logfactory.java

这是使用java语言设计和实现一个日志库Customer Logging Library（CLL）。

/*
 * 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;


import java.io.BufferedReader;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Enumeration;
import java.util.Hashtable;
import java.util.Properties;


/**
 * <p>Factory for creating {@link Log} instances, with discovery and
 * configuration features similar to that employed by standard Java APIs
 * such as JAXP.</p>
 * 
 * <p><strong>IMPLEMENTATION NOTE</strong> - This implementation is heavily
 * based on the SAXParserFactory and DocumentBuilderFactory implementations
 * (corresponding to the JAXP pluggability APIs) found in Apache Xerces.</p>
 *
 * @author Craig R. McClanahan
 * @author Costin Manolache
 * @author Richard A. Sitze
 * @version $Revision: 593798 $ $Date: 2007-11-10 18:40:43 +0100 $
 */

public abstract class LogFactory {
    // Implementation note re AccessController usage
    //
    // It is important to keep code invoked via an AccessController to small
    // auditable blocks. Such code must carefully evaluate all user input
    // (parameters, system properties, config file contents, etc). As an 
    // example, a Log implementation should not write to its logfile
    // with an AccessController anywhere in the call stack, otherwise an
    // insecure application could configure the log implementation to write
    // to a protected file using the privileges granted to JCL rather than
    // to the calling application.
    //
    // Under no circumstance should a non-private method return data that is
    // retrieved via an AccessController. That would allow an insecure app
    // to invoke that method and obtain data that it is not permitted to have.
    //
    // Invoking user-supplied code with an AccessController set is not a major
    // issue (eg invoking the constructor of the class specified by 
    // HASHTABLE_IMPLEMENTATION_PROPERTY). That class will be in a different
    // trust domain, and therefore must have permissions to do whatever it
    // is trying to do regardless of the permissions granted to JCL. There is
    // a slight issue in that untrusted code may point that environment var
    // to another trusted library, in which case the code runs if both that
    // lib and JCL have the necessary permissions even when the untrusted
    // caller does not. That's a pretty hard route to exploit though.


    // ----------------------------------------------------- Manifest Constants

    /**
     * The name (<code>priority</code>) of the key in the config file used to 
     * specify the priority of that particular config file. The associated value 
     * is a floating-point number; higher values take priority over lower values.
     */
    public static final String PRIORITY_KEY = "priority";

    /**
     * The name (<code>use_tccl</code>) of the key in the config file used 
     * to specify whether logging classes should be loaded via the thread 
     * context class loader (TCCL), or not. By default, the TCCL is used.
     */
    public static final String TCCL_KEY = "use_tccl";

    /**
     * The name (<code>org.apache.commons.logging.LogFactory</code>) of the property 
     * used to identify the LogFactory implementation
     * class name. This can be used as a system property, or as an entry in a
     * configuration properties file.
     */
    public static final String FACTORY_PROPERTY =
        "org.apache.commons.logging.LogFactory";

    /**
     * The fully qualified class name of the fallback <code>LogFactory</code>
     * implementation class to use, if no other can be found.
     */
    public static final String FACTORY_DEFAULT =
        "org.apache.commons.logging.impl.LogFactoryImpl";

    /**
     * The name (<code>commons-logging.properties</code>) of the properties file to search for.
     */
    public static final String FACTORY_PROPERTIES =
        "commons-logging.properties";

    /**
     * JDK1.3+ <a href="http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html#Service%20Provider">
     * 'Service Provider' specification</a>.
     * 
     */
    protected static final String SERVICE_ID =
        "META-INF/services/org.apache.commons.logging.LogFactory";

    /**
     * The name (<code>org.apache.commons.logging.diagnostics.dest</code>) 
     * of the property used to enable internal commons-logging
     * diagnostic output, in order to get information on what logging
     * implementations are being discovered, what classloaders they 
     * are loaded through, etc.
     * <p>
     * If a system property of this name is set then the value is
     * assumed to be the name of a file. The special strings
     * STDOUT or STDERR (case-sensitive) indicate output to
     * System.out and System.err respectively.
     * <p>
     * Diagnostic logging should be used only to debug problematic
     * configurations and should not be set in normal production use.
     */
    public static final String DIAGNOSTICS_DEST_PROPERTY =
        "org.apache.commons.logging.diagnostics.dest";

    /**
     * When null (the usual case), no diagnostic output will be
     * generated by LogFactory or LogFactoryImpl. When non-null,
     * interesting events will be written to the specified object.
     */
    private static PrintStream diagnosticsStream = null;
    
