Java字节码实例分析
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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; 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
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
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。这里是创新互联,小编将为大家推送更多相关知识点的文章,欢迎关注!
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