logfactory.java

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

                        // An incompatible ClassLoader was used to load the 
                        // implementation. 
                        // As the same classes
                        // must be available in multiple class loaders,
                        // it is very likely that multiple JCL jars are present.
                        // The most likely fix for this
                        // problem is to remove the extra JCL jars from the 
                        // ClassLoader hierarchy. 
                        //
                        if (isDiagnosticsEnabled()) {
                            logDiagnostic(
                                    "Factory class " + logFactoryClass.getName()
                                + " loaded from classloader " + objectId(logFactoryClass.getClassLoader())
                                + " does not extend '" + LogFactory.class.getName()
                                + "' as loaded by this classloader.");
                            logHierarchy("[BAD CL TREE] ", classLoader);
                        }
                    }
                    
                    return (LogFactory) logFactoryClass.newInstance();

                } catch (ClassNotFoundException ex) {
                    if (classLoader == thisClassLoader) {
                        // Nothing more to try, onwards.
                        if (isDiagnosticsEnabled()) {
                            logDiagnostic(
                                    "Unable to locate any class called '" + factoryClass
                                    + "' via classloader " + objectId(classLoader));
                        }
                        throw ex;
                    }
                    // ignore exception, continue
                } catch (NoClassDefFoundError e) {
                    if (classLoader == thisClassLoader) {
                        // Nothing more to try, onwards.
                        if (isDiagnosticsEnabled()) {
                            logDiagnostic(
                                    "Class '" + factoryClass + "' cannot be loaded"
                                    + " via classloader " + objectId(classLoader)
                                    + " - it depends on some other class that cannot"
                                    + " be found.");
                        }
                        throw e;
                    }
                    // ignore exception, continue
                } catch(ClassCastException e) {
                    if (classLoader == thisClassLoader) {
                        // There's no point in falling through to the code below that
                        // tries again with thisClassLoader, because we've just tried
                        // loading with that loader (not the TCCL). Just throw an
                        // appropriate exception here.

                        final boolean implementsLogFactory = implementsLogFactory(logFactoryClass);
                        
                        //
                        // Construct a good message: users may not actual expect that a custom implementation 
                        // has been specified. Several well known containers use this mechanism to adapt JCL 
                        // to their native logging system. 
                        // 
                        String msg = 
                            "The application has specified that a custom LogFactory implementation should be used but " +
                            "Class '" + factoryClass + "' cannot be converted to '"
                            + LogFactory.class.getName() + "'. ";
                        if (implementsLogFactory) {
                            msg = msg + "The conflict is caused by the presence of multiple LogFactory classes in incompatible classloaders. " +
                                "Background can be found in http://commons.apache.org/logging/tech.html. " +
                                "If you have not explicitly specified a custom LogFactory then it is likely that " +
                                "the container has set one without your knowledge. " +
                                "In this case, consider using the commons-logging-adapters.jar file or " +
                                "specifying the standard LogFactory from the command line. ";
                        } else {
                            msg = msg + "Please check the custom implementation. ";
                        }
                        msg = msg + "Help can be found @http://commons.apache.org/logging/troubleshooting.html.";
                        
                        if (isDiagnosticsEnabled()) {
                            logDiagnostic(msg);
                        }
                        
                        ClassCastException ex = new ClassCastException(msg);
                        throw ex;
                    }
                    
                    // Ignore exception, continue. Presumably the classloader was the
                    // TCCL; the code below will try to load the class via thisClassLoader.
                    // This will handle the case where the original calling class is in
                    // a shared classpath but the TCCL has a copy of LogFactory and the
                    // specified LogFactory implementation; we will fall back to using the
                    // LogFactory implementation from the same classloader as this class.
                    //
                    // Issue: this doesn't handle the reverse case, where this LogFactory
                    // is in the webapp, and the specified LogFactory implementation is
                    // in a shared classpath. In that case:
                    // (a) the class really does implement LogFactory (bad log msg above)
                    // (b) the fallback code will result in exactly the same problem.
                }
            }

            /* At this point, either classLoader == null, OR
             * classLoader was unable to load factoryClass.
             *
             * In either case, we call Class.forName, which is equivalent
             * to LogFactory.class.getClassLoader().load(name), ie we ignore
             * the classloader parameter the caller passed, and fall back
             * to trying the classloader associated with this class. See the
             * javadoc for the newFactory method for more info on the 
             * consequences of this.
             *
             * Notes:
             * * LogFactory.class.getClassLoader() may return 'null'
             *   if LogFactory is loaded by the bootstrap classloader.
             */
            // Warning: must typecast here & allow exception
            // to be generated/caught & recast properly.
            if (isDiagnosticsEnabled()) {
                logDiagnostic(
                    "Unable to load factory class via classloader " 
                    + objectId(classLoader)
                    + " - trying the classloader associated with this LogFactory.");
            }
            logFactoryClass = Class.forName(factoryClass);
            return (LogFactory) logFactoryClass.newInstance();
        } catch (Exception e) {
            // Check to see if we've got a bad configuration
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Unable to create LogFactory instance.");
            }
            if (logFactoryClass != null
                && !LogFactory.class.isAssignableFrom(logFactoryClass)) {
                
                return new LogConfigurationException(
                    "The chosen LogFactory implementation does not extend LogFactory."
                    + " Please check your configuration.",
                    e);
            }
            return new LogConfigurationException(e);
        }
    }

