JSF Managed Bean auto-create?

问题

Is it possible to have a JSF managed bean be automatically created?

For example I have several session scoped beans. Sometimes it becomes necessary to access these instances in code (rather than just in JSF) this is done by:

PageBean pageBean = (PageBean) FacesContext.getCurrentInstance().getExternalContext().getSessionMap().get("pages");

However if no page has already been visited which calls to '#{pages}' this resolves to null ... is there anyway to get JSF to create a bean when the scope 'begins'? So in this case ideally when a user session begins the 'pages' bean would be instantiated in the session immediately?

回答1:

Use Application#evaluateExpressionGet() instead. It will create bean when not done yet.

FacesContext context = FacesContext.getCurrentInstance();
Bean bean = (Bean) context.getApplication().evaluateExpressionGet(context, "#{bean}", Bean.class);

Where "bean" is the managed bean name and Bean.class is the appropriate backing bean class.

You can if necessary wrap this up in a helper method so that casting is unnecessary (the JSF boys didn't take benefit of generics and the Class parameter in evaluateExpressionGet):

public static <T> T findBean(String managedBeanName, Class<T> beanClass) {
    FacesContext context = FacesContext.getCurrentInstance();
    return beanClass.cast(context.getApplication().evaluateExpressionGet(context, "#{" + managedBeanName + "}", beanClass));
}

which can be used as:

Bean bean = findBean("bean", Bean.class);

Or without the type, but with a @SuppressWarnings:

@SuppressWarnings("unchecked")
public static <T> T findBean(String managedBeanName) {
    FacesContext context = FacesContext.getCurrentInstance();
    return (T) context.getApplication().evaluateExpressionGet(context, "#{" + managedBeanName + "}", Object.class);
}

which can be used as:

Bean bean = findBean("bean");

Update: the above is by the way JSF 1.2 specific. Here's the way for JSF 1.1 or older, using the currently deprecated Application#createValueBinding():

FacesContext context = FacesContext.getCurrentInstance();
Bean bean = (Bean) context.getApplication().createValueBinding("#{bean}").getValue(context);

回答2:

What about this solution:

public static Object getBean(String beanName)
{          
    Object returnObject = FacesContext.getCurrentInstance().getELContext().getELResolver().getValue(FacesContext.getCurrentInstance().getELContext(), null, beanName);  
    if (returnObject == null)  
        System.out.println("Bean with name " + beanName + " was not found. Check the faces-config.xml file if the given bean name is ok.");          
    return returnObject;
}

By this way you can even avoid the Bean.class parameter.

回答3:

One mechanism is to inject the bean into the bean you want to refer to into another bean, as demonstrated with expensiveBean here:

  <managed-bean>
    <managed-bean-name>requestBean</managed-bean-name>
    <managed-bean-class>lifetime.RequestBean</managed-bean-class>
    <managed-bean-scope>request</managed-bean-scope>
    <managed-property>
      <property-name>cachedAsset</property-name>
      <property-class>lifetime.ExpensiveBean</property-class>
      <value>#{expensiveBean}</value>
    </managed-property>
  </managed-bean>

This isn't very "lazy", but it can be convenient.

回答4:

Question: will using

FacesContext context = FacesContext.getCurrentInstance();

Bean bean = (Bean) context.getApplication().evaluateExpressionGet(context, "#{bean}", Bean.class);

cause a new Bean to be instantiated each time the code runs through these statements? Or will it simply refer to the same instance initially created?

来源：https://stackoverflow.com/questions/2050309/jsf-managed-bean-auto-create

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java

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managed

javabeans