Java字节码深入解析

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本文将对Java字节码进行解析，以及一些效率问题。

一：Java字节代码的组织形式

类文件{

OxCAFEBABE，小版本号，大版本号，常量池大小，常量池数组，访问控制标记，当前类信息，父类信息，实现的接口个数，实现的接口信息数组，域个数，域信息数组，方法个数，方法信息数组，属性个数，属性信息数组

}

二：查看方法 — javap命令

例子：有一个Java类Demo.java

public class Demo {

private String str1;

private String str2;

private int num1;

private int num2;

public static final String STATIC_DATA = “hello world”;

private void sayHello1（）{

System.out.println（”this is method1…”）；

}

private void sayHello2（）{

System.out.println（”this is method2…”）；

}

public void sayHello3（）{

System.out.println（”this is method3…”）；

}

}

通过jdk自带的反编译工具命令 javap 可以查看class文件的字节码信息

D:\&;javap -verbose Demo 》 Demo.txt

Demo.txt：

Compiled from “Demo.java”

public class Demo extends java.lang.Object

SourceFile: “Demo.java”

minor version: 0

major version: 49

Constant pool:

const #1 = class #2; // Demo

const #2 = Asciz Demo;

const #3 = class #4; // java/lang/Object

const #4 = Asciz java/lang/Object;

const #5 = Asciz str1;

const #6 = Asciz Ljava/lang/String;;

const #7 = Asciz str2;

const #8 = Asciz num1;

const #9 = Asciz I;

const #10 = Asciz num2;

const #11 = Asciz STATIC_DATA;

const #12 = Asciz ConstantValue;

const #13 = String #14; // hello world

const #14 = Asciz hello world;

const #15 = Asciz <init>;

const #16 = Asciz （）V;

const #17 = Asciz Code;

const #18 = Method #3.#19; // java/lang/Object.”<init>”:（）V

const #19 = NameAndType #15:#16;// “<init>”:（）V

const #20 = Asciz LineNumberTable;

const #21 = Asciz LocalVariableTable;

const #22 = Asciz this;

const #23 = Asciz LDemo;;

const #24 = Asciz sayHello1;

const #25 = Field #26.#28; // java/lang/System.out:Ljava/io/PrintStream;

const #26 = class #27; // java/lang/System

const #27 = Asciz java/lang/System;

const #28 = NameAndType #29:#30;// out:Ljava/io/PrintStream;

const #29 = Asciz out;

const #30 = Asciz Ljava/io/PrintStream;;

const #31 = String #32; // this is method1…

const #32 = Asciz this is method1…；

const #33 = Method #34.#36; // java/io/PrintStream.println:（Ljava/lang/String;）V

const #34 = class #35; // java/io/PrintStream

const #35 = Asciz java/io/PrintStream;

const #36 = NameAndType #37:#38;// println:（Ljava/lang/String;）V

const #37 = Asciz println;

const #38 = Asciz （Ljava/lang/String;）V;

const #39 = Asciz sayHello2;

const #40 = String #41; // this is method2…

const #41 = Asciz this is method2…；

const #42 = Asciz sayHello3;

const #43 = String #44; // this is method3…

const #44 = Asciz this is method3…；

const #45 = Asciz SourceFile;

const #46 = Asciz Demo.java;

{

public static final java.lang.String STATIC_DATA;

Constant value: String hello world

public Demo（）；

Code:

Stack=1, Locals=1, Args_size=1

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怀着淡定从容的心态去面对，也就没有了真正意义上的寂寞了。