Java之字节码(1) - 深入解析

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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 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。