Beginning Java EE 6 with GlassFish 3, Second Edition

CHAPTER 7 ■ SESSION BEANS AND THE TIMER SERVICE• @Lock(LockType.WRITE): A method associated with an exclusive lock will not allowconcurrent invocations until the method’s processing is completed. For example, if aclient C1 invokes a method with an exclusive lock, client C2 will not be able to invokethe method until C1 has finished.• @Lock(LockType.READ): A method associated with a shared lock will allow any numberof other concurrent invocations to the bean’s instance. For example, two clients, C1and C2, can access simultaneously a method with a shared lock.The @Lock annotation can be specified on the class, the methods, or both. Specifying on the classmeans that it applies to all methods. If the concurrency locking attribute is not specified, it is assumed tobe @Lock(WRITE) by default. The code in Listing 7-6 shows CacheEJB with a READ lock in the bean class.This implies that all methods will have READ concurrency except getFromCache(), which is overridden byWRITE.Listing 7-6. A Singleton Session Bean with CMC@Singleton@Lock(LockType.READ)public class CacheEJB {private Map cache = new HashMap();public void addToCache(Long id, Object object) {if (!cache.containsKey(id))cache.put(id, object);}public void removeFromCache(Long id) {if (cache.containsKey(id))cache.remove(id);}}@AccessTimeout(value = 20, unit = TimeUnit.SECONDS)@Lock(LockType.WRITE)public Object getFromCache(Long id) {if (cache.containsKey(id))return cache.get(id);elsereturn null;}In Listing 7-6, the getFromCache() method uses an @AccessTimeout annotation. When a concurrentaccess is blocked, a timeout can be specified to reject a request if the lock is not acquired within a certaintime. If a getFromCache() invocation is locked for more than 20 seconds, the client will get aConcurrentAccessTimeoutException.212