LogFactory (Logging 1.1 API)

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org.apache.commons.logging
Class LogFactory

java.lang.Object
  org.apache.commons.logging.LogFactory

Direct Known Subclasses:: LogFactoryImpl

public abstract class LogFactory
extends java.lang.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.

Version:: $Revision: 853148 $ $Date: 2006-05-03 21:58:34 +0100 (Wed, 03 May 2006) $
Author:: Craig R. McClanahan, Costin Manolache, Richard A. Sitze

Field Summary
`static java.lang.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.
`protected static java.util.Hashtable`	`factories` The previously constructed `LogFactory` instances, keyed by the `ClassLoader` with which it was created.
`static java.lang.String`	`FACTORY_DEFAULT` The fully qualified class name of the fallback `LogFactory` implementation class to use, if no other can be found.
`static java.lang.String`	`FACTORY_PROPERTIES` The name (`commons-logging.properties`) of the properties file to search for.
`static java.lang.String`	`FACTORY_PROPERTY` The name (`org.apache.commons.logging.LogFactory`) of the property used to identify the LogFactory implementation class name.
`static java.lang.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.
`protected static LogFactory`	`nullClassLoaderFactory` Prevously constructed `LogFactory` instance as in the `factories` map, but for the case where `getClassLoader` returns `null`.
`static java.lang.String`	`PRIORITY_KEY` The name (`priority`) of the key in the config file used to specify the priority of that particular config file.
`protected static java.lang.String`	`SERVICE_ID` JDK1.3+ 'Service Provider' specification.
`static java.lang.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.

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

Method Summary
`protected static java.lang.Object`	`createFactory(java.lang.String factoryClass, java.lang.ClassLoader classLoader)` Implements the operations described in the javadoc for newFactory.
`protected static java.lang.ClassLoader`	`directGetContextClassLoader()` Return the thread context class loader if available; otherwise return null.
`abstract java.lang.Object`	`getAttribute(java.lang.String name)` Return the configuration attribute with the specified name (if any), or `null` if there is no such attribute.
`abstract java.lang.String[]`	`getAttributeNames()` Return an array containing the names of all currently defined configuration attributes.
`protected static java.lang.ClassLoader`	`getClassLoader(java.lang.Class clazz)` Safely get access to the classloader for the specified class.
`protected static java.lang.ClassLoader`	`getContextClassLoader()` Calls LogFactory.directGetContextClassLoader under the control of an AccessController class.
`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.
`abstract Log`	`getInstance(java.lang.Class clazz)` Convenience method to derive a name from the specified class and call `getInstance(String)` with it.
`abstract Log`	`getInstance(java.lang.String name)` Construct (if necessary) and return a `Log` instance, using the factory's current set of configuration attributes.
`static Log`	`getLog(java.lang.Class clazz)` Convenience method to return a named logger, without the application having to care about factories.
`static Log`	`getLog(java.lang.String name)` 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.
`protected static void`	`logRawDiagnostic(java.lang.String msg)` Write the specified message to the internal logging destination.
`protected static LogFactory`	`newFactory(java.lang.String factoryClass, java.lang.ClassLoader classLoader)` Method provided for backwards compatibility; see newFactory version that takes 3 parameters.
`protected static LogFactory`	`newFactory(java.lang.String factoryClass, java.lang.ClassLoader classLoader, java.lang.ClassLoader contextClassLoader)` Return a new instance of the specified `LogFactory` implementation class, loaded by the specified class loader.
`static java.lang.String`	`objectId(java.lang.Object o)` Returns a string that uniquely identifies the specified object, including its class.
`abstract void`	`release()` Release any internal references to previously created `Log` instances returned by this factory.
`static void`	`release(java.lang.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 void`	`releaseAll()` Release any internal references to previously created `LogFactory` instances, after calling the instance method `release()` on each of them.
`abstract void`	`removeAttribute(java.lang.String name)` Remove any configuration attribute associated with the specified name.
`abstract void`	`setAttribute(java.lang.String name, java.lang.Object value)` Set the configuration attribute with the specified name.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

PRIORITY_KEY

public static final java.lang.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.

