3.4. 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-aware
ApplicationContext
).
The scopes supported out of the box are listed below:
Table 3.4. Bean scopes
| Scope | Description |
|---|---|
|
Scopes a single bean definition to a single object instance per Spring IoC container. |
|
|
Scopes a single bean definition to any number of object instances. |
|
|
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 |
|
|
Scopes a single bean definition to the lifecycle of a HTTP |
|
|
Scopes a single bean definition to the lifecycle of a global HTTP |
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 create
exactly 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 hardcodes 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
Section 3.5.1, “Lifecycle interfaces”
.)
When using singleton-scoped beans that have dependencies on beans that are scoped as prototypes, please be aware that
dependencies 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, you are referred to the section entitled
Section 3.3.8, “Method Injection”
![[Note]](http://static.springframework.org/spring/docs/2.0.x/images/admons/note.png)
|
Backwards compatibility note: specifying the lifecycle scope in XML |
|---|---|
|
If you are referencing the
To be totally clear about this, this means that if you use the ” |
|
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.
|
|
Note |
|---|---|
|
The scopes that are described in the following paragraphs are |
|
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
not
required 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 Spring
DispatcherServlet
, or
DispatcherPortlet
, then no special setup is necessary:
DispatcherServlet
and
DispatcherPortlet
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 session
scope 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.
Being able to define a bean scoped to a HTTP request or
Session
(or indeed
a custom scope
of your own devising) is all very well, but one of the main value-adds of the Spring IoC container is that it manages not only the instantiation of your objects (beans), but also the wiring up of collaborators (or dependencies). If you want to inject a (for example) HTTP request scoped bean into another bean, you will need to inject an AOP proxy in place of the scoped bean. That is, you need to inject a proxy object that exposes the same public interface as the scoped object, but that is smart enough to be able to retrieve the real, target object from the relevant scope (for example a HTTP request) and delegate method calls onto the real object.
|
|
Note |
|---|---|
|
You |
|
Let’s look at the configuration that is required to effect this; the configuration is not hugely complex (it takes just one line), but it is important to understand the “
why
” as well as the “
how
” behind it.
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd">
<!-- a HTTP Session-scoped bean exposed as a proxy -->
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
<!-- this next element effects the proxying of the surrounding bean -->
<aop:scoped-proxy/>
</bean>
<!-- a singleton-scoped bean injected with a proxy to the above bean -->
<bean id="userService" class="com.foo.SimpleUserService">
<!-- a reference to the proxied 'userPreferences' bean -->
<property name="userPreferences" ref="userPreferences"/>
</bean>
</beans>
To create such a proxy, you need only to insert a child
<aop:scoped-proxy/>
element into a scoped bean definition (you may also need the CGLIB library on your classpath so that the container can effect class-based proxying; you will also need to be using
Appendix A,
XML Schema-based configuration
). So, just why do you need this
<aop:scoped-proxy/>
element in the definition of beans scoped at the
request
,
session
,
globalSession
and ‘
insert your custom scope here
‘ level? The reason is best explained by picking apart the following bean definition (please note that the following
'userPreferences'
bean definition as it stands is
incomplete
):
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
From the above configuration it is evident that the singleton bean
'userManager'
is being injected with a reference to the HTTP
Session
-scoped bean
'userPreferences'
. The salient point here is that the
'userManager'
bean is a singleton… it will be instantiated
exactly once
per container, and its dependencies (in this case only one, the
'userPreferences'
bean) will also only be injected (once!). This means that the
'userManager'
will (conceptually) only ever operate on the exact same
'userPreferences'
object, that is the one that it was originally injected with. This is
not
what you want when you inject a HTTP
Session
-scoped bean as a dependency into a collaborating object (typically). Rather, what we
do
want is a single
'userManager'
object, and then, for the lifetime of a HTTP
Session
, we want to see and use a
'userPreferences'
object that is specific to said HTTP
Session
.
Rather what you need then is to inject some sort of object that exposes the exact same public interface as the
UserPreferences
class (ideally an object that
is a
UserPreferences
instance) and that is smart enough to be able to go off and fetch the
real
UserPreferences
object from whatever underlying scoping mechanism we have chosen (HTTP request,
Session
, etc.). We can then safely inject this proxy object into the
'userManager'
bean, which will be blissfully unaware that the
UserPreferences
reference that it is holding onto is a proxy. In the case of this example, when a
UserManager
instance invokes a method on the dependency-injected
UserPreferences
object, it is really invoking a method on the proxy… the proxy will then go off and fetch the real
UserPreferences
object from (in this case) the HTTP
Session
, and delegate the method invocation onto the retrieved real
UserPreferences
object.
