common-logging--源码之SimpleLog

• common-logging源码Log接口

在common-logging的源码中，将log核心类抽象成了一个Log接口。

这里贴出Log接口的源码：

/*
 * 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
 *
 *      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.
 */

package org.apache.commons.logging;

/**
 * A simple logging interface abstracting logging APIs.  In order to be
 * instantiated successfully by {@link LogFactory}, classes that implement
 * this interface must have a constructor that takes a single String
 * parameter representing the "name" of this Log.
 * <p>
 * The six logging levels used by <code>Log</code> are (in order):
 * <ol>
 * <li>trace (the least serious)</li>
 * <li>debug</li>
 * <li>info</li>
 * <li>warn</li>
 * <li>error</li>
 * <li>fatal (the most serious)</li>
 * </ol>
 * The mapping of these log levels to the concepts used by the underlying
 * logging system is implementation dependent.
 * The implementation should ensure, though, that this ordering behaves
 * as expected.
 * <p>
 * Performance is often a logging concern.
 * By examining the appropriate property,
 * a component can avoid expensive operations (producing information
 * to be logged).
 * <p>
 * For example,
 * <pre>
 *    if (log.isDebugEnabled()) {
 *        ... do something expensive ...
 *        log.debug(theResult);
 *    }
 * </pre>
 * <p>
 * Configuration of the underlying logging system will generally be done
 * external to the Logging APIs, through whatever mechanism is supported by
 * that system.
 *
 * @version $Id: Log.java 1606045 2014-06-27 12:11:56Z tn $
 */
public interface Log {

    /**
     * Logs a message with debug log level.
     *
     * @param message log this message
     */
    void debug(Object message);

    /**
     * Logs an error with debug log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void debug(Object message, Throwable t);

    /**
     * Logs a message with error log level.
     *
     * @param message log this message
     */
    void error(Object message);

    /**
     * Logs an error with error log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void error(Object message, Throwable t);

    /**
     * Logs a message with fatal log level.
     *
     * @param message log this message
     */
    void fatal(Object message);

    /**
     * Logs an error with fatal log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void fatal(Object message, Throwable t);

    /**
     * Logs a message with info log level.
     *
     * @param message log this message
     */
    void info(Object message);

    /**
     * Logs an error with info log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void info(Object message, Throwable t);

    /**
     * Is debug logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than debug.
     *
     * @return true if debug is enabled in the underlying logger.
     */
    boolean isDebugEnabled();

    /**
     * Is error logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than error.
     *
     * @return true if error is enabled in the underlying logger.
     */
    boolean isErrorEnabled();

    /**
     * Is fatal logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than fatal.
     *
     * @return true if fatal is enabled in the underlying logger.
     */
    boolean isFatalEnabled();

    /**
     * Is info logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than info.
     *
     * @return true if info is enabled in the underlying logger.
     */
    boolean isInfoEnabled();

    /**
     * Is trace logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than trace.
     *
     * @return true if trace is enabled in the underlying logger.
     */
    boolean isTraceEnabled();

    /**
     * Is warn logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than warn.
     *
     * @return true if warn is enabled in the underlying logger.
     */
    boolean isWarnEnabled();

    /**
     * Logs a message with trace log level.
     *
     * @param message log this message
     */
    void trace(Object message);

    /**
     * Logs an error with trace log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void trace(Object message, Throwable t);

    /**
     * Logs a message with warn log level.
     *
     * @param message log this message
     */
    void warn(Object message);

    /**
     * Logs an error with warn log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void warn(Object message, Throwable t);
}

• common-logging源码simpleLog类

前面的博客中，我自己模拟写了一套log4j，这里贴出common-logging中的一个简单例子源码，大致的代码比我前面写的有点差不多，只不过几个值得性能上注意的点还是值得学习的。

/*
 * 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
 *
 *      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.
 */

package org.apache.commons.logging.impl;

import java.io.InputStream;
import java.io.Serializable;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Properties;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogConfigurationException;

