logfactoryimpl.java

comming logging 的源码 可以封装log4j等日志系统。 源码中使用了很多的设计模式

     * or a configuration property has been set in a commons-logging.properties
     * file.
     * 
     * @return the value associated with the property, or null.
     */
    private String getConfigurationValue(String property) {
        if (isDiagnosticsEnabled()) {
            logDiagnostic("[ENV] Trying to get configuration for item " + property);
        }

        Object valueObj =  getAttribute(property);
        if (valueObj != null) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("[ENV] Found LogFactory attribute [" + valueObj + "] for " + property);
            }
            return valueObj.toString();
        }
        
        if (isDiagnosticsEnabled()) {
            logDiagnostic("[ENV] No LogFactory attribute found for " + property);
        }

        try {
            // warning: minor security hole here, in that we potentially read a system
            // property that the caller cannot, then output it in readable form as a
            // diagnostic message. However it's only ever JCL-specific properties
            // involved here, so the harm is truly trivial. 
            String value = getSystemProperty(property, null);
            if (value != null) {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic("[ENV] Found system property [" + value + "] for " + property);
                }
                return value;
            }

            if (isDiagnosticsEnabled()) {
                logDiagnostic("[ENV] No system property found for property " + property);
            }
        } catch (SecurityException e) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("[ENV] Security prevented reading system property " + property);
            }
        }

        if (isDiagnosticsEnabled()) {
            logDiagnostic("[ENV] No configuration defined for item " + property);
        }

        return null;
    }
    
    /**
     * Get the setting for the user-configurable behaviour specified by key.
     * If nothing has explicitly been set, then return dflt.  
     */
    private boolean getBooleanConfiguration(String key, boolean dflt) {
        String val = getConfigurationValue(key);
        if (val == null)
            return dflt;
        return Boolean.valueOf(val).booleanValue();
    }

    /**
     * Initialize a number of variables that control the behaviour of this
     * class and that can be tweaked by the user. This is done when the first
     * logger is created, not in the constructor of this class, because we
     * need to give the user a chance to call method setAttribute in order to
     * configure this object.
     */
    private void initConfiguration() {
        allowFlawedContext = getBooleanConfiguration(ALLOW_FLAWED_CONTEXT_PROPERTY, true);
        allowFlawedDiscovery = getBooleanConfiguration(ALLOW_FLAWED_DISCOVERY_PROPERTY, true);
        allowFlawedHierarchy = getBooleanConfiguration(ALLOW_FLAWED_HIERARCHY_PROPERTY, true);
    }
  

    /**
     * Attempts to create a Log instance for the given category name.
     * Follows the discovery process described in the class javadoc.
     * 
     * @param logCategory the name of the log category
     * 
     * @throws LogConfigurationException if an error in discovery occurs, 
     * or if no adapter at all can be instantiated
     */
    private Log discoverLogImplementation(String logCategory)
    throws LogConfigurationException
    {
        if (isDiagnosticsEnabled()) {
            logDiagnostic("Discovering a Log implementation...");
        }
        
        initConfiguration();
        
        Log result = null;
        
        // See if the user specified the Log implementation to use
        String specifiedLogClassName = findUserSpecifiedLogClassName();

        if (specifiedLogClassName != null) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Attempting to load user-specified log class '" + 
                    specifiedLogClassName + "'...");
            }
            
            result = createLogFromClass(specifiedLogClassName,
                                        logCategory,
                                        true);
            if (result == null) {
                StringBuffer messageBuffer =  new StringBuffer("User-specified log class '");
                messageBuffer.append(specifiedLogClassName);
                messageBuffer.append("' cannot be found or is not useable.");
                
