Beginning Java EE 6 with GlassFish 3, Second Edition

CHAPTER 8 ■ CALLBACKS AND INTERCEPTORS• The method can have public, private, protected, or package-level access, butmust not be static or final.• A method may be annotated with multiple annotations (the init() method shownlater in Listing 8-2 is annotated with @PostConstruct and @PostActivate).However, only one annotation of a given type may be present on a bean (you can’thave two @PostConstruct annotations in the same session bean, for example).• A callback method can access the beans’ environment entries (see the“Environment Naming Context” section in Chapter 7).These callbacks are typically used to allocate and/or release the bean’s resources. As an example,Listing 8-1 shows the singleton bean CacheEJB using a @PostConstruct annotation to initialize its cache.Straight after creating the single instance of the CacheEJB, the container invokes the initCache() method.Listing 8-1. A Singleton Initializing Its Cache with the @PostConstruct Annotation@Singletonpublic class CacheEJB {private Map cache = new HashMap();@PostConstructprivate void initCache() {// Initializes the cache}}public Object getFromCache(Long id) {if (cache.containsKey(id))return cache.get(id);elsereturn null;}Listing 8-2 shows a snippet of code for a stateful bean. The container maintains conversationalstate, which can include heavy-duty resources such as a database connection. Because it is expensive toopen a database connection, it should be shared across calls but released when the bean is idle (orpassivated).After creating an instance of a stateful bean, the container injects the reference of a datasource tothe ds attribute. Once the injection is done, the container will call the designated @PostConstructmethod (init()), which creates a database connection. If the container happens to passivate theinstance, the close() method will be invoked (@PrePassivate). The purpose of this is to close the JDBCconnection, which holds native resources and is no longer needed during passivation. When the clientinvokes a business method on the bean, the container activates it and calls the init() method again(@PostActivate). When the client invokes the checkout() method (annotated with the @Removeannotation), the container removes the instance but first will call the close() method again(@PreDestroy).242