    /**
     * Determines whether the given class actually implements <code>LogFactory</code>.
     * Diagnostic information is also logged.
     * <p>
     * <strong>Usage:</strong> to diagnose whether a classloader conflict is the cause
     * of incompatibility. The test used is whether the class is assignable from
     * the <code>LogFactory</code> class loaded by the class's classloader.
     * @param logFactoryClass <code>Class</code> which may implement <code>LogFactory</code>
     * @return true if the <code>logFactoryClass</code> does extend
     * <code>LogFactory</code> when that class is loaded via the same
     * classloader that loaded the <code>logFactoryClass</code>.
     */
    private static boolean implementsLogFactory(Class logFactoryClass) {
        boolean implementsLogFactory = false;
        if (logFactoryClass != null) {
            try {
                ClassLoader logFactoryClassLoader = logFactoryClass.getClassLoader();
                if (logFactoryClassLoader == null) {
                    logDiagnostic("[CUSTOM LOG FACTORY] was loaded by the boot classloader");
                } else {
                    logHierarchy("[CUSTOM LOG FACTORY] ", logFactoryClassLoader);
                    Class factoryFromCustomLoader
                        = Class.forName("org.apache.commons.logging.LogFactory", false, logFactoryClassLoader);
                    implementsLogFactory = factoryFromCustomLoader.isAssignableFrom(logFactoryClass);
                    if (implementsLogFactory) {
                        logDiagnostic("[CUSTOM LOG FACTORY] " + logFactoryClass.getName()
                                + " implements LogFactory but was loaded by an incompatible classloader.");
                    } else {
                        logDiagnostic("[CUSTOM LOG FACTORY] " + logFactoryClass.getName()
                                + " does not implement LogFactory.");
                    }
                }
            } catch (SecurityException e) {
                //
                // The application is running within a hostile security environment.
                // This will make it very hard to diagnose issues with JCL.
                // Consider running less securely whilst debugging this issue.
                //
                logDiagnostic("[CUSTOM LOG FACTORY] SecurityException thrown whilst trying to determine whether " +
                        "the compatibility was caused by a classloader conflict: "
                        + e.getMessage());
            } catch (LinkageError e) {
                //
                // This should be an unusual circumstance.
                // LinkageError's usually indicate that a dependent class has incompatibly changed.
                // Another possibility may be an exception thrown by an initializer.
                // Time for a clean rebuild?
                //
                logDiagnostic("[CUSTOM LOG FACTORY] LinkageError thrown whilst trying to determine whether " +
                        "the compatibility was caused by a classloader conflict: "
                        + e.getMessage());
            } catch (ClassNotFoundException e) {
                //
                // LogFactory cannot be loaded by the classloader which loaded the custom factory implementation.
                // The custom implementation is not viable until this is corrected.
                // Ensure that the JCL jar and the custom class are available from the same classloader.
                // Running with diagnostics on should give information about the classloaders used
                // to load the custom factory.
                //
                logDiagnostic("[CUSTOM LOG FACTORY] LogFactory class cannot be loaded by classloader which loaded the " +
                        "custom LogFactory implementation. Is the custom factory in the right classloader?");
            }
        }
        return implementsLogFactory;
    }

    /**
     * Applets may run in an environment where accessing resources of a loader is
     * a secure operation, but where the commons-logging library has explicitly
     * been granted permission for that operation. In this case, we need to 
     * run the operation using an AccessController.
     */
    private static InputStream getResourceAsStream(final ClassLoader loader,
                                                   final String name)
    {
        return (InputStream)AccessController.doPrivileged(
            new PrivilegedAction() {
                public Object run() {
                    if (loader != null) {
                        return loader.getResourceAsStream(name);
                    } else {
                        return ClassLoader.getSystemResourceAsStream(name);
                    }
                }
            });
    }

    /**
     * Given a filename, return an enumeration of URLs pointing to
     * all the occurrences of that filename in the classpath.
     * <p>
     * This is just like ClassLoader.getResources except that the
     * operation is done under an AccessController so that this method will
     * succeed when this jarfile is privileged but the caller is not.
     * This method must therefore remain private to avoid security issues.
     * <p>
     * If no instances are found, an Enumeration is returned whose
     * hasMoreElements method returns false (ie an "empty" enumeration).
     * If resources could not be listed for some reason, null is returned.
     */
    private static Enumeration getResources(final ClassLoader loader,
            final String name)
    {
        PrivilegedAction action = 
            new PrivilegedAction() {
                public Object run() {
                    try {
                        if (loader != null) {
                            return loader.getResources(name);
                        } else {
                            return ClassLoader.getSystemResources(name);
                        }
                    } catch(IOException e) {
                        if (isDiagnosticsEnabled()) {
                            logDiagnostic(
                                "Exception while trying to find configuration file "
                                + name + ":" + e.getMessage());
                        }
                        return null;
                    } catch(NoSuchMethodError e) {
                        // we must be running on a 1.1 JVM which doesn't support
                        // ClassLoader.getSystemResources; just return null in
                        // this case.
                        return null;
                    }
                }
            };
        Object result = AccessController.doPrivileged(action);
        return (Enumeration) result;
    }

    /**
     * Given a URL that refers to a .properties file, load that file.
     * This is done under an AccessController so that this method will
     * succeed when this jarfile is privileged but the caller is not.
     * This method must therefore remain private to avoid security issues.
     * <p>
     * Null is returned if the URL cannot be opened.
     */
    private static Properties getProperties(final URL url) {
        PrivilegedAction action = 
            new Pr