Java字节码实例分析

这篇文章主要讲解了"Java字节码实例分析"，文中的讲解内容简单清晰，易于学习与理解，下面请大家跟着小编的思路慢慢深入，一起来研究和学习"Java字节码实例分析"吧！

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."":()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:      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。***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。

感谢各位的阅读，以上就是"Java字节码实例分析"的内容了，经过本文的学习后，相信大家对Java字节码实例分析这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是，小编将为大家推送更多相关知识点的文章，欢迎关注！