org.springframework.beans.factory.access
Class SingletonBeanFactoryLocator

java.lang.Object
  org.springframework.beans.factory.access.SingletonBeanFactoryLocator

All Implemented Interfaces:

BeanFactoryLocator

Direct Known Subclasses:

ContextSingletonBeanFactoryLocator

public class SingletonBeanFactoryLocator

extends java.lang.Object

implements BeanFactoryLocator

Keyed-singleton implementation of BeanFactoryLocator, which leverages existing Spring constructs. This is normally accessed through DefaultLocatorFactory, but may also be used directly.

Please see the warning in BeanFactoryLocator's JavaDoc about appropriate usage of singleton style BeanFactoryLocator implementations. It is the opinion of the Spring team that the use of this class and similar classes is unecessary except (sometimes) for a small amount of glue code. Excessive usage will lead to code that is more tightly coupled, and harder to modify or test.

In this implementation, a BeanFactory is built up from one or more XML definition files, accessed as resources. The default name of the resource file(s) is 'beanRefFactory.xml', which is used when the instance is obtained with the no-arg getInstance() method. Using getInstance(String selector) will return a singleton instance which will use a name for the resource file(s) which is the specificed selector argument, instead of the default. The purpose of this BeanFactory is to create and hold a copy of one or more 'real' BeanFactory or Application Context instances, and allow those to be obtained either directly or via an alias. As such, it provides a level of indirection, and allows multiple pieces of code, which are not able to work in a Dependency Injection fashion, to refer to and use the same target BeanFactory/ApplicationContext instance(s), by different names.

Consider an example application scenario:

com.mycompany.myapp.util.applicationContext.xml - ApplicationContext definition file which defines beans for 'util' layer.
com.mycompany.myapp.dataaccess-applicationContext.xml - ApplicationContext definition file which defines beans for 'data access' layer. Depends on the above
com.mycompany.myapp.services.applicationContext.xml - ApplicationContext definition file which defines beans for 'services' layer. Depends on the above.

In an ideal scenario, these would be combined to create one ApplicationContext, or created as three hierarchical ApplicationContexts, by one piece of code somewhere at application startup (perhaps a Servlet filter), from which all other code in the application would flow, obtained as beans from the context(s). However when third party code enters into the picture, things can get problematic. If the third party code needs to create user classes, which should normally be obtained from a Spring BeanFactory/ApplicationContext, but can handle only newInstance() style object creation, then some extra work is required to actually access and use object from a BeanFactory/ApplicationContext. One solutions is to make the class created by the third party code be just a stub or proxy, which gets the real object from a BeanFactory/ApplicationContext, and delegates to it. However, it is is not normally workable for the stub to create the BeanFactory on each use, as depending on what is inside it, that can be an expensive operation. Additionally, there is a fairly tight coupling between the stub and the name of the definition resource for the BeanFactory/ApplicationContext. This is where SingletonBeanFactoryLocator comes in. The stub can obtain a SingletonBeanFactoryLocator instance, which is effectively a singleton, and ask it for an appropriate BeanFactory. A subsequent invocation (assuming the same classloader is involved) by the stub or another piece of code, will obtain the same instance. The simple aliasing mechanism allows the context to be asked for by a name which is appropriate for (or describes) the user. The deployer can match alias names to actual context names.

Another use of SingletonBeanFactoryLocator, is to demand-load/use one or more BeanFactories/ApplicationContexts. Because the definiiton can contain one of more BeanFactories/ApplicationContexts, which can be independent or in a hierarchy, if they are set to lazy-initialize, they will only be created when actually requested for use.

Given the above-mentioned three ApplicationContexts, consider the simplest SingletonBeanFactoryLocator usage scenario, where there is only one single beanRefFactory.xml definition file:

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
 
 <beans>
 
   <bean id="com.mycompany.myapp"
         class="org.springframework.context.support.ClassPathXmlApplicationContext">
     <constructor-arg>
       <list>
         <value>com.mycompany.myapp.util.applicationContext.xml</value>
         <value>com.mycompany.myapp.dataaccess.applicationContext.xml</value>
         <value>com.mycompany.myapp.dataaccess.services.xml</value>
       </list>
     </constructor-arg>
   </bean>
 
 </beans>
 

The client code is as simple as:

