Save This Page
Home » commons-logging-1.1.1-src » org.apache.commons » logging » [javadoc | source]
org.apache.commons.logging
abstract public class: LogFactory [javadoc | source]
java.lang.Object
   org.apache.commons.logging.LogFactory

Direct Known Subclasses:
    Log4jFactory, LogFactoryImpl

Factory for creating Log instances, with discovery and configuration features similar to that employed by standard Java APIs such as JAXP.

IMPLEMENTATION NOTE - This implementation is heavily based on the SAXParserFactory and DocumentBuilderFactory implementations (corresponding to the JAXP pluggability APIs) found in Apache Xerces.

Field Summary
public static final  String PRIORITY_KEY    The name (priority) 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 TCCL_KEY    The name (use_tccl) 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 FACTORY_PROPERTY    The name (org.apache.commons.logging.LogFactory) 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_DEFAULT    The fully qualified class name of the fallback LogFactory implementation class to use, if no other can be found. 
public static final  String FACTORY_PROPERTIES    The name (commons-logging.properties) of the properties file to search for. 
protected static final  String SERVICE_ID    JDK1.3+ 'Service Provider' specification
public static final  String DIAGNOSTICS_DEST_PROPERTY    The name (org.apache.commons.logging.diagnostics.dest) 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.

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.

Diagnostic logging should be used only to debug problematic configurations and should not be set in normal production use. 

public static final  String HASHTABLE_IMPLEMENTATION_PROPERTY   

Setting this system property (org.apache.commons.logging.LogFactory.HashtableImpl) value allows the Hashtable used to store classloaders to be substituted by an alternative implementation.

Note: LogFactory will print:

[ERROR] LogFactory: Load of custom hashtable failed
to system error and then continue using a standard Hashtable.

Usage: Set this property when Java is invoked and LogFactory will attempt to load a new instance of the given implementation class. For example, running the following ant scriplet:

<java classname="${test.runner}" fork="yes" failonerror="${test.failonerror}">
...
<sysproperty
key="org.apache.commons.logging.LogFactory.HashtableImpl"
value="org.apache.commons.logging.AltHashtable"/>
</java>
will mean that LogFactory will load an instance of org.apache.commons.logging.AltHashtable.

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 ;)

 
protected static  Hashtable factories    The previously constructed LogFactory instances, keyed by the ClassLoader with which it was created. 
protected static  LogFactory nullClassLoaderFactory    Prevously constructed LogFactory instance as in the factories map, but for the case where getClassLoader returns null. This can happen when:
  • using JDK1.1 and the calling code is loaded via the system classloader (very common)
  • using JDK1.2+ and the calling code is loaded via the boot classloader (only likely for embedded systems work).
Note that factories is a Hashtable (not a HashMap), and hashtables don't allow null as a key. 
Constructor:
 protected LogFactory() 
Method from org.apache.commons.logging.LogFactory Summary:
createFactory,   directGetContextClassLoader,   getAttribute,   getAttributeNames,   getClassLoader,   getContextClassLoader,   getFactory,   getInstance,   getInstance,   getLog,   getLog,   isDiagnosticsEnabled,   logRawDiagnostic,   newFactory,   newFactory,   objectId,   release,   release,   releaseAll,   removeAttribute,   setAttribute
Methods from java.lang.Object:
equals,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from org.apache.commons.logging.LogFactory Detail:
 protected static Object createFactory(String factoryClass,
    ClassLoader classLoader) 
    Implements the operations described in the javadoc for newFactory.
 protected static ClassLoader directGetContextClassLoader() throws LogConfigurationException 
    Return the thread context class loader if available; otherwise return null.

    Most/all code should call getContextClassLoaderInternal rather than calling this method directly.

    The thread context class loader is available for JDK 1.2 or later, if certain security conditions are met.

    Note that no internal logging is done within this method because this method is called every time LogFactory.getLogger() is called, and we don't want too much output generated here.

 abstract public Object getAttribute(String name)
    Return the configuration attribute with the specified name (if any), or null if there is no such attribute.
 abstract public String[] getAttributeNames()
    Return an array containing the names of all currently defined configuration attributes. If there are no such attributes, a zero length array is returned.
 protected static ClassLoader getClassLoader(Class clazz) 
    Safely get access to the classloader for the specified class.

    Theoretically, calling getClassLoader can throw a security exception, and so should be done under an AccessController in order to provide maximum flexibility. However in practice people don't appear to use security policies that forbid getClassLoader calls. So for the moment all code is written to call this method rather than Class.getClassLoader, so that we could put AccessController stuff in this method without any disruption later if we need to.

    Even when using an AccessController, however, this method can still throw SecurityException. Commons-logging basically relies on the ability to access classloaders, ie a policy that forbids all classloader access will also prevent commons-logging from working: currently this method will throw an exception preventing the entire app from starting up. Maybe it would be good to detect this situation and just disable all commons-logging? Not high priority though - as stated above, security policies that prevent classloader access aren't common.

    Note that returning an object fetched via an AccessController would technically be a security flaw anyway; untrusted code that has access to a trusted JCL library could use it to fetch the classloader for a class even when forbidden to do so directly.

 protected static ClassLoader getContextClassLoader() throws LogConfigurationException 
    Returns the current context classloader.

