Weld SiteCommunity Documentation
So far, we’ve seen a few examples of scope type annotations. The scope of a bean determines the lifecycle of instances of the bean. The scope also determines which clients refer to which instances of the bean. According to the CDI specification, a scope determines:
- When a new instance of any bean with that scope is created
- When an existing instance of any bean with that scope is destroyed
- Which injected references refer to any instance of a bean with that scope
For example, if we have a session-scoped bean, CurrentUser, all beans
that are called in the context of the same HttpSession will see the
same instance of CurrentUser. This instance will be automatically
created the first time a CurrentUser is needed in that session, and
automatically destroyed when the session ends.
JPA entities aren’t a great fit for this model. Entities have their
whole own lifecycle and identity model which just doesn’t map naturally
to the model used in CDI. Therefore, we recommend against treating
entities as CDI beans. You’re certainly going to run into problems if
you try to give an entity a scope other than the default scope
@Dependent. The client proxy will get in the way if you try to pass an
injected instance to the JPA EntityManager.
CDI defines four built-in scopes:
@RequestScoped
@SessionScoped
@ApplicationScoped
@ConversationScoped
For a web application that uses CDI, any servlet request has access to active request, session and application scopes. Furthermore, since CDI 1.1 the conversation context is active during every servlet request.
The request and application scopes are also active:
@PostConstruct callback of any bean
If the application tries to invoke a bean with a scope that does not
have an active context, a ContextNotActiveException is thrown by the
container at runtime.
Managed beans with scope @SessionScoped or @ConversationScoped must
be serializable, since the container passivates the HTTP session from
time to time.
Three of the four built-in scopes should be extremely familiar to every Java EE developer, so let’s not waste time discussing them here. One of the scopes, however, is new.
The conversation scope is a bit like the traditional session scope in that it holds state associated with a user of the system, and spans multiple requests to the server. However, unlike the session scope, the conversation scope:
A conversation represents a task—a unit of work from the point of view of the user. The conversation context holds state associated with what the user is currently working on. If the user is doing multiple things at the same time, there are multiple conversations.
The conversation context is active during any servlet request (since CDI 1.1). Most conversations are destroyed at the end of the request. If a conversation should hold state across multiple requests, it must be explicitly promoted to a long-running conversation.
CDI provides a built-in bean for controlling the lifecycle of conversations in a CDI application. This bean may be obtained by injection:
@Inject Conversation conversation;
To promote the conversation associated with the current request to a
long-running conversation, call the begin() method from application
code. To schedule the current long-running conversation context for
destruction at the end of the current request, call end().
In the following example, a conversation-scoped bean controls the conversation with which it is associated:
import javax.enterprise.inject.Produces;
import javax.inject.Inject;
import javax.persistence.PersistenceContextType.EXTENDED;
@ConversationScoped @Stateful
public class OrderBuilder {
private Order order;
private @Inject Conversation conversation;
private @PersistenceContext(type = EXTENDED) EntityManager em;
@Produces public Order getOrder() {
return order;
}
public Order createOrder() {
order = new Order();
conversation.begin();
return order;
}
public void addLineItem(Product product, int quantity) {
order.add(new LineItem(product, quantity));
}
public void saveOrder(Order order) {
em.persist(order);
conversation.end();
}
@Remove
public void destroy() {}
}
This bean is able to control its own lifecycle through use of the
Conversation API. But some other beans have a lifecycle which depends
completely upon another object.
The conversation context automatically propagates with any JSF faces request (JSF form submission) or redirect. It does not automatically propagate with non-faces requests, for example, navigation via a link.
We can force the conversation to propagate with a non-faces request by
including the unique identifier of the conversation as a request
parameter. The CDI specification reserves the request parameter named
cid for this use. The unique identifier of the conversation may be
obtained from the Conversation object, which has the EL bean name
javax.enterprise.context.conversation.
Therefore, the following link propagates the conversation:
<a href="/addProduct.jsp?cid=#{javax.enterprise.context.conversation.id}">Add Product</a>
It’s probably better to use one of the link components in JSF 2:
<h:link outcome="/addProduct.xhtml" value="Add Product">
<f:param name="cid" value="#{javax.enterprise.context.conversation.id}"/>
</h:link>
The conversation context propagates across redirects, making it very easy to implement the common POST-then-redirect pattern, without resort to fragile constructs such as a "flash" object. The container automatically adds the conversation id to the redirect URL as a request parameter.
In certain scenarios it may be desired to suppress propagation of a
long-running conversation. The conversationPropagation request
parameter (introduced in CDI 1.1) may be used for this purpose. If the
conversationPropagation request parameter has the value none , the
container will not reassociate the existing conversation but will
instead associate the request with a new transient conversation even
though the conversation id was propagated.
The container is permitted to destroy a conversation and all state held in its context at any time in order to conserve resources. A CDI implementation will normally do this on the basis of some kind of timeout—though this is not required by the specification. The timeout is the period of inactivity before the conversation is destroyed (as opposed to the amount of time the conversation is active).
The Conversation object provides a method to set the timeout. This is
a hint to the container, which is free to ignore the setting.
conversation.setTimeout(timeoutInMillis);
The conversation management is not always smooth. For example, if the
propagated conversation cannot be restored, the
javax.enterprise.context.NonexistentConversationException is thrown.