/**
 * Simple implementation of Log that sends all enabled log messages,
 * for all defined loggers, to System.err.  The following system properties
 * are supported to configure the behavior of this logger:
 * <ul>
 * <li><code>org.apache.commons.logging.simplelog.defaultlog</code> -
 *     Default logging detail level for all instances of SimpleLog.
 *     Must be one of ("trace", "debug", "info", "warn", "error", or "fatal").
 *     If not specified, defaults to "info". </li>
 * <li><code>org.apache.commons.logging.simplelog.log.xxxxx</code> -
 *     Logging detail level for a SimpleLog instance named "xxxxx".
 *     Must be one of ("trace", "debug", "info", "warn", "error", or "fatal").
 *     If not specified, the default logging detail level is used.</li>
 * <li><code>org.apache.commons.logging.simplelog.showlogname</code> -
 *     Set to <code>true</code> if you want the Log instance name to be
 *     included in output messages. Defaults to <code>false</code>.</li>
 * <li><code>org.apache.commons.logging.simplelog.showShortLogname</code> -
 *     Set to <code>true</code> if you want the last component of the name to be
 *     included in output messages. Defaults to <code>true</code>.</li>
 * <li><code>org.apache.commons.logging.simplelog.showdatetime</code> -
 *     Set to <code>true</code> if you want the current date and time
 *     to be included in output messages. Default is <code>false</code>.</li>
 * <li><code>org.apache.commons.logging.simplelog.dateTimeFormat</code> -
 *     The date and time format to be used in the output messages.
 *     The pattern describing the date and time format is the same that is
 *     used in <code>java.text.SimpleDateFormat</code>. If the format is not
 *     specified or is invalid, the default format is used.
 *     The default format is <code>yyyy/MM/dd HH:mm:ss:SSS zzz</code>.</li>
 * </ul>
 * <p>
 * In addition to looking for system properties with the names specified
 * above, this implementation also checks for a class loader resource named
 * <code>"simplelog.properties"</code>, and includes any matching definitions
 * from this resource (if it exists).
 *
 * @version $Id: SimpleLog.java 1435115 2013-01-18 12:40:19Z tn $
 */
public class SimpleLog implements Log, Serializable {

    /** Serializable version identifier. */
    private static final long serialVersionUID = 136942970684951178L;

    // ------------------------------------------------------- Class Attributes

    /** All system properties used by <code>SimpleLog</code> start with this */
    static protected final String systemPrefix = "org.apache.commons.logging.simplelog.";

    /** Properties loaded from simplelog.properties */
    static protected final Properties simpleLogProps = new Properties();

    /** The default format to use when formating dates */
    static protected final String DEFAULT_DATE_TIME_FORMAT = "yyyy/MM/dd HH:mm:ss:SSS zzz";

    /** Include the instance name in the log message? */
    static volatile protected boolean showLogName = false;

    /** Include the short name ( last component ) of the logger in the log
     *  message. Defaults to true - otherwise we'll be lost in a flood of
     *  messages without knowing who sends them.
     */
    static volatile protected boolean showShortName = true;

    /** Include the current time in the log message */
    static volatile protected boolean showDateTime = false;

    /** The date and time format to use in the log message */
    static volatile protected String dateTimeFormat = DEFAULT_DATE_TIME_FORMAT;

    /**
     * Used to format times.
     * <p>
     * Any code that accesses this object should first obtain a lock on it,
     * ie use synchronized(dateFormatter); this requirement was introduced
     * in 1.1.1 to fix an existing thread safety bug (SimpleDateFormat.format
     * is not thread-safe).
     */
    static protected DateFormat dateFormatter = null;

    // ---------------------------------------------------- Log Level Constants

    /** "Trace" level logging. */
    public static final int LOG_LEVEL_TRACE  = 1;
    /** "Debug" level logging. */
    public static final int LOG_LEVEL_DEBUG  = 2;
    /** "Info" level logging. */
    public static final int LOG_LEVEL_INFO   = 3;
    /** "Warn" level logging. */
    public static final int LOG_LEVEL_WARN   = 4;
    /** "Error" level logging. */
    public static final int LOG_LEVEL_ERROR  = 5;
    /** "Fatal" level logging. */
    public static final int LOG_LEVEL_FATAL  = 6;