    /**
     * A string that gets prefixed to every message output by the
     * logDiagnostic method, so that users can clearly see which
     * LogFactory class is generating the output.
     */
    private static String diagnosticPrefix;
    
    /**
     * <p>Setting this system property 
     * (<code>org.apache.commons.logging.LogFactory.HashtableImpl</code>) 
     * value allows the <code>Hashtable</code> used to store
     * classloaders to be substituted by an alternative implementation.
     * </p>
     * <p>
     * <strong>Note:</strong> <code>LogFactory</code> will print:
     * <code><pre>
     * [ERROR] LogFactory: Load of custom hashtable failed</em>
     * </pre></code>
     * to system error and then continue using a standard Hashtable.
     * </p>
     * <p>
     * <strong>Usage:</strong> Set this property when Java is invoked
     * and <code>LogFactory</code> will attempt to load a new instance 
     * of the given implementation class.
     * For example, running the following ant scriplet:
     * <code><pre>
     *  &lt;java classname="${test.runner}" fork="yes" failonerror="${test.failonerror}"&gt;
     *     ...
     *     &lt;sysproperty 
     *        key="org.apache.commons.logging.LogFactory.HashtableImpl"
     *        value="org.apache.commons.logging.AltHashtable"/&gt;
     *  &lt;/java&gt;
     * </pre></code>
     * will mean that <code>LogFactory</code> will load an instance of
     * <code>org.apache.commons.logging.AltHashtable</code>.
     * </p>
     * <p>
     * A typical use case is to allow a custom
     * Hashtable implementation using weak references to be substituted.
     * This will allow classloaders to be garbage collected without
     * the need to release them (on 1.3+ JVMs only, of course ;)
     * </p>
     */
    public static final String HASHTABLE_IMPLEMENTATION_PROPERTY =
        "org.apache.commons.logging.LogFactory.HashtableImpl";
    /** Name used to load the weak hashtable implementation by names */
    private static final String WEAK_HASHTABLE_CLASSNAME = 
        "org.apache.commons.logging.impl.WeakHashtable";

    /**
     * A reference to the classloader that loaded this class. This is the
     * same as LogFactory.class.getClassLoader(). However computing this
     * value isn't quite as simple as that, as we potentially need to use
     * AccessControllers etc. It's more efficient to compute it once and
     * cache it here.
     */
    private static ClassLoader thisClassLoader;
    
    // ----------------------------------------------------------- Constructors


    /**
     * Protected constructor that is not available for public use.
     */
    protected LogFactory() {
    }
    
    // --------------------------------------------------------- Public Methods


    /**
     * Return the configuration attribute with the specified name (if any),
     * or <code>null</code> if there is no such attribute.
     *
     * @param name Name of the attribute to return
     */
    public abstract Object getAttribute(String name);


    /**
     * Return an array containing the names of all currently defined
     * configuration attributes.  If there are no such attributes, a zero
     * length array is returned.
     */
    public abstract String[] getAttributeNames();


    /**
     * Convenience method to derive a name from the specified class and
     * call <code>getInstance(String)</code> with it.
     *
     * @param clazz Class for which a suitable Log name will be derived
     *
     * @exception LogConfigurationException if a suitable <code>Log</code>
     *  instance cannot be returned
     */
    public abstract Log getInstance(Class clazz)
        throws LogConfigurationException;


    /**
     * <p>Construct (if necessary) and return a <code>Log</code> instance,
     * using the factory's current set of configuration attributes.</p>
     *
     * <p><strong>NOTE</strong> - Depending upon the implementation of
     * the <code>LogFactory</code> you are using, the <code>Log</code>
     * instance you are returned may or may not be local to the current
     * application, and may or may not be returned again on a subsequent
     * call with the same name argument.</p>
     *
     * @param name Logical name of the <code>Log</code> instance to be
     *  returned (the meaning of this name is only known to the underlying
     *  logging implementation that is being wrapped)
     *
     * @exception LogConfigurationException if a suitable <code>Log</code>
     *  instance cannot be returned
     */
    public abstract Log getInstance(String name)
        throws LogConfigurationException;


    /**
     * Release any internal references to previously created {@link Log}
     * instances returned by this factory.  This is useful in environments
     * like servlet containers, which implement application reloading by
     * throwing away a ClassLoader.  Dangling references to objects in that
     * class loader would prevent garbage collection.
     */
    public abstract void release();


    /**
     * Remove any configuration attribute associated with the specified name.
     * If there is no such attribute, no action is taken.
     *
     * @param name Name of the attribute to remove