Java字节码深入解析

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。

原文链接：http://15838341661-139-com.iteye.com/blog/1287866

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