Modify the class file inside the jar package

Error message

Exception in thread “main” java.lang.SecurityException: class “xxx.xxx.xx”‘s signer information does not match signer information of other classes in the same package
at java.lang.ClassLoader.checkCerts(ClassLoader.java:898)
at java.lang.ClassLoader.preDefineClass(ClassLoader.java:668)
at java.lang.ClassLoader.defineClass(ClassLoader.java:761)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)

Since the jar package signature is encrypted, you can see *RSA, *.SF files under the MATE-INF directory of the jar using decompiler or other software such as idea, you need to remove these files to replace the class files inside the jar package.

files in the jar package.

Remove jar package signature

https://stackoverflow.com/questions/7757083/how-do-i-unsign-a-jar/34331144#34331144

The code is as follows

import java.io.BufferedReader;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Enumeration;
import java.util.zip.ZipEntry;
import java.util.zip.ZipFile;
import java.util.zip.ZipOutputStream;

public class JarUnsigner {

  private static final String MANIFEST = "META-INF/MANIFEST.MF";

  public static void main(String[] args){
    String infile = “xxx.jar”;  
    String outfile = "xxx-unnsigned.jar";  
    if ((new File(outfile)).exists()){
      System.out.println("Output file already exists:" + outfile);
      System.exit(1);
    }
    try{
      ZipFile zipFile = new ZipFile(infile);
      final ZipOutputStream zos = new ZipOutputStream(new FileOutputStream(outfile));
      for (Enumeration e = zipFile.entries(); e.hasMoreElements();) {
        ZipEntry entryIn = (ZipEntry) e.nextElement();

        if (! exclude_file( entryIn.getName() ) ) {

          /* copy the entry as-is */
          zos.putNextEntry( new ZipEntry( entryIn.getName() ));
          InputStream is = zipFile.getInputStream(entryIn);
          byte[] buf = new byte[1024];
          int len;
          while ((len = (is.read(buf))) > 0) {
            zos.write(buf, 0, len);
          }
          zos.closeEntry();

        } else {

          if (MANIFEST.equals(entryIn.getName())){
            /* if MANIFEST, adjust the entry */
            zos.putNextEntry(new ZipEntry(MANIFEST));

            // manifest entries until first empty line. i.e. the 'MainAttributes' section
            // (this method is used so to keep the formatting exactly the same)
            InputStream mIS = zipFile.getInputStream(entryIn);
            BufferedReader in = new BufferedReader(new InputStreamReader(mIS));
            String line = in.readLine();
            byte[] mNL = "\n".getBytes("UTF-8");
            while( line != null && !line.trim().isEmpty() ) {
              zos.write( line.getBytes("UTF-8"));
              zos.write( mNL );
              line = in.readLine();
            }
            zos.write( mNL );
            zos.closeEntry();

          }else{
            /* else: Leave out the Signature files */
          }

        }

      }
      zos.close();
      System.out.println("Successfully unsigned " + outfile);

    }catch(IOException ex){
      System.err.println("Error for file: " + infile);
      ex.printStackTrace();
      System.exit(1);
    }
  }

  /**
   * Exclude .SF signature file
   * Exclude .RSA and DSA (signed version of .SF file) 
   * Exclude SIG-  files  (unknown sign types for signed .SF file)
   * Exclude Manifest file
   * @param filename
   * @return 
   */
  public static boolean exclude_file(String filename){
    return filename.equals("META-INF/MANIFEST.MF") ||
           filename.startsWith("META-INF/SIG-") || 
           filename.startsWith("META-INF/") && ( filename.endsWith(".SF") || filename.endsWith(".RSA") || filename.endsWith(".DSA") );
  }

}

After running the jar, you can modify the class files in the jar package.

There is a simpler way, open the jar with a compression software, delete the related *.SF, *.RSA, *.DSA files