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Javassist (JAVA programming ASSISTant) makes Java bytecode manipulation simple. It is a class library for editing bytecodes in Java.
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/*
 * Javassist, a Java-bytecode translator toolkit.
 * Copyright (C) 1999- Shigeru Chiba. All Rights Reserved.
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License.  Alternatively, the contents of this file may be used under
 * the terms of the GNU Lesser General Public License Version 2.1 or later,
 * or the Apache License Version 2.0.
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 */

package javassist.convert;

import javassist.CannotCompileException;
import javassist.CtClass;
import javassist.bytecode.CodeAttribute;
import javassist.bytecode.CodeIterator;
import javassist.bytecode.ConstPool;
import javassist.bytecode.Descriptor;
import javassist.bytecode.StackMap;
import javassist.bytecode.StackMapTable;

final public class TransformNew extends Transformer {
    private int nested;
    private String classname, trapClass, trapMethod;

    public TransformNew(Transformer next,
                 String classname, String trapClass, String trapMethod) {
        super(next);
        this.classname = classname;
        this.trapClass = trapClass;
        this.trapMethod = trapMethod;
    }

    @Override
    public void initialize(ConstPool cp, CodeAttribute attr) {
        nested = 0;
    }

    /**
     * Replace a sequence of
     *    NEW classname
     *    DUP
     *    ...
     *    INVOKESPECIAL
     * with
     *    NOP
     *    NOP
     *    ...
     *    INVOKESTATIC trapMethod in trapClass
     */
    @Override
    public int transform(CtClass clazz, int pos, CodeIterator iterator,
                         ConstPool cp) throws CannotCompileException
    {
        int index;
        int c = iterator.byteAt(pos);
        if (c == NEW) {
            index = iterator.u16bitAt(pos + 1);
            if (cp.getClassInfo(index).equals(classname)) {
                if (iterator.byteAt(pos + 3) != DUP)
                    throw new CannotCompileException(
                                "NEW followed by no DUP was found");

                iterator.writeByte(NOP, pos);
                iterator.writeByte(NOP, pos + 1);
                iterator.writeByte(NOP, pos + 2);
                iterator.writeByte(NOP, pos + 3);
                ++nested;

                StackMapTable smt
                    = (StackMapTable)iterator.get().getAttribute(StackMapTable.tag);
                if (smt != null)
                    smt.removeNew(pos);

                StackMap sm
                    = (StackMap)iterator.get().getAttribute(StackMap.tag);
                if (sm != null)
                    sm.removeNew(pos);
            }
        }
        else if (c == INVOKESPECIAL) {
            index = iterator.u16bitAt(pos + 1);
            int typedesc = cp.isConstructor(classname, index);
            if (typedesc != 0 && nested > 0) {
                int methodref = computeMethodref(typedesc, cp);
                iterator.writeByte(INVOKESTATIC, pos);
                iterator.write16bit(methodref, pos + 1);
                --nested;
            }
        }

        return pos;
    }

    private int computeMethodref(int typedesc, ConstPool cp) {
        int classIndex = cp.addClassInfo(trapClass);
        int mnameIndex = cp.addUtf8Info(trapMethod);
        typedesc = cp.addUtf8Info(
                Descriptor.changeReturnType(classname,
                                            cp.getUtf8Info(typedesc)));
        return cp.addMethodrefInfo(classIndex,
                        cp.addNameAndTypeInfo(mnameIndex, typedesc));
    }
}