    /** Enable all logging levels */
    public static final int LOG_LEVEL_ALL    = LOG_LEVEL_TRACE - 1;

    /** Enable no logging levels */
    public static final int LOG_LEVEL_OFF    = LOG_LEVEL_FATAL + 1;

    // ------------------------------------------------------------ Initializer

    private static String getStringProperty(String name) {
        String prop = null;
        try {
            prop = System.getProperty(name);
        } catch (SecurityException e) {
            // Ignore
        }
        return prop == null ? simpleLogProps.getProperty(name) : prop;
    }

    private static String getStringProperty(String name, String dephault) {
        String prop = getStringProperty(name);
        return prop == null ? dephault : prop;
    }

    private static boolean getBooleanProperty(String name, boolean dephault) {
        String prop = getStringProperty(name);
        return prop == null ? dephault : "true".equalsIgnoreCase(prop);
    }

    // Initialize class attributes.
    // Load properties file, if found.
    // Override with system properties.
    static {
        // Add props from the resource simplelog.properties
        InputStream in = getResourceAsStream("simplelog.properties");
        if(null != in) {
            try {
                simpleLogProps.load(in);
                in.close();
            } catch(java.io.IOException e) {
                // ignored
            }
        }

        showLogName = getBooleanProperty(systemPrefix + "showlogname", showLogName);
        showShortName = getBooleanProperty(systemPrefix + "showShortLogname", showShortName);
        showDateTime = getBooleanProperty(systemPrefix + "showdatetime", showDateTime);

        if(showDateTime) {
            dateTimeFormat = getStringProperty(systemPrefix + "dateTimeFormat",
                                               dateTimeFormat);
            try {
                dateFormatter = new SimpleDateFormat(dateTimeFormat);
            } catch(IllegalArgumentException e) {
                // If the format pattern is invalid - use the default format
                dateTimeFormat = DEFAULT_DATE_TIME_FORMAT;
                dateFormatter = new SimpleDateFormat(dateTimeFormat);
            }
        }
    }

    // ------------------------------------------------------------- Attributes

    /** The name of this simple log instance */
    protected volatile String logName = null;
    /** The current log level */
    protected volatile int currentLogLevel;
    /** The short name of this simple log instance */
    private volatile String shortLogName = null;

    // ------------------------------------------------------------ Constructor

    /**
     * Construct a simple log with given name.
     *
     * @param name log name
     */
    public SimpleLog(String name) {
        logName = name;

        // Set initial log level
        // Used to be: set default log level to ERROR
        // IMHO it should be lower, but at least info ( costin ).
        setLevel(SimpleLog.LOG_LEVEL_INFO);

        // Set log level from properties
        String lvl = getStringProperty(systemPrefix + "log." + logName);
        int i = String.valueOf(name).lastIndexOf(".");
        while(null == lvl && i > -1) {
            name = name.substring(0,i);
            lvl = getStringProperty(systemPrefix + "log." + name);
            i = String.valueOf(name).lastIndexOf(".");
        }

        if(null == lvl) {
            lvl =  getStringProperty(systemPrefix + "defaultlog");
        }

        if("all".equalsIgnoreCase(lvl)) {
            setLevel(SimpleLog.LOG_LEVEL_ALL);
        } else if("trace".equalsIgnoreCase(lvl)) {
            setLevel(SimpleLog.LOG_LEVEL_TRACE);
        } else if("debug".equalsIgnoreCase(lvl)) {
            setLevel(SimpleLog.LOG_LEVEL_DEBUG);
        } else if("info".equalsIgnoreCase(lvl)) {
            setLevel(SimpleLog.LOG_LEVEL_INFO);
        } else if("warn".equalsIgnoreCase(lvl)) {
            setLevel(SimpleLog.LOG_LEVEL_WARN);
        } else if("error".equalsIgnoreCase(lvl)) {
            setLevel(SimpleLog.LOG_LEVEL_ERROR);
        } else if("fatal".equalsIgnoreCase(lvl)) {
            setLevel(SimpleLog.LOG_LEVEL_FATAL);
        } else if("off".equalsIgnoreCase(lvl)) {
            setLevel(SimpleLog.LOG_LEVEL_OFF);
        }
    }

