最近在學習逆向分析和反編譯，無意之中瞭解到了JClassLib。JClassLib不但是一個位元組碼閱讀器而且還包含一個類庫允許開發者讀取,修改,寫入Java Class檔案與位元組碼。其他的用途我就不說了，先看一下效果。

package com.qx;

public class Main {
	public static void main(String[] args) {
		print();
	}
	
	static void print() {
		System.out.println("google");
	}
}
 

package cc.cc.c;

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.ConstantUtf8Info;

public class Main {
	public static void main(String[] args) throws Exception {

		String filePath = "C:\\Main.class";
		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 == 21) {//剛剛找到的是21位置
					ConstantUtf8Info uInfo = (ConstantUtf8Info) infos[i]; //剛剛那裡是CONSTANT_Utf-8_info所以這裡要用這個
					uInfo.setBytes("baidu".getBytes());
					infos[i] = uInfo;
				}
			}
		}
		//這種方式也可以，一樣的
/*		if(infos[count] != null) {
			ConstantUtf8Info uInfo = (ConstantUtf8Info) infos[i]; //剛剛那裡是CONSTANT_Utf-8_info所以這裡要用這個
			uInfo.setBytes("baidu".getBytes());
			infos[count] = uInfo;
		}*/
		
		cf.setConstantPool(infos);
		fis.close();
		File f = new File(filePath);
		ClassFileWriter.writeToFile(f, cf);
	}
}