Or if there are concurrent requests for a one long-running conversation,
`javax.enterprise.context.BusyConversationException ` is thrown. For
such cases, developer has no opportunity to deal with the exception by
default, as the conversation associated with a Servlet request is
determined at the beginning of the request before calling any service()
method of any servlet in the web application, even before calling any of
the filters in the web application and before the container calls any
ServletRequestListener or AsyncListener in the web application.
To be allowed to handle the exceptions, a filter defined in the CDI 1.1 with the name ` CDI Conversation Filter ` can be used. By mapping the ` CDI Conversation Filter ` in the web.xml just after some other filters, we are able to catch the exceptions in them since the ordering in the web.xml specifies the ordering in which the filters will be called (described in the servlet specification).
In the following example, a filter MyFilter checks for the BusyConversationException thrown during the conversation association. In the web.xml example, the filter is mapped before the CDI Conversation Filter.
public class MyFilter implements Filter {
...
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
try {
chain.doFilter(request, response);
} catch (BusyConversationException e) {
response.setContentType("text/plain");
response.getWriter().print("BusyConversationException");
}
}
...
To make it work, we need to map our MyFilter before the CDI Conversation Filter in the web.xml file.
<filter-mapping>
<filter-name>My Filter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter-mapping>
<filter-name>CDI Conversation Filter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
The mapping of the CDI Conversation Filter determines when Weld reads
the cid request parameter. This process forces request body parsing.
If your application relies on setting a custom character encoding for
the request or parsing the request body itself by reading an
InputStream or Reader, make sure that this is performed in a filter
that executes before the CDI Conversation Filter is executed. See
this FAQ page for details.
Alternatively, the lazy conversation context initialization (see below)
may be used.
In addition to the four built-in scopes, CDI also supports two
pseudo-scopes. The first is the singleton pseudo-scope, which we
specify using the annotation @Singleton.
Unlike the other scopes, which belong to the package
javax.enterprise.context, the @Singleton annotation is defined in
the package javax.inject.
You can guess what "singleton" means here. It means a bean that is
instantiated once. Unfortunately, there’s a little problem with this
pseudo-scope. Beans with scope @Singleton don’t have a proxy object.
Clients hold a direct reference to the singleton instance. So we need to
consider the case of a client that can be serialized, for example, any
bean with scope @SessionScoped or @ConversationScoped, any dependent
object of a bean with scope @SessionScoped or @ConversationScoped,
or any stateful session bean.
Now, if the singleton instance is a simple, immutable, serializable object like a string, a number or a date, we probably don’t mind too much if it gets duplicated via serialization. However, that makes it no stop being a true singleton, and we may as well have just declared it with the default scope.
There are several ways to ensure that the singleton bean remains a singleton when its client gets serialized:
writeResolve() and readReplace()
(as defined by the Java serialization specification),
Instance<X> where X is the
bean type of the singleton bean.
A fourth, better solution is to instead use @ApplicationScoped,
allowing the container to proxy the bean, and take care of serialization
problems automatically.
Finally, CDI features the so-called dependent pseudo-scope. This is the default scope for a bean which does not explicitly declare a scope type.
For example, this bean has the scope type @Dependent:
public class Calculator { ... }
An instance of a dependent bean is never shared between different clients or different injection points. It is strictly a dependent object of some other object. It is instantiated when the object it belongs to is created, and destroyed when the object it belongs to is destroyed.
If a Unified EL expression refers to a dependent bean by EL name, an instance of the bean is instantiated every time the expression is evaluated. The instance is not reused during any other expression evaluation.
If you need to access a bean directly by EL name in a JSF page, you
probably need to give it a scope other than @Dependent. Otherwise, any
value that gets set to the bean by a JSF input will be lost immediately.
That’s why CDI features the @Model stereotype; it lets you give a bean
a name, and set its scope to @RequestScoped in one stroke. If you need
to access a bean that really has to have the scope @Dependent from a
JSF page, inject it into a different bean, and expose it to EL via a
getter method.
Beans with scope @Dependent don’t need a proxy object. The client
holds a direct reference to its instance.
CDI makes it easy to obtain a dependent instance of a bean, even if the bean is already declared as a bean with some other scope type.
The built-in qualifier @New allows us to obtain a dependent object of
a specified class.
@Inject @New Calculator calculator;
The class must be a valid managed bean or session bean, but need not be an enabled bean.
This works even if Calculator is already declared with a different
scope type, for example:
@ConversationScoped
public class Calculator { ... }
So the following injected attributes each get a different instance of
Calculator:
public class PaymentCalc {
@Inject Calculator calculator;
@Inject @New Calculator newCalculator;
}
The calculator field has a conversation-scoped instance of
Calculator injected. The newCalculator field has a new instance of
Calculator injected, with a lifecycle that is bound to the owning
PaymentCalc.
This feature is particularly useful with producer methods, as we’ll see in Chapter 8, Producer methods.
The @New qualifier was deprecated in CDI 1.1. CDI applications are
encouraged to inject @Dependent scoped beans instead.