 BeanFactoryLocator bfl = SingletonBeanFactoryLocator.getInstance();
 BeanFactoryReference bf = bfl.useFactory("com.mycompany.myapp");
 // now use some bean from factory 
 MyClass zed = bf.getFactory().getBean("mybean");
 

Another relatively simple variation of the beanRefFactory.xml definition file could be:

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
 
 <beans>
 
   <bean id="com.mycompany.myapp.util" lazy-init="true"
         class="org.springframework.context.support.ClassPathXmlApplicationContext">
     <constructor-arg>
       <value>com.mycompany.myapp.util.applicationContext.xml</value>
     </constructor-arg>
   </bean>
 
   <!-- child of above -->
   <bean id="com.mycompany.myapp.dataaccess" lazy-init="true"
         class="org.springframework.context.support.ClassPathXmlApplicationContext">
     <constructor-arg>
       <list><value>com.mycompany.myapp.dataaccess.applicationContext.xml</value></list>
     </constructor-arg>
     <constructor-arg>
       <ref bean="com.mycompany.myapp.util"/>
     </constructor-arg>
   </bean>
 
   <!-- child of above -->
   <bean id="com.mycompany.myapp.services" lazy-init="true"
         class="org.springframework.context.support.ClassPathXmlApplicationContext">
     <constructor-arg>
       <list><value>com.mycompany.myapp.dataaccess.services.xml</value></value>
     </constructor-arg>
     <constructor-arg>
       <ref bean="com.mycompany.myapp.dataaccess"/>
     </constructor-arg>
   </bean>
 
   <!-- define an alias -->
   <bean id="com.mycompany.myapp.mypackage"
         class="java.lang.String">
     <constructor-arg>
       <value>com.mycompany.myapp.services</value>
     </constructor-arg>
   </bean>
 
 </beans>
 

In this example, there is a hierarchy of three contexts created. The (potential) advantage is that if the lazy flag is set to true, a context will only be created if it's actually used. If there is some code that is only needed some of the time, this mechanism can save some resources. Additionally, an alias to the last context has been created. Aliases allow usage of the idiom where client code asks for a context with an id which represents the package or module the code is in, and the actual definition file(s) for the SingletonBeanFactoryLocator maps that id to a real context id.

A final example is more complex, with a beanRefFactory.xml for every module. All the files are automatically combined to create the final definition.
beanRefFactory.xml file inside jar for util module:

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
 
 <beans>
   <bean id="com.mycompany.myapp.util" lazy-init="true"
        class="org.springframework.context.support.ClassPathXmlApplicationContext">
     <constructor-arg>
       <value>com.mycompany.myapp.util.applicationContext.xml</value>
     </constructor-arg>
   </bean>
 </beans>
 

beanRefFactory.xml file inside jar for data-access module:

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
 
 <beans>
   <!-- child of util -->
   <bean id="com.mycompany.myapp.dataaccess" lazy-init="true"
        class="org.springframework.context.support.ClassPathXmlApplicationContext">
     <constructor-arg>
       <list><value>com.mycompany.myapp.dataaccess.applicationContext.xml</value></list>
     </constructor-arg>
     <constructor-arg>
       <ref bean="com.mycompany.myapp.util"/>
     </constructor-arg>
   </bean>
 </beans>
 

beanRefFactory.xml file inside jar for services module:

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
 
 <beans>
   <!-- child of data-access -->
   <bean id="com.mycompany.myapp.services" lazy-init="true"
        class="org.springframework.context.support.ClassPathXmlApplicationContext">
     <constructor-arg>
       <list><value>com.mycompany.myapp.dataaccess.services.xml</value></list>
     </constructor-arg>
     <constructor-arg>
       <ref bean="com.mycompany.myapp.dataaccess"/>
     </constructor-arg>
   </bean>
 </beans>
 

beanRefFactory.xml file inside jar for mypackage module. This doesn't create any of its own contexts, but allows the other ones to be referred to be a name known to this module:

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
 
 <beans>
   <!-- define an alias -->
   <bean id="com.mycompany.myapp.mypackage"
         class="java.lang.String">
     <constructor-arg>
       <value>com.mycompany.myapp.services</value>
     </constructor-arg>
   </bean>
 
 </beans>
 

Version:

$Revision: 1.13 $

Author:

Colin Sampaleanu

See Also:

DefaultLocatorFactory

Field Summary

static java.lang.String	BEANS_REFS_XML_NAME
protected static org.apache.commons.logging.Log	logger

Constructor Summary

protected	SingletonBeanFactoryLocator() Constructor which uses the default "beanRefFactory.xml", as the name of the definition file(s).
protected	SingletonBeanFactoryLocator(java.lang.String resourceName) Constructor which uses the the specified name as the name of the definition file(s).