    // -------------------------------------------------------- Properties

    /**
     * Set logging level.
     *
     * @param currentLogLevel new logging level
     */
    public void setLevel(int currentLogLevel) {
        this.currentLogLevel = currentLogLevel;
    }

    /**
     * Get logging level.
     */
    public int getLevel() {
        return currentLogLevel;
    }

    // -------------------------------------------------------- Logging Methods

    /**
     * Do the actual logging.
     * <p>
     * This method assembles the message and then calls <code>write()</code>
     * to cause it to be written.
     *
     * @param type One of the LOG_LEVEL_XXX constants defining the log level
     * @param message The message itself (typically a String)
     * @param t The exception whose stack trace should be logged
     */
    protected void log(int type, Object message, Throwable t) {
        // Use a string buffer for better performance
        final StringBuffer buf = new StringBuffer();

        // Append date-time if so configured
        if(showDateTime) {
            final Date now = new Date();
            String dateText;
            synchronized(dateFormatter) {
                dateText = dateFormatter.format(now);
            }
            buf.append(dateText);
            buf.append(" ");
        }

        // Append a readable representation of the log level
        switch(type) {
            case SimpleLog.LOG_LEVEL_TRACE: buf.append("[TRACE] "); break;
            case SimpleLog.LOG_LEVEL_DEBUG: buf.append("[DEBUG] "); break;
            case SimpleLog.LOG_LEVEL_INFO:  buf.append("[INFO] ");  break;
            case SimpleLog.LOG_LEVEL_WARN:  buf.append("[WARN] ");  break;
            case SimpleLog.LOG_LEVEL_ERROR: buf.append("[ERROR] "); break;
            case SimpleLog.LOG_LEVEL_FATAL: buf.append("[FATAL] "); break;
        }

        // Append the name of the log instance if so configured
        if(showShortName) {
            if(shortLogName == null) {
                // Cut all but the last component of the name for both styles
                final String slName = logName.substring(logName.lastIndexOf(".") + 1);
                shortLogName = slName.substring(slName.lastIndexOf("/") + 1);
            }
            buf.append(String.valueOf(shortLogName)).append(" - ");
        } else if(showLogName) {
            buf.append(String.valueOf(logName)).append(" - ");
        }

        // Append the message
        buf.append(String.valueOf(message));

        // Append stack trace if not null
        if(t != null) {
            buf.append(" <");
            buf.append(t.toString());
            buf.append(">");

            final java.io.StringWriter sw = new java.io.StringWriter(1024);
            final java.io.PrintWriter pw = new java.io.PrintWriter(sw);
            t.printStackTrace(pw);
            pw.close();
            buf.append(sw.toString());
        }

        // Print to the appropriate destination
        write(buf);
    }

    /**
     * Write the content of the message accumulated in the specified
     * <code>StringBuffer</code> to the appropriate output destination.  The
     * default implementation writes to <code>System.err</code>.
     *
     * @param buffer A <code>StringBuffer</code> containing the accumulated
     *  text to be logged
     */
    protected void write(StringBuffer buffer) {
        System.err.println(buffer.toString());
    }

    /**
     * Is the given log level currently enabled?
     *
     * @param logLevel is this level enabled?
     */
    protected boolean isLevelEnabled(int logLevel) {
        // log level are numerically ordered so can use simple numeric
        // comparison
        return logLevel >= currentLogLevel;
    }

    // -------------------------------------------------------- Log Implementation

    /**
     * Logs a message with
     * <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_DEBUG</code>.
     *
     * @param message to log
     * @see org.apache.commons.logging.Log#debug(Object)
     */
    public final void debug(Object message) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_DEBUG)) {
            log(SimpleLog.LOG_LEVEL_DEBUG, message, null);
        }
    }

