Load a Byte Array into a Memory Class Loader

Question

I am wondering how I can load a byte array into a memory URLClassLoader? The byte array is the decrypted bytes of a jar file (as seen below)!

Most of the memory class loaders are using ClassLoader and not URLClassLoader! I need it to be using URLClassLoader.

    byte[] fileB = Util.crypt.getFileBytes(inputFile);
    byte[] dec;
    dec = Util.crypt.decrypt(fileB, "16LENGTHLONGKEYX".getBytes());
    //Load bytes into memory and load a class here?

Thanks!

You might have to stand on your head and create a URLStreamHandlerFactory and all the gorp behind it. Or else extend URLClassLoader so you can invoke defineClass. — Hot Licks
– Hot Licks, Commented Mar 23, 2012 at 1:15

Chris White · Accepted Answer · 2012-03-23 00:55:30Z

2

Have you looked at the NetworkClassLoader example in the ClassLoader javadocs:

http://docs.oracle.com/javase/6/docs/api/index.html?java/lang/ClassLoader.html

Using this as a base, you just need to implement the loadClassData method, which will pull the desired resource from decrypted jar bytes. You can wrap the decrypted bytes with a JarInputStream(new ByteArrayInputStream(dec)), then iterate through the jar entries until you find the resource / class you're interested in and then return the jar entry's byte array

answered Mar 23, 2012 at 0:55

Chris White

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2 Comments

Timothy Hanes Over a year ago

Interesting, although I am not loading the jars directly from a network. All the jars are actually stored locally in the temp directory and downloaded every time you run the program. I am using "new URLClassLoader(jars)" (jars being an array list of all the jar files) to create the class loader, that seemed to work, although using a normal ClassLoader and adding them didn't.

Chris White Over a year ago

I'm not saying that this will load it from a network, it's just some example code you can amend for your purposes. You can call your EncryptedJarClassLoader, add as many jars to an internal collection as you want and assuming you have a way of decrypting them, you should be able to follow the above

MrJames · Accepted Answer · 2012-03-23 01:11:32Z

1

I will post here a implementation I had done in the past:

// main
String className = "tests.compiler.DynamicCompilationHelloWorldEtc";
//...
ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); 
File classesDir = new File(tempDir);
CustomClassLoader ccl = new CustomClassLoader(classLoader, classesDir);         
if (ccl != null) {
    Class clazz = ccl.loadClass(className);
///...
}

My Custom ClassLoader:

package tests.classloader;

import java.io.File;
import java.io.FileInputStream;
import java.nio.ByteBuffer;

public class CustomClassLoader extends ClassLoader {

    private File classesDir;

    public CustomClassLoader(ClassLoader parent, File classesDir) {
       super(parent);      

       this.classesDir = classesDir;
    }

    public Class findClass(String name) {
       byte[] data = loadDataFromAny(name);
       return defineClass(name, data, 0, data.length);
    }

    private byte[] loadDataFromAny(String name) {

        name = name.replace('.', '/');
        name = name + ".class";

        byte[] ret = null;

        try {
            File f = new File(classesDir.getAbsolutePath(), name);
            FileInputStream fis = new FileInputStream(f);

            ByteBuffer bb = ByteBuffer.allocate(4*1024); 
            byte[] buf = new byte[1024];
            int readedBytes = -1; 

            while ((readedBytes = fis.read(buf)) != -1) {
                bb.put(buf, 0, readedBytes);
            }

            ret = bb.array();           
        }
        catch (Exception e) {
            e.printStackTrace();
        }

        return ret;
    }
}

answered Mar 23, 2012 at 1:11

MrJames

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2 Comments

Timothy Hanes Over a year ago

Let me try this and I'll get back to you! This may actually work.

MrJames Over a year ago

I didn't post all the code, so probably you need to change/correct some lines. The original code contained a dynamic Java compiler that receives a String with code, compiles it and loads it using this ClassLoader.

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