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Papaya for Testing Log4J: Utilities for Java 1.8+ written by Kevin Connor ARPE
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package com.googlecode.kevinarpe.papaya.testing.logging.log4j;

/*
 * #%L
 * This file is part of Papaya.
 * %%
 * Copyright (C) 2013 - 2014 Kevin Connor ARPE ([email protected])
 * %%
 * Papaya 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 3 of the License, or
 * (at your option) any later version.
 * 
 * GPL Classpath Exception:
 * This project is subject to the "Classpath" exception as provided in
 * the LICENSE file that accompanied this code.
 * 
 * Papaya 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 Papaya.  If not, see .
 * #L%
 */

import com.google.common.collect.ImmutableList;
import com.googlecode.kevinarpe.papaya.annotation.FullyTested;
import com.googlecode.kevinarpe.papaya.testing.logging.CapturingLogger;
import org.apache.log4j.Appender;
import org.apache.log4j.Logger;
import org.apache.log4j.spi.LoggingEvent;
import org.mockito.ArgumentCaptor;

import java.util.List;

import static org.mockito.Mockito.atLeastOnce;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;

/**
 * Convenient base class for testing classes that employ Log4J for logging.
 *
 * Subclasses should call methods in this order:
 * 
 *     {@link #addMockAppender()} - capture logging events
 *     {@link #removeMockAppender()} - cleanup
 *     {@link #getLoggingEventList()} - perform logging event analysis
 * 
 *
 * @author Kevin Connor ARPE ([email protected])
 *
 * @see http://slackhacker.com/2009/12/08/testing-logging-behaviour-in-four-code-lines-flat/
 * @see CapturingLogger
 * @see LoggingEvent
 */
@FullyTested
public class Log4JTestBase
implements CapturingLogger {

    private final Appender _mockAppender;

    /**
     * Creates a new mock logger.  Must be added to root logger via {@link #addMockAppender()}.
     *
     * @see #addMockAppender()
     * @see #removeMockAppender()
     */
    protected Log4JTestBase() {
        _mockAppender = mock(Appender.class);
    }

    /**
     * Appends a mock logger to the root logger.  Logging events are only captured after calling
     * this method.
     *
     * Logging events are accessible via {@link #getLoggingEventList()}.
     *
     * @see Logger#getRootLogger()
     * @see Logger#addAppender(Appender)
     * @see #removeMockAppender()
     * @see #getLoggingEventList()
     */
    protected final void addMockAppender() {
        Logger rootLogger = Logger.getRootLogger();
        rootLogger.addAppender(_mockAppender);
    }

    /**
     * Removes the mock logger from the root logger.  Logging events are not capturing after calling
     * this method.
     *
     * Logging events are accessible via {@link #getLoggingEventList()}.
     *
     * @see Logger#getRootLogger()
     * @see Logger#removeAppender(Appender)
     * @see #addMockAppender()
     * @see #getLoggingEventList()
     */
    protected final void removeMockAppender() {
        Logger rootLogger = Logger.getRootLogger();
        rootLogger.removeAppender(_mockAppender);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public final List getLoggingEventList() {
        ArgumentCaptor argumentCaptor = ArgumentCaptor.forClass(LoggingEvent.class);
        List loggingEventList = null;
        try {
            verify(_mockAppender, atLeastOnce()).doAppend(argumentCaptor.capture());
            loggingEventList = argumentCaptor.getAllValues();
        }
        catch (Throwable ignore) {
            loggingEventList = ImmutableList.of();
        }
        return loggingEventList;
    }
}