    /**
     * Logs a message with
     * <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_DEBUG</code>.
     *
     * @param message to log
     * @param t log this cause
     * @see org.apache.commons.logging.Log#debug(Object, Throwable)
     */
    public final void debug(Object message, Throwable t) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_DEBUG)) {
            log(SimpleLog.LOG_LEVEL_DEBUG, message, t);
        }
    }

    /**
     * Logs a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_TRACE</code>.
     *
     * @param message to log
     * @see org.apache.commons.logging.Log#trace(Object)
     */
    public final void trace(Object message) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_TRACE)) {
            log(SimpleLog.LOG_LEVEL_TRACE, message, null);
        }
    }

    /**
     * Logs a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_TRACE</code>.
     *
     * @param message to log
     * @param t log this cause
     * @see org.apache.commons.logging.Log#trace(Object, Throwable)
     */
    public final void trace(Object message, Throwable t) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_TRACE)) {
            log(SimpleLog.LOG_LEVEL_TRACE, message, t);
        }
    }

    /**
     * Logs a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_INFO</code>.
     *
     * @param message to log
     * @see org.apache.commons.logging.Log#info(Object)
     */
    public final void info(Object message) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_INFO)) {
            log(SimpleLog.LOG_LEVEL_INFO,message,null);
        }
    }

    /**
     * Logs a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_INFO</code>.
     *
     * @param message to log
     * @param t log this cause
     * @see org.apache.commons.logging.Log#info(Object, Throwable)
     */
    public final void info(Object message, Throwable t) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_INFO)) {
            log(SimpleLog.LOG_LEVEL_INFO, message, t);
        }
    }

    /**
     * Logs a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_WARN</code>.
     *
     * @param message to log
     * @see org.apache.commons.logging.Log#warn(Object)
     */
    public final void warn(Object message) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_WARN)) {
            log(SimpleLog.LOG_LEVEL_WARN, message, null);
        }
    }

    /**
     * Logs a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_WARN</code>.
     *
     * @param message to log
     * @param t log this cause
     * @see org.apache.commons.logging.Log#warn(Object, Throwable)
     */
    public final void warn(Object message, Throwable t) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_WARN)) {
            log(SimpleLog.LOG_LEVEL_WARN, message, t);
        }
    }

    /**
     * Logs a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_ERROR</code>.
     *
     * @param message to log
     * @see org.apache.commons.logging.Log#error(Object)
     */
    public final void error(Object message) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_ERROR)) {
            log(SimpleLog.LOG_LEVEL_ERROR, message, null);
        }
    }

    /**
     * Logs a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_ERROR</code>.
     *
     * @param message to log
     * @param t log this cause
     * @see org.apache.commons.logging.Log#error(Object, Throwable)
     */
    public final void error(Object message, Throwable t) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_ERROR)) {
            log(SimpleLog.LOG_LEVEL_ERROR, message, t);
        }
    }

    /**
     * Log a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_FATAL</code>.
     *
     * @param message to log
     * @see org.apache.commons.logging.Log#fatal(Object)
     */
    public final void fatal(Object message) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_FATAL)) {
            log(SimpleLog.LOG_LEVEL_FATAL, message, null);
        }
    }

    /**
     * Logs a message with <code>org.apache.commons.logging.impl.SimpleLog.LOG_LEVEL_FATAL</code>.
     *
     * @param message to log
     * @param t log this cause
     * @see org.apache.commons.logging.Log#fatal(Object, Throwable)
     */
    public final void fatal(Object message, Throwable t) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_FATAL)) {
            log(SimpleLog.LOG_LEVEL_FATAL, message, t);
        }
    }

    /**
     * Are debug messages currently enabled?
     * <p>
     * This allows expensive operations such as <code>String</code>
     * concatenation to be avoided when the message will be ignored by the
     * logger.
     */
    public final boolean isDebugEnabled() {
        return isLevelEnabled(SimpleLog.LOG_LEVEL_DEBUG);
    }

