Lifecycle of FacesContext?

问题

While going through the javadoc of FacesContext, I came across this sentence

The instance remains active until its release() method is called, after which no further references to this instance are allowed. While a FacesContext instance is active, it must not be referenced from any thread other than the one upon which the servlet container executing this web application utilizes for the processing of this request

Does this mean that FacesContext will never go for garbage collection, and the instance will be destroyed only when the current-webapplication stops (server is stopped)?

Is FacesContext following singleton pattern? In that case, how will it behave when multiple request and coming for rendering response simultaneously as it serves only one request per time?

回答1:

Does this mean that FacesContext will never go for garbage collection, and the instance will be destroyed only when the current-webapplication stops (server is stopped)?

No, you read it wrongly. The FacesContext lives as long as a single HTTP request. It will (actually, "can" is a better word) not immediately be GC'ed if you incorrectly reference it anywhere in your own code beyond its scope. E.g. as a property of a session scoped managed bean which lives longer than a single HTTP request:

@ManagedBean
@SessionScoped
public class BadSessionBean {

    // Bad Example! Never do this! Not threadsafe and instance can't be GC'ed by end of request!
    private FacesContext context = FacesContext.getCurrentInstance();

}

If you aren't doing that anywhere in your code, and thus you're always obtaining the current instance in method local scope, then it will have the chance to be properly GC'ed.

@ManagedBean
@SessionScoped
public class GoodSessionBean {

    public void someMethod() {
        // OK! Declared in method local scope and thus threadsafe.
        FacesContext context = FacesContext.getCurrentInstance();
    }

}

Please note that this GC behavior is not specific to JSF/FacesContext, it's just specific to basic Java in general.

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Is FacesContext following singleton pattern? In that case, how will it behave when multiple request and coming for rendering response simultaneously as it serves only one request per time?

No, it's definitely not a singleton. It's a ThreadLocal instance which is created by FacesServlet right after the service() method is entered and destroyed by the FacesServlet right before the service() method is left. Thus, there's only one instance per request (and thus not per application). Note that one HTTP request counts as one separate thread. There can be multiple threads (read: requests) and thus there can be multiple instances of the FacesContext during application's lifetime. Its main pattern is the facade pattern, but that's further not related to it being a ThreadLocal.

See also:

• java threadlocal singleton - what is it?
• How to initialize an API in servlet environment
• Examples of GoF Design Patterns in Java's core libraries

来源：https://stackoverflow.com/questions/16229901/lifecycle-of-facescontext