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。
