src/examples/Helloify.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

import org.apache.bcel.classfile.ClassParser;
import org.apache.bcel.classfile.Code;
import org.apache.bcel.classfile.ConstantClass;
import org.apache.bcel.classfile.ConstantPool;
import org.apache.bcel.classfile.ConstantUtf8;
import org.apache.bcel.classfile.JavaClass;
import org.apache.bcel.classfile.Method;
import org.apache.bcel.classfile.Utility;
import org.apache.bcel.generic.ConstantPoolGen;
import org.apache.bcel.generic.GETSTATIC;
import org.apache.bcel.generic.INVOKESPECIAL;
import org.apache.bcel.generic.INVOKEVIRTUAL;
import org.apache.bcel.generic.InstructionHandle;
import org.apache.bcel.generic.InstructionList;
import org.apache.bcel.generic.MethodGen;
import org.apache.bcel.generic.PUSH;

/**
 * Read class file(s) and patch all of its methods, so that they print "hello" and their name and signature before doing
 * anything else.
 */
public final class Helloify {

    private static String className;
    private static ConstantPoolGen cp;
    private static int out; // reference to System.out
    private static int println; // reference to PrintStream.println

    /**
     * Change class name to <old_name>_hello
     */
    private static void helloifyClassName(final JavaClass javaClass) {
        className = javaClass.getClassName() + "_hello";
        int index = javaClass.getClassNameIndex();

        index = ((ConstantClass) cp.getConstant(index)).getNameIndex();
        cp.setConstant(index, new ConstantUtf8(Utility.packageToPath(className)));
    }

    /**
     * Patch a method.
     */
    private static Method helloifyMethod(Method m) {
        final Code code = m.getCode();
        final int flags = m.getAccessFlags();
        final String name = m.getName();

        // Sanity check
        if (m.isNative() || m.isAbstract() || code == null) {
            return m;
        }

        // Create instruction list to be inserted at method start.
        final String mesg = "Hello from " + Utility.methodSignatureToString(m.getSignature(), name, Utility.accessToString(flags));
        final InstructionList patch = new InstructionList();
        patch.append(new GETSTATIC(out));
        patch.append(new PUSH(cp, mesg));
        patch.append(new INVOKEVIRTUAL(println));

        final MethodGen mg = new MethodGen(m, className, cp);
        final InstructionList il = mg.getInstructionList();
        final InstructionHandle[] ihs = il.getInstructionHandles();

        if (name.equals("<init>")) { // First let the super or other constructor be called
            for (int j = 1; j < ihs.length; j++) {
                if (ihs[j].getInstruction() instanceof INVOKESPECIAL) {
                    il.append(ihs[j], patch); // Should check: method name == "<init>"
                    break;
                }
            }
        } else {
            il.insert(ihs[0], patch);
        }

        // Stack size must be at least 2, since the println method takes 2 argument.
        if (code.getMaxStack() < 2) {
            mg.setMaxStack(2);
        }

        m = mg.getMethod();

        il.dispose(); // Reuse instruction handles

        return m;
    }

    public static void main(final String[] argv) throws Exception {
        for (final String arg : argv) {
            if (arg.endsWith(JavaClass.EXTENSION)) {
                final JavaClass javaClass = new ClassParser(arg).parse();
                final ConstantPool constants = javaClass.getConstantPool();
                final String fileName = arg.substring(0, arg.length() - 6) + "_hello.class";
                cp = new ConstantPoolGen(constants);

                helloifyClassName(javaClass);

                out = cp.addFieldref("java.lang.System", "out", "Ljava/io/PrintStream;");
                println = cp.addMethodref("java.io.PrintStream", "println", "(Ljava/lang/String;)V");
                // Patch all methods.
                final Method[] methods = javaClass.getMethods();

                for (int j = 0; j < methods.length; j++) {
                    methods[j] = helloifyMethod(methods[j]);
                }

                // Finally dump it back to a file.
                javaClass.setConstantPool(cp.getFinalConstantPool());
                javaClass.dump(fileName);
            }
        }
    }
}