javap——查看class文件的方法

有时候为了研究Javac的原理，要去看看class文件的内容是如何组织的，这时候很有必要查看class文件。方法有很多种，这里推荐使用JDK自带的javap工具。 
首先建立如下源码：

public class HelloWorld{
    public static void main(String[] args){
        System.out.println("hello world!");
    }
}

使用javac编译器把源文件编译好，再接着使用javap工具查看class文件，命令如下 
javap -verbose HelloWorld

这是DOS窗口的运行结果复制出来如下图：

F:>javac HelloWorld.java

F:>javap -verbose HelloWorld
Classfile /F:/HelloWorld.class
  Last modified 2017-12-24; size 426 bytes
  MD5 checksum 4efac412ef483c8a3fe7489c87d15c8c
  Compiled from "HelloWorld.java"
public class HelloWorld
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#15         // java/lang/Object."<init>":()V
   #2 = Fieldref           #16.#17        // java/lang/System.out:Ljava/io/Print
Stream;
   #3 = String             #18            // hello world!
   #4 = Methodref          #19.#20        // java/io/PrintStream.println:(Ljava/
lang/String;)V
   #5 = Class              #21            // HelloWorld
   #6 = Class              #22            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               main
  #12 = Utf8               ([Ljava/lang/String;)V
  #13 = Utf8               SourceFile
  #14 = Utf8               HelloWorld.java
  #15 = NameAndType        #7:#8          // "<init>":()V
  #16 = Class              #23            // java/lang/System
  #17 = NameAndType        #24:#25        // out:Ljava/io/PrintStream;
  #18 = Utf8               hello world!
  #19 = Class              #26            // java/io/PrintStream
  #20 = NameAndType        #27:#28        // println:(Ljava/lang/String;)V
  #21 = Utf8               HelloWorld
  #22 = Utf8               java/lang/Object
  #23 = Utf8               java/lang/System
  #24 = Utf8               out
  #25 = Utf8               Ljava/io/PrintStream;
  #26 = Utf8               java/io/PrintStream
  #27 = Utf8               println
  #28 = Utf8               (Ljava/lang/String;)V
{
  public HelloWorld();
    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 1: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljav
a/io/PrintStream;
         3: ldc           #3                  // String hello world!
         5: invokevirtual #4                  // Method java/io/PrintStream.prin
tln:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 3: 0
        line 4: 8
}
SourceFile: "HelloWorld.java"

F:>

这样就是JVM规范的class文件了。结合这个对比源码可以深刻的认识到Javac的作用所在。