jclasslib与javassist案例

1、.有时候，我们在没有java源程序的情况下，想改变.CLASS文件的部分内容输出或者改变跳转流，怎么办呢？ 介绍2个java hacker的工具javassist 以及jclasslib。下载javassist-3.1RC2和jclasslib_windows_3_0。解压后，将2个JAR 加入classpath内。原文件HelloWorld.javapackage com.unmi; public class HelloWorld public static void main(String args) System.out.println("Hello,gorld!")

2、; 想修改输出的”hello world”,用jclasslib.exe 检测可以看到属于常量池的23。 如下代码就针对23 做调整。package com.unmi;import javassist.ClassPool;import javassist.CtClass;import javassist.CtMethod;import java.io.*;import org.gjt.jclasslib.io.ClassFileWriter;import org.gjt.jclasslib.structures.CPInfo;import org.gjt.jclasslib.structure

3、s.ClassFile;import org.gjt.jclasslib.structures.constants.ConstantStringInfo;import org.gjt.jclasslib.structures.constants.ConstantUtf8Info;public class JclasslibTest public static void main(String args) throws Exception String filePath = "D:java_projcomunmiHelloWorld.class" FileInputStrea

4、m fis = new FileInputStream( filePath); DataInput di = new DataInputStream(fis); ClassFile cf = new ClassFile(); cf.read(di); CPInfo infos = cf.getConstantPool(); int count = infos.length; for (int i = 0; i < count; i+) if (infosi != null) System.out.print(i); System.out.print(" = "); S

5、ystem.out.print(infosi.getVerbose(); System.out.print(" = "); System.out.println(infosi.getTagVerbose(); if(i = 23) ConstantUtf8Info uInfo = (ConstantUtf8Info)infosi; uInfo.setString("china alibaba "); cf.setConstantPool(infos); fis.close(); File f = new File(filePath + "c&q

6、uot;); ClassFileWriter.writeToFile(f, cf); D:java_proj>javac -cp %CLASSPATH%;. com/unmi/JclasslibTest.javaD:java_proj>java com.unmi.JclasslibTest1 = com/unmi/HelloWorld = CONSTANT_Class_info2 = com/unmi/HelloWorld = CONSTANT_Utf8_info3 = java/lang/Object = CONSTANT_Class_info4 = java/lang/Obje

7、ct = CONSTANT_Utf8_info5 = <init> = CONSTANT_Utf8_info6 = ()V = CONSTANT_Utf8_info7 = Code = CONSTANT_Utf8_info8 = java/lang/Object.<init> = CONSTANT_Methodref_info9 = <init>()V = CONSTANT_NameAndType_info10 = LineNumberTable = CONSTANT_Utf8_info11 = LocalVariableTable = CONSTANT_U

8、tf8_info12 = this = CONSTANT_Utf8_info13 = Lcom/unmi/HelloWorld; = CONSTANT_Utf8_info14 = main = CONSTANT_Utf8_info15 = (Ljava/lang/String;)V = CONSTANT_Utf8_info16 = java/lang/System.out = CONSTANT_Fieldref_info17 = java/lang/System = CONSTANT_Class_info18 = java/lang/System = CONSTANT_Utf8_info19

9、= outLjava/io/PrintStream; = CONSTANT_NameAndType_info20 = out = CONSTANT_Utf8_info21 = Ljava/io/PrintStream; = CONSTANT_Utf8_info22 = Hello,gorld! = CONSTANT_String_info23 = Hello,gorld! = CONSTANT_Utf8_info24 = java/io/PrintStream.println = CONSTANT_Methodref_info25 = java/io/PrintStream = CONSTAN

10、T_Class_info26 = java/io/PrintStream = CONSTANT_Utf8_info27 = println(Ljava/lang/String;)V = CONSTANT_NameAndType_info28 = println = CONSTANT_Utf8_info29 = (Ljava/lang/String;)V = CONSTANT_Utf8_info30 = args = CONSTANT_Utf8_info31 = Ljava/lang/String; = CONSTANT_Utf8_info32 = SourceFile = CONSTANT_Utf8_info33 = HelloWorld.java = CONSTANT_Utf8_info更改生成的HelloWorld.classc 为HelloWorld.class执行输出为D:java_proj>java com.unmi.HelloWorldchina alibaba 另外，也可以修改if 语句跳转Jclas