See Also:: Constant Field Values

TCCL_KEY

public static final java.lang.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.

See Also:: Constant Field Values

FACTORY_PROPERTY

public static final java.lang.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.

See Also:: Constant Field Values

FACTORY_DEFAULT

public static final java.lang.String FACTORY_DEFAULT

The fully qualified class name of the fallback LogFactory implementation class to use, if no other can be found.

See Also:: Constant Field Values

FACTORY_PROPERTIES

public static final java.lang.String FACTORY_PROPERTIES

The name (commons-logging.properties) of the properties file to search for.

See Also:: Constant Field Values

SERVICE_ID

protected static final java.lang.String SERVICE_ID

JDK1.3+ 'Service Provider' specification.

See Also:: Constant Field Values

DIAGNOSTICS_DEST_PROPERTY

public static final java.lang.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.

See Also:: Constant Field Values

HASHTABLE_IMPLEMENTATION_PROPERTY

public static final java.lang.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 ;)

See Also:: Constant Field Values

factories

protected static java.util.Hashtable factories

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

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:

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 Detail

LogFactory

protected LogFactory()

Protected constructor that is not available for public use.

Method Detail

getAttribute

public abstract java.lang.Object getAttribute(java.lang.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 java.lang.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.

getInstance

public abstract Log getInstance(java.lang.Class clazz)
                         throws LogConfigurationException

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(java.lang.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.

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

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.

removeAttribute

public abstract void removeAttribute(java.lang.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(java.lang.String name,
                                  java.lang.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

getFactory

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.

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

getLog

public static Log getLog(java.lang.Class clazz)
                  throws LogConfigurationException

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(java.lang.String name)
                  throws LogConfigurationException

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

release

public static void release(java.lang.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.

getClassLoader

protected static java.lang.ClassLoader getClassLoader(java.lang.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 java.lang.ClassLoader getContextClassLoader()
                                                      throws LogConfigurationException

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.; java.lang.SecurityException - if the current java security policy doesn't allow this class to access the context classloader.

directGetContextClassLoader

protected static java.lang.ClassLoader directGetContextClassLoader()
                                                            throws LogConfigurationException

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.; java.lang.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

newFactory

protected static LogFactory newFactory(java.lang.String factoryClass,
                                       java.lang.ClassLoader classLoader,
                                       java.lang.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.

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(java.lang.String factoryClass,
                                       java.lang.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.

createFactory

protected static java.lang.Object createFactory(java.lang.String factoryClass,
                                                java.lang.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

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(java.lang.String msg)

Write the specified message to the internal logging destination.

Parameters:: msg - is the diagnostic message to be output.
Since:: 1.1

objectId

public static java.lang.String objectId(java.lang.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

Overview

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PREV CLASS NEXT CLASS

FRAMES NO FRAMES

SUMMARY: NESTED | FIELD | CONSTR | METHOD

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org.apache.commons.logging Class LogFactory

PRIORITY_KEY

TCCL_KEY

FACTORY_PROPERTY

FACTORY_DEFAULT

FACTORY_PROPERTIES

SERVICE_ID

DIAGNOSTICS_DEST_PROPERTY

HASHTABLE_IMPLEMENTATION_PROPERTY

factories

nullClassLoaderFactory

LogFactory

getAttribute

getAttributeNames

getInstance

getInstance

release

removeAttribute

setAttribute

getFactory

getLog

getLog

release

releaseAll

getClassLoader

getContextClassLoader

directGetContextClassLoader

newFactory

ClassLoader conflicts

newFactory

createFactory

isDiagnosticsEnabled

logRawDiagnostic

objectId

org.apache.commons.logging
Class LogFactory