Java Bukkit load .jar file from InputStream

Question

Hello Stackoverflow community,

i have a question about java and bukkit specific. I have a encrypted plugin and i dont want to save the decrypted file on disk. So i used InputStream. But my problem now is how to inject this file into minecraft ( bukkit ). Is there any Custom Classloader available ? I searched alot but i dont find any working solution.

I decrypt an encrypted plugin with AES-128:

    FileInputStream fin; 
    CipherInputStream cin;
    int nread = 0;
    byte [] inbuf = new byte [MAX_FILE_BUF];

    fin = new FileInputStream (input);
    cin = new CipherInputStream (fin, mDecipher);
    ByteArrayOutputStream baos = new ByteArrayOutputStream();

    while ((nread = cin.read (inbuf)) > 0 )
    {
        byte[] trimbuf = new byte [nread];
        for (int i = 0; i < nread; i++)
        {
            trimbuf[i] = inbuf[i];
        }
        baos.write(trimbuf);
    }

So now i tried to load the file with InputStream

    ByteArrayOutputStream out = new ByteArrayOutputStream();
    InputStream is2 = new ByteArrayInputStream(baos.toByteArray());
    JarInputStream in = new JarInputStream(is2);

This is working well at this point. Now i want to load this InputStream "is2" into bukkit server.

Answer 1

Because the way class loading works in java, you can not directly load a class from a inputstream like you have. What you can do, is:

• Load every file from the jar to the memory
• Use a custom classloader that loads the files from the memory
• Use that classloader to load your plugin

The first step is easy, we are going to make a Map<String,byte[]> for the files that come from the jar file.

Map<String,byte[]> map = new HashMap<>();
ZipEntry entry;
byte[] read = new byte[1024];
while((entry = in.getNextEntry()) != null) {
    ByteArrayOutputStream r = new ByteArrayOutputStream(Math.max(128, entry.getSize()));
    int i;
    while((i = in.read(read) >= 0)
        r.write(read, 0, i);
    is.close();
    map.put(entry.getName(), r.toByteArray());
}

Having our map of filename -> byte array of data, we can then implement our custom class loader:

public class MappedJarClassLoader extends ClassLoader {
     Map<String,byte[]> map = new HashMap<>();

     public MappedJarClassLoader (ClassLoader parent, Map<String,byte[]> map) {
         super(parent);
         this.map = map;
     }

     public Class findClass(String name) throws ClassNotFoundException {
         byte[] b = map.get(name.replace('/', '$').replace('.', File.separatorChar));
         if(b == null)
             throw new ClassNotFoundException(name);
         return defineClass(name, b, 0, b.length);
     }
}

Because of the estrictions in bukkit, we cannot place the class extending JavaPlugin inside our encrypted part of the jar. Instead of doing that, place a class inside the encrypted jar that has a constructor that accepts the main instance, and load the class in the onEnable using:

mappedJarClassLoader.loadClass("path.to.new.class").getConstructor(getClass()).newInstance(this)