Enterprise JavaBeans EJB component types

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1 Enterprise JavaBeans EJB component types

2 Recommended book Introduction to EJB 3

3 EJB 3.1 component example package examples; import public class HelloBean { public String hello() { return Hello, World! ; Introduction to EJB 4

4 Introduction to EJB 5

5 Essential EJB 3.x characteristics Business logic may be separated from middleware services invocation (security, transactions, ) implicit middleware new middleware can be added without changing components source code Component programming model is not EJB specific components are simple POJO classes Component (as a simple class) can be used in different frameworks or with plain Java Introduction to EJB 6

6 EJB component types Service components (stateless) Technical name: Stateless session beans Session components (stateful) Technical name: Stateful session beans Singleton components Technical name: Singleton session beans Messaging components Technical name: Message-driven beans Introduction to EJB 7

7 Stateless session beans A stateless session bean is a bean that holds conversations that span a single method call. After any method call, the container may choose to destroy a stateless session bean, or recreate it, clearing itself out of all information pertaining to past invocations Stateless session beans can contain state that is not specific to any one client (e.g., DB connection) You can keep this state in a private variables. So long as you re willing to lose the data in your private variable at any time, you ll be fine Introduction to EJB 8

8 Stateless session bean public class HelloBean { public String hello(string name) { return Hello, +name; public String goodbye(string name) { return Bye, +name; Introduction to EJB 9

9 Stateful session beans A stateful session bean is a bean that is designed to service business processes that span multiple method requests or transactions. To accomplish this, stateful session beans retain state on behalf of an individual client. If a stateful session bean s state is changed during a method invocation, that same state will be available to that same client upon the following invocation. It is highly recommended to implement at least one method annotated (if CDI is not used) After client executes this method, container destroys the stateful session bean instance If CDI is being used, do not implement/call method annotated Introduction to EJB 10

10 Stateful session bean public class MyBean { private String name; public void setname(string name) { this.name = name; public String sayhello() { return Hello, + this.name; public String saygoodbye() { return Bye, + this.name; Introduction to EJB 11

11 Thread management The EJB container takes care of servicing concurrent requests from clients without making the programmer to write multithreading code to handle them No two threads may access a single instance of Stateless or Stateful component EJB containers provide built-in thread management support It can instantiate multiple instances of the bean maintain a pool of bean instances to efficiently service the concurrent client requests EJB component programmer does not have to write thread-safe code This is valid for Stateless and Stateful components only Singleton components are being discussed further Introduction to EJB 12

12 Singleton session beans Singleton session beans: are identified by annotation and are instantiated once per application The existing instance is shared by clients and supports concurrent access By default the container is responsible for deciding whenever a singleton bean is created The developer can instruct the container to initialize the bean during application startup, using annotation permits defining dependencies on other singleton beans The container must initialize all startup singletons before starting to deliver any client requests Introduction to EJB 13

13 Singleton session beans A singleton bean maintains state between client invocations This state does not survive application shutdown In order to deal with concurrent client invocations the developer must define a concurrency strategy: Container-managed concurrency (CMC) - the container manages the access to the bean instance. This is the default strategy. may be used to control CMC Bean-managed concurrency (BMC) - the container takes no intervention in managing concurrency, allowing concurrent access to the bean, and deferring client invocation synchronization to the developer. Class must be annotated With BMCs it is legal to use synchronization primitives like synchronized and volatile in order to coordinate multiple threads from different clients Introduction to EJB 14

14 Container-managed concurrency The container manages concurrency using Each method is associated to a read or a write lock. A READ lock indicates that the method can be accessed by many concurrent invocations. A WRITE lock indicates that the method can only be invoked by one client at a time. By default the value of the locking attribute is WRITE. Whenever a method with write locking has concurrent accesses, the container allows one of them to execute it and holds the other ones until the method becomes available. The clients on hold will wait indefinitely, unless annotation is used, which allows to define a maximum time (in milliseconds) that a client will wait, throwing a ConcurrentAccessTimeoutException as soon as the timeout is reached. Introduction to EJB 15

