Two of the most elementary and important packages in Spring are the org.springframework.beans and org.springframework.context packages. Code in these packages provides the basis for Spring's Inversion of Control (alternately called Dependency Injection) features. The BeanFactory provides an advanced configuration mechanism capable of managing beans (objects) of any nature, using potentially any kind of storage facility. The ApplicationContext builds on top of the BeanFactory and adds other functionality such as easier integration with Springs AOP features, message resource handling (for use in internationalization), event propagation, declarative mechanisms to create the ApplicationContext and optional parent contexts, and application-layer specific contexts such as the WebApplicationContext, among other enhancements.
In short, the BeanFactory provides the configuration framework and basic functionality, while the ApplicationContext adds enhanced capabilities to it, some of them perhaps more J2EE and enterprise-centric. In general, an ApplicationContext is a complete superset of a BeanFactory, and any description of BeanFactory capabilities and behavior should be considered to apply to ApplicationContexts as well.
Users are sometimes unsure whether BeanFactory or ApplicationContext are best suited for use in a particular situation. Normally when building most applications in a J2EE-environment, the best option is to use the ApplicationContext, since it offers all the features of the BeanFactory and adds on to it in terms of features, while also allowing a more declarative approach to use of some functionality, which is generally desirable. The main usage scenario when you might prefer to use the BeanFactory is when memory usage is the greatest concern (such as in an applet where every last kilobyte counts), and you don't need all the features of the ApplicationContext.
This chapter is roughly divided into two parts, the first part covering the basic principles that apply to both the BeanFactory and the ApplicationContext. The second part will cover some of the features that only apply to the ApplicationContext.
The BeanFactory is the actual container which instantiates, configures, and manages a number of beans. These beans typically collaborate with one another, and thus have dependencies between themselves. These dependencies are reflected in the configuration data used by the BeanFactory (although some dependencies may not be visible as configuration data, but rather be a function of programmatic interactions between beans at runtime).
A BeanFactory is represented by the interface org.springframework.beans.factory.BeanFactory, for which there are multiple implementations. The most commonly used simple BeanFactory implementation is org.springframework.beans.factory.xml.XmlBeanFactory. (This should be qualified with the reminder that ApplicationContexts are a subclass of BeanFactory, and most users ultimately prefer to use XML variants of ApplicationContext).
Although for most scenarios, almost all user code managed by the BeanFactory does not have to be aware of the BeanFactory, the BeanFactory does have to be instantiated somehow. This can happen via explicit user code such as:
InputStream is = new FileInputStream("beans.xml");
XmlBeanFactory factory = new XmlBeanFactory(is);or
ClassPathResource res = new ClassPathResource("beans.xml");
XmlBeanFactory factory = new XmlBeanFactory(res);or
ClassPathXmlApplicationContext appContext = new ClassPathXmlApplicationContext(
new String[] {"applicationContext.xml", "applicationContext-part2.xml"});
// of course, an ApplicationContext is just a BeanFactory
BeanFactory factory = (BeanFactory) appContext;For many usage scenarios, user code will not have to instantiate the BeanFactory, since Spring Framework code will do it. For example, the web layer provides support code to load a Spring ApplicationContext automatically as part of the normal startup process of a J2EE web-app. This declarative process is described here:
While programmatic manipulation of BeanFactories will be described later, the following sections will concentrate on describing the configuration of BeanFactories.
A BeanFactory configuration consists of, at its most basic level, definitions of one or more beans that the BeanFactory must manage. In an XmlBeanFactory, these are configured as one or more bean elements inside a top-level beans element.
<?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="..." class="...">
...
</bean>
<bean id="..." class="...">
...
</bean>
...
</beans>Bean definitions inside a DefaultListableBeanFactory variant (like XmlBeanFactory) are represented as BeanDefinition objects, which contain (among other information) the following details:
a class name: this is normally the actual implementation class of the bean being described in the bean definition. However, if the bean is to be constructed by calling a static factory method instead of using a normal constructor, this will actually be the class name of the factory class.
bean behavioral configuration elements, which state how the bean should behave in the container (i.e. prototype or singleton, autowiring mode, dependency checking mode, initialization and destruction methods)
constructor arguments and property values to set in the newly created bean. An example would be the number of connections to use in a bean that manages a connection pool (either specified as a property or as a constructor argument), or the pool size limit.
other beans a bean needs to do its work, i.e. collaborators (also specified as properties or as constructor arguments). These can also be called dependencies.
The concepts listed above directly translate to a set of elements the bean definition consists of. Some of these element groups are listed below, along with a link to further documentation about each of them.
Table 3.1. Bean definition explanation
| Feature | More info |
|---|---|
| class | Section 3.2.3, “The bean class” |
| id and name | Section 3.2.4, “The bean identifiers (id and name)” |
| singleton or prototype | Section 3.2.5, “To singleton or not to singleton” |
| constructor arguments | Section 3.3.1, “Setting bean properties and collaborators” |
| bean properties | Section 3.3.1, “Setting bean properties and collaborators” |
| autowiring mode | Section 3.3.6, “Autowiring collaborators” |
| dependency checking mode | Section 3.3.7, “Checking for dependencies” |
| initialization method | Section 3.4.1, “Lifecycle interfaces” |
| destruction method | Section 3.4.1, “Lifecycle interfaces” |
Note that a bean definition is represented by the real interface org.springframework.beans.factory.config.BeanDefinition, and its various sub-interfaces and implementations. However, it is very unlikely that most user code would ever work with a BeanDefinition.
Besides bean definitions which contain information on how to create a bean, a bean factory can also allow to register existing bean instances. DefaultListableBeanFactory supports this through the registerSingleton method, as defined by the org.springframework.beans.factory.config.ConfigurableBeanFactory interface. Typical applications purely work with bean definitions, though.
The class attribute is normally mandatory (see Section 3.2.3.3, “Bean creation via instance factory method” and Section 3.5, “Abstract and child bean definitions” for the two exception) and is used for one of two purposes. In the much more common case where the BeanFactory itself directly creates the bean by calling its constructor (equivalent to Java code calling new), the class attribute specifies the class of the bean to be constructed. In the less common case where the BeanFactory calls a static, so-called factory method on a class to create the bean, the class attribute specifies the actual class containing the static factory method. (the type of the returned bean from the static factory method may be the same class or another class entirely, it doesn't matter).
When creating a bean using the constructor approach, all normal classes are usable by Spring and compatible with Spring. That is, the class being created does not need to implement any specific interfaces or be coded in a specific fashion. Just specifying the bean class should be enough. However, depending on what type of IoC you are going to use for that specific bean, you may need a default (empty) constructor.
Additionally, the BeanFactory isn't limited to just managing true JavaBeans, it is also able to manage virtually any class you want it to manage. Most people using Spring prefer to have actual JavaBeans (having just a default (no-argument) constructor and appropriate setters and getters modeled after the properties) in the BeanFactory, but it it's also possible to have more exotic non-bean-style classes in your BeanFactory. If, for example, you need to use a legacy connection pool that absolutely does not adhere to the JavaBean specification, no worries, Spring can manage it as well.
Using the XmlBeanFactory you can specify your bean class as follows:
<bean id="exampleBean"
class="examples.ExampleBean"/>
<bean name="anotherExample"
class="examples.ExampleBeanTwo"/> The mechanism for supplying (optional) arguments to the constructor, or setting properties of the object instance after it has been constructed, will be described shortly.
When defining a bean which is to be created using a static factory method, along with the class attribute which specifies the class containing the static factory method, another attribute named factory-method is needed to specify the name of the factory method itself. Spring expects to be able to call this method (with an optional list of arguments as described later) and get back a live object, which from that point on is treated as if it had been created normally via a constructor. One use for such a bean definition is to call static factories in legacy code.
Following is an example of a bean definition which specifies that the bean is to be created by calling a factory-method. Note that the definition does not specify the type (class) of the returned object, only the class containing the factory method. In this example, createInstance must be a static method.
<bean id="exampleBean"
class="examples.ExampleBean2"
factory-method="createInstance"/>The mechanism for supplying (optional) arguments to the factory method, or setting properties of the object instance after it has been returned from the factory, will be described shortly.
Quite similar to using a static factory method to create a bean, is the use of an instance (non-static) factory method, where a factory method of an existing bean from the factory is called to create the new bean.
To use this mechanism, the class attribute must be left empty, and the factory-bean attribute must specify the name of a bean in the current or an ancestor bean factory which contains the factory method. The factory method itself should still be set via the factory-method attribute.
Following is an example:
<!-- The factory bean, which contains a method called
createInstance -->
<bean id="myFactoryBean"
class="...">
...
</bean>
<!-- The bean to be created via the factory bean -->
<bean id="exampleBean"
factory-bean="myFactoryBean"
factory-method="createInstance"/>Although the mechanisms for setting bean properties are still to be discussed, one implication of this approach is that the factory bean itself can be managed and configured via Dependency Injection, by the container.
Every bean has one or more ids (also called identifiers, or names; these terms refer to the same thing). These ids must be unique within the BeanFactory or ApplicationContext the bean is hosted in. A bean will almost always have only one id, but if a bean has more than one id, the extra ones can essentially be considered aliases.
