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Problem loading custom Java class

I am sending the class object from the client side to the server. Each time the server needs to load the class object sent by the client , instead of reusing it , it follows the model of parent delegation (when it was loaded during the 1st iteration).

I am trying to use a custom server-side class loader whose loadClass(String)just calls findClass()instead of checking with the parent hierarchy. To achieve this, I do the following:

  • Generate byte [] by reading the client-side .class file as follows:
Class cl = com.example.XYZ.class;
String path = cl.getName().replace('.', '/') + ".class";
InputStream is = cl.getClassLoader().getResourceAsStream(path);
ByteArrayOutputStream bos = new ByteArrayOutputStream();
int data = -1;
while((data=is.read())!=-1)
  bos.write(data);
byte[] classBinaryData = bos.toByteArray();

I am sending classBinaryDatato the server.

  1. , byte[], , , MD5, , defineClass findClass.

( , [] .class)

Incompatible magic value ..... in class file <Unknown>

com/example/XYZ (wrong name: com/example/XYZ), defineClass

/.

+4
4

[] .

, , .

class CustomClassLoader extends ClassLoader {

    public Class loadTheClass(String name, byte[] bytes) {

        return defineClass(name, bytes, 0, bytes.length);
    }
}

,

CustomClassLoader ccl = new CustomClassLoader();
        Class cz = ccl.loadTheClass("com.example.XYZ", classBinaryData);
        Object o = cz.newInstance();
  • , '.' '/' .
  • , .
+3

. .

- .

, ; 4 . ( 0xCAFEBABE), .

, , , , .

+1

1. Dot

com/example/XYZ ( : com/example/XYZ), defineClass

, .. com.example.XYZ

Class clazz = classLoader.loadCustomClass("com.example.XYZ", bytes);

2. ( )

.....

, . , java.lang.ClassFormatError. , 0xCAFEBABE ( ) .

, .

  • com.basaki.model.Book Base64 .
  • testLoadingClassWithCorrectMagicNumber Base64 . - .

  • testLoadingClassWithIncorrectCorrectMagicNumber ( Base64) , c b. 0xCAFEBABE 0xBAFEBABE. ,

    java.lang.ClassFormatError: 3137256126 com/basaki/model/Book

    public class LoadingBookFromBinaryArrayTest {

    private static class MyCustomClassLoader extends ClassLoader {

        public Class loadCustomClass(String name, byte[] bytes) {
            return defineClass(name, bytes, 0, bytes.length);
        }
    }

    public static String BOOK_CLAZZ = "yv66vgAAADQAHQoABQAYCQAEABkJAAQAGgcAGwcAHAEABXRpdGxlAQASTGphdmEvbGFuZy9TdHJpbmc7AQAGYXV0aG9yAQAGPGluaXQ-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";

    @Test
    public void testLoadingClassWithCorrectMagicNumber() throws IllegalAccessException, InstantiationException, DecoderException {
        byte[] bytes = Base64.getUrlDecoder().decode(BOOK_CLAZZ);
        MyCustomClassLoader classLoader = new MyCustomClassLoader();
        Class clazz = classLoader.loadCustomClass("com.basaki.model.Book", bytes);
    }

    @Test(expected = ClassFormatError.class)
    public void testLoadingClassWithIncorrectCorrectMagicNumber() throws IllegalAccessException, InstantiationException, DecoderException {
        byte[] bytes = Base64.getUrlDecoder().decode(BOOK_CLAZZ);
        String hex = Hex.encodeHexString(bytes);
        System.out.println(hex);

        // changing magic number 0xCAFEBABE to invalid 0xBAFEBABE
        String malHex = "b" + hex.substring(1, hex.length());
        System.out.println(malHex);
        byte[] malBytes = Hex.decodeHex(malHex.toCharArray());

        MyCustomClassLoader classLoader = new MyCustomClassLoader();
        Class clazz = classLoader.loadCustomClass("com.basaki.model.Book", bytes9);
    }
}
+1

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package org.valhalla.classloader;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.Socket;

public class RemoteClassLoader extends ClassLoader {

    private Socket socket;
    private DataOutputStream dos;
    private DataInputStream dis;

    public RemoteClassLoader(Socket socket, ClassLoader parent) {
        super(parent);
        this.socket = socket;
        OutputStream os;
        InputStream is;
        try {
            os = socket.getOutputStream();
            is = socket.getInputStream();
        } catch (IOException e) {
            throw new RuntimeException("Unable to get Socket output stream", e);
        }
        dos = new DataOutputStream(os);
        dis = new DataInputStream(is);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        Class<?> clz = null;
        System.out.println("Looking up class: " + name);
        synchronized(this.getClassLoadingLock(name)) {
            try {
                System.out.println("Sending request for class: " + name);
                dos.writeUTF(name);
                boolean success = dis.readBoolean();
                System.out.println("Action was " + success);
                if (success) {
                    // Get bytes;
                    System.out.println("Reading size of class file");
                    int len = dis.readInt();
                    System.out.println("Size of class is " + len);
                    byte data[] = new byte[len];
                    int cur, size = 0;
                    for (cur = 0 ; cur < len ;  cur += size) {
                        size = dis.read(data, cur, len - cur);
                        System.out.println("Read size: " + size);
                    }
                    System.out.println("Completed reading class file for class " + name);
                    return defineClass(name, data, 0, len);
                }
            } catch (IOException e) {
                throw new ClassNotFoundException("Class: " + name + " was not found", e);
            }
        }
        return clz;
    }

    public void close() {
        try {
            if (socket != null && socket.isClosed() == false) {
                this.socket.close();
            }
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
    }
}

. , , , , .