15 Singleton session public class CounterBean implements Counter { private int void init() { count = public int getcount() { public int incrementcount() { return ++count; Introduction to EJB 16

16 Memory management: stateless session bean pooling Any stateless session bean instance can service any client request. These are instances of one and the same component Introduction to EJB 17

17 Memory management: stateful session beans Only effect of pooling Bean s state does not allow to reuse bean for other client The same problem as with multiple programs in OS how to reuse RAM Swapping For stateful session beans this is called passivation and activation Introduction to EJB 18

18 Passivation of a stateful bean Introduction to EJB 19

19 Activation of a stateful bean Introduction to EJB 20

20 Memory management: stateful session bean Annotates the method that will be called before passivation of the Annotates the method that will be called after activation of the bean Introduction to EJB 21

21 Memory management: conclusions Stateless and stateful session beans: Pros: no need to care about concurrency we do not have to write thread-safe code Cons: bigger memory consumption (by component pools) Singleton beans: Pros: low memory consumption a single component s instance only Cons: programmer has to deal with Locks and concurrency issues Introduction to EJB 22

22 Life-cycle management: Annotated method is called after the instance of a component was Annotated method is called before the instance of a component will be destroyed Introduction to EJB 23

23 Asynchronous communication It can be used with all types of session beans. The spec defines that with asynchronous invocations the control must return to the client before the container dispatches the invocation to the bean instance. This takes the use of session beans into a whole new level, permitting the developer to take benefit from the potential of having asynchronous invocations to session beans, allowing a client to trigger parallel processing flows Introduction to EJB 24

24 Asynchronous communication: SessionContext public Future<String> saybye() { String bye = executelongquery(); if (!ctx.wascancelcalled()){ bye += executesecondlongquery(); return new AsyncResult<String>(bye); Introduction to EJB 25

25 Asynchronous communication: Bye byeejb; public void invokesaybye() { Future<String> futstr = byeejb.saybye();... Do other important things... String result = futstr.get(); Introduction to EJB 26

26 Timer public class TimerEJB public void itismonday(timer timer) public void itisendofmonth(timer timer) {... Introduction to EJB 27

27 Timer service Schedule syntax is similar to UNIX/Linux cron tool Examples: Every Tuesday at = "7", minute = "30", dayofweek = "Tue") From Monday to Friday, at 7, 15 and = "7, 15, 20", dayofweek = "Mon-Fri") Every hour on = "*", dayofweek = "0") Last Friday of December, at = "12", dayofmonth = "Last Fri", month="dec") Dynamic schedules are possible too: ScheduleExpression expression=new ScheduleExpression(); expression.second("*/1").minute("*").hour("*"); Introduction to EJB 28

28 How component B can call component // or other scopes public class A // public class B private A // public class B private A a; public void mb() { a.ma();... public void mb() { a.ma();... Introduction to EJB 29

29 How component B can call component means that for each component s B instance a new instance of component A will be created It does not care and other scope annotations May be used without CDI (CDI) looks at the component s A scope, takes corresponding context and: if finds instance of component A, uses it otherwise create a new instance of A, puts it to this context, and returns to component B Introduction to EJB 30

30 Embeddable EJB container public class DesktopApp { public static void main(string args[]) { EJBContainer ec = EJBContainer.createEJBContainer(); Context ctx = ec.getcontext(); ByeEJB byeejb = (ByeEJB)ctx.lookup("java:global/bye/ByeEJB"); String msg = byeejb.saybye(); ec.close(); Introduction to EJB 31

31 Conclusions: EJB container provided middleware services 1. Memory management component pooling 2. Life-cycle management 3. Thread management 4. Asynchronous communication 5. Timer Service 6. Embeddable EJB container 7. Distribution transparency 8. Declarative transactions 9. Declarative security Introduction to EJB 32

Introduction. No-Interface View. Discuss this Article. April 2009

Introduction. No-Interface View. Discuss this Article. April 2009 April 2009 Discuss this Article Introduction Enterprise Java Beans (EJB) is a server-side component architecture for the Java Enterprise Edition (Java EE) platform, aiming to enable rapid and simplified