                // Mistyping or misspelling names is a common fault.
                // Construct a good error message, if we can
                if (specifiedLogClassName != null) {
                    informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_LOG4J_LOGGER);
                    informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_JDK14_LOGGER);
                    informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_LUMBERJACK_LOGGER);
                    informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_SIMPLE_LOGGER);
                }
                throw new LogConfigurationException(messageBuffer.toString());
            }
            
            return result;
        }
        
        // No user specified log; try to discover what's on the classpath
        //
        // Note that we deliberately loop here over classesToDiscover and
        // expect method createLogFromClass to loop over the possible source
        // classloaders. The effect is:
        //   for each discoverable log adapter
        //      for each possible classloader
        //          see if it works
        //
        // It appears reasonable at first glance to do the opposite: 
        //   for each possible classloader
        //     for each discoverable log adapter
        //        see if it works
        //
        // The latter certainly has advantages for user-installable logging
        // libraries such as log4j; in a webapp for example this code should
        // first check whether the user has provided any of the possible
        // logging libraries before looking in the parent classloader. 
        // Unfortunately, however, Jdk14Logger will always work in jvm>=1.4,
        // and SimpleLog will always work in any JVM. So the loop would never
        // ever look for logging libraries in the parent classpath. Yet many
        // users would expect that putting log4j there would cause it to be
        // detected (and this is the historical JCL behaviour). So we go with
        // the first approach. A user that has bundled a specific logging lib
        // in a webapp should use a commons-logging.properties file or a
        // service file in META-INF to force use of that logging lib anyway,
        // rather than relying on discovery.
        
        if (isDiagnosticsEnabled()) {
            logDiagnostic(
                "No user-specified Log implementation; performing discovery" +
                " using the standard supported logging implementations...");
        }
        for(int i=0; (i<classesToDiscover.length) && (result == null); ++i) {
            result = createLogFromClass(classesToDiscover[i], logCategory, true);
        }
        
        if (result == null) {
            throw new LogConfigurationException
                        ("No suitable Log implementation");
        }
        
        return result;        
    }


    /**
     * Appends message if the given name is similar to the candidate.
     * @param messageBuffer <code>StringBuffer</code> the message should be appended to, 
     * not null
     * @param name the (trimmed) name to be test against the candidate, not null
     * @param candidate the candidate name (not null)
     */
    private void informUponSimilarName(final StringBuffer messageBuffer, final String name, 
            final String candidate) {
        if (name.equals(candidate)) {
            // Don't suggest a name that is exactly the same as the one the
            // user tried...
            return;
        }

        // If the user provides a name that is in the right package, and gets
        // the first 5 characters of the adapter class right (ignoring case),
        // then suggest the candidate adapter class name.
        if (name.regionMatches(true, 0, candidate, 0, PKG_LEN + 5)) {
            messageBuffer.append(" Did you mean '");
            messageBuffer.append(candidate);
            messageBuffer.append("'?");
        }
    }
    
    
    /**
     * Checks system properties and the attribute map for 
     * a Log implementation specified by the user under the 
     * property names {@link #LOG_PROPERTY} or {@link #LOG_PROPERTY_OLD}.
     * 
     * @return classname specified by the user, or <code>null</code>
     */
    private String findUserSpecifiedLogClassName()
    {
        if (isDiagnosticsEnabled()) {
            logDiagnostic("Trying to get log class from attribute '" + LOG_PROPERTY + "'");
        }
        String specifiedClass = (String) getAttribute(LOG_PROPERTY);

        if (specifiedClass == null) { // @deprecated
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Trying to get log class from attribute '" + 
                              LOG_PROPERTY_OLD + "'");
            }
            specifiedClass = (String) getAttribute(LOG_PROPERTY_OLD);
        }

        if (specifiedClass == null) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Trying to get log class from system property '" + 
                          LOG_PROPERTY + "'");
            }
            try {
                specifiedClass = getSystemProperty(LOG_PROPERTY, null);
            } catch (SecurityException e) {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic("No access allowed to system property '" + 
                        LOG_PROPERTY + "' - " + e.getMessage());
                }
            }
        }

        if (specifiedClass == null) { // @deprecated
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Trying to get log class from system property '" + 
                          LOG_PROPERTY_OLD + "'");
            }
            try {
                specifiedClass = getSystemProperty(LOG_PROPERTY_OLD, null);
            } catch (SecurityException e) {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic("No access allowed to system property '" + 
                        LOG_PROPERTY_OLD + "' - " + e.getMessage());
                }
            }
        }
        
