org.apache.commons.logging

Class LogFactory

public abstract class LogFactory extends Object

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
static StringDIAGNOSTICS_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.
protected static Hashtablefactories
The previously constructed LogFactory instances, keyed by the ClassLoader with which it was created.
static StringFACTORY_DEFAULT
The fully qualified class name of the fallback LogFactory implementation class to use, if no other can be found.
static StringFACTORY_PROPERTIES
The name (commons-logging.properties) of the properties file to search for.
static StringFACTORY_PROPERTY
The name (org.apache.commons.logging.LogFactory) of the property used to identify the LogFactory implementation class name.
static StringHASHTABLE_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.

protected static LogFactorynullClassLoaderFactory
Prevously constructed LogFactory instance as in the factories map, but for the case where getClassLoader returns null.
static StringPRIORITY_KEY
The name (priority) of the key in the config file used to specify the priority of that particular config file.
protected static StringSERVICE_ID
static StringTCCL_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.
Constructor Summary
protected LogFactory()
Protected constructor that is not available for public use.
Method Summary
protected static ObjectcreateFactory(String factoryClass, ClassLoader classLoader)
Implements the operations described in the javadoc for newFactory.
protected static ClassLoaderdirectGetContextClassLoader()
Return the thread context class loader if available; otherwise return null.
abstract ObjectgetAttribute(String name)
Return the configuration attribute with the specified name (if any), or null if there is no such attribute.
abstract String[]getAttributeNames()
Return an array containing the names of all currently defined configuration attributes.
protected static ClassLoadergetClassLoader(Class clazz)
Safely get access to the classloader for the specified class.
protected static ClassLoadergetContextClassLoader()
Calls LogFactory.directGetContextClassLoader under the control of an AccessController class.
static LogFactorygetFactory()

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

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.

static LoggetLog(Class clazz)
Convenience method to return a named logger, without the application having to care about factories.
static LoggetLog(String name)
Convenience method to return a named logger, without the application having to care about factories.
protected static booleanisDiagnosticsEnabled()
Indicates true if the user has enabled internal logging.
protected static voidlogRawDiagnostic(String msg)
Write the specified message to the internal logging destination.
protected static LogFactorynewFactory(String factoryClass, ClassLoader classLoader, ClassLoader contextClassLoader)
Return a new instance of the specified LogFactory implementation class, loaded by the specified class loader.
protected static LogFactorynewFactory(String factoryClass, ClassLoader classLoader)
Method provided for backwards compatibility; see newFactory version that takes 3 parameters.
static StringobjectId(Object o)
Returns a string that uniquely identifies the specified object, including its class.
abstract voidrelease()
Release any internal references to previously created Log instances returned by this factory.
static voidrelease(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.
static voidreleaseAll()
Release any internal references to previously created LogFactory instances, after calling the instance method release() on each of them.
abstract voidremoveAttribute(String name)
Remove any configuration attribute associated with the specified name.
abstract voidsetAttribute(String name, Object value)
Set the configuration attribute with the specified name.

Field Detail

DIAGNOSTICS_DEST_PROPERTY

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.

factories

protected static Hashtable factories
The previously constructed LogFactory instances, keyed by the ClassLoader with which it was created.

FACTORY_DEFAULT

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.

FACTORY_PROPERTIES

public static final String FACTORY_PROPERTIES
The name (commons-logging.properties) of the properties file to search for.

FACTORY_PROPERTY

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.

HASHTABLE_IMPLEMENTATION_PROPERTY

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 ;)

nullClassLoaderFactory

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: Note that factories is a Hashtable (not a HashMap), and hashtables don't allow null as a key.

PRIORITY_KEY

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.

SERVICE_ID

protected static final String SERVICE_ID
JDK1.3+ 'Service Provider' specification.

TCCL_KEY

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.

Constructor Detail

LogFactory

protected LogFactory()
Protected constructor that is not available for public use.