In an XmlBeanFactory (including ApplicationContext variants), you use the id or name attributes to specify the bean id(s), and at least one id must be specified in one or both of these attributes. The id attribute allows you to specify one id, and as it is marked in the XML DTD (definition document) as a real XML element ID attribute, the parser is able to do some extra validation when other elements point back to this one. As such, it is the preferred way to specify a bean id. However, the XML spec does limit the characters which are legal in XML IDs. This is usually not really a constraint, but if you have a need to use one of these characters, or want to introduce other aliases to the bean, you may also or instead specify one or more bean ids (separated by a comma (,) or semicolon (;) via the name attribute.
Beans are defined to be deployed in one of two modes: singleton or non-singleton. (The latter is also called a prototype, although the term is used loosely as it doesn't quite fit). When a bean is a singleton, only one shared instance of the bean will be managed and all requests for beans with an id or ids matching that bean definition will result in that one specific bean instance being returned.
The non-singleton, prototype mode of a bean deployment results in the creation of a new bean instance every time a request for that specific bean is done. This is ideal for situations where for example each user needs an independent user object or something similar.
Beans are deployed in singleton mode by default, unless you specify otherwise. Keep in mind that by changing the type to non-singleton (prototype), each request for a bean will result in a newly created bean and this might not be what you actually want. So only change the mode to prototype when absolutely necessary.
In the example below, two beans are declared of which one is defined as a singleton, and the other one is a non-singleton (prototype). exampleBean is created each and every time a client asks the BeanFactory for this bean, while yetAnotherExample is only created once; a reference to the exact same instance is returned on each request for this bean.
<bean id="exampleBean"
class="examples.ExampleBean" singleton="false"/>
<bean name="yetAnotherExample"
class="examples.ExampleBeanTwo" singleton="true"/>Note: when deploying a bean in the prototype mode, the lifecycle of the bean changes slightly. By definition, Spring cannot manage the complete lifecycle of a non-singleton/prototype bean, since after it is created, it is given to the client and the container does not keep track of it at all any longer. You can think of Spring's role when talking about a non-singleton/prototype bean as a replacement for the 'new' operator. Any lifecycle aspects past that point have to be handled by the client. The lifecycle of a bean in the BeanFactory is further described in Section 3.4.1, “Lifecycle interfaces”.
Inversion of Control has already been referred to as Dependency Injection. The basic principle is that beans define their dependencies (i.e. the other objects they work with) only through constructor arguments, arguments to a factory method, or properties which are set on the object instance after it has been constructed or returned from a factory method. Then, it is the job of the container to actually inject those dependencies when it creates the bean. This is fundamentally the inverse (hence the name Inversion of Control) of the bean instantiating or locating its dependencies on its own using direct construction of classes, or something like the Service Locator pattern. While we will not elaborate too much on the advantages of Dependency Injection, it becomes evident upon usage that code gets much cleaner and reaching a higher grade of decoupling is much easier when beans do not look up their dependencies, but are provided with them, and additionally do not even know where the dependencies are located and of what actual type they are.
As touched on in the previous paragraph, Inversion of Control/Dependency Injection exists in two major variants:
setter-based dependency injection is realized by calling setters on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean. Beans defined in the BeanFactory that use setter-based dependency injection are true JavaBeans. Spring generally advocates usage of setter-based dependency injection, since a large number of constructor arguments can get unwieldy, especially when some properties are optional.
constructor-based dependency injection is realized by invoking a constructor with a number of arguments, each representing a collaborator or property. Additionally, calling a static factory method with specific arguments, to construct the bean, can be considered almost equivalent, and the rest of this text will consider arguments to a constructor and arguments to a static factory method similarly. Although Spring generally advocates usage of setter-based dependency injection for most situations, it does fully support the constructor-based approach as well, since you may wish to use it with pre-existing beans which provide only multi-argument constructors, and no setters. Additionally, for simpler beans, some people prefer the constructor approach as a means of ensuring beans cannot be constructed in an invalid state.
The BeanFactory supports both of these variants for injecting dependencies into beans it manages. (It in fact also supports injecting setter-based dependencies after some dependencies have already been supplied via the constructor approach.) The configuration for the dependencies comes in the form of a BeanDefinition, which is used together with JavaBeans PropertyEditors to know how to convert properties from one format to another. The actual values being passed around are done in the form of PropertyValue objects. However, most users of Spring will not be dealing with these classes directly (i.e. programmatically), but rather with an XML definition file which will be converted internally into instances of these classes, and used to load an entire BeanFactory or ApplicationContext.
Bean dependency resolution generally happens as follows:
The BeanFactory is created and initialized with a configuration which describes all the beans. Most Spring users use a BeanFactory or ApplicationContext variant which supports XML format configuration files.
Each bean has dependencies expressed in the form of properties, constructor arguments, or arguments to the static-factory method when that is used instead of a normal constructor. These dependencies will be provided to the bean, when the bean is actually created.
Each property or constructor-arg is either an actual definition of the value to set, or a reference to another bean in the BeanFactory. In the case of the ApplicationContext, the reference can be to a bean in a parent ApplicationContext.
Each property or constructor argument which is a value must be able to be converted from whatever format it was specified in, to the actual type of that property or constructor argument. By default Spring can convert a value supplied in string format to all built-in types, such as int, long, String, boolean, etc. Additionally, when talking about the XML based BeanFactory variants (including the ApplicationContext variants), these have built-in support for defining Lists, Maps, Sets, and Properties collection types. Additionally, Spring uses JavaBeans PropertyEditor definitions to be able to convert string values to other, arbitrary types. (You can provide the BeanFactory with your own PropertyEditor definitions to be able to convert your own custom types; more information about PropertyEditors and how to manually add custom ones, can be found in Section 3.9, “Registering additional custom PropertyEditors”). When a bean property is a Java Class type, Spring allows you to specify the value for that property as a string value which is the name of the class, and the ClassEditor PropertyEditor, which is built-in, will take care of converting that class name to an actual Class instance.
It is important to realize that Spring validates the configuration of each bean in the BeanFactory when the BeanFactory is created, including the validation that properties which are bean references are actually referring to valid beans (i.e. the beans being referred to are also defined in the BeanFactory, or in the case of ApplicationContext, a parent context). However, the bean properties themselves are not set until the bean is actually created. For beans which are singleton and set to be pre-instantiated (such as singleton beans in an ApplicationContext), creation happens at the time that the BeanFactory is created, but otherwise this is only when the bean is requested. When a bean actually has to be created, this will potentially cause a graph of other beans to be created, as its dependencies and its dependencies' dependencies (and so on) are created and assigned.
You can generally trust Spring to do the right thing. It will pick up configuration issues, including references to non-existent beans and circular dependencies, at BeanFactory load-time. It will actually set properties and resolve dependencies (i.e. create those dependencies if needed) as late as possible, which is when the bean is actually created. This does mean that a BeanFactory which has loaded correctly, can later generate an exception when you request a bean, if there is a problem creating that bean or one of its dependencies. This could happen if the bean throws an exception as a result of a missing or invalid property, for example. This potentially delayed visibility of some configuration issues is why ApplicationContext by default pre-instantiates singleton beans. At the cost of some upfront time and memory to create these beans before they are actually needed, you find out about configuration issues when the ApplicationContext is created, not later. If you wish, you can still override this default behavior and set any of these singleton beans to lazy-load (not be pre-instantiated).
Some examples:
First, an example of using the BeanFactory for setter-based dependency injection. Below is a small part of an XmlBeanFactory configuration file specifying some bean definitions. Following is the code for the actual main bean itself, showing the appropriate setters declared.
<bean id="exampleBean" class="examples.ExampleBean">
<property name="beanOne"><ref bean="anotherExampleBean"/></property>
<property name="beanTwo"><ref bean="yetAnotherBean"/></property>
<property name="integerProperty"><value>1</value></property>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {
private AnotherBean beanOne;
private YetAnotherBean beanTwo;
private int i;
public void setBeanOne(AnotherBean beanOne) {
this.beanOne = beanOne;
}
public void setBeanTwo(YetAnotherBean beanTwo) {
this.beanTwo = beanTwo;
}
public void setIntegerProperty(int i) {
this.i = i;
}
}As you can see, setters have been declared to match against the properties specified in the XML file. (The properties from the XML file, directly relate to the PropertyValues object from the RootBeanDefinition)
Now, an example of using the BeanFactory for IoC type 3 (constructor-based dependency injection). Below is a snippet from an XML configuration that specifies constructor arguments and the actual bean code, showing the constructor:
<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg><ref bean="anotherExampleBean"/></constructor-arg>
<constructor-arg><ref bean="yetAnotherBean"/></constructor-arg>
<constructor-arg type="int"><value>1</value></constructor-arg>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {
private AnotherBean beanOne;
private YetAnotherBean beanTwo;
private int i;
public ExampleBean(AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
this.beanOne = anotherBean;
this.beanTwo = yetAnotherBean;
this.i = i;
}
}As you can see, the constructor arguments specified in the bean definition will be used to pass in as arguments to the constructor of the ExampleBean.
Now consider a variant of this where instead of using a constructor, Spring is told to call a static factory method to return an instance of the object.:
<bean id="exampleBean" class="examples.ExampleBean"
factory-method="createInstance">
<constructor-arg><ref bean="anotherExampleBean"/></constructor-arg>
<constructor-arg><ref bean="yetAnotherBean"/></constructor-arg>
<constructor-arg><value>1</value></constructor-arg>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {
...
