap反编译class文件

一个普通的Java类：

package org.ccnt.concurrence;

public class VolatileTest {

    public static volatile int race = 0;
    
    public static void increate() {
        race ++;
    }
    
    private static final int THREAD_COUNT = 20;
    
    public static void main(String[] args) {
        Thread[] threads = new Thread[THREAD_COUNT];
        for (int i=0; i<THREAD_COUNT; i++) {
            threads[i] = new Thread(new Runnable() {
                
                @Override
                public void run() {
                    for (int i=0; i<10000; i++){
                        increate();
                    }
                }
            });
            threads[i].start();
        }
        
        while (Thread.activeCount() > 1) {
            Thread.yield();
        }
        
        System.out.println(race);
    }
}

现在我们使用JDK带的反编译工具javap来反编译VilatileTest.class，先执行下面的命令：

javap org.ccnt.concurrence.VolatileTest > VolatileTest.jad

VolatieTest.jad

Compiled from "VolatileTest.java"
public class org.ccnt.concurrence.VolatileTest {
  public static volatile int race;
  static {};
  public org.ccnt.concurrence.VolatileTest();
  public static void increate();
  public static void main(java.lang.String[]);
}

查看字节码：

javap -c org.ccnt.concurrence.VolatileTest > VolatileTest.txt

VolatileTest.txt

Compiled from "VolatileTest.java"
public class org.ccnt.concurrence.VolatileTest {
  public static volatile int race;

  static {};
    Code:
       0: iconst_0      
       1: putstatic     #13                 // Field race:I
       4: return        

  public org.ccnt.concurrence.VolatileTest();
    Code:
       0: aload_0       
       1: invokespecial #18                 // Method java/lang/Object."<init>":()V
       4: return        

  public static void increate();
    Code:
       0: getstatic     #13                 // Field race:I
       3: iconst_1      
       4: iadd          
       5: putstatic     #13                 // Field race:I
       8: return        

  public static void main(java.lang.String[]);
    Code:
       0: bipush        20
       2: anewarray     #25                 // class java/lang/Thread
       5: astore_1      
       6: iconst_0      
       7: istore_2      
       8: goto          37
      11: aload_1       
      12: iload_2       
      13: new           #25                 // class java/lang/Thread
      16: dup           
      17: new           #27                 // class org/ccnt/concurrence/VolatileTest$1
      20: dup           
      21: invokespecial #29                 // Method org/ccnt/concurrence/VolatileTest$1."<init>":()V
      24: invokespecial #30                 // Method java/lang/Thread."<init>":(Ljava/lang/Runnable;)V
      27: aastore       
      28: aload_1       
      29: iload_2       
      30: aaload        
      31: invokevirtual #33                 // Method java/lang/Thread.start:()V
      34: iinc          2, 1
      37: iload_2       
      38: bipush        20
      40: if_icmplt     11
      43: goto          49
      46: invokestatic  #36                 // Method java/lang/Thread.yield:()V
      49: invokestatic  #39                 // Method java/lang/Thread.activeCount:()I
      52: iconst_1      
      53: if_icmpgt     46
      56: getstatic     #43                 // Field java/lang/System.out:Ljava/io/PrintStream;
      59: getstatic     #13                 // Field race:I
      62: invokevirtual #49                 // Method java/io/PrintStream.println:(I)V
      65: return        
}