Method to edit a method inside a .jar file using Javassist

EditMethod.java

import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URI;
import java.nio.file.FileSystem;
import java.nio.file.FileSystems;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardCopyOption;
import java.util.HashMap;
import java.util.Map;
import java.util.jar.JarFile;
import java.util.zip.ZipEntry;

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.NotFoundException;

public class EditMethod {
	public static void modifyJar(String pathToJAR, String pathToClassInsideJAR, String methodName, String type, String codeToSetMethod) throws IOException, CannotCompileException, NotFoundException {
		//REQUIRES JAVASSIST
		JarFile jarFile = new JarFile(pathToJAR);
		ZipEntry zipEntry = jarFile.getEntry(pathToClassInsideJAR);
		InputStream fis = jarFile.getInputStream(zipEntry);
		
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.makeClass(fis);
		fis.close();
		jarFile.close();
		//Type should be in this format: "()Ljava/lang/String;" (example for String)
		CtMethod cm = cc.getMethod(methodName, type);
		cm.setBody(codeToSetMethod);
		//Replace Windows's Stupid backslashes
		String classFileName = pathToClassInsideJAR.replace("\\", "/").substring(0, pathToClassInsideJAR.lastIndexOf('/'));
		DataOutputStream out = new DataOutputStream(new FileOutputStream(classFileName));
                cc.getClassFile().write(out);
        
                Map<String, String> launchenv = new HashMap<>(); 
                URI launchuri = URI.create("jar:"+new File(pathToJAR).toURI());
		launchenv.put("create", "true");
		try (FileSystem zipfs = FileSystems.newFileSystem(launchuri, launchenv)) {
		    Path externalClassFile = Paths.get(classFileName);
		    Path pathInJarfile = zipfs.getPath(pathToClassInsideJAR);          
		    // copy a file into the zip file
		    Files.copy( externalClassFile,pathInJarfile, 
			    StandardCopyOption.REPLACE_EXISTING ); 
		}
	}
}

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