Stateful Session Beans

Transcription

1 Berner Fachhochschule Technik und Informatik Stateful Session Beans Course Multi Tier Business Applications with Java EE Prof. Dr. Eric Dubuis Berner Fachhochschule Biel

2 Content Characteristics of stateful session beans Business interface, bean implementation Bean's life cycle Conversational state Activation and passivation Callback annotations Client view April 14, 2011 MTA with Java EE / Stateful Session Beans 2

3 Characteristics of a Stateful Session Beans Executes on behalf of a single client. A component that preserves state information between method calls (conversational state). Can be transaction-aware. Does not represent directly shared data in the database, although it may access and update such data. Is relatively short-lived. Can be viewed as an extension of application clients that hold (client) state information. A typical EJB container provides a scalable runtime environment to execute a large number of (stateful and/or stateless) session objects concurrently. But: Pooling is not as simple as with stateless session beans (container issue) activation/passivation (containers decide on strategy; LRU used most often) April 14, 2011 MTA with Java EE / Stateful Session Beans 3

4 What are Stateful Session Beans Used For? To encapsulate some of the business logic and conversional state, and to combine these in a workflow. To present a simplified interface to clients. For example, a shopping cart April 14, 2011 MTA with Java EE / Stateful Session Beans 4

5 A Stateful Session Bean Example: The Business Interface To illustrate the mechanism of a stateful session bean, we define an interface for a remote calculator engine. Notice that the engine can memorize a value: /** * Defines a calculator component with a memory register. */ (omitted here to be technology-independent) public interface Calculator { public double add(double x, double y); public double subtract(double x, double y); public double multiply(double x, double y); public double divide(double x, double y); public void memoryadd(double x); public void memorysubtract(double x); public double memoryread(); public void memoryclear(); public void close(); } Remarks: Methods of the form memoryxx() manipulate a memory register located in the stateful session bean. Method close() tells the bean when the client is done with it. April 14, 2011 MTA with Java EE / Stateful Session Beans 5

6 The Stateful Calculator Bean Class I The implementation of the preceding interface is straightforward. EJB3 technology comes in with the help of EJB annotations (in bold). import javax.ejb.remote; import javax.ejb.remove; = { Calculator.class }) public class CalculatorBean implements Calculator { private double memorycell = 0.0; Implementation of interface Calculator: not mandatory, but recommended public double add(double x, double y) { return x + y; } // Similar for subtract(), multiply(), and divide(). //... to be continued April 14, 2011 MTA with Java EE / Stateful Session Beans 6

7 The Stateful Calculator Bean Class You add the business methods. Some of them manipulate the bean's memory cell: // Methods that manipulate the memory cell: public void