/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 */

/*

 * build         @BUILD_TAG_PLACEHOLDER@

 *

 * @COPYRIGHT_MINI_LEGAL_NOTICE_PLACEHOLDER@

 */

import java.io.*;

import java.lang.reflect.*;

/*

  ClassLoader that knows how to fabricate exactly one class.  The name

  of the class is defined by the parameter singleClassName.  When

  asked to load a class, this loader first delegates to its parent as

  usual, then, if that doesn't find the class and if the name is the

  same as singleClassName, the class is fabricated.  It is a public

  class with no fields or methods and a single public no-arg

  constructor that simply calls its parent's no-arg constructor.  This

  means that the parent must have a public or protected no-arg

  constructor.

 */

public class SingleClassLoader extends ClassLoader {

    SingleClassLoader(String singleClassName, Class superclass,

                      ClassLoader parent) {

        super(parent);

        Constructor superConstr;

        try {

            superConstr = superclass.getDeclaredConstructor(new Class[0]);

        } catch (NoSuchMethodException e) {

            throw new IllegalArgumentException("Superclass must have no-arg " +

                                               "constructor");

        }

        int superConstrMods = superConstr.getModifiers();

        if ((superConstrMods & (Modifier.PUBLIC|Modifier.PROTECTED)) == 0) {

            final String msg =

                "Superclass no-arg constructor must be public or protected";

            throw new IllegalArgumentException(msg);

        }

        this.singleClassName = singleClassName;

        final Class c;

        try {

            c = makeClass(superclass);

        } catch (IOException e) {

            throw new RuntimeException(e.toString());

        }

        this.singleClass = c;

    }

    private Class makeClass(Class superclass) throws IOException {

        final String superName = superclass.getName();

        ByteArrayOutputStream bout = new ByteArrayOutputStream();

        dout = new DataOutputStream(bout);

        String thisNameInternal = singleClassName.replace('.', '/');

        String superNameInternal = superName.replace('.', '/');

        dout.writeInt(0xcafebabe); // magic

        dout.writeInt(0x0003002d); // major 45 minor 3

        dout.writeShort(10);       // cpool count (incl virtual 0 element)

        cpoolIndex = 1;

        int thisNameConst = writeUTFConst(thisNameInternal);

        int thisClassConst = writeClassConst(thisNameConst);

        int superNameConst = writeUTFConst(superNameInternal);

        int superClassConst = writeClassConst(superNameConst);

        int initNameConst = writeUTFConst("<init>");

        int voidNoArgSigConst = writeUTFConst("()V");

        int codeNameConst = writeUTFConst("Code");

        int superConstructorNameAndTypeConst =

            writeNameAndTypeConst(initNameConst, voidNoArgSigConst);

        int superConstructorMethodRefConst =

            writeMethodRefConst(superClassConst,

                                superConstructorNameAndTypeConst);

        dout.writeShort(Modifier.PUBLIC | 0x20 /*SUPER*/);

        dout.writeShort(thisClassConst);

        dout.writeShort(superClassConst);

        dout.writeInt(0);   // n interfaces, n fields

        dout.writeShort(1); // n methods

        // <init> method

        dout.writeShort(Modifier.PUBLIC);

        dout.writeShort(initNameConst);

        dout.writeShort(voidNoArgSigConst);

        dout.writeShort(1); // attr count

        // Code attribute

        dout.writeShort(codeNameConst);

        dout.writeInt(17);   // len

        dout.writeShort(1);  // max stack

        dout.writeShort(1);  // max locals

        dout.writeInt(5);    // code len

        dout.writeByte(42);  // aload_0

        dout.writeByte(183); // invokespecial

        dout.writeShort(superConstructorMethodRefConst);

        dout.writeByte(177); // return

        dout.writeShort(0);  // 0 catches

        dout.writeShort(0);  // 0 method attrs

        dout.writeShort(0);  // 0 class attrs

        dout.close();

        byte[] classBytes = bout.toByteArray();

        dout = null;

        return

            defineClass(singleClassName, classBytes, 0, classBytes.length);

    }

    protected Class findClass(String name) throws ClassNotFoundException {

        if (name.equals(singleClassName))

            return singleClass;

        else

            throw new ClassNotFoundException(name);

    }

    private int writeUTFConst(String s) throws IOException {

        dout.writeByte(1);

        dout.writeUTF(s);

        return cpoolIndex++;

    }

    private int writeClassConst(int nameIndex) throws IOException {

        dout.writeByte(7);

        dout.writeShort((short) nameIndex);

        return cpoolIndex++;

    }

    private int writeNameAndTypeConst(int nameIndex, int typeIndex)

            throws IOException {

        dout.writeByte(12);

        dout.writeShort((short) nameIndex);

        dout.writeShort((short) typeIndex);

        return cpoolIndex++;

    }

    private int writeMethodRefConst(int classIndex, int nameAndTypeIndex)

            throws IOException {

        dout.writeByte(10);

        dout.writeShort((short) classIndex);

        dout.writeShort((short) nameAndTypeIndex);

        return cpoolIndex++;

    }

    private final String singleClassName;

    private final Class singleClass;

    private DataOutputStream dout;

    private int cpoolIndex;

}