    /**
     * Are error messages currently enabled?
     * <p>
     * This allows expensive operations such as <code>String</code>
     * concatenation to be avoided when the message will be ignored by the
     * logger.
     */
    public final boolean isErrorEnabled() {
        return isLevelEnabled(SimpleLog.LOG_LEVEL_ERROR);
    }

    /**
     * Are fatal messages currently enabled?
     * <p>
     * This allows expensive operations such as <code>String</code>
     * concatenation to be avoided when the message will be ignored by the
     * logger.
     */
    public final boolean isFatalEnabled() {
        return isLevelEnabled(SimpleLog.LOG_LEVEL_FATAL);
    }

    /**
     * Are info messages currently enabled?
     * <p>
     * This allows expensive operations such as <code>String</code>
     * concatenation to be avoided when the message will be ignored by the
     * logger.
     */
    public final boolean isInfoEnabled() {
        return isLevelEnabled(SimpleLog.LOG_LEVEL_INFO);
    }

    /**
     * Are trace messages currently enabled?
     * <p>
     * This allows expensive operations such as <code>String</code>
     * concatenation to be avoided when the message will be ignored by the
     * logger.
     */
    public final boolean isTraceEnabled() {
        return isLevelEnabled(SimpleLog.LOG_LEVEL_TRACE);
    }

    /**
     * Are warn messages currently enabled?
     * <p>
     * This allows expensive operations such as <code>String</code>
     * concatenation to be avoided when the message will be ignored by the
     * logger.
     */
    public final boolean isWarnEnabled() {
        return isLevelEnabled(SimpleLog.LOG_LEVEL_WARN);
    }

    /**
     * Return the thread context class loader if available.
     * Otherwise return null.
     *
     * The thread context class loader is available for JDK 1.2
     * or later, if certain security conditions are met.
     *
     * @exception LogConfigurationException if a suitable class loader
     * cannot be identified.
     */
    private static ClassLoader getContextClassLoader() {
        ClassLoader classLoader = null;

        try {
            // Are we running on a JDK 1.2 or later system?
            final Method method = Thread.class.getMethod("getContextClassLoader", (Class[]) null);

            // Get the thread context class loader (if there is one)
            try {
                classLoader = (ClassLoader)method.invoke(Thread.currentThread(), (Class[]) null);
            } catch (IllegalAccessException e) {
                // ignore
            } catch (InvocationTargetException e) {
                /**
                 * InvocationTargetException is thrown by 'invoke' when
                 * the method being invoked (getContextClassLoader) throws
                 * an exception.
                 *
                 * getContextClassLoader() throws SecurityException when
                 * the context class loader isn't an ancestor of the
                 * calling class's class loader, or if security
                 * permissions are restricted.
                 *
                 * In the first case (not related), we want to ignore and
                 * keep going.  We cannot help but also ignore the second
                 * with the logic below, but other calls elsewhere (to
                 * obtain a class loader) will trigger this exception where
                 * we can make a distinction.
                 */
                if (e.getTargetException() instanceof SecurityException) {
                    // ignore
                } else {
                    // Capture 'e.getTargetException()' exception for details
                    // alternate: log 'e.getTargetException()', and pass back 'e'.
                    throw new LogConfigurationException
                        ("Unexpected InvocationTargetException", e.getTargetException());
                }
            }
        } catch (NoSuchMethodException e) {
            // Assume we are running on JDK 1.1
            // ignore
        }

        if (classLoader == null) {
            classLoader = SimpleLog.class.getClassLoader();
        }

        // Return the selected class loader
        return classLoader;
    }

    private static InputStream getResourceAsStream(final String name) {
        return (InputStream)AccessController.doPrivileged(
            new PrivilegedAction() {
                public Object run() {
                    ClassLoader threadCL = getContextClassLoader();

                    if (threadCL != null) {
                        return threadCL.getResourceAsStream(name);
                    } else {
                        return ClassLoader.getSystemResourceAsStream(name);
                    }
                }
            });
    }
}

