修改jar的.class文件，并重新打包

使用javassist修改.class文件，并重新打包

Javassist是一款字节码编辑工具，可以直接编辑和生成Java生成的字节码，以达到对.class文件进行动态修改的效果。熟练使用这套工具，可以让Java编程更接近与动态语言编程。

下面实现如何修改jar包里的方法

Javassist下载地址 查看

1.准备jar包

my/Solution.java

package my.tools;

public class Solution{

public int lengthOfLongestSubstring(String s) {

int longSub = 0;

if(s.isEmpty()) return 0;

if(s.length()==1) return 1;

char[] arr = s.toCharArray();

String str = String.valueOf(arr[0]);

longSub = 1;

for(int i=1;i

int pos = str.indexOf(arr[i]);

if (pos !=-1) {

str = str.substring(pos+1)+arr[i];

}else{

str+=arr[i];

}

if(str.length()>longSub) longSub = str.length();

}

return longSub;

}

public int strStr(String haystack, String needle) {

if(needle.isEmpty()||needle.equals(haystack)) return 0;

int l=needle.length();

int r = haystack.length()-l;

for(int i=0;i

String tempStr=haystack.substring(i,l+i);

if(tempStr.equals(needle))

return i;

}

return -1;

}

}

View Code

my/Encrypt.java

package my.tools;

import javax.crypto.Cipher;

import javax.crypto.SecretKey;

import javax.crypto.SecretKeyFactory;

import javax.crypto.spec.DESKeySpec;

import javax.crypto.spec.IvParameterSpec;

import java.math.BigInteger;

import java.net.URLDecoder;

import java.security.MessageDigest;

import java.security.NoSuchAlgorithmException;

public class Encrypt {

public static String md5(String plainText) {

byte[] secretBytes = null;

try {

MessageDigest md = MessageDigest.getInstance("MD5");

md.update(plainText.getBytes());

secretBytes = md.digest();

} catch (NoSuchAlgorithmException e) {

throw new RuntimeException("dont have algorithm");

}

String md5code = new BigInteger(1, secretBytes).toString(16);

for (int i = 0; i < 32 - md5code.length(); i++) {

md5code = "0" + md5code;

}

return md5code;

}

public static String decrypt(String message, String key) throws Exception {

byte[] bytesrc = convertHexString(message);

Cipher cipher = Cipher.getInstance("DES/CBC/PKCS5Padding");

DESKeySpec desKeySpec = new DESKeySpec(key.getBytes("UTF-8"));

SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DES");

SecretKey secretKey = keyFactory.generateSecret(desKeySpec);

IvParameterSpec iv = new IvParameterSpec(key.getBytes("UTF-8"));

cipher.init(2, secretKey, iv);

byte[] retByte = cipher.doFinal(bytesrc);

return new String(retByte);

}

public static byte[] convertHexString(String ss) {

byte[] digest = new byte[ss.length() / 2];

for (int i = 0; i < digest.length; i++) {

String byteString = ss.substring(2 * i, 2 * i + 2);

int byteValue = Integer.parseInt(byteString, 16);

digest[i] = ((byte) byteValue);

}

return digest;

}

public static String decode(String value) {

try {

return URLDecoder.decode(decrypt(value, "Yst@2_BI"), "utf-8");

} catch (Exception e) {

e.printStackTrace();

}

return null;

}

}

View Code

新建resource

编译 javac  -sourcepath my my\tools\*.java -d resource

META-INF/MANIFEST.MF

Manifest-Version: 1.0

Created-By: 1.8.0_151 (Oracle Corporation)

进入resource打包  jar -cvfm mytools.jar META-INF\MANIFEST.MF *

使用

import my.tools.Solution;

public class Main {

public static void main(String[] args) {

System.out.println(new Solution().strStr("aacabseew", "ab"));

}

}

输出：3

实现的查找第二个字符串出现的位置

2.修改编译后的class

导入下载的Javassist里的javassist.jar

import javassist.ClassPool;

import javassist.CtClass;

import javassist.CtMethod;

public class Main {

public static void main(String[] args) {

try {

ClassPool.getDefault().insertClassPath("data/mytools.jar");

CtClass c2 = ClassPool.getDefault().getCtClass("my.tools.Solution");

CtMethod[] ms = c2.getDeclaredMethods();

for (CtMethod c : ms) {

System.out.println(c.getName());

CtClass[] ps = c.getParameterTypes();

for (CtClass cx : ps) {

System.out.println("\t" + cx.getName());

}

if (c.getName().equals("strStr") && ps.length == 2

&& ps[0].getName().equals("java.lang.String")

&& ps[1].getName().equals("java.lang.String")) {

c.setBody("{System.out.println($1);return 0;}");

}

}

c2.writeFile();

} catch (Exception e) {

e.printStackTrace();

}

}

}

运行后会在项目根目录下生成修改后的class文件

3.生成新的jar

将以前的jar包修改mytools.zip

解压mytools.zip为mytools1，将刚才生成的my/tools/Solution.class替换掉mytools1里的Solution.class，

压缩mytools1为mytools1.zip

 修改文件名为mytools1.jar

导入新的jar包

测试

import my.tools.Solution;

public class Main {

public static void main(String[] args) {

System.out.println(new Solution().strStr("aacabseew","ab"));

}

}

输出

aacabseew0

 完成

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