Why JSF calls getters multiple times

Question

Let's say I specify an outputText component like this:

<h:outputText value="#{ManagedBean.someProperty}"/>

If I print a log message when the getter for someProperty is called and load the page, it is trivial to notice that the getter is being called more than once per request (twice or three times is what happened in my case):

DEBUG 2010-01-18 23:31:40,104 (ManagedBean.java:13) - Getting some property
DEBUG 2010-01-18 23:31:40,104 (ManagedBean.java:13) - Getting some property

If the value of someProperty is expensive to calculate, this can potentially be a problem.

I googled a bit and figured this is a known issue. One workaround was to include a check and see if it had already been calculated:

private String someProperty;

public String getSomeProperty() {
    if (this.someProperty == null) {
        this.someProperty = this.calculatePropertyValue();
    }
    return this.someProperty;
}

The main problem with this is that you get loads of boilerplate code, not to mention private variables that you might not need.

What are the alternatives to this approach? Is there a way to achieve this without so much unnecessary code? Is there a way to stop JSF from behaving in this way?

Thanks for your input!

Answer 1

You could probably use AOP to create some sort of Aspect that cached the results of our getters for a configurable amount of time. This would prevent you from needing to copy-and-paste boilerplate code in dozens of accessors.

Answer 2

Getters are solely there to access bean properties, not to do some business logic. There you have the bean constructor, initialization blocks or event methods for. All are executed only once during bean's life and that's exactly what you want.

I've summarized all different right ways to preset/load a property.

public class Bean {

    // Direct initialization.
    private SomeObject someProperty = loadSomeProperty();

    {
        // Or in init block (applies to all constructors, not so relevant in javabeans).
        this.someProperty = loadSomeProperty();
    }

    public Bean() {
        // Or in constructor.
        this.someProperty = loadSomeProperty();
    }

    @PostConstruct
    public void init() {
        // Or in @PostConstruct (Will be invoked AFTER construction and dependency/property injection).
        this.someProperty = loadSomeProperty();
    }

    public void change(ValueChangeEvent event) {
        // Or in some FacesEvent method.
        this.someProperty = loadSomeProperty();
    }

    public String submit() {
        // Or in Action method.
        this.someProperty = loadSomeProperty();
        return "outcome";
    }

    public SomeObject getSomeProperty() {
        // Just keep getter untouched. It isn't intented to do business logic!
        return this.someProperty;
    }

}

A getter will normally be called one or two times per JSF request-response cycle (learn it here), but it can get up (much) higher, especially when used in UIData components or here and there in a boolean expression like the rendered attribute. Once again, don't use a getter for other purposes than just returning the data.

Answer 3

If the value of someProperty is expensive to calculate, this can potentially be a problem.

This is what we call a premature optimization. In the rare case that a profiler tells you that the calculation of a property is so extraordinarily expensive that calling it three times rather than once has a significant performance impact, you add caching as you describe. But unless you do something really stupid like factoring primes or accessing a databse in a getter, your code most likely has a dozen worse inefficiencies in places you've never thought about.

Answer 4

With JSF 2.0 you can attach a listener to a system event

<h:outputText value="#{ManagedBean.someProperty}">
   <f:event type="preRenderView" listener="#{ManagedBean.loadSomeProperty}" />
</h:outputText>

Alternatively you can enclose the JSF page in an f:view tag

<f:view>
   <f:event type="preRenderView" listener="#{ManagedBean.loadSomeProperty}" />

      .. jsf page here...

<f:view>