Java字节码深入解析

会计考友 · 发表于 2012-8-4 12:44:44

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 ;
const #16 = Asciz ()V;
const #17 = Asciz Code;
const #18 = Method    #3.#19; //  java/lang/Object."":()V
const #19 = NameAndType #15:#16;//  "":()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."

会计考友 · 发表于 2012-8-4 12:44:45

":()V
4:    return
  LineNumberTable:
line 2: 0
  LocalVariableTable:
Start  Length  Slot  Name Signature
0    5    0 this    LDemo;</p>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
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();
}
}

　　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."":()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."":(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:    areturnwｗw.ExamＷ.CoM
}

会计考友 · 发表于 2012-8-4 12:44:46

　　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；</p>　　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。

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[专业语言] Java字节码深入解析

Java字节码深入解析

Java字节码深入解析