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An auxiliary findbugs.sourceforge.net plugin for java bug detectors that fall outside the narrow scope of detectors to be packaged with the product itself.

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/*
 * fb-contrib - Auxiliary detectors for Java programs
 * Copyright (C) 2005-2018 Dave Brosius
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */
package com.mebigfatguy.fbcontrib.detect;

import org.apache.bcel.classfile.Code;
import org.apache.bcel.classfile.Method;
import org.apache.bcel.generic.Type;

import com.mebigfatguy.fbcontrib.utils.BugType;
import com.mebigfatguy.fbcontrib.utils.StopOpcodeParsingException;

import edu.umd.cs.findbugs.BugInstance;
import edu.umd.cs.findbugs.BugReporter;
import edu.umd.cs.findbugs.BytecodeScanningDetector;
import edu.umd.cs.findbugs.OpcodeStack;
import edu.umd.cs.findbugs.ba.ClassContext;

/**
 * looks for methods that are defined to return Boolean, but return null. This thus allows three return values, Boolean.FALSE, Boolean.TRUE and null. If three
 * values intended, it would be more clear to just create an enumeration with three values and return that type.
 */
public class TristateBooleanPattern extends BytecodeScanningDetector {
    private BugReporter bugReporter;
    private OpcodeStack stack;

    /**
     * constructs a TBP detector given the reporter to report bugs on
     *
     * @param bugReporter
     *            the sync of bug reports
     */
    public TristateBooleanPattern(BugReporter bugReporter) {
        this.bugReporter = bugReporter;
    }

    /**
     * implements the visitor to allocate the opcode stack
     *
     * @param classContext
     *            the context object of the currently parsed class
     */
    @Override
    public void visitClassContext(ClassContext classContext) {
        try {
            stack = new OpcodeStack();
            super.visitClassContext(classContext);
        } finally {
            stack = null;
        }
    }

    /**
     * implements the visitor to filter out methods that don't return Boolean,
     *
     *
     * @param obj
     *            the context object for the currently parsed code block
     */
    @Override
    public void visitCode(Code obj) {
        try {
            Method m = getMethod();
            Type retType = m.getReturnType();
            if ("Ljava/lang/Boolean;".equals(retType.getSignature())) {
                stack.resetForMethodEntry(this);
                super.visitCode(obj);
            }
        } catch (StopOpcodeParsingException e) {
            // if the method was already reported
        }
    }

    /**
     * implements the visitor to look for null returns
     */
    @Override
    public void sawOpcode(int seen) {
        try {

            stack.precomputation(this);

            if ((seen == ARETURN) && (stack.getStackDepth() > 0)) {
                OpcodeStack.Item item = stack.getStackItem(0);
                if (item.isNull()) {
                    bugReporter.reportBug(new BugInstance(this, BugType.TBP_TRISTATE_BOOLEAN_PATTERN.name(), NORMAL_PRIORITY).addClass(this).addMethod(this)
                            .addSourceLine(this));
                    throw new StopOpcodeParsingException();
                }
            }
        } finally {
            stack.sawOpcode(this, seen);
        }
    }

}