Posted in: Bean scopes in Spring
Bean scopes
When you create a bean definition what you are actually creating is a recipe for creating actual instances of the class defined by that bean definition. The idea that a bean definition is a recipe is important, because it means that, just like a class, you can potentially have many object instances created from a single recipe.
You can control not only the various dependencies and configuration values that are to be plugged into an object that is created from a particular bean definition, but also the scope of the objects created from a particular bean definition. This approach is very powerful and gives you the flexibility to choose the scope of the objects you create through configuration instead of having to 'bake in' the scope of an object at the Java class level. Beans can be defined to be deployed in one of a number of scopes: out of the box, the Spring Framework supports exactly five scopes (of which three are available only if you are using a web-awareApplicationContext).
The scopes supported out of the box are listed below:
Bean scopes
| Scope | Description |
|---|---|
singleton | Scopes a single bean definition to a single object instance per Spring IoC container. |
prototype | Scopes a single bean definition to any number of object instances. |
request | Scopes a single bean definition to the lifecycle of a single HTTP request; that is each and every HTTP request will have its own instance of a bean created off the back of a single bean definition. Only valid in the context of a web-aware Spring ApplicationContext. |
session | Scopes a single bean definition to the lifecycle of a HTTP Session. Only valid in the context of a web-aware SpringApplicationContext. |
global session | Scopes a single bean definition to the lifecycle of a g lobal HTTP Session. Typically only valid when used in a portlet context. Only valid in the context of a web-aware Spring ApplicationContext. |
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 by the Spring container.
To put it another way, when you define a bean definition and it is scoped as a singleton, then the Spring IoC container will createexactly one instance of the object defined by that bean definition. This single instance will be stored in a cache of such singleton beans, and all subsequent requests and references for that named bean will result in the cached object being returned.
Please be aware that Spring's concept of a singleton bean is quite different from the Singleton pattern as defined in the seminal Gang of Four (GoF) patterns book. The GoF Singleton hard codes the scope of an object such that one and only one instance of a particular class will ever be created per ClassLoader. The scope of the Spring singleton is best described as per container and per bean. This means that if you define one bean for a particular class in a single Spring container, then the Spring container will create one and only one instance of the class defined by that bean definition. The singleton scope is the default scope in Spring. To define a bean as a singleton in XML, you would write configuration like so:
<bean id="accountService" class="com.foo.DefaultAccountService"/>
<!-- the following is equivalent, though redundant (singleton scope is the default); using spring-beans-2.0.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" scope="singleton"/>
<!-- the following is equivalent and preserved for backward compatibility in spring-beans.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" singleton="true"/>The non-singleton, prototype scope of bean deployment results in the creation of a new bean instance every time a request for that specific bean is made (that is, it is injected into another bean or it is requested via a programmatic getBean() method call on the container). As a rule of thumb, you should use the prototype scope for all beans that are stateful, while the singleton scope should be used for stateless beans.
The following diagram illustrates the Spring prototype scope. Please note that a DAO would not typically be configured as a prototype, since a typical DAO would not hold any conversational state; it was just easier for this author to reuse the core of the singleton diagram.
To define a bean as a prototype in XML, you would write configuration like so:
<!-- using spring-beans-2.0.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" scope="prototype"/>
<!-- the following is equivalent and preserved for backward compatibility in spring-beans.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" singleton="false"/>There is one quite important thing to be aware of when deploying a bean in the prototype scope, in that the lifecycle of the bean changes slightly. Spring does not manage the complete lifecycle of a prototype bean: the container instantiates, configures, decorates and otherwise assembles a prototype object, hands it to the client and then has no further knowledge of that prototype instance. This means that while initialization lifecycle callback methods will be called on all objects regardless of scope, in the case of prototypes, any configured destruction lifecycle callbacks will not be called. It is the responsibility of the client code to clean up prototype scoped objects and release any expensive resources that the prototype bean(s) are holding onto. (One possible way to get the Spring container to release resources used by prototype-scoped beans is through the use of a custom bean post-processor which would hold a reference to the beans that need to be cleaned up.)
In some respects, you can think of the Spring containers role when talking about a prototype-scoped bean as somewhat of a replacement for the Java 'new' operator. All lifecycle aspects past that point have to be handled by the client. (The lifecycle of a bean in the Spring container is further described in the section entitled “Lifecycle callbacks
When using singleton-scoped beans that have dependencies on beans that are scoped as prototypes, please be aware thatdependencies are resolved at instantiation time. This means that if you dependency inject a prototype-scoped bean into a singleton-scoped bean, a brand new prototype bean will be instantiated and then dependency injected into the singleton bean... but that is all. That exact same prototype instance will be the sole instance that is ever supplied to the singleton-scoped bean, which is fine if that is what you want.