Method Summary

protected BeanFactory	createDefinition(java.lang.String[] resources) Actually creates definition in the form of a BeanFactory, given an array of URLs representing resources which should be combined.
protected void	destroyDefinition(BeanFactory groupDef, java.lang.String resourceName) Destroy definition in separate method so subclass may work with other definition types
protected java.util.Collection	getAllDefinitionResources(java.lang.String resourceName) Method which returns resources (as URLs) which make up the definition of one bean factory/application context.
static BeanFactoryLocator	getInstance() Returns an instance which uses the default "beanRefFactory.xml", as the name of the definition file(s).
static BeanFactoryLocator	getInstance(java.lang.String selector) Returns an instance which uses the the specified selector, as the name of the definition file(s).
BeanFactoryReference	useBeanFactory(java.lang.String factoryKey) Use the BeanFactory (or derived class such as ApplicationContext) specified by the factoryKey parameter.

Methods inherited from class java.lang.Object

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

Field Detail

BEANS_REFS_XML_NAME

public static final java.lang.String BEANS_REFS_XML_NAME

See Also:

Constant Field Values

logger

protected static final org.apache.commons.logging.Log logger

Constructor Detail

SingletonBeanFactoryLocator

protected SingletonBeanFactoryLocator()

Constructor which uses the default "beanRefFactory.xml", as the name of the definition file(s). All resources returned by the definition classloader's getResources() method with this name will be combined to create a definition.

SingletonBeanFactoryLocator

protected SingletonBeanFactoryLocator(java.lang.String resourceName)

Constructor which uses the the specified name as the name of the definition file(s). All resources returned by the definition classloader's getResources() method with this name will be combined to create a definition definition.

Method Detail

getInstance

public static BeanFactoryLocator getInstance()
                                      throws FatalBeanException

Returns an instance which uses the default "beanRefFactory.xml", as the name of the definition file(s). All resources returned by the current thread's context classloader's getResources() method with this name will be combined to create a definition, which is just a BeanFactory.

Throws:

FatalBeanException

getInstance

public static BeanFactoryLocator getInstance(java.lang.String selector)
                                      throws FatalBeanException

Returns an instance which uses the the specified selector, as the name of the definition file(s). All resources returned by the current thread's context classloader's getResources() method with this name will be combined to create a definition, which is just a a BeanFactory.

Parameters:

selector - the name of the resource(s) which will be read and combine to form the definition for the SingletonBeanFactoryLocator instance

Throws:

FatalBeanException

useBeanFactory

public BeanFactoryReference useBeanFactory(java.lang.String factoryKey)
                                    throws BeansException

Description copied from interface: BeanFactoryLocator

Use the BeanFactory (or derived class such as ApplicationContext) specified by the factoryKey parameter. The definition is possibly loaded/created as needed.

Specified by:

useBeanFactory in interface BeanFactoryLocator

Parameters:

factoryKey - a resource name specifying which BeanFactory the BeanFactoryLocator should return for usage. The actual meaning of the resource name is specific to the actual implementation of BeanFactoryLocator.

Returns:

the BeanFactory instance, wrapped as a BeanFactoryReference object

Throws:

BeansException - if there is an error loading or accessing the BeanFactory

createDefinition

protected BeanFactory createDefinition(java.lang.String[] resources)
                                throws BeansException

Actually creates definition in the form of a BeanFactory, given an array of URLs representing resources which should be combined. This is split out as a separate method so that subclasses can override the actual type uses (to be an ApplicationContext, for example).

Throws:

BeansException

destroyDefinition

protected void destroyDefinition(BeanFactory groupDef,
                                 java.lang.String resourceName)
                          throws BeansException

Destroy definition in separate method so subclass may work with other definition types

Throws:

BeansException

getAllDefinitionResources

protected java.util.Collection getAllDefinitionResources(java.lang.String resourceName)
                                                  throws java.io.IOException

Method which returns resources (as URLs) which make up the definition of one bean factory/application context.

Protected so that test cases may subclass this class and override this method to avoid the need for multiple classloaders to test multi-file capability in the rest of the class.

Throws:

java.io.IOException