借助jclasslib与javassist修改java+class字节码

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。

jclasslib_windows_3_0.exe在JAVA_HOME/lib下安装

原文件HelloWorld.java

package com.unmi;

public class HelloWorld {

public static void main(String[] args) {

System.out.println(\

} }

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.structures.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 = \ FileInputStream 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 (infos[i] != null) { System.out.print(i); System.out.print(\

System.out.print(infos[i].getVerbose()); System.out.print(\

System.out.println(infos[i].getTagVerbose()); if(i == 23){

ConstantUtf8Info uInfo = (ConstantUtf8Info)infos[i]; uInfo.setString(\

} } }

cf.setConstantPool(infos); fis.close();

File f = new File(filePath + \ ClassFileWriter.writeToFile(f, cf); } } }

D:\\java_proj>javac -cp %CLASSPATH%;. com/unmi/JclasslibTest.java

D:\\java_proj>java com.unmi.JclasslibTest 1 = com/unmi/HelloWorld = CONSTANT_Class_info

2 = com/unmi/HelloWorld = CONSTANT_Utf8_info 3 = java/lang/Object = CONSTANT_Class_info 4 = java/lang/Object = CONSTANT_Utf8_info 5 = = CONSTANT_Utf8_info 6 = ()V = CONSTANT_Utf8_info 7 = Code = CONSTANT_Utf8_info

8 = java/lang/Object. = CONSTANT_Methodref_info 9 = ()V = CONSTANT_NameAndType_info 10 = LineNumberTable = CONSTANT_Utf8_info 11 = LocalVariableTable = CONSTANT_Utf8_info 12 = this = CONSTANT_Utf8_info

13 = Lcom/unmi/HelloWorld; = CONSTANT_Utf8_info 14 = main = CONSTANT_Utf8_info

15 = ([Ljava/lang/String;)V = CONSTANT_Utf8_info 16 = java/lang/System.out = CONSTANT_Fieldref_info 17 = java/lang/System = CONSTANT_Class_info 18 = java/lang/System = CONSTANT_Utf8_info

19 = outLjava/io/PrintStream; = CONSTANT_NameAndType_info 20 = out = CONSTANT_Utf8_info

21 = Ljava/io/PrintStream; = CONSTANT_Utf8_info 22 = Hello,gorld! = CONSTANT_String_info 23 = Hello,gorld! = CONSTANT_Utf8_info

24 = java/io/PrintStream.println = CONSTANT_Methodref_info 25 = java/io/PrintStream = CONSTANT_Class_info 26 = java/io/PrintStream = CONSTANT_Utf8_info

27 = println(Ljava/lang/String;)V = CONSTANT_NameAndType_info 28 = println = CONSTANT_Utf8_info

29 = (Ljava/lang/String;)V = CONSTANT_Utf8_info 30 = args = CONSTANT_Utf8_info

31 = [Ljava/lang/String; = CONSTANT_Utf8_info 32 = SourceFile = CONSTANT_Utf8_info 33 = 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.HelloWorld china alibaba inc.com

另外，也可以修改if 语句跳转

Jclasslib 观测到jvm op code 如下

现在想更改>= 为<,怎么办？ 查询Opcode Mnemonics by Opcode

http://java.sun.com/docs/books/jvms/second_edition/html/Mnemonics.doc.html 并查到 >= 的操作码为 0xA1

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