, . , .

package org.valhalla.client;

import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.InputStream;
import java.net.Socket;

public class ClientConnection {

    private Socket socket;

    public ClientConnection(Socket socket) {
        this.socket = socket;
    }

    public void process() {
        try {
            DataInputStream dis = new DataInputStream(socket.getInputStream());
            DataOutputStream dos = new DataOutputStream(socket.getOutputStream());
            String name = null;
            while ((name = dis.readUTF()) != null && name.length() > 0) {
                System.out.println("Looking up class: " + name);
                InputStream resource = ClassLoader.getSystemResourceAsStream(name.replace('.', '/') + ".class");
                if (resource == null) {
                    System.out.println("Class not found: " + name);
                    dos.writeBoolean(false);
                    continue;
                }
                System.out.println("Found class: " + name);
                try {
                    byte buf[] = new byte[1024];
                    ByteArrayOutputStream bos = new ByteArrayOutputStream();
                    int size = 0;
                    while ((size = resource.read(buf)) > 0) {
                        bos.write(buf, 0, size);
                    }
                    byte clz[] = bos.toByteArray();
                    dos.writeBoolean(true);
                    System.out.println("Sendding class size: " + clz.length);
                    dos.writeInt(clz.length);
                    System.out.println("Sending class bytes");
                    dos.write(clz);
                    System.out.println("Sent class bytes");
                } catch (Throwable t) {
                    t.printStackTrace();
                    dos.writeBoolean(false);
                }
            }
        } catch (Throwable t) {
            t.printStackTrace();
        } finally {
            if (socket != null && socket.isClosed() == false) {
                try { socket.close(); } catch(Throwable t) {}
            }
        }
    }

}

, , , . , , .

package org.valhalla.classloader;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.Method;
import java.net.ServerSocket;
import java.net.Socket;

public class RemoteClassLoaderServer {

    public static void main(String[] args) {
        if (args.length < 1) {
            System.out.println("syntax error:  missing port");
            System.exit(1);
        }
        int port = 0;
        try {
            port = Integer.parseInt(args[0]);
        } catch(NumberFormatException nfe) {
            System.out.println("Invalid port number: " + args[1]);
            System.exit(2);
        }

        if (port < 0) {
            System.out.println("Port cannot be negative: " + port);
        }

        ServerSocket server = null;

        try {
            server = new ServerSocket(port);
        } catch (IOException e) {
            System.out.println("Unable to create server socket for port: " + port);
            System.exit(3);
        }

        Socket s = null;
        try {
            s = server.accept();
            InputStream is = s.getInputStream();
            DataInputStream dis = new DataInputStream(is);
            System.out.println("Waiting for class name");
            String name = dis.readUTF();
            System.out.println("Received class name: " + name);
            RemoteClassLoader rcl = new RemoteClassLoader(s, RemoteClassLoaderServer.class.getClassLoader());
            System.out.println("Finding class: " + name);
            Class<?> clz = rcl.loadClass(name);
            Method m = clz.getMethod("main", String[].class);
            System.out.println("Executing main method");
            m.invoke(null, new Object[] { new String[0] });
            System.out.println("done");
            new DataOutputStream(s.getOutputStream()).writeUTF("");
        } catch (Throwable e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            if (s != null && s.isClosed() == false) {
                try { s.close(); } catch(Throwable t) {}
            }
        }
    }

}


package org.valhalla.client;

import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.net.UnknownHostException;

public class ClientMain {

    public static void main(String[] args) {
        int port = Integer.parseInt(args[0]);
        try {
            Socket socket = new Socket("localhost", port);
            System.out.println("Opened socket at port: " + port);
            String name = Main.class.getName();
            new DataOutputStream(socket.getOutputStream()).writeUTF(name);
            System.out.println("Sent Class name: " + name);
            ClientConnection conn = new ClientConnection(socket);
            conn.process();
        } catch (UnknownHostException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

.

org.valhalla.client;

public class Main {

    public static void main(String args[]) {
        Client client = new Client();
        client.execute();
    }

}

.

org.valhalla.client;

public class Client {

    public void execute() {
        System.out.println("######### We are calling the Client class execute method #####");
    }

}

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0

Source: https://habr.com/ru/post/1690153/

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