Java字节码深入解析-白红宇

Java字节码深入解析

阅读量：5889 次

发布时间：2019-06-19

本文共 5322 字，大约阅读时间需要 17 分钟。

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

0: aload_0

1: invokespecial #18; //Method java/lang/Object."<init>":()V

4: return

LineNumberTable:

line 2: 0

LocalVariableTable:

Start Length Slot Name Signature

0 5 0 this LDemo;

public void sayHello3();

Code:

Stack=2, Locals=1, Args_size=1

0: getstatic #25; //Field java/lang/System.out:Ljava/io/PrintStream;

3: ldc #43; //String this is method3...

5: invokevirtual #33; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

8: return

LineNumberTable:

line 17: 0

line 18: 8

LocalVariableTable:

Start Length Slot Name Signature

0 9 0 this LDemo;

}

解析：

　　1、版本号 major version： 49 //java版本 jdk1.6显示的是50， jdk1.5显示的是49，jdk1.4显示的是58 ，高版本能执行低版本的class文件

　　2、常量池Constant pool

　　Method：方法

　　Field：字段

　　String：字符串

　　Asciz：签名如<init>由jvm调用，其他是不能够去调用它的

　　NameAndType：变量名的类型

　　Class：类

　　通过字节码，我们可以看到Demo类继承于java.lang.Object，如果类中没有显式声明构造函数的话，编译器会插入一个缺省无参的构造函数(构造函数在JVM级别是显示成<init>的普通函数)。

　　三：检测代码的效率问题

　　学习Java的过程中，都会了解到字符串合并时要用到StringBuffer 来代替String，那下面就来通过Java字节码来验证两种方式的效率性。

　　例子：一个Java类 TestString.java

<strong>public class TestString {

public String testString(String str1, String str2){

return str1 + str2;

}

public String testStringBuffer(StringBuffer sb, String str){

return sb.append(str).toString();

}

</strong>

　　javap –c TestString 后字节码信息：

Compiled from "TestString.java"

public class TestString extends java.lang.Object{

public TestString();

Code:

0: aload_0

1: invokespecial #8; //Method java/lang/Object."<init>":()V

4: return

public java.lang.String testString(java.lang.String, java.lang.String);

Code:

0: new #16; //class java/lang/StringBuilder

3: dup

4: aload_1

5: invokestatic #18; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;

8: invokespecial #24; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V

11: aload_2

12: invokevirtual #27; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

15: invokevirtual #31; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;

18: areturn

public java.lang.String testStringBuffer(java.lang.StringBuffer, java.lang.String);

Code:

0: aload_1

1: aload_2

2: invokevirtual #40; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;

5: invokevirtual #45; //Method java/lang/StringBuffer.toString:()Ljava/lang/String;

8: areturn

}

　　从上面编译后的字节码信息可以看出来，方法testString 调用了五个方法：new 、invokestatic 、invokespecial 和两个invokevirtual ；而testStringBuffer 方法只调用了两个invokevirtual 方法。第一个方法比第二个方法多做了好多工作，其效率当然是要低的。而且我们从java/lang/StringBuilder.append：（Ljava/lang/String；）Ljava/lang/StringBuilder；

　　可以看出来其实对于String字符串合并，内部还是转化为StringBuilder的方法调用，这是因为String是长度不可变的，所以不如直接采用StringBuilder（与StringBuffer 长度都是可变的，只不过前者是非线程安全，后者是线程安全）进行字符串合并。

转载于:https://blog.51cto.com/cfkjava/884924

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