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ProGuard is a free Java class file shrinker, optimizer, and obfuscator. It removes unused classes, fields, methods, and attributes. It then optimizes the bytecode. It then renames the remaining classes, fields, and methods using short meaningless names. Finally, it preverifies the processed code for Java 6 or for Java Micro Edition.
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/*
 * ProGuard -- shrinking, optimization, obfuscation, and preverification
 *             of Java bytecode.
 *
 * Copyright (c) 2002-2009 Eric Lafortune ([email protected])
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the Free
 * Software Foundation; either version 2 of the License, or (at your option)
 * any later version.
 *
 * This program 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 for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 */
package proguard.optimize.info;

import proguard.classfile.*;
import proguard.classfile.attribute.*;
import proguard.classfile.attribute.visitor.AttributeVisitor;
import proguard.classfile.instruction.*;
import proguard.classfile.util.SimplifiedVisitor;
import proguard.classfile.visitor.*;

/**
 * This ClassPoolVisitor marks all methods that have side effects.
 *
 * @see ReadWriteFieldMarker
 * @see NoSideEffectMethodMarker
 * @author Eric Lafortune
 */
public class SideEffectMethodMarker
extends      SimplifiedVisitor
implements   ClassPoolVisitor,
             ClassVisitor,
             MemberVisitor,
             AttributeVisitor
{
    // A reusable object for checking whether instructions have side effects.
    private final SideEffectInstructionChecker sideEffectInstructionChecker = new SideEffectInstructionChecker(false);

    // Parameters and values for visitor methods.
    private int     newSideEffectCount;
    private boolean hasSideEffects;


    // Implementations for ClassPoolVisitor.

    public void visitClassPool(ClassPool classPool)
    {
        // Go over all classes and their methods, marking if they have side
        // effects, until no new cases can be found.
        do
        {
            newSideEffectCount = 0;

            // Go over all classes and their methods once.
            classPool.classesAccept(this);
        }
        while (newSideEffectCount > 0);
    }


    // Implementations for ClassVisitor.

    public void visitProgramClass(ProgramClass programClass)
    {
        // Go over all methods.
        programClass.methodsAccept(this);
    }


    // Implementations for MemberVisitor.

    public void visitProgramMethod(ProgramClass programClass, ProgramMethod programMethod)
    {
        if (!hasSideEffects(programMethod) &&
            !NoSideEffectMethodMarker.hasNoSideEffects(programMethod))
        {
            // Initialize the return value.
            hasSideEffects =
                (programMethod.getAccessFlags() &
                 (ClassConstants.INTERNAL_ACC_NATIVE |
                  ClassConstants.INTERNAL_ACC_SYNCHRONIZED)) != 0;

            // Look further if the method hasn't been marked yet.
            if (!hasSideEffects)
            {
                // Investigate the actual code.
                programMethod.attributesAccept(programClass, this);
            }

            // Mark the method depending on the return value.
            if (hasSideEffects)
            {
                markSideEffects(programMethod);

                newSideEffectCount++;
            }
        }
    }


    // Implementations for AttributeVisitor.

    public void visitAnyAttribute(Clazz clazz, Attribute attribute) {}


    public void visitCodeAttribute(Clazz clazz, Method method, CodeAttribute codeAttribute)
    {
        // Remember whether the code has any side effects.
        hasSideEffects = hasSideEffects(clazz, method, codeAttribute);
    }


    // Small utility methods.

    /**
     * Returns whether the given code has any side effects.
     */
    private boolean hasSideEffects(Clazz         clazz,
                                   Method        method,
                                   CodeAttribute codeAttribute)
    {
        byte[] code   = codeAttribute.code;
        int    length = codeAttribute.u4codeLength;

        // Go over all instructions.
        int offset = 0;
        do
        {
            // Get the current instruction.
            Instruction instruction = InstructionFactory.create(code, offset);

            // Check if it may be throwing exceptions.
            if (sideEffectInstructionChecker.hasSideEffects(clazz,
                                                            method,
                                                            codeAttribute,
                                                            offset,
                                                            instruction))
            {
                return true;
            }

            // Go to the next instruction.
            offset += instruction.length(offset);
        }
        while (offset < length);

        return false;
    }


    private static void markSideEffects(Method method)
    {
        MethodOptimizationInfo info = MethodOptimizationInfo.getMethodOptimizationInfo(method);
        if (info != null)
        {
            info.setSideEffects();
        }
    }


    public static boolean hasSideEffects(Method method)
    {
        MethodOptimizationInfo info = MethodOptimizationInfo.getMethodOptimizationInfo(method);
        return info == null ||
               info.hasSideEffects();
    }
}