[Log4J]一个只打Console并且设有最高级别的Appender

/*
 * Copyright 1999-2005 The Apache Software Foundation.
 * 
 * Licensed 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
 * 
 *      http://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 java.io.IOException;
import java.io.OutputStream;

import org.apache.log4j.Layout;
import org.apache.log4j.Level;
import org.apache.log4j.WriterAppender;
import org.apache.log4j.helpers.LogLog;
import org.apache.log4j.spi.LoggingEvent;

/**
 * ConsoleAppender appends log events to <code>System.out</code> or
 * <code>System.err</code> using a layout specified by the user. The default
 * target is <code>System.out</code>.
 * 
 * @author Ceki Gülcü
 * @author Curt Arnold
 * @since 1.1
 */
public class MyAppender extends WriterAppender {

	public static final String SYSTEM_OUT = "System.out";
	public static final String SYSTEM_ERR = "System.err";

	protected String target = SYSTEM_OUT;

	/**
	 * Determines if the appender honors reassignments of System.out or
	 * System.err made after configuration.
	 */
	private boolean follow = false;

	/**
	 * Constructs an unconfigured appender.
	 */
	public MyAppender() {
	}

	/**
	 * Creates a configured appender.
	 * 
	 * @param layout
	 *            layout, may not be null.
	 */
	public MyAppender(Layout layout) {
		this(layout, SYSTEM_OUT);
	}

	/**
	 * Creates a configured appender.
	 * 
	 * @param layout
	 *            layout, may not be null.
	 * @param targetStr
	 *            target, either "System.err" or "System.out".
	 */
	public MyAppender(Layout layout, String target) {
		setLayout(layout);
		setTarget(target);
		activateOptions();
	}

	/**
	 * Sets the value of the <b>Target</b> option. Recognized values are
	 * "System.out" and "System.err". Any other value will be ignored.
	 * */
	public void setTarget(String value) {
		String v = value.trim();

		if (SYSTEM_OUT.equalsIgnoreCase(v)) {
			target = SYSTEM_OUT;
		} else if (SYSTEM_ERR.equalsIgnoreCase(v)) {
			target = SYSTEM_ERR;
		} else {
			targetWarn(value);
		}
	}

	/**
	 * Returns the current value of the <b>Target</b> property. The default
	 * value of the option is "System.out".
	 * 
	 * See also {@link #setTarget}.
	 * */
	public String getTarget() {
		return target;
	}

	/**
	 * Sets whether the appender honors reassignments of System.out or
	 * System.err made after configuration.
	 * 
	 * @param newValue
	 *            if true, appender will use value of System.out or System.err
	 *            in force at the time when logging events are appended.
	 * @since 1.2.13
	 */
	public final void setFollow(final boolean newValue) {
		follow = newValue;
	}

	/**
	 * Gets whether the appender honors reassignments of System.out or
	 * System.err made after configuration.
	 * 
	 * @return true if appender will use value of System.out or System.err in
	 *         force at the time when logging events are appended.
	 * @since 1.2.13
	 */
	public final boolean getFollow() {
		return follow;
	}

	@Override
	public void append(LoggingEvent event) {

		// Reminder: the nesting of calls is:
		//
		// doAppend()
		// - check threshold
		// - filter
		// - append();
		// - checkEntryConditions();
		// - subAppend();
		//[yangyh]
		if (event.getLevel().isGreaterOrEqual(Level.WARN)) {
			return;
		}
		if (!checkEntryConditions()) {
			return;
		}
		subAppend(event);
	}

	void targetWarn(String val) {
		LogLog.warn("[" + val + "] should be System.out or System.err.");
		LogLog.warn("Using previously set target, System.out by default.");
	}

	/**
	 * Prepares the appender for use.
	 */
	@Override
	public void activateOptions() {
		if (follow) {
			if (target.equals(SYSTEM_ERR)) {
				setWriter(createWriter(new SystemErrStream()));
			} else {
				setWriter(createWriter(new SystemOutStream()));
			}
		} else {
			if (target.equals(SYSTEM_ERR)) {
				setWriter(createWriter(System.err));
			} else {
				setWriter(createWriter(System.out));
			}
		}
		super.activateOptions();
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	protected final void closeWriter() {
		if (follow) {
			super.closeWriter();
		}
	}

	/**
	 * An implementation of OutputStream that redirects to the current
	 * System.err.
	 * 
	 */
	private static class SystemErrStream extends OutputStream {
		public SystemErrStream() {
		}

		@Override
		public void close() {
		}

		@Override
		public void flush() {
			System.err.flush();
		}

		@Override
		public void write(final byte[] b) throws IOException {
			System.err.write(b);
		}

		@Override
		public void write(final byte[] b, final int off, final int len) throws IOException {
			System.err.write(b, off, len);
		}

		@Override
		public void write(final int b) throws IOException {
			System.err.write(b);
		}
	}

	/**
	 * An implementation of OutputStream that redirects to the current
	 * System.out.
	 * 
	 */
	private static class SystemOutStream extends OutputStream {
		public SystemOutStream() {
		}

		@Override
		public void close() {
		}

		@Override
		public void flush() {
			System.out.flush();
		}

		@Override
		public void write(final byte[] b) throws IOException {
			System.out.write(b);
		}

		@Override
		public void write(final byte[] b, final int off, final int len) throws IOException {
			System.out.write(b, off, len);
		}

		@Override
		public void write(final int b) throws IOException {
			System.out.write(b);
		}
	}

}

　　log4j.properties:

### set log levels ###
log4j.rootLogger = debug , stderr, D, A1

log4j.appender.stdout = org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target = System.out
log4j.appender.stdout.layout = org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern = %d{ABSOLUTE} %5p %c{1}:%L - %m%n

log4j.appender.stderr = org.apache.log4j.ConsoleAppender
log4j.appender.stderr.Target = System.err
log4j.appender.stderr.Threshold = WARN
log4j.appender.stderr.layout = org.apache.log4j.PatternLayout
log4j.appender.stderr.layout.ConversionPattern = %d{ABSOLUTE} %5p %c{1}:%L - %m%n

log4j.appender.A1 = com.baidu.tv.video.fetcher.log.MyAppender
log4j.appender.A1.Target = System.out
log4j.appender.A1.Threshold = DEBUG
log4j.appender.A1.layout = org.apache.log4j.PatternLayout
log4j.appender.A1.layout.ConversionPattern = %d{ABSOLUTE} %5p %c{1}:%L - %m%n

log4j.appender.D = org.apache.log4j.DailyRollingFileAppender
log4j.appender.D.File = logs/error.log
log4j.appender.D.Append = true
log4j.appender.D.Threshold = WARN
log4j.appender.D.layout = org.apache.log4j.PatternLayout
log4j.appender.D.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} [ %t:%r ] - [ %p ] %m%n

貌似现在的Appender只有最低级别：Threshold
这里重写了WriterAppander的append方法