Method Detail

createFactory

protected static Object createFactory(String factoryClass, ClassLoader classLoader)
Implements the operations described in the javadoc for newFactory.

Parameters: factoryClass classLoader used to load the specified factory class. This is expected to be either the TCCL or the classloader which loaded this class. Note that the classloader which loaded this class might be "null" (ie the bootloader) for embedded systems.

Returns: either a LogFactory object or a LogConfigurationException object.

Since: 1.1

directGetContextClassLoader

protected static ClassLoader directGetContextClassLoader()
Return the thread context class loader if available; otherwise return null.

Most/all code should call getContextClassLoader 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.

Throws: LogConfigurationException if a suitable class loader cannot be identified. SecurityException if the java security policy forbids access to the context classloader from one of the classes in the current call stack.

Since: 1.1

getAttribute

public abstract Object getAttribute(String name)
Return the configuration attribute with the specified name (if any), or null if there is no such attribute.

Parameters: name Name of the attribute to return

getAttributeNames

public abstract 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.

getClassLoader

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.

Since: 1.1

getContextClassLoader

protected static ClassLoader getContextClassLoader()
Calls LogFactory.directGetContextClassLoader under the control of an AccessController class. This means that java code running under a security manager that forbids access to ClassLoaders will still work if this class is given appropriate privileges, even when the caller doesn't have such privileges. Without using an AccessController, the the entire call stack must have the privilege before the call is allowed.

Returns: the context classloader associated with the current thread, or null if security doesn't allow it.

Throws: LogConfigurationException if there was some weird error while attempting to get the context classloader. SecurityException if the current java security policy doesn't allow this class to access the context classloader.

getFactory

public static LogFactory getFactory()

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.

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.

Throws: LogConfigurationException if the implementation class is not available or cannot be instantiated.

getInstance

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

Parameters: clazz Class for which a suitable Log name will be derived

Throws: LogConfigurationException if a suitable Log instance cannot be returned

getInstance

public abstract Log getInstance(String name)

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.

Parameters: name Logical name of the Log instance to be returned (the meaning of this name is only known to the underlying logging implementation that is being wrapped)

Throws: LogConfigurationException if a suitable Log instance cannot be returned

getLog

public static Log getLog(Class clazz)
Convenience method to return a named logger, without the application having to care about factories.

Parameters: clazz Class from which a log name will be derived

Throws: LogConfigurationException if a suitable Log instance cannot be returned

getLog

public static Log getLog(String name)
Convenience method to return a named logger, without the application having to care about factories.

Parameters: name Logical name of the Log instance to be returned (the meaning of this name is only known to the underlying logging implementation that is being wrapped)

Throws: LogConfigurationException if a suitable Log instance cannot be returned

isDiagnosticsEnabled

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.

Returns: true if calls to logDiagnostic will have any effect.

Since: 1.1

logRawDiagnostic

protected static final void logRawDiagnostic(String msg)
Write the specified message to the internal logging destination.

Parameters: msg is the diagnostic message to be output.

Since: 1.1

newFactory

protected static LogFactory newFactory(String factoryClass, ClassLoader classLoader, ClassLoader contextClassLoader)
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.

Parameters: factoryClass Fully qualified name of the LogFactory implementation class classLoader ClassLoader from which to load this class contextClassLoader is the context that this new factory will manage logging for.

Throws: LogConfigurationException if a suitable instance cannot be created

Since: 1.1

newFactory

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.

objectId

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.

Parameters: o may be null.

Returns: a string of form classname@hashcode, or "null" if param o is null.

Since: 1.1

release

public abstract 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.

release

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.

Parameters: classLoader ClassLoader for which to release the LogFactory

releaseAll

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.

removeAttribute

public abstract void removeAttribute(String name)
Remove any configuration attribute associated with the specified name. If there is no such attribute, no action is taken.

Parameters: name Name of the attribute to remove

setAttribute

public abstract 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).

Parameters: name Name of the attribute to set value Value of the attribute to set, or null to remove any setting for this attribute