知识大全 Java字节码深入解析

Posted 2022-07-19 字节

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　　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 str ; private String str ; private int num ; private int num ; public static final String STATIC_DATA = hello world ;

　　private void sayHello () System out println( this is method ); private void sayHello () System out println( this is method ); public void sayHello () System out println( this is method ); 　　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: major version:

　　Constant pool:const # = class # ; // Democonst # = Asciz Demo;const # = class # ; // java/lang/Objectconst # = Asciz java/lang/Object;const # = Asciz str ;const # = Asciz Ljava/lang/String;;const # = Asciz str ;const # = Asciz num ;const # = Asciz I;const # = Asciz num ;const # = Asciz STATIC_DATA;const # = Asciz ConstantValue;const # = String # ; // hello worldconst # = Asciz hello world;const # = Asciz <init>;const # = Asciz ()V;const # = Asciz Code;const # = Method # # ; // java/lang/Object <init> :()Vconst # = NameAndType # :# ;// <init> :()Vconst # = Asciz LineNumberTable;const # = Asciz LocalVariableTable;const # = Asciz this;const # = Asciz LDemo;;const # = Asciz sayHello ;const # = Field # # ; // java/lang/System out:Ljava/io/PrintStream;const # = class # ; // java/lang/Systemconst # = Asciz java/lang/System;const # = NameAndType # :# ;// out:Ljava/io/PrintStream;const # = Asciz out;const # = Asciz Ljava/io/PrintStream;;const # = String # ; // this is method const # = Asciz this is method ;const # = Method # # ; // java/io/PrintStream println:(Ljava/lang/String;)Vconst # = class # ; // java/io/PrintStreamconst # = Asciz java/io/PrintStream;const # = NameAndType # :# ;// println:(Ljava/lang/String;)Vconst # = Asciz println;const # = Asciz (Ljava/lang/String;)V;const # = Asciz sayHello ;const # = String # ; // this is method const # = Asciz this is method ;const # = Asciz sayHello ;const # = String # ; // this is method const # = Asciz this is method ;const # = Asciz SourceFile;const # = Asciz Demo java;

　　public static final java lang String STATIC_DATA; Constant value: String hello worldpublic Demo(); Code: Stack= Locals= Args_size= : aload_ : invokespecial # ; //Method java/lang/Object <init> :()V : return LineNumberTable: line : LocalVariableTable: Start Length Slot Name Signature this LDemo;

　　public void sayHello (); Code: Stack= Locals= Args_size= : getstatic # ; //Field java/lang/System out:Ljava/io/PrintStream; : ldc # ; //String this is method : invokevirtual # ; //Method java/io/PrintStream println:(Ljava/lang/String;)V : return LineNumberTable: line : line : LocalVariableTable: Start Length Slot Name Signature this LDemo;

　　解析

　　版本号 major version //java版本 jdk 显示的是 jdk 显示的是 jdk 显示的是高版本能执行低版本的class文件

　　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 str String str ) return str + str ; 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 Objectpublic TestString(); Code: : aload_ : invokespecial # ; //Method java/lang/Object <init> :()V : return

　　public java lang String testString(java lang String java lang String); Code: : new # ; //class java/lang/StringBuilder : dup : aload_ : invokestatic # ; //Method java/lang/String valueOf:(Ljava/lang/Object;)Ljava/lang/String; : invokespecial # ; //Method java/lang/StringBuilder <init> :(Ljava/lang/String;)V : aload_ : invokevirtual # ; //Method java/lang/StringBuilder append:(Ljava/lang/String;)Ljava/lang/StringBuilder; : invokevirtual # ; //Method java/lang/StringBuilder toString:()Ljava/lang/String; : areturn

　　public java lang String testStringBuffer(java lang StringBuffer java lang String); Code: : aload_ : aload_ : invokevirtual # ; //Method java/lang/StringBuffer append:(Ljava/lang/String;)Ljava/lang/StringBuffer; : invokevirtual # ; //Method java/lang/StringBuffer toString:()Ljava/lang/String; : 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

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