logfactory.java

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

     */
    public abstract void removeAttribute(String name);


    /**
     * Set the configuration attribute with the specified name.  Calling
     * this with a <code>null</code> value is equivalent to calling
     * <code>removeAttribute(name)</code>.
     *
     * @param name Name of the attribute to set
     * @param value Value of the attribute to set, or <code>null</code>
     *  to remove any setting for this attribute
     */
    public abstract void setAttribute(String name, Object value);


    // ------------------------------------------------------- Static Variables


    /**
     * The previously constructed <code>LogFactory</code> instances, keyed by
     * the <code>ClassLoader</code> with which it was created.
     */
    protected static Hashtable factories = null;
    
    /**
     * Prevously constructed <code>LogFactory</code> instance as in the
     * <code>factories</code> map, but for the case where
     * <code>getClassLoader</code> returns <code>null</code>.
     * This can happen when:
     * <ul>
     * <li>using JDK1.1 and the calling code is loaded via the system
     *  classloader (very common)</li>
     * <li>using JDK1.2+ and the calling code is loaded via the boot
     *  classloader (only likely for embedded systems work).</li>
     * </ul>
     * Note that <code>factories</code> is a <i>Hashtable</i> (not a HashMap),
     * and hashtables don't allow null as a key.
     */
    protected static LogFactory nullClassLoaderFactory = null;

    /**
     * Create the hashtable which will be used to store a map of
     * (context-classloader -> logfactory-object). Version 1.2+ of Java
     * supports "weak references", allowing a custom Hashtable class
     * to be used which uses only weak references to its keys. Using weak
     * references can fix memory leaks on webapp unload in some cases (though
     * not all). Version 1.1 of Java does not support weak references, so we
     * must dynamically determine which we are using. And just for fun, this
     * code also supports the ability for a system property to specify an
     * arbitrary Hashtable implementation name.
     * <p>
     * Note that the correct way to ensure no memory leaks occur is to ensure
     * that LogFactory.release(contextClassLoader) is called whenever a 
     * webapp is undeployed.
     */
    private static final Hashtable createFactoryStore() {
        Hashtable result = null;
        String storeImplementationClass;
        try {
            storeImplementationClass = getSystemProperty(HASHTABLE_IMPLEMENTATION_PROPERTY, null);
        } catch(SecurityException ex) {
            // Permissions don't allow this to be accessed. Default to the "modern"
            // weak hashtable implementation if it is available.
            storeImplementationClass = null;
        }

        if (storeImplementationClass == null) {
            storeImplementationClass = WEAK_HASHTABLE_CLASSNAME;
        }
        try {
            Class implementationClass = Class.forName(storeImplementationClass);
            result = (Hashtable) implementationClass.newInstance();
            
        } catch (Throwable t) {
            // ignore
            if (!WEAK_HASHTABLE_CLASSNAME.equals(storeImplementationClass)) {
                // if the user's trying to set up a custom implementation, give a clue
                if (isDiagnosticsEnabled()) {
                    // use internal logging to issue the warning
                    logDiagnostic("[ERROR] LogFactory: Load of custom hashtable failed");
                } else {
                    // we *really* want this output, even if diagnostics weren't
                    // explicitly enabled by the user.
                    System.err.println("[ERROR] LogFactory: Load of custom hashtable failed");
                }
            }
        }
        if (result == null) {
            result = new Hashtable();
        }
        return result;
    }


    // --------------------------------------------------------- Static Methods

    /** Utility method to safely trim a string. */
    private static String trim(String src) {
    	if (src == null) {
    		return null;
    	}
    	return src.trim();
    }

    /**
     * <p>Construct (if necessary) and return a <code>LogFactory</code>
     * instance, using the following ordered lookup procedure to determine
     * the name of the implementation class to be loaded.</p>
     * <ul>
     * <li>The <code>org.apache.commons.logging.LogFactory</code> system
     *     property.</li>
     * <li>The JDK 1.3 Service Discovery mechanism</li>
     * <li>Use the properties file <code>commons-logging.properties</code>
     *     file, if found in the class path of this class.  The configuration
     *     file is in standard <code>java.util.Properties</code> format and
     *     contains the fully qualified name of the implementation class
     *     with the key being the system property defined above.</li>
     * <li>Fall back to a default implementation class
     *     (<code>org.apache.commons.logging.impl.LogFactoryImpl</code>).</li>
     * </ul>
     *
     * <p><em>NOTE</em> - If the properties file method of identifying the
     * <code>LogFactory</code> implementation class is utilized, all of the
     * properties defined in this file will be set as configuration attributes
     * on the corresponding <code>LogFactory</code> instance.</p>
     * 
     * <p><em>NOTE</em> - In a multithreaded environment it is possible 
     * that two different instances will be returned for the same 
     * classloader environment. 
     * </p>
     *
     * @exception LogConfigurationException if the implementation class is not
     *  available or cannot be instantiated.
     */
    public static LogFactory getFactory() throws LogConfigurationException {
        // Identify the class loader we will be using
        ClassLoader contextClassLoader = getContextClassLoaderInternal();

        if (contextClassLoader == null) {
            // This is an odd enough situation to report about. This
            // output will be a nuisance on JDK1.1, as the system
            // classloader is null in that environment.
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Context classloader is null.");
            }
        }