That is why you need the following, correct and complete, configuration when injecting
request-
,
session-
, and
globalSession-scoped
beans into collaborating objects:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
<aop:scoped-proxy/>
</bean>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
By default, when the Spring container is creating a proxy for a bean that is marked up with the
<aop:scoped-proxy/>
element,
a CGLIB-based class proxy will be created
. This means that you need to have the CGLIB library on the classpath of your application.
Note: CGLIB proxies will only intercept public method calls!
Do not call non-public methods on such a proxy; they will not be delegated to the scoped target object.
You can choose to have the Spring container create ‘standard’ JDK interface-based proxies for such scoped beans by specifying ‘
false
‘ for the value of the ‘
proxy-target-class
‘ attribute of the
<aop:scoped-proxy/>
element. Using JDK interface-based proxies does mean that you don’t need any additional libraries on your application’s classpath to effect such proxying, but it does mean that the class of the scoped bean must implement at least one interface, and
all
of the collaborators into which the scoped bean is injected must be referencing the bean via one of its interfaces.
<!-- DefaultUserPreferences implements the UserPreferences interface -->
<bean id="userPreferences" class="com.foo.DefaultUserPreferences" scope="session">
<aop:scoped-proxy proxy-target-class="false"/>
</bean>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
The section entitled
Section 6.6, “Proxying mechanisms”
may also be of some interest with regard to understanding the nuances of choosing whether class-based or interface-based proxying is right for you.
As of Spring 2.0, the bean scoping mechanism in Spring is extensible. This means that you are not limited to just the bean scopes that Spring provides out of the box; you can define your own scopes, or even redefine the existing scopes (although that last one would probably be considered bad practice – please note that you
cannot
override the built-in
singleton
and
prototype
scopes).
Scopes are defined by the
org.springframework.beans.factory.config.Scope
interface. This is the interface that you will need to implement in order to integrate your own custom scope(s) into the Spring container, and is described in detail below. You may wish to look at the
Scope
implementations that are supplied with the Spring Framework itself for an idea of how to go about implementing your own. The
Scope JavaDoc
explains the main class to implement when you need your own scope in more detail too.
The
Scope
interface has four methods dealing with getting objects from the scope, removing them from the scope and allowing them to be ‘destroyed’ if needed.
The first method should return the object from the underlying scope. The session scope implementation for example will return the session-scoped bean (and if it does not exist, return a new instance of the bean, after having bound it to the session for future reference).
Object get(String name, ObjectFactory objectFactory)
The second method should remove the object from the underlying scope. The session scope implementation for example, removes the session-scoped bean from the underlying session. The object should be returned (you are allowed to return null if the object with the specified name wasn’t found)
Object remove(String name)
The third method is used to register callbacks the scope should execute when it is destroyed or when the specified object in the scope is destroyed. Please refer to the JavaDoc or a Spring scope implementation for more information on destruction callbacks.
void registerDestructionCallback(String name, Runnable destructionCallback)
The last method deals with obtaining the conversation identifier for the underlying scope. This identifier is different for each scope. For a session for example, this can be the session identifier.
String getConversationId()
After you have written and tested one or more custom
Scope
implementations, you then need to make the Spring container aware of your new scope(s). The central method to register a new
Scope
with the Spring container is declared on the
ConfigurableBeanFactory
interface (implemented by most of the concrete
BeanFactory
implementations that ship with Spring); this central method is displayed below:
void registerScope(String scopeName, Scope scope);
The first argument to the
registerScope(..)
method is the unique name associated with a scope; examples of such names in the Spring container itself are
'singleton'
and
'prototype'
. The second argument to the
registerScope(..)
method is an actual instance of the custom
Scope
implementation that you wish to register and use.
Let’s assume that you have written your own custom
Scope
implementation, and you have registered it like so:
// note: the ThreadScope class does not ship with the Spring Framework
Scope customScope = new ThreadScope();
beanFactory.registerScope("thread", scope);
You can then create bean definitions that adhere to the scoping rules of your custom
Scope
like so:
<bean id="..." class="..." scope="thread"/>
If you have your own custom
Scope
implementation(s), you are not just limited to only programmatic registration of the custom scope(s). You can also do the
Scope
registration declaratively, using the
CustomScopeConfigurer
class.
The declarative registration of custom
Scope
implementations using the
CustomScopeConfigurer
class is shown below:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd">
<bean class="org.springframework.beans.factory.config.CustomScopeConfigurer">
<property name="scopes">
<map>
<entry key="thread">
<bean class="com.foo.ThreadScope"/>
</entry>
</map>
</property>
</bean>
<bean id="bar" class="x.y.Bar" scope="thread">
<property name="name" value="Rick"/>
<aop:scoped-proxy/>
</bean>
<bean id="foo" class="x.y.Foo">
<property name="bar" ref="bar"/>
</bean>
</beans>
需要研究一下Bean Scope问题， 留个记号