// a private constructor
private ExampleBean(...) {
...
}
// a static factory method
// the arguments to this method can be considered the dependencies of the bean that
// is returned, regardless of how those arguments are actually used.
public static ExampleBean createInstance(
AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
ExampleBean eb = new ExampleBean(...);
// some other operations
...
return eb;
}
}Note that arguments to the static factory method are supplied via constructor-arg elements, exactly the same as if a constructor had actually been used. These arguments are optional. Also, it is important to realize that the type of the class being returned by the factory method does not have to be of the same type as the class which contains the static factory method, although in this example it is. An instance (non-static) factory method, mentioned previously, would be used in an essentially identical fashion (aside from the use of the factory-bean attribute instead of the class attribute), so will not be detailed here.
Constructor argument resolution matching occurs using the argument's type. When another bean is referenced, the type is known, and matching can occur. When a simple type is used, such as <value>true<value>, Spring cannot determine the type of the value, and so cannot match by type without help. Consider the following class, which is used for the following two sections:
package examples;
public class ExampleBean {
private int years; //No. of years to the calculate the Ultimate Answer
private String ultimateAnswer; //The Answer to Life, the Universe, and Everything
public ExampleBean(int years, String ultimateAnswer) {
this.years = years;
this.ultimateAnswer = ultimateAnswer;
}
}The above scenario can use type matching with simple types by explicitly specifying the type of the constructor argument using the type attribute. For example:
<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg type="int"><value>7500000</value></constructor-arg>
<constructor-arg type="java.lang.String"><value>42</value></constructor-arg>
</bean> Constructor arguments can have their index specified explicitly by use of the index attribute. For example:
<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg index="0"><value>7500000</value></constructor-arg>
<constructor-arg index="1"><value>42</value></constructor-arg>
</bean> As well as solving the ambiguity problem of multiple simple values, specifying an index also solves the problem of ambiguity where a constructor may have two arguments of the same type. Note that the index is 0 based.
Specifying a constructor argument index is the preferred way of performing constructor IoC.
As mentioned in the previous section, bean properties and constructor arguments can be defined as either references to other managed beans (collaborators), or values defined inline. The XmlBeanFactory supports a number of sub-element types within its property and constructor-arg elements for this purpose.
The value element specifies a property or constructor argument as a human-readable string representation. As mentioned in detail previously, JavaBeans PropertyEditors are used to convert these string values from a java.lang.String to the actual property or argument type.
<beans>
<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<!-- results in a setDriverClassName(String) call -->
<property name="driverClassName">
<value>com.mysql.jdbc.Driver</value>
</property>
<property name="url">
<value>jdbc:mysql://localhost:3306/mydb</value>
</property>
<property name="username">
<value>root</value>
</property>
</bean>
</beans> The null element is used to handle null values. Spring treats empty arguments for properties and the like as empty Strings. The following XmlBeanFactory configuration:
<bean class="ExampleBean">
<property name="email"><value></value></property>
</bean> results in the email property being set to "", equivalent to the java code: exampleBean.setEmail(""). The special <null> element may be used to indicate a null value, so that:
<bean class="ExampleBean">
<property name="email"><null/></property>
</bean> is equivalent to the java code: exampleBean.setEmail(null).
The list, set, map, and props elements allow properties and arguments of Java type List, Set, Map, and Properties, respectively, to be defined and set.
<beans>
...
<bean id="moreComplexObject" class="example.ComplexObject">
<!-- results in a setPeople(java.util.Properties) call -->
<property name="people">
<props>
<prop key="HarryPotter">The magic property</prop>
<prop key="JerrySeinfeld">The funny property</prop>
</props>
</property>
<!-- results in a setSomeList(java.util.List) call -->
<property name="someList">
<list>
<value>a list element followed by a reference</value>
<ref bean="myDataSource"/>
</list>
</property>
<!-- results in a setSomeMap(java.util.Map) call -->
<property name="someMap">
<map>
<entry key="yup an entry">
<value>just some string</value>
</entry>
<entry key="yup a ref">
<ref bean="myDataSource"/>
</entry>
</map>
</property>
<!-- results in a setSomeSet(java.util.Set) call -->
<property name="someSet">
<set>
<value>just some string</value>
<ref bean="myDataSource"/>
</set>
</property>
</bean>
</beans>Note that the value of a Map entry, or a set value, can also again be any of the elements:
(bean | ref | idref | list | set | map | props | value | null)
A bean element inside the property element is used to define a bean value inline, instead of referring to a bean defined elsewhere in the BeanFactory. The inline bean definition does not need to have any id defined.
<bean id="outer" class="...">
<!-- Instead of using a reference to target, just use an inner bean -->
<property name="target">
<bean class="com.mycompany.PersonImpl">
<property name="name"><value>Tony</value></property>
<property name="age"><value>51</value></property>
</bean>
</property>
</bean>An idref element is simply a shorthand and error-proof way to set a property to the String id or name of another bean in the container.
<bean id="theTargetBean" class="...">
</bean>
<bean id="theClientBean" class="...">
<property name="targetName">
<idref bean="theTargetBean"/>
</property>
</bean>This is exactly equivalent at runtime to the following fragment:
<bean id="theTargetBean" class="...">
</bean>
<bean id="theClientBean" class="...">
<property name="targetName">
<value>theTargetBean</value>
</property>
</bean>The main reason the first form is preferable to the second is that using the idref tag will allow Spring to validate at deployment time that the other bean actually exists. In the second variation, the class who's targetName property is forced to do its own validation, and that will only happen when that class is actually instantiated by Spring, possibly long after the container is actually deployed.
Additionally, if the bean being referred to is in the same actual XML file, and the bean name is the bean id, the local attribute may be used, which will allow the XML parser itself to validate the bean name even earlier, at XML document parse time.
<property name="targetName">
<idref local="theTargetBean"/>
</property>The ref element is the final element allowed inside a property definition element. It is used to set the value of the specified property to be a reference to another bean managed by the container, a collaborator, so to speak. As mentioned in a previous section, the referred-to bean is considered to be a dependency of the bean who's property is being set, and will be initialized on demand as needed (if it is a singleton bean it may have already been initialized by the container) before the property is set. All references are ultimately just a reference to another object, but there are 3 variations on how the id/name of the other object may be specified, which determines how scoping and validation is handled.
Specifying the target bean by using the bean attribute of the ref tag is the most general form, and will allow creating a reference to any bean in the same BeanFactory/ApplicationContext (whether or not in the same XML file), or parent BeanFactory/ApplicationContext. The value of the bean attribute may be the same as either the id attribute of the target bean, or one of the values in the name attribute of the target bean.
<ref bean="someBean"/>
Specifying the target bean by using the local attribute leverages the ability of the XML parser to validate XML id references within the same file. The value of the local attribute must be the same as the id attribute of the target bean. The XML parser will issue an error if no matching element is found in the same file. As such, using the local variant is the best choice (in order to know about errors are early as possible) if the target bean is in the same XML file.
<ref local="someBean"/>
Specifying the target bean by using the parent attribute allows a reference to be created to a bean which is in a parent BeanFactory (or ApplicationContext) of the current BeanFactory (or ApplicationContext). The value of the parent attribute may be the same as either the id attribute of the target bean, or one of the values in the name attribute of the target bean, and the target bean must be in a parent BeanFactory or ApplicationContext to the current one. The main use of this bean reference variant is when there is a need to wrap an existing bean in a parent context with some sort of proxy (which may have the same name as the parent), and needs the original object so it may wrap it.
<ref parent="someBean"/>
For most users, the majority of the beans in the container will be singletons. When a singleton bean needs to collaborate with (use) another singleton bean, or a non-singleton bean needs to collaborate with another non-singleton bean, the typical and common approach of handling this dependency by defining one bean to be a property of the other, is quite adequate. There is however a problem when the bean lifecycles are different. Consider a singleton bean A which needs to use a non-singleton (prototype) bean B, perhaps on each method invocation on A. The container will only create the singleton bean A once, and thus only get the opportunity to set its properties once. There is no opportunity for the container to provide bean A with a new instance of bean B every time one is needed.
One solution to this problem is to forgo some inversion of control. Bean A can be aware of the container (as described here) by implementing BeanFactoryAware, and use programmatic means (as described here) to ask the container via a getBean("B") call for (a new) bean B every time it needs it. This is generally not a desirable solution since the bean code is then aware of and coupled to Spring.
Method Injection, an advanced feature of the BeanFactory, allows this use case to be handled in a clean fashion, along with some other scenarios.
Lookup method injection refers to the ability of the container to override abstract or concrete methods on managed beans in the container, to return the result of looking up another named bean in the container. The lookup will typically be of a non-singleton bean as per the scenario described above (although it can also be a singleton). Spring implements this through a dynamically generated subclass overriding the method, using bytecode generation via the CGLIB library.
In the client class containing the method to be injected, the method definition must be an abstract (or concrete) definition in this form:
protected abstract SingleShotHelper createSingleShotHelper();
If the method is not abstract, Spring will simply override the existing implementation. In the XmlBeanFactory case, you instruct Spring to inject/override this method to return a particular bean from the container, by using the lookup-method element inside the bean definition. For example:
<!-- a stateful bean deployed as a prototype (non-singleton) -->
<bean id="singleShotHelper class="..." singleton="false">
</bean>
<!-- myBean uses singleShotHelper -->
<bean id="myBean" class="...">
<lookup-method name="createSingleShotHelper"
bean="singleShotHelper"/>
<property>
...
</property>
</bean>The bean identified as myBean will call its own method createSingleShotHelper whenever it needs a new instance of the singleShotHelper bean. It is important to note that the person deploying the beans must be careful to deploy singleShotHelper as a non-singleton (if that is actually what is needed). If it is deployed as a singleton (either explicitly, or relying on the default true setting for this flag), the same instance of singleShotHelper will be returned each time!
Note that lookup method injection can be combined with Constructor Injection (supplying optional constructor arguments to the bean being constructed), and also with Setter Injection (settings properties on the bean being constructed).
A less commonly useful form of method injection than Lookup Method Injection is the ability to replace arbitrary methods in a managed bean with another method implementation. Users may safely skip the rest of this section (which describes this somewhat advanced feature), until this functionality is actually needed.
In an XmlBeanFactory, the replaced-method element may be used to replace an existing method implementation with another, for a deployed bean. Consider the following class, with a method computeValue, which we want to override:
...
public class MyValueCalculator {
public String computeValue(String input) {
... some real code
}
... some other methods
}A class implementing the org.springframework.beans.factory.support.MethodReplacer interface is needed to provide the new method definition.
/** meant to be used to override the existing computeValue
implementation in MyValueCalculator */
public class ReplacementComputeValue implements MethodReplacer {
public Object reimplement(Object o, Method m, Object[] args) throws Throwable {
// get the input value, work with it, and return a computed result
String input = (String) args[0];
...
return ...;
}The BeanFactory deployment definition to deploy the original class and specify the method override would look like:
<bean id="myValueCalculator class="x.y.z.MyValueCalculator">
<!-- arbitrary method replacement -->
<replaced-method name="computeValue" replacer="replacementComputeValue">
<arg-type>String</arg-type>
</replaced-method>
</bean>
<bean id="replacementComputeValue" class="a.b.c.ReplaceMentComputeValue">
</bean>One or more contained arg-type elements within the replaced-method element may be used to indicate the method signature of the method being overridden. Note that the signature for the arguments is actually only needed in the case that the method is actually overloaded and there are multiple variants within the class. For convenience, the type string for an argument may be a substring of the fully qualified type name. For example, all the following would match java.lang.String.
java.lang.String
String
StrSince the number of arguments is often enough to distinguish between each possible choice, this shortcut can save a lot of typing, by just using the shortest string which will match an argument.
For most situations, the fact that a bean is a dependency of another is expressed simply by the fact that one bean is set as a property of another. This is typically done with the ref element in the XmlBeanFactory. In a variation of this, sometimes a bean which is aware of the container is simply given the id of its dependency (using a string value or alternately the idref element, which evaluates the same as a string value). The first bean then programmatically asks the container for its dependency. In either case, the dependency is properly initialized before the dependent bean.
For the relatively infrequent situations where dependencies between beans are less direct (for example, when a static initializer in a class needs to be triggered, such as database driver registration), the depends-on element may be used to explicitly force one or more beans to be initialized before the bean using this element is initialized.
Following is an example configuration:
<bean id="beanOne" class="ExampleBean" depends-on="manager">
<property name="manager"><ref local="manager"/></property>
</bean>
<bean id="manager" class="ManagerBean"/>A BeanFactory is able to autowire relationships between collaborating beans. This means it's possible to automatically let Spring resolve collaborators (other beans) for your bean by inspecting the contents of the BeanFactory. The autowiring functionality has five modes. Autowiring is specified per bean and can thus be enabled for some beans, while other beans won't be autowired. Using autowiring, it is possible to reduce or eliminate the need to specify properties or constructor arguments, saving a significant amount of typing.[1]In an XmlBeanFactory, the autowire mode for a bean definition is specified by using the autowire attribute of the bean element. The following values are allowed.
Table 3.2. Autowiring modes
| Mode | Explanation |
|---|---|
| no | No autowiring at all. Bean references must be defined via a ref element. This is the default, and changing this is discouraged for larger deployments, since explicitly specifying collaborators gives greater control and clarity. To some extent, it is a form of documentation about the structure of a system. |
| byName | Autowiring by property name. This option will inspect the BeanFactory and look for a bean named exactly the same as the property which needs to be autowired. For example, if you have a bean definition which is set to autowire by name, and it contains a master property (that is, it has a setMaster(...) method), Spring will look for a bean definition named master, and use it to set the property. |
| byType | Allows a property to be autowired if there is exactly one bean of the property type in the BeanFactory. If there is more than one, a fatal exception is thrown, and this indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set. If this is not desirable, setting the dependency-check="objects" attribute value specifies that an error should be thrown in this case. |
| constructor | This is analogous to byType, but applies to constructor arguments. If there isn't exactly one bean of the constructor argument type in the bean factory, a fatal error is raised. |
| autodetect | Chooses constructor or byType through introspection of the bean class. If a default constructor is found, byType gets applied. |
Note that explicit dependencies in property and constructor-arg elements always override autowiring. Autowire behavior can be combined with dependency checking, which will be performed after all autowiring has been completed.
It's important to understand the pros and cons around autowiring. Some advantages of autowiring:
It can significantly reduce the volume of configuration required. (However, mechanisms such as the use of a configuration "template," discussed elsewhere in this chapter, are also valuable here.)
It can cause configuration to keep itself up to date as your objects evolve. For example, if you need to add an additional dependency to a class, that dependency can be satisfied automatically without the need to modify configuration. Thus there may be a strong case for autowiring during development, without ruling out the option of switching to explicit wiring when the code base becomes more stable.
Some disadvantages of autowiring:
It's more magical than explicit wiring. Although, as noted in the above table, Spring is careful to avoid guessing in case of ambiguity which might have unexpected results, the relationships between your Spring-managed objects is no longer explicitly documented.
Wiring information may not be available to tools that may generate documentation from a Spring application context.
Autowiring by type will only work when there is a single bean definition of the type specified by the setter method or constructor argument. You need to use explicit wiring if there is any potential ambiguity.
There is no "wrong" or "right" answer in all cases. We recommend a degree of consistency across a project. For example, if autowiring is not used in general, it might be confusing to developers to use it just to one or two bean definitions.
Spring has the ability to try to check for the existence of unresolved dependencies of a bean deployed into the BeanFactory. These are JavaBeans properties of the bean, which do not have actual values set for them in the bean definition, or alternately provided automatically by the autowiring feature.
This feature is sometimes useful when you want to ensure that all properties (or all properties of a certain type) are set on a bean. Of course, in many cases a bean class will have default values for many properties, or some properties do not apply to all usage scenarios, so this feature is of limited use. Dependency checking can also be enabled and disabled per bean, just as with the autowiring functionality. The default is to not check dependencies. Dependency checking can be handled in several different modes. In an XmlBeanFactory, this is specified via the dependency-check attribute in a bean definition, which may have the following values.
Table 3.3. Dependency checking modes
| Mode | Explanation |
|---|---|
| none | No dependency checking. Properties of the bean which have no value specified for them are simply not set. |
| simple | Dependency checking is performed for primitive types and collections (everything except collaborators, i.e. other beans) |
| object | Dependency checking is performed for collaborators |
| all | Dependency checking is done for collaborators, primitive types and collections |
Spring provides several marker interfaces to change the behavior of your bean in the BeanFactory. They include InitializingBean and DisposableBean. Implementing these interfaces will result in the BeanFactory calling afterPropertiesSet() for the former and destroy() for the latter to allow the bean to perform certain actions upon initialization and destruction.
Internally, Spring uses BeanPostProcessors to process any marker interfaces it can find and call the appropriate methods. If you need custom features or other lifecycle behavior Spring doesn't offer out-of-the-box, you can implement a BeanPostProcessor yourself. More information about this can be found in Section 3.7, “Customizing beans with BeanPostprocessors”.
All the different lifecycle marker interfaces are described below. In one of the appendices, you can find diagram that show how Spring manages beans and how those lifecycle features change the nature of your beans and how they are managed.
Implementing the org.springframework.beans.factory.InitializingBean allows a bean to perform initialization work after all necessary properties on the bean are set by the BeanFactory. The InitializingBean interface specifies exactly one method:
* Invoked by a BeanFactory after it has set all bean properties supplied
* (and satisfied BeanFactoryAware and ApplicationContextAware).
* <p>This method allows the bean instance to perform initialization only
* possible when all bean properties have been set and to throw an
* exception in the event of misconfiguration.
* @throws Exception in the event of misconfiguration (such
* as failure to set an essential property) or if initialization fails.
*/
void afterPropertiesSet() throws Exception;Note: generally, the use of the InitializingBean marker interface can be avoided (and is discouraged since it unnecessarily couples the code to Spring). A bean definition provides support for a generic initialization method to be specified. In the case of the XmlBeanFactory, this is done via the init-method attribute. For example, the following definition:
<bean id="exampleInitBean" class="examples.ExampleBean" init-method="init"/>
public class ExampleBean {
public void init() {
// do some initialization work
}
}Is exactly the same as:
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements InitializingBean {
public void afterPropertiesSet() {
// do some initialization work
}
}but does not couple the code to Spring.
Implementing the org.springframework.beans.factory.DisposableBean interface allows a bean to get a callback when the BeanFactory containing it is destroyed. The DisposableBean interface specifies one method:
/**
* Invoked by a BeanFactory on destruction of a singleton.
* @throws Exception in case of shutdown errors.
* Exceptions will get logged but not re-thrown to allow
* other beans to release their resources too.
*/
void destroy() throws Exception;
Note: generally, the use of the DisposableBean marker interface can be avoided (and is discouraged since it unnecessarily couples the code to Spring). A bean definition provides support for a generic destroy method to be specified. In the case of the XmlBeanFactory, this is done via the destroy-method attribute. For example, the following definition:
<bean id="exampleInitBean" class="examples.ExampleBean" destroy-method="cleanup"/>
public class ExampleBean {
public void cleanup() {
// do some destruction work (like closing connection)
}
}Is exactly the same as:
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements DisposableBean {
public void destroy() {
// do some destruction work
}
}but does not couple the code to Spring.
Important note: when deploying a bean in the prototype mode, the lifecycle of the bean changes slightly. By definition, Spring cannot manage the complete lifecycle of a non-singleton/prototype bean, since after it is created, it is given to the client and the container does not keep track of it at all any longer. You can think of Spring's role when talking about a non-singleton/prototype bean as a replacement for the 'new' operator. Any lifecycle aspects past that point have to be handled by the client. The lifecycle of a bean in the BeanFactory is further described in Section 3.4.1, “Lifecycle interfaces”.
A class which implements the org.springframework.beans.factory.BeanFactoryAware interface is provided with a reference to the BeanFactory that created it, when it is created by that BeanFactory.
public interface BeanFactoryAware {
/**
* Callback that supplies the owning factory to a bean instance.
* <p>Invoked after population of normal bean properties but before an init
* callback like InitializingBean's afterPropertiesSet or a custom init-method.
* @param beanFactory owning BeanFactory (may not be null).
* The bean can immediately call methods on the factory.
* @throws BeansException in case of initialization errors
* @see BeanInitializationException
*/
void setBeanFactory(BeanFactory beanFactory) throws BeansException;
}This allows beans to manipulate the BeanFactory that created them programmatically, through the org.springframework.beans.factory.BeanFactory interface, or by casting the reference to a known subclass of this which exposes additional functionality. Primarily this would consist of programmatic retrieval of other beans. While there are cases when this capability is useful, it should generally be avoided, since it couples the code to Spring, and does not follow the Inversion of Control style, where collaborators are provided to beans as properties.
If a bean implements the org.springframework.beans.factory.BeanNameAware interface and is deployed in a BeanFactory, the BeanFactory will call the bean through this interface to inform the bean of the id it was deployed under. The callback will be Invoked after population of normal bean properties but before an init callback like InitializingBean's afterPropertiesSet or a custom init-method.
The org.springframework.beans.factory.FactoryBean interface is to be implemented by objects that are themselves factories. The BeanFactory interface provides three method:
Object getObject(): has to return an instance of the object this factory creates. The instance can possibly be shared (depending on whether this factory returns singletons or prototypes).
boolean isSingleton(): has to return true if this FactoryBean returns singletons, false otherwise
Class getObjectType(): has to return either the object type returned by the getObject() method or null if the type isn't known in advance
A bean definition potentially contains a large amount of configuration information, including container specific information (i.e. initialization method, static factory method name, etc.) and constructor arguments and property values. A child bean definition is a bean definition which inherits configuration data from a parent definition. It is then able to override some values, or add others, as needed. Using parent and child bean definitions can potentially save a lot of typing. Effectively, this is a form of templating.
When working with a BeanFactory programmatically, child bean definitions are represented by the ChildBeanDefinition class. Most users will never work with them on this level, instead configuring bean definitions declaratively in something like the XmlBeanFactory. In an XmlBeanFactory bean definition, a child bean definition is indicated simply by using the parent attribute, specifying the parent bean as the value of this attribute.
<bean id="inheritedTestBean" abstract="true"
class="org.springframework.beans.TestBean">
<property name="name"><value>parent</value></property>
<property name="age"><value>1</value></property>
</bean>
<bean id="inheritsWithDifferentClass" class="org.springframework.beans.DerivedTestBean"
parent="inheritedTestBean" init-method="initialize">
<property name="name"><value>override</value></property>
<!-- age should inherit value of 1 from parent -->
</bean>A child bean definition will use the bean class from the parent definition if none is specified, but can also override it. In the latter case, the child bean class must be compatible with the parent, i.e. it must accept the parent's property values.
A child bean definition will inherit constructor argument values, property values and method overrides from the parent, with the option to add new values. If init method, destroy method and/or static factory method are specified, they will override the corresponding parent settings.
The remaining settings will always be taken from the child definition: depends on, autowire mode, dependency check, singleton, lazy init.
Note that in the example above, we have explicitly marked the parent bean definition as abstract by using the abstract attribute. In the case that the parent definition does not specify a class:
<bean id="inheritedTestBeanWithoutClass">
<property name="name"><value>parent</value></property>
<property name="age"><value>1</value></property>
</bean>
<bean id="inheritsWithClass" class="org.springframework.beans.DerivedTestBean"
parent="inheritedTestBeanWithoutClass" init-method="initialize">
<property name="name"><value>override</value></property>
<!-- age should inherit value of 1 from parent -->
</bean>the parent bean cannot get instantiated on its own since it is incomplete, and it's also considered abstract. When a definition is considered abstract like this (explicitly or implicitly), it's usable just as a pure template or abstract bean definition that will serve as parent definition for child definitions. Trying to use such an abstract parent bean on its own (by referring to it as a ref property of another bean, or doing an explicit getBean() call with the parent bean id, will result in an error. Similarly, the container's internal preInstantiateSingletons method will completely ignore bean definitions which are considered abstract.
Important Note: Application contexts (but not simple bean factories) will by default pre-instantiate all singletons. Therefore it is important (at least for singleton beans) that if you have a (parent) bean definition which you intend to use only as a template, and this definition specifies a class, you must make sure to set the abstract attribute to true, otherwise the application context will actually pre-instantiate it.
A BeanFactory is essentially nothing more than the interface for an advanced factory capable of maintaining a registry of different beans and their dependencies. The BeanFactory enables you to read bean definitions and access them using the bean factory. When using just the BeanFactory you would create one and read in some bean definitions in the XML format as follows:
InputStream is = new FileInputStream("beans.xml");
XmlBeanFactory factory = new XmlBeanFactory(is);Basically that's all there is to it. Using getBean(String) you can retrieve instances of your beans. You'll get a reference to the same bean if you defined it as a singleton (the default) or you'll get a new instance each time if you set singleton to false. The client-side view of the BeanFactory is surprisingly simple. The BeanFactory interface has only five methods for clients to call:
boolean containsBean(String): returns true if the BeanFactory contains a bean definition or bean instance that matches the given name
Object getBean(String): returns an instance of the bean registered under the given name. Depending on how the bean was configured by the BeanFactory configuration, either a singleton and thus shared instance or a newly created bean will be returned. A BeansException will be thrown when either the bean could not be found (in which case it'll be a NoSuchBeanDefinitionException), or an exception occurred while instantiating and preparing the bean
Object getBean(String,Class): returns a bean, registered under the given name. The bean returned will be cast to the given Class. If the bean could not be cast, corresponding exceptions will be thrown (BeanNotOfRequiredTypeException). Furthermore, all rules of the getBean(String) method apply (see above)
boolean isSingleton(String): determines whether or not the bean definition or bean instance registered under the given name is a singleton or a prototype. If no bean corresponding to the given name could not be found, an exception will be thrown (NoSuchBeanDefinitionException)
String[] getAliases(String): Return the aliases for the given bean name, if any were defined in the bean definition
Sometimes there is a need to ask a BeanFactory for an actual FactoryBean instance itself, not the bean it produces. This may be done by prepending the bean id with & when calling the getBean method of BeanFactory (including ApplicationContext). So for a given FactoryBean with an id myBean, invoking getBean("myBean") on the BeanFactory will return the product of the FactoryBean, but invoking getBean("&myBean") will return the FactoryBean instance itself.
A bean post-processor is a java class which implements the org.springframework.beans.factory.config.BeanPostProcessor interface, which consists of two callback methods. When such a class is registered as a post-processor with the BeanFactory, for each bean instance that is created by the BeanFactory, the post-processor will get a callback from the BeanFactory before any initialization methods (afterPropertiesSet and any declared init method) are called, and also afterwords. The post-processor is free to do what it wishes with the bean, including ignoring the callback completely. A bean post-processor will typically check for marker interfaces, or do something such as wrap a bean with a proxy. Some Spring helper classes are implemented as bean post-processors.
It is important to know that a BeanFactory treats bean post-processors slightly differently than an ApplicationContext. An ApplicationContext will automatically detect any beans which are deployed into it which implement the BeanPostProcessor interface, and register them as post-processors, to be then called appropriately by the factory on bean creation. Nothing else needs to be done other than deploying the post-processor in a similar fashion to any other bean. On the other hand, when using plain BeanFactories, bean post-processors have to manually be explicitly registered, with a code sequence such as the following:
ConfigurableBeanFactory bf = new .....; // create BeanFactory ... // now register some beans // now register any needed BeanPostProcessors MyBeanPostProcessor pp = new MyBeanPostProcessor(); bf.addBeanPostProcessor(pp); // now start using the factory ...
Since this manual registration step is not convenient, and ApplictionContexts are functionally supersets of BeanFactories, it is generally recommended that ApplicationContext variants are used when bean post-processors are needed.
A bean factory post-processor is a java class which implements the org.springframework.beans.factory.config.BeanFactoryPostProcessor interface. It is executed manually (in the case of the BeanFactory) or automatically (in the case of the ApplicationContext) to apply changes of some sort to an entire BeanFactory, after it has been constructed. Spring includes a number of pre-existing bean factory post-processors, such as PropertyResourceConfigurer and PropertyPlaceHolderConfigurer, both described below, and BeanNameAutoProxyCreator, very useful for wrapping other beans transactionally or with any other kind of proxy, as described later in this manual. The BeanFactoryPostProcessor can be used to add custom editors (as also mentioned in Section 3.9, “Registering additional custom PropertyEditors”).
In a BeanFactory, the process of applying a BeanFactoryPostProcessor is manual, and will be similar to this:
XmlBeanFactory factory = new XmlBeanFactory(new FileSystemResource("beans.xml"));
// create placeholderconfigurer to bring in some property
// values from a Properties file
PropertyPlaceholderConfigurer cfg = new PropertyPlaceholderConfigurer();
cfg.setLocation(new FileSystemResource("jdbc.properties"));
// now actually do the replacement
cfg.postProcessBeanFactory(factory);An ApplicationContext will detect any beans which are deployed into it which implement the BeanFactoryPostProcessor interface, and automatically use them as bean factory post-processors, at the appropriate time. Nothing else needs to be done other than deploying these post-processor in a similar fashion to any other bean.
Since this manual step is not convenient, and ApplictionContexts are functionally supersets of BeanFactories, it is generally recommended that ApplicationContext variants are used when bean factory post-processors are needed.
The PropertyPlaceholderConfigurer, implemented as a bean factory post-processor, is used to externalize some property values from a BeanFactory definition, into another separate file in Java Properties format. This is useful to allow the person deploying an application to customize some key properties (for example database URLs, usernames and passwords), without the complexity or risk of modifying the main XML definition file or files for the BeanFactory.
Consider a fragment from a BeanFactory definition, where a DataSource with placeholder values is defined:
In the example below, a datasource is defined, and we will configure some properties from an external Properties file. At runtime, we will apply a PropertyPlaceholderConfigurer to the BeanFactory which will replace some properties of the datasource:
<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<property name="driverClassName"><value>${jdbc.driverClassName}</value></property>
<property name="url"><value>${jdbc.url}</value></property>
<property name="username"><value>${jdbc.username}</value></property>
<property name="password"><value>${jdbc.password}</value></property>
</bean>The actual values come from another file in Properties format:
jdbc.driverClassName=org.hsqldb.jdbcDriver jdbc.url=jdbc:hsqldb:hsql://production:9002 jdbc.username=sa jdbc.password=root
To use this with a BeanFactory, the bean factory post-processor is manually executed on it:
XmlBeanFactory factory = new XmlBeanFactory(new FileSystemResource("beans.xml"));
PropertyPlaceholderConfigurer cfg = new PropertyPlaceholderConfigurer();
cfg.setLocation(new FileSystemResource("jdbc.properties"));
cfg.postProcessBeanFactory(factory);Note that ApplicationContexts are able to automatically recognize and apply beans deployed in them which implement BeanFactoryPostProcessor. This means that as described here, applying PropertyPlaceholderConfiguer is much more convenient when using an ApplicationContext. For this reason, it is recommended that users wishing to use this or other bean factory postprocessors use an ApplicationContext instead of a BeanFactory.
The PropertyPlaceHolderConfigurer doesn't only look for properties in the Properties file you specify, but also checks against the Java System properties if it cannot find a property you are trying to use. This behavior can be customized by setting the systemPropertiesMode property of the configurer. It has three values, one to tell the configurer to always override, one to let it never override and one to let it override only if the property cannot be found in the properties file specified. Please consult the JavaDoc for the PropertiesPlaceHolderConfigurer for more information.
The PropertyOverrideConfigurer, another bean factory post-processor, is similar to the PropertyPlaceholderConfigurer, but in contrast to the latter, the original definitions can have default values or no values at all for bean properties. If an overriding Properties file does not have an entry for a certain bean property, the default context definition is used.
Note that the bean factory definition is not aware of being overridden, so it is not immediately obvious when looking at the XML definition file that the override configurer is being used. In case that there are multiple PropertyOverrideConfigurers that define different values for the same bean property, the last one will win (due to the overriding mechanism).
Properties file configuration lines are expected to be in the format:
beanName.property=valueAn example properties file could look like:
dataSource.driverClassName=com.mysql.jdbc.Driver dataSource.url=jdbc:mysql:mydb
This example file would be usable against a BeanFactory definition which contains a bean in it called dataSource, which has driver and url properties.
When setting bean properties as a string value, a BeanFactory ultimately uses standard JavaBeans PropertyEditors to convert these Strings to the complex type of the property. Spring pre-registers a number of custom PropertyEditors (for example, to convert a classname expressed as a string into a real Class object). Additionally, Java's standard JavaBeans PropertyEditor lookup mechanism allows a PropertyEditor for a class to be simply named appropriately and placed in the same package as the class it provides support for, to be found automatically.
If there is a need to register other custom PropertyEditors, there are several mechanisms available.
The most manual approach, which is not normally convenient or recommended, is to simply use the registerCustomEditor() method of the ConfigurableBeanFactory interface, assuming you have a BeanFactory reference.
The more convenient mechanism is to use a special bean factory post-processor called CustomEditorConfigurer. Although bean factory post-processors can be used semi-manually with BeanFactories, this one has a nested property setup, so it is strongly recommended that, as described here, it is used with the ApplicationContext, where it may be deployed in similar fashion to any other bean, and automatically detected and applied.
Note that all bean factories and application contexts automatically use a number of built-in property editors, through their use of something called a BeanWrapper to handle property conversions. The standard property editors that the BeanWrapper registers are listed in the next chapter. Additionally, ApplicationContexts also override or add an additional 3 editors to handle resource lookups in a manner appropriate to the specific application context type. Thee are: InputStreamEditor, ResourceEditor and URLEditor.
While the beans package provides basic functionality for managing and manipulating beans, often in a programmatic way, the context package adds ApplicationContext, which enhances BeanFactory functionality in a more framework-oriented style. Many users will use ApplicationContext in a completely declarative fashion, not even having to create it manually, but instead relying on support classes such as ContextLoader to automatically start an ApplicationContext as part of the normal startup process of a J2EE web-app. Of course, it is still possible to programmatically create an ApplicationContext.
The basis for the context package is the ApplicationContext interface, located in the org.springframework.context package. Deriving from the BeanFactory interface, it provides all the functionality of BeanFactory. To allow working in a more framework-oriented fashion, using layering and hierarchical contexts, the context package also provides the following:
MessageSource, providing access to messages in, i18n-style
Access to resources, such as URLs and files
Event propagation to beans implementing the ApplicationListener interface
Loading of multiple (hierarchical) contexts, allowing each to be focused on one particular layer, for example the web layer of an application
As the ApplicationContext includes all functionality of the BeanFactory, it is generally recommended that it be used over the BeanFactory, except for a few limited situations such as perhaps in an Applet, where memory consumption might be critical, and a few extra kilobytes might make a difference. The following sections described functionality which ApplicationContext adds to basic BeanFactory capabilities.
As already stated in the previous section, the ApplicationContext has a couple of features that distinguish it from the BeanFactory. Let us review them one-by-one.
The ApplicationContext interface extends an interface called MessageSource, and therefore provides messaging (i18n or internationalization) functionality. Together with the NestingMessageSource, capable of resolving hierarchical messages, these are the basic interfaces Spring provides to do message resolution. Let's quickly review the methods defined there:
String getMessage (String code, Object[] args, String default, Locale loc): the basic method used to retrieve a message from the MessageSource. When no message is found for the specified locale, the default message is used. Any arguments passed in are used as replacement values, using the MessageFormat functionality provided by the standard library.
String getMessage (String code, Object[] args, Locale loc): essentially the same as the previous method, but with one difference: no default message can be specified; if the message cannot be found, a NoSuchMessageException is thrown.
String getMessage(MessageSourceResolvable resolvable, Locale locale): all properties used in the methods above are also wrapped in a class named MessageSourceResolvable, which you can use via this method.
When an ApplicationContext gets loaded, it automatically searches for a MessageSource bean defined in the context. The bean has to have the name messageSource. If such a bean is found, all calls to the methods described above will be delegated to the message source that was found. If no message source was found, the ApplicationContext attempts to see if it has a parent containing a bean with the same name. If so, it uses that bean as the MessageSource. If it can't find any source for messages, an empty StaticMessageSource will be instantiated in order to be able to accept calls to the methods defined above.
Spring currently provides two MessageSource implementations. These are the ResourceBundleMessageSource and the StaticMessageSource. Both implement NestingMessageSource in order to do nested messaging. The StaticMessageSource is hardly ever used but provides programmatic ways to add messages to the source. The ResourceBundleMessageSource is more interesting and is the one we will provides an example for:
<beans>
<bean id="messageSource"
class="org.springframework.context.support.ResourceBundleMessageSource">
<property name="basenames">
<list>
<value>format</value>
<value>exceptions</value>
<value>windows</value>
</list>
</property>
</bean>
</beans> This assumes you have three resource bundles defined on your classpath called format, exceptions and windows. Using the JDK standard way of resolving messages through ResourceBundles, any request to resolve a message will be handled. TODO: SHOW AN EXAMPLE
Event handling in the ApplicationContext is provided through the ApplicationEvent class and ApplicationListener interface. If a bean which implements the ApplicationListener interface is deployed into the context, every time an ApplicationEvent gets published to the ApplicationContext, that bean will be notified. Essentially, this is the standard Observer design pattern. Spring provides three standard events:
Table 3.4. Built-in Events
| Event | Explanation |
|---|---|
| ContextRefreshedEvent | Event published when the ApplicationContext is initialized or refreshed. Initialized here means that all beans are loaded, singletons are pre-instantiated and the ApplicationContext is ready for use |
| ContextClosedEvent | Event published when the ApplicationContext is closed, using the close() method on the ApplicationContext. Closed here means that singletons are destroyed |
| RequestHandledEvent | A web-specific event telling all beans that a HTTP request has been serviced (i.e. this will be published after the request has been finished). Note that this event is only applicable for web applications using Spring's DispatcherServlet |
Implementing custom events can be done as well. Simply call the publishEvent() method on the ApplicationContext, specifying a parameter which is an instance of your custom event class implementing ApplicationEvent. Event listeners receive events synchronously. This means the publishEvent() method blocks until all listeners have finished processing the event. Furthermore, when a listener receives an event it operates inside the transaction context of the publisher, if a transaction context is available.
Let's look at an example. First, the ApplicationContext:
<bean id="emailer" class="example.EmailBean">
<property name="blackList">
<list>
<value>[email protected]</value>
<value>[email protected]</value>
<value>[email protected]</value>
</list>
</property>
</bean>
<bean id="blackListListener" class="example.BlackListNotifier">
<property name="notificationAddress">
<value>[email protected]</value>
</property>
</bean>and then, the actual beans:
public class EmailBean implements ApplicationContextAware {
/** the blacklist */
private List blackList;
public void setBlackList(List blackList) {
this.blackList = blackList;
}
public void setApplicationContext(ApplicationContext ctx) {
this.ctx = ctx;
}
public void sendEmail(String address, String text) {
if (blackList.contains(address)) {
BlackListEvent evt = new BlackListEvent(address, text);
ctx.publishEvent(evt);
return;
}
// send email
}
}
public class BlackListNotifier implement ApplicationListener {
/** notification address */
private String notificationAddress;
public void setNotificationAddress(String notificationAddress) {
this.notificationAddress = notificationAddress;
}
public void onApplicationEvent(ApplicationEvent evt) {
if (evt instanceof BlackListEvent) {
// notify appropriate person
}
}
}Of course, this particular example could probably be implemented in better ways (perhaps by using AOP features), but it should be sufficient to illustrate the basic event mechanism.
Many applications need to access resources. Resources could include files, but also things like web pages or NNTP newsfeeds. Spring provides a clean and transparent way of accessing resources in a protocol independent way. The ApplicationContext interface includes a method (getResource(String)) to take care of this.
The Resource class defines a couple of methods that are shared across all Resource implementations:
Table 3.5. Resource functionality
| Method | Explanation |
|---|---|
| getInputStream() | Opens an InputStream on the resource and returns it |
| exists() | Checks if the resource exists, returning false if it doesn't |
| isOpen() | Will return true is multiple streams cannot be opened for this resource. This will be false for some resources, but file-based resources for instance, cannot be read multiple times concurrently |
| getDescription() | Returns a description of the resource, often the fully qualified file name or the actual URL |
A couple of Resource implementations are provided by Spring. They all need a String representing the actual location of the resource. Based upon that String, Spring will automatically choose the right Resource implementation for you. When asking an ApplicationContext for a resource first of all Spring will inspect the resource location you're specifying and look for any prefixes. Depending on the implementation of the ApplicationContext more or less Resource implementations are available. Resources can best be configured by using the ResourceEditor and for example the XmlBeanFactory.
The BeanFactory already offers a number of mechanisms to control the lifecycle of beans deployed in it (such as marker interfaces like InitializingBean or DisposableBean, their configuration only equivalents such as the init-method and destroy-method attributes in an XmlBeanFactory config, and bean post-processors. In an ApplicationContext, all of these still work, but additional mechanisms are added for customizing behavior of beans and the container.
All marker interfaces available with BeanFactories still work. The ApplicationContext does add one extra marker interface which beans may implement, org.springframework.context.ApplicationContextAware. A bean which implements this interface and is deployed into the context will be called back on creation of the bean, using the interface's setApplicationContext() method, and provided with a reference to the context, which may be stored for later interaction with the context.
Bean post-processors, java classes which implement the org.springframework.beans.factory.config.BeanPostProcessor interface, have already been mentioned. It is worth mentioning again here though, that post-processors are much more convenient to use in ApplicationContexts than in plain BeanFactories. In an ApplicationContext, any deployed bean which implements the above marker interface is automatically detected and registered as a bean post-processor, to be called appropriately at creation time for each bean in the factory.
Bean factory post-processors, java classes which implement the org.springframework.beans.factory.config.BeanFactoryPostProcessor interface, have already been mentioned. It is worth mentioning again here though, that bean factory post-processors are much more convenient to use in ApplicationContexts than in plain BeanFactories. In an ApplicationContext, any deployed bean which implements the above marker interface is automatically detected as a bean factory post-processor, to be called at the appropriate time.
The PropertyPlaceholderConfigurer has already been described, as used with a BeanFactory. It is worth mentioning here though, that it is generally more convenient to use it with an ApplicationContext, since the context will automatically recognize and apply any bean factory post-processors, such as this one, when they are simply deployed into it like any other bean. There is no need for a manual step to execute it.
<!-- property placeholder post-processor -->
<bean id="placeholderConfig"
class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="location"><value>jdbc.properties</value></property>
</bean>As previously mentioned, standard JavaBeans PropertyEditors are used to convert property values expressed as strings to the actual complex type of the property. CustomEditorConfigurer, a bean factory post-processor, may be used to conveniently add support for additional PropertyEditors to an ApplicationContext.
Consider a user class ExoticType, and another class DependsOnExoticType which needs ExoticType set as a property:
public class ExoticType {
private String name;
public ExoticType(String name) {
this.name = name;
}
}
public class DependsOnExoticType {
private ExoticType type;
public void setType(ExoticType type) {
this.type = type;
}
}When things are properly set up, we want to be able to assign the type property as a string, which a PropertyEditor will behind the scenes convert into a real ExoticType object.:
<bean id="sample" class="example.DependsOnExoticType">
<property name="type"><value>aNameForExoticType</value></property>
</bean>The PropertyEditor could look similar to this:
// converts string representation to ExoticType object
public class ExoticTypeEditor extends PropertyEditorSupport {
private String format;
public void setFormat(String format) {
this.format = format;
}
public void setAsText(String text) {
if (format != null && format.equals("upperCase")) {
text = text.toUpperCase();
}
ExoticType type = new ExoticType(text);
setValue(type);
}
}Finally, we use CustomEditorConfigurer to register the new PropertyEditor with the ApplicationContext, which will then be able to use it as needed.:
<bean id="customEditorConfigurer"
class="org.springframework.beans.factory.config.CustomEditorConfigurer">
<property name="customEditors">
<map>
<entry key="example.ExoticType">
<bean class="example.ExoticTypeEditor">
<property name="format">
<value>upperCase</value>
</property>
</bean>
</entry>
</map>
</property>
</bean>PropertyPathFactoryBean is a FactoryBean that evaluates a property path on a given target object. The target object can be specified directly or via a bean name. This value may then be used in another bean definition as a property value or constructor argument.
Here's an example where a path is used against another bean, by name:
// target bean to be referenced by name
<bean id="person" class="org.springframework.beans.TestBean" singleton="false">
<property name="age"><value>10</value></property>
<property name="spouse">
<bean class="org.springframework.beans.TestBean">
<property name="age"><value>11</value></property>
</bean>
</property>
</bean>
// will result in 11, which is the value of property 'spouse.age' of bean 'person'
<bean id="theAge" class="org.springframework.beans.factory.config.PropertyPathFactoryBean">
<property name="targetBeanName"><value>person</value></property>
<property name="propertyPath"><value>spouse.age</value></property>
</bean>In this example, a path is evaluated against an inner bean:
// will result in 12, which is the value of property 'age' of the inner bean
<bean id="theAge" class="org.springframework.beans.factory.config.PropertyPathFactoryBean">
<property name="targetObject">
<bean class="org.springframework.beans.TestBean">
<property name="age"><value>12</value></property>
</bean>
</property>
<property name="propertyPath"><value>age</value></property>
</bean>There is also a shortcut form, where the bean name is the property path.
// will result in 10, which is the value of property 'age' of bean 'person' <bean id="person.age" class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/>
This form does mean that there is no choice in the name of the bean, any reference to it will also have to use the same id, which is the path. Of curse, if used as an inner bean, there is no need to refer to it at all:
<bean id="..." class="...">
<proprty name="age">
<bean id="person.age"
class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/>
</property>
</bean>The result type may be specifically set in the actual definition. This is not necessary for most use cases, but can be of use for some. Please see the JavaDocs for more info on this feature.
FileRetrievingFactoryBean is a FactoryBean which retrieves a static or non-static field value. It is typically used for retrieving public static final constants, which may then be used to set a property value or constructor arg for another bean.
Here's an example which shows how a static field is exposed, by using the staticField property:
<bean id="myField"
class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean">
<property name="staticField"><value>java.sql.Connection.TRANSACTION_SERIALIZABLE</value></property>
</bean>There's also a convenience usage form where the static field is specified as a bean name:
<bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE"
class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean"/>This means there is no longer any choice in what the bean id is (so any other bean that refers to it will also have to use this longer name), but this form is very concise to define, and very convenient to use as an inner bean since the id doesn't have to be specified for the bean reference:
<bean id="..." class="...">
<proprty name="isolation">
<bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE"
class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean"/>
</property>
</bean>It's also possible to access a non-static field of another bean, as described in the JavaDocs.
it is sometimes necessary to call a static or non-static method in one class, just to perform some sort of initialization, before some other class is used. Additionally, it is sometimes necessary to set a property on a bean, as the result of a method call on another bean in the container, or a static method call on any arbitrary class. For both of these purposes, a helper class called MethodInvokingFactoryBean may be used. This is a FactoryBean which returns a value which is the result of a static or instance method invocation.
We would however recommend that for the second use case, factory-methods, described previously, are a better all around choice.
An example (in an XML based BeanFactory definition) of a bean definition which uses this class to force some sort of static initialization:
<bean id="force-init" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean"> <property name="staticMethod"><value>com.example.MyClass.initialize</value></property> </bean> <bean id="bean1" class="..." depends-on="force-init"> ... </bean>
Note that the definition for bean1 has used the depends-on attribute to refer to the force-init bean, which will trigger initializing force-init first, and thus calling the static initializer method, when bean1 is first initialized.
Here's an example of a bean definition which uses this class to call a static factory method:
<bean id="myClass" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean"> <property name="staticMethod"><value>com.whatever.MyClassFactory.getInstance</value></property> </bean>
An example of calling a static method then an instance method to get at a Java System property. Somewhat verbose, but it works.
<bean id="sysProps" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
<property name="targetClass"><value>java.lang.System</value></property>
<property name="targetMethod"><value>getProperties</value></property>
</bean>
<bean id="javaVersion" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
<property name="targetObject"><ref local="sysProps"/></property>
<property name="targetMethod"><value>getProperty</value></property>
<property name="arguments">
<list>
<value>java.version</value>
</list>
</property>
</bean>Note that as it is expected to be used mostly for accessing factory methods, MethodInvokingFactoryBean by default operates in a singleton fashion. The first request by the container for the factory to produce an object will cause the specified method invocation, whose return value will be cached and returned for the current and subsequent requests. An internal singleton property of the factory may be set to false, to cause it to invoke the target method each time it is asked for an object.
A static target method may be specified by setting the targetMethod property to a String representing the static method name, with targetClass specifying the Class that the static method is defined on. Alternatively, a target instance method may be specified, by setting the targetObject property as the target object, and the targetMethod property as the name of the method to call on that target object. Arguments for the method invocation may be specified by setting the args property.
It's often useful to split up container definitions into multiple XML files. One way to then load an application context which is configured from all these XML fragments is to use the application context constructor which takes multiple Resource locations. With a bean factory, a bean definition reader can be used multiple times to read definitions from each file in turn.
Generally, the Spring team prefers the above approach, since it keeps container configurations files unaware of the fact that they are being combined with others. However, an alternate approach is to from one XML bean definition file, use one or more instances of the import element to load definitions from one or more other files. Any import elements must be placed before bean elements in the file doing the importing. Let's look at a sample:
<beans> <import resource="services.xml"/> <import resource="resources/messageSource.xml"/> <import resource="/resources/themeSource.xml"/> <bean id="bean1" class="..."/> <bean id="bean2" class="..."/> . . .
In this example, external bean definitions are being loaded from 3 files, services.xml, messageSource.xml, and themeSource.xml. All location paths are considered relative to the definition file doing the importing, so services.xml in this case must be in the same directory or classpath location as the file doing the importing, while messageSource.xml and themeSource.xml must be in a resources location below the location of the importing file. As you can see, a leading slash is actually ignored, but given that these are considered relative paths, it is probably better form not to use the slash at all.
The contents of the files being imported must be fully valid XML bean definition files according to the DTD, including the top level beans element.
As opposed to the BeanFactory, which will often be created programmatically, ApplicationContexts can be created declaratively using for example a ContextLoader. Of course you can also create ApplicationContexts programmatically using one of the ApplicationContext implementations. First, let's examine the ContextLoader and its implementations.
The ContextLoader has two implementations: the ContextLoaderListener and the ContextLoaderServlet. They both have the same functionality but differ in that the listener cannot be used in Servlet 2.2 compatible containers. Since the Servlet 2.4 specification, listeners are required to initialize after startup of a web application. A lot of 2.3 compatible containers already implement this feature. It is up to you as to which one you use, but all things being equal you should probably prefer ContextLoaderListener; for more information on compatibility, have a look at the JavaDoc for the ContextLoaderServlet.
You can register an ApplicationContext using the ContextLoaderListener as follows:
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/daoContext.xml /WEB-INF/applicationContext.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<!-- OR USE THE CONTEXTLOADERSERVLET INSTEAD OF THE LISTENER
<servlet>
<servlet-name>context</servlet-name>
<servlet-class>org.springframework.web.context.ContextLoaderServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
-->The listener inspects the contextConfigLocation parameter. If it doesn't exist, it'll use /WEB-INF/applicationContext.xml as a default. When it does exist, it'll separate the String using predefined delimiters (comma, semi-colon and space) and use the values as locations where application contexts will be searched for. The ContextLoaderServlet can - as said - be used instead of the ContextLoaderListener. The servlet will use the contextConfigLocation parameter just as the listener does.
The majority of the code inside an application is best written in a Dependency Injection (Inversion of Control) style, where that code is served out of a BeanFactory or ApplicationContext container, has its own dependencies supplied by the container when it is created, and is completely unaware of the container. However, for the small glue layers of code that are sometimes needed to tie other code together, there is sometimes a need for singleton (or quasi-singleton) style access to a BeanFactory or ApplicationContext. For example, third party code may try to construct new objects directly (Class.forName() style), without the ability to force it to get these objects out of a BeanFactory. If the object constructed by the third party code is just a small stub or proxy, which then uses a singleton style access to a BeanFactory/ApplicationContext to get a real object to delegate to, then inversion of control has still been achieved for the majority of the code (the object coming out of the BeanFactory); thus most code is still unaware of the container or how it is accessed, and remains uncoupled from other code, with all ensuing benefits. EJBs may also use this stub/proxy approach to delegate to a plain java implementation object, coming out of a BeanFactory. While the BeanFactory ideally does not have to be a singleton, it may be unrealistic in terms of memory usage or initialization times (when using beans in the BeanFactory such as a Hibernate SessionFactory) for each bean to use its own, non-singleton BeanFactory.
As another example, in a complex J2EE apps with multiple layers (i.e. various JAR files, EJBs, and WAR files packaged as an EAR), with each layer having its own ApplicationContext definition (effectively forming a hierarchy), the preferred approach when there is only one web-app (WAR) in the top hierarchy is to simply create one composite ApplicationContext from the multiple XML definition files from each layer. All the ApplicationContext variants may be constructed from multiple definition files in this fashion. However, if there are multiple sibling web-apps at the top of the hierarchy, it is problematic to create an ApplicationContext for each web-app which consists of mostly identical bean definitions from lower layers, as there may be issues due to increased memory usage, issues with creating multiple copies of beans which take a long time to initialize (i.e. a Hibernate SessionFactory), and possible issues due to side-effects. As an alternative, classes such as ContextSingletonBeanFactoryLocator or SingletonBeanFactoryLocator may be used to demand load multiple hierarchical (i.e. one is a parent of another) BeanFactories or ApplicationContexts in an effectively singleton fashion, which may then be used as the parents of the web-app ApplicationContexts. The result is that bean definitions for lower layers are loaded only as needed, and loaded only once.
You can see a detailed example of using SingletonBeanFactoryLocator and ContextSingletonBeanFactoryLocator by viewing their respective JavaDocs.
As mentioned in the chapter on EJBs, the Spring convenience base classes for EJBs normally use a non-singleton BeanFactoryLocator implementation, which is easily replaced by the use of SingletonBeanFactoryLocator and ContextSingletonBeanFactoryLocator if there is a need.