    In versions prior to 1.1, this method did not use an AccessController. In version 1.1, an AccessController wrapper was incorrectly added to this method, causing a minor security flaw.

    In version 1.1.1 this change was reverted; this method no longer uses an AccessController. User code wishing to obtain the context classloader must invoke this method via AccessController.doPrivileged if it needs support for that.

 public static LogFactory getFactory() throws LogConfigurationException 

    Construct (if necessary) and return a LogFactory instance, using the following ordered lookup procedure to determine the name of the implementation class to be loaded.

    • The org.apache.commons.logging.LogFactory system property.
    • The JDK 1.3 Service Discovery mechanism
    • Use the properties file commons-logging.properties file, if found in the class path of this class. The configuration file is in standard java.util.Properties format and contains the fully qualified name of the implementation class with the key being the system property defined above.
    • Fall back to a default implementation class (org.apache.commons.logging.impl.LogFactoryImpl).

    NOTE - If the properties file method of identifying the LogFactory implementation class is utilized, all of the properties defined in this file will be set as configuration attributes on the corresponding LogFactory instance.

    NOTE - In a multithreaded environment it is possible that two different instances will be returned for the same classloader environment.

 abstract public Log getInstance(Class clazz) throws LogConfigurationException
    Convenience method to derive a name from the specified class and call getInstance(String) with it.
 abstract public Log getInstance(String name) throws LogConfigurationException

    Construct (if necessary) and return a Log instance, using the factory's current set of configuration attributes.

    NOTE - Depending upon the implementation of the LogFactory you are using, the Log 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.

 public static Log getLog(Class clazz) throws LogConfigurationException 
    Convenience method to return a named logger, without the application having to care about factories.
 public static Log getLog(String name) throws LogConfigurationException 
    Convenience method to return a named logger, without the application having to care about factories.
 protected static boolean isDiagnosticsEnabled() 
    Indicates true if the user has enabled internal logging.

    By the way, sorry for the incorrect grammar, but calling this method areDiagnosticsEnabled just isn't java beans style.

 protected static final  void logRawDiagnostic(String msg) 
    Write the specified message to the internal logging destination.
 protected static LogFactory newFactory(String factoryClass,
    ClassLoader classLoader) 
    Method provided for backwards compatibility; see newFactory version that takes 3 parameters.

    This method would only ever be called in some rather odd situation. Note that this method is static, so overriding in a subclass doesn't have any effect unless this method is called from a method in that subclass. However this method only makes sense to use from the getFactory method, and as that is almost always invoked via LogFactory.getFactory, any custom definition in a subclass would be pointless. Only a class with a custom getFactory method, then invoked directly via CustomFactoryImpl.getFactory or similar would ever call this. Anyway, it's here just in case, though the "managed class loader" value output to the diagnostics will not report the correct value.

 protected static LogFactory newFactory(String factoryClass,
    ClassLoader classLoader,
    ClassLoader contextClassLoader) throws LogConfigurationException 
    Return a new instance of the specified LogFactory implementation class, loaded by the specified class loader. If that fails, try the class loader used to load this (abstract) LogFactory.

    ClassLoader conflicts

    Note that there can be problems if the specified ClassLoader is not the same as the classloader that loaded this class, ie when loading a concrete LogFactory subclass via a context classloader.

    The problem is the same one that can occur when loading a concrete Log subclass via a context classloader.

    The problem occurs when code running in the context classloader calls class X which was loaded via a parent classloader, and class X then calls LogFactory.getFactory (either directly or via LogFactory.getLog). Because class X was loaded via the parent, it binds to LogFactory loaded via the parent. When the code in this method finds some LogFactoryYYYY class in the child (context) classloader, and there also happens to be a LogFactory class defined in the child classloader, then LogFactoryYYYY will be bound to LogFactory@childloader. It cannot be cast to LogFactory@parentloader, ie this method cannot return the object as the desired type. Note that it doesn't matter if the LogFactory class in the child classloader is identical to the LogFactory class in the parent classloader, they are not compatible.

    The solution taken here is to simply print out an error message when this occurs then throw an exception. The deployer of the application must ensure they remove all occurrences of the LogFactory class from the child classloader in order to resolve the issue. Note that they do not have to move the custom LogFactory subclass; that is ok as long as the only LogFactory class it can find to bind to is in the parent classloader.

 public static String objectId(Object o) 
    Returns a string that uniquely identifies the specified object, including its class.

    The returned string is of form "classname@hashcode", ie is the same as the return value of the Object.toString() method, but works even when the specified object's class has overidden the toString method.

 abstract public  void release()
    Release any internal references to previously created 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 static  void release(ClassLoader classLoader) 
    Release any internal references to previously created LogFactory instances that have been associated with the specified class loader (if any), after calling the instance method release() on each of them.
 public static  void releaseAll() 
    Release any internal references to previously created LogFactory instances, after calling the instance method release() on each of them. 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.
 abstract public  void removeAttribute(String name)
    Remove any configuration attribute associated with the specified name. If there is no such attribute, no action is taken.
 abstract public  void setAttribute(String name,
    Object value)
    Set the configuration attribute with the specified name. Calling this with a null value is equivalent to calling removeAttribute(name).