org.sape.carbon.core.config.cache
Class DoubleCheckConfigurationCache

java.lang.Object
  |
  +--org.sape.carbon.core.config.cache.DoubleCheckConfigurationCache

All Implemented Interfaces:

ConfigurationCache, EventListener, NodeEventListener

public class DoubleCheckConfigurationCache

extends Object

implements ConfigurationCache, NodeEventListener

This implementation uses the infamous double-check lock to minimize the synchronization required. Multi-threaded read access to the cache will not cause contention. Adding new configurations to the cache is synchronized.

Caution: Double-check locking is not bulletproof. There is a small possibility when using optimizing compilers or multi-processor machines that stale configurations or invalid configurations are returned when multiple threads concurrently access the same configuration when that configuration is being loaded, changed, or removed.

This implementation uses 2 maps to store configuration data, an active map and a background map. When the cache is modified, the background map is updated first. Then the maps are swapped making the background map the active one. Then the background (used to be active) is updated. This is done to make use of the fact that reference assignment is atomic and prevent map from being accessed while they are being updated.

Configuration objects are cached lazily, i.e. they are cached after they are requested the first time. Once a configuration object is cached, the cache will listen for Node events on the configuration. If a node event occurs, the entire cache is cleared. This is done to prevent inconsistencies between this cache and caches internal to the configuration objects themselves in the case of nested configurations.

Copyright 2002 Sapient

Since:

carbon 1.1

Version:

$Revision: 1.10 $($Author: dvoet $ / $Date: 2003/05/05 21:21:16 $)

Author:

Douglas Voet, Oct 24, 2002

Field Summary

private Map	activeCache Active cache used to return value.
private Map	backgroundCache Background cache used for loading.
private ConfigurationService	configService Holds the config service being cached.
private int	entranceCount Holds the entrance count for re-entrant calls.
private Map	loadingConfigurations Map of loaded configuration.
private org.apache.commons.logging.Log	log Provides a handle to Apache-commons logger

Constructor Summary

DoubleCheckConfigurationCache(ConfigurationService configService)
Constructs a new DoubleCheckConfigurationCache around the given service.

Method Summary

private void	activateBackgroundCache() Makes the background cache active and the active cache the background.
Configuration	getConfiguration(String name) Gets back a configuration with the given name.
void	nodeChanged(Node changedNode) Updates the configuration cache with the new instance of the node.
void	nodeRemoved(String removedNodeName) Removes a given node from the cache.

Methods inherited from class java.lang.Object

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

Field Detail

log

private org.apache.commons.logging.Log log

Provides a handle to Apache-commons logger

activeCache

private Map activeCache

Active cache used to return value.

backgroundCache

private Map backgroundCache

Background cache used for loading.

configService

private ConfigurationService configService

Holds the config service being cached.

entranceCount

private int entranceCount

Holds the entrance count for re-entrant calls.

loadingConfigurations

private Map loadingConfigurations

Map of loaded configuration.

Constructor Detail

DoubleCheckConfigurationCache

public DoubleCheckConfigurationCache(ConfigurationService configService)

Constructs a new DoubleCheckConfigurationCache around the given service.

Parameters:

configService - service to provide a double-check cache for.

Method Detail

getConfiguration

public Configuration getConfiguration(String name)

Gets back a configuration with the given name. This will either return the configuration from the cache or load it if needed.

Specified by:

getConfiguration in interface ConfigurationCache

Parameters:

name - the name of the configuration to load

Returns:

configuration at the given name

Throws:

ConfigurationNotFoundException - indicates there is no configuration at the given name

nodeChanged

public void nodeChanged(Node changedNode)

Updates the configuration cache with the new instance of the node.

Specified by:

nodeChanged in interface NodeEventListener

Parameters:

changedNode - the node that has been changed

nodeRemoved

public void nodeRemoved(String removedNodeName)

Removes a given node from the cache.

Specified by:

nodeRemoved in interface NodeEventListener

Parameters:

removedNodeName - node to remove from the cache

activateBackgroundCache

private void activateBackgroundCache()

Makes the background cache active and the active cache the background.