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 ;
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."
Java字节码深入解析
":()V4: return
LineNumberTable:
line 2: 0
LocalVariableTable:
StartLengthSlotName 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:
StartLengthSlotName 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
}
Java字节码深入解析
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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