        // Remove any whitespace; it's never valid in a classname so its
        // presence just means a user mistake. As we know what they meant,
        // we may as well strip the spaces.
        if (specifiedClass != null) {
            specifiedClass = specifiedClass.trim();
        }

        return specifiedClass;
    }

    
    /**
     * Attempts to load the given class, find a suitable constructor,
     * and instantiate an instance of Log.
     * 
     * @param logAdapterClassName classname of the Log implementation
     * 
     * @param logCategory  argument to pass to the Log implementation's
     * constructor
     * 
     * @param affectState  <code>true</code> if this object's state should
     * be affected by this method call, <code>false</code> otherwise.
     * 
     * @return  an instance of the given class, or null if the logging
     * library associated with the specified adapter is not available.
     *                          
     * @throws LogConfigurationException if there was a serious error with
     * configuration and the handleFlawedDiscovery method decided this
     * problem was fatal.
     */                                  
    private Log createLogFromClass(String logAdapterClassName,
                                   String logCategory,
                                   boolean affectState) 
            throws LogConfigurationException {       

        if (isDiagnosticsEnabled()) {
            logDiagnostic("Attempting to instantiate '" + logAdapterClassName + "'");
        }
        
        Object[] params = { logCategory };
        Log logAdapter = null;
        Constructor constructor = null;
        
        Class logAdapterClass = null;
        ClassLoader currentCL = getBaseClassLoader();
        
        for(;;) {
            // Loop through the classloader hierarchy trying to find
            // a viable classloader.
            logDiagnostic(
                    "Trying to load '"
                    + logAdapterClassName
                    + "' from classloader "
                    + objectId(currentCL));
            try {
                if (isDiagnosticsEnabled()) {
                    // Show the location of the first occurrence of the .class file
                    // in the classpath. This is the location that ClassLoader.loadClass
                    // will load the class from -- unless the classloader is doing
                    // something weird. 
                    URL url;
                    String resourceName = logAdapterClassName.replace('.', '/') + ".class";
                    if (currentCL != null) {
                        url = currentCL.getResource(resourceName );
                    } else {
                        url = ClassLoader.getSystemResource(resourceName + ".class");
                    }

                    if (url == null) {
                        logDiagnostic("Class '" + logAdapterClassName + "' [" + resourceName + "] cannot be found.");
                    } else {
                        logDiagnostic("Class '" + logAdapterClassName + "' was found at '" + url + "'");
                    }
                }

                Class c = null;
                try {
                    c = Class.forName(logAdapterClassName, true, currentCL);
                } catch (ClassNotFoundException originalClassNotFoundException) {
                    // The current classloader was unable to find the log adapter 
                    // in this or any ancestor classloader. There's no point in
                    // trying higher up in the hierarchy in this case..
                    String msg = "" + originalClassNotFoundException.getMessage();
                    logDiagnostic(
                        "The log adapter '"
                        + logAdapterClassName
                        + "' is not available via classloader " 
                        + objectId(currentCL)
                        + ": "
                        + msg.trim());
                    try {
                        // Try the class classloader.
                        // This may work in cases where the TCCL
                        // does not contain the code executed or JCL.
                        // This behaviour indicates that the application 
                        // classloading strategy is not consistent with the
                        // Java 1.2 classloading guidelines but JCL can
                        // and so should handle this case.
                        c = Class.forName(logAdapterClassName);
                    } catch (ClassNotFoundException secondaryClassNotFoundException) {
                        // no point continuing: this adapter isn't available
                        msg = "" + secondaryClassNotFoundException.getMessage();
                        logDiagnostic(
                            "The log adapter '"
                            + logAdapterClassName
                            + "' is not available via the LogFactoryImpl class classloader: "
                            + msg.trim());
                        break;
                    }
                }
                
                constructor = c.getConstructor(logConstructorSignature);
                Object o = constructor.newInstance(params);

                // Note that we do this test after trying to create an instance
                // [rather than testing Log.class.isAssignableFrom(c)] so that
                // we don't complain about Log hierarchy problems when the
                // adapter couldn't be instantiated anyway.
                if (o instanceof Log) {
                    logAdapterClass = c;
                    logAdapter = (Log) o;
                    break;
                }
                
                // Oops, we have a potential problem here. An adapter class