        // Return any previously registered factory for this class loader
        LogFactory factory = getCachedFactory(contextClassLoader);
        if (factory != null) {
            return factory;
        }

        if (isDiagnosticsEnabled()) {
            logDiagnostic(
                    "[LOOKUP] LogFactory implementation requested for the first time for context classloader "
                    + objectId(contextClassLoader));
            logHierarchy("[LOOKUP] ", contextClassLoader);
        }

        // Load properties file.
        //
        // If the properties file exists, then its contents are used as
        // "attributes" on the LogFactory implementation class. One particular
        // property may also control which LogFactory concrete subclass is
        // used, but only if other discovery mechanisms fail..
        //
        // As the properties file (if it exists) will be used one way or 
        // another in the end we may as well look for it first.

        Properties props = getConfigurationFile(contextClassLoader, FACTORY_PROPERTIES);

        // Determine whether we will be using the thread context class loader to
        // load logging classes or not by checking the loaded properties file (if any).
        ClassLoader baseClassLoader = contextClassLoader;
        if (props != null) {
            String useTCCLStr = props.getProperty(TCCL_KEY);
            if (useTCCLStr != null) {
                // The Boolean.valueOf(useTCCLStr).booleanValue() formulation
                // is required for Java 1.2 compatability.
                if (Boolean.valueOf(useTCCLStr).booleanValue() == false) {
                    // Don't use current context classloader when locating any
                    // LogFactory or Log classes, just use the class that loaded
                    // this abstract class. When this class is deployed in a shared
                    // classpath of a container, it means webapps cannot deploy their
                    // own logging implementations. It also means that it is up to the
                    // implementation whether to load library-specific config files
                    // from the TCCL or not.
                    baseClassLoader = thisClassLoader; 
                }
            }
        }

        // Determine which concrete LogFactory subclass to use.
        // First, try a global system property
        if (isDiagnosticsEnabled()) {
            logDiagnostic(
                    "[LOOKUP] Looking for system property [" + FACTORY_PROPERTY 
                    + "] to define the LogFactory subclass to use...");
        }
        
        try {
            String factoryClass = getSystemProperty(FACTORY_PROPERTY, null);
            if (factoryClass != null) {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic(
                            "[LOOKUP] Creating an instance of LogFactory class '" + factoryClass
                            + "' as specified by system property " + FACTORY_PROPERTY);
                }
                
                factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);
            } else {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic(
                            "[LOOKUP] No system property [" + FACTORY_PROPERTY 
                            + "] defined.");
                }
            }
        } catch (SecurityException e) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic(
                        "[LOOKUP] A security exception occurred while trying to create an"
                        + " instance of the custom factory class"
                        + ": [" + trim(e.getMessage())
                        + "]. Trying alternative implementations...");
            }
            ;  // ignore
        } catch(RuntimeException e) {
            // This is not consistent with the behaviour when a bad LogFactory class is
            // specified in a services file.
            //
            // One possible exception that can occur here is a ClassCastException when
            // the specified class wasn't castable to this LogFactory type.
            if (isDiagnosticsEnabled()) {
                logDiagnostic(
                        "[LOOKUP] An exception occurred while trying to create an"
                        + " instance of the custom factory class"
                        + ": [" + trim(e.getMessage())
                        + "] as specified by a system property.");
            }
            throw e;
        }


        // Second, try to find a service by using the JDK1.3 class
        // discovery mechanism, which involves putting a file with the name
        // of an interface class in the META-INF/services directory, where the
        // contents of the file is a single line specifying a concrete class 
        // that implements the desired interface.

        if (factory == null) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic(
                        "[LOOKUP] Looking for a resource file of name [" + SERVICE_ID
                        + "] to define the LogFactory subclass to use...");
            }
            try {
                InputStream is = getResourceAsStream(contextClassLoader,
                                                     SERVICE_ID);

                if( is != null ) {
                    // This code is needed by EBCDIC and other strange systems.
                    // It's a fix for bugs reported in xerces
                    BufferedReader rd;
                    try {
                        rd = new BufferedReader(new InputStreamReader(is, "UTF-8"));
                    } catch (java.io.UnsupportedEncodingException e) {
                        rd = new BufferedReader(new InputStreamReader(is));
                    }

                    String factoryClassName = rd.readLine();
                    rd.close();

                    if (factoryClassName != null &&
                        ! "".equals(factoryClassName)) {
                        if (isDiagnosticsEnabled()) {
                            logDiagnostic(
                                    "[LOOKUP]  Creating an instance of LogFactory class " + factoryClassName
                                    + " as specified by file '" + SERVICE_ID 
                                    + "' which was present in the path of the context"
                                    + " classloader.");
                        }
                        factory = newFactory(factoryClassName, baseClassLoader, contextClassLoader );
                    }