• simpleLog类值得借鉴的地方

1，simpleLog也同样定义了几个静态常量用来保存从配置文件中读取出来的日志输出控制字段，在这里用了volatile关键字修饰，避免了线程问题。关于volatile关键字我前面的博客里面有，在写完自己的一套web框架后，我会认真的整理下并发编程，关于这块短板以后会补充起来。

2，simpleLog类中几个输出日志的方法，比如debug()，info()等在调用核心log()方法之前都先加一个逻辑控制。

 public final void debug(Object message) {
        if (isLevelEnabled(SimpleLog.LOG_LEVEL_DEBUG)) {
            log(SimpleLog.LOG_LEVEL_DEBUG, message, null);
        }
    }

代码相对比较严谨，而且这里加一个开关也控制了不必要的性能消耗，当然这个开关也直接拎出来了，我们在实际的业务代码中写日志输出的时候也可以调用来加上控制，提高性能，比如减少一些输出日志的字符串连接等等。

 protected boolean isLevelEnabled(int logLevel) {
        // log level are numerically ordered so can use simple numeric
        // comparison
        return logLevel >= currentLogLevel;
    }

3，从properties文件中读取配置的几个方法，特别是读取Boolean类型属性的方法，以后我们就不要老土的用Boolean.booleanValue()了，用一个字符串.equals()就OK。

private static boolean getBooleanProperty(String name, boolean dephault) {
        String prop = getStringProperty(name);
        return prop == null ? dephault : "true".equalsIgnoreCase(prop);
    }

4，加载配置文件获得一个输入流的方法，使用了JDK的几个安全类，这里贴出源码，以后也可以直接拿来使用。

private static ClassLoader getContextClassLoader() {
        ClassLoader classLoader = null;

        try {
            // Are we running on a JDK 1.2 or later system?
            final Method method = Thread.class.getMethod("getContextClassLoader", (Class[]) null);

            // Get the thread context class loader (if there is one)
            try {
                classLoader = (ClassLoader)method.invoke(Thread.currentThread(), (Class[]) null);
            } catch (IllegalAccessException e) {
                // ignore
            } catch (InvocationTargetException e) {
                /**
                 * InvocationTargetException is thrown by 'invoke' when
                 * the method being invoked (getContextClassLoader) throws
                 * an exception.
                 *
                 * getContextClassLoader() throws SecurityException when
                 * the context class loader isn't an ancestor of the
                 * calling class's class loader, or if security
                 * permissions are restricted.
                 *
                 * In the first case (not related), we want to ignore and
                 * keep going.  We cannot help but also ignore the second
                 * with the logic below, but other calls elsewhere (to
                 * obtain a class loader) will trigger this exception where
                 * we can make a distinction.
                 */
                if (e.getTargetException() instanceof SecurityException) {
                    // ignore
                } else {
                    // Capture 'e.getTargetException()' exception for details
                    // alternate: log 'e.getTargetException()', and pass back 'e'.
                    throw new LogConfigurationException
                        ("Unexpected InvocationTargetException", e.getTargetException());
                }
            }
        } catch (NoSuchMethodException e) {
            // Assume we are running on JDK 1.1
            // ignore
        }

        if (classLoader == null) {
            classLoader = SimpleLog.class.getClassLoader();
        }

        // Return the selected class loader
        return classLoader;
    }

    private static InputStream getResourceAsStream(final String name) {
        return (InputStream)AccessController.doPrivileged(
            new PrivilegedAction() {
                public Object run() {
                    ClassLoader threadCL = getContextClassLoader();

                    if (threadCL != null) {
                        return threadCL.getResourceAsStream(name);
                    } else {
                        return ClassLoader.getSystemResourceAsStream(name);
                    }
                }
            });
    }

• 总结

Commons Logging的设计比较简单，它定义了一个Log接口，所有它支持的日志系统都有相应的Log实现类，如Log4JLogger、Jdk14Logger、Jdk13LumberjackLogger、SimpleLog、NoOpLog、AvalonLogger、LogKitLogger等类，在

LogFactory中定义了一定的规则，从而根据当前的环境和配置取得特定的Log子类实例。关于它是如何去获取特定Log实现的，我下一篇博客会整理到。