However, sometimes what you actually want is for the singleton-scoped bean to be able to acquire a brand new instance of the prototype-scoped bean again and again and again at runtime. In that case it is no use just dependency injecting a prototype-scoped bean into your singleton bean, because as explained above, that only happens once when the Spring container is instantiating the singleton bean and resolving and injecting its dependencies. If you are in the scenario where you need to get a brand new instance of a (prototype) bean again and again and again at runtime,
If you are referencing the 'spring-beans.dtd' DTD in a bean definition file(s), and you are being explicit about the lifecycle scope of your beans you must use the "singleton" attribute to express the lifecycle scope (remembering that the singleton lifecycle scope is the default). If you are referencing the 'spring-beans-2.0.dtd' DTD or the Spring 2.0 XSD schema, then you will need to use the "scope" attribute (because the "singleton" attribute was removed from the definition of the new DTD and XSD files in favor of the "scope" attribute). To be totally clear about this, this means that if you use the "singleton" attribute in an XML bean definition then you must be referencing the 'spring-beans.dtd' DTD in that file. If you are using the "scope" attribute then you must be referencing either the 'spring-beans-2.0.dtd' DTD or the 'spring-beans-2.5.xsd' XSD in that file. |
The other scopes
The other scopes, namely request, session, and global session are for use only in web-based applications (and can be used irrespective of which particular web application framework you are using, if indeed any). In the interest of keeping related concepts together in one place in the reference documentation, these scopes are described here.
The scopes that are described in the following paragraphs are only available if you are using a web-aware SpringApplicationContext implementation (such as XmlWebApplicationContext). If you try using these next scopes with regular Spring IoC containers such as the XmlBeanFactory orClassPathXmlApplicationContext, you will get an IllegalStateException complaining about an unknown bean scope. | |
In order to support the scoping of beans at the request, session, and global session levels (web-scoped beans), some minor initial configuration is required before you can set about defining your bean definitions. Please note that this extra setup is notrequired if you just want to use the 'standard' scopes (namely singleton and prototype).
Now as things stand, there are a couple of ways to effect this initial setup depending on your particular Servlet environment...
If you are accessing scoped beans within Spring Web MVC, i.e. within a request that is processed by the SpringDispatcherServlet, or DispatcherPortlet, then no special setup is necessary: DispatcherServlet andDispatcherPortlet already expose all relevant state.
When using a Servlet 2.4+ web container, with requests processed outside of Spring's DispatcherServlet (e.g. when using JSF or Struts), you need to add the following javax.servlet.ServletRequestListener to the declarations in your web application's 'web.xml' file.
<web-app>
...
<listener>
<listener-class>org.springframework.web.context.request.RequestContextListener</listener-class>
</listener>
...
</web-app>If you are using an older web container (Servlet 2.3), you will need to use the provided javax.servlet.Filter implementation. Find below a snippet of XML configuration that has to be included in the 'web.xml' file of your web application if you want to have access to web-scoped beans in requests outside of Spring's DispatcherServlet on a Servlet 2.3 container. (The filter mapping depends on the surrounding web application configuration and so you will have to change it as appropriate.)
<web-app>
..
<filter>
<filter-name>requestContextFilter</filter-name>
<filter-class>org.springframework.web.filter.RequestContextFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>requestContextFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
...
</web-app>That's it. DispatcherServlet, RequestContextListener and RequestContextFilter all do exactly the same thing, namely bind the HTTP request object to the Thread that is servicing that request. This makes beans that are request- and session-scoped available further down the call chain.
Consider the following bean definition:
<bean id="loginAction" class="com.foo.LoginAction" scope="request"/>With the above bean definition in place, the Spring container will create a brand new instance of the LoginAction bean using the'loginAction' bean definition for each and every HTTP request. That is, the 'loginAction' bean will be effectively scoped at the HTTP request level. You can change or dirty the internal state of the instance that is created as much as you want, safe in the knowledge that other requests that are also using instances created off the back of the same 'loginAction' bean definition will not be seeing these changes in state since they are particular to an individual request. When the request is finished processing, the bean that is scoped to the request will be discarded.
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>With the above bean definition in place, the Spring container will create a brand new instance of the UserPreferences bean using the 'userPreferences' bean definition for the lifetime of a single HTTP Session. In other words, the 'userPreferences'bean will be effectively scoped at the HTTP Session level. Just like request-scoped beans, you can change the internal state of the instance that is created as much as you want, safe in the knowledge that other HTTP Session instances that are also using instances created off the back of the same 'userPreferences' bean definition will not be seeing these changes in state since they are particular to an individual HTTP Session. When the HTTP Session is eventually discarded, the bean that is scoped to that particular HTTP Session will also be discarded.
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="globalSession"/>The global session scope is similar to the standard HTTP Session scope (described immediately above), and really only makes sense in the context of portlet-based web applications. The portlet specification defines the notion of a global Session that is shared amongst all of the various portlets that make up a single portlet web application. Beans defined at the global sessionscope are scoped (or bound) to the lifetime of the global portlet Session.
Please note that if you are writing a standard Servlet-based web application and you define one or more beans as having global session scope, the standard HTTP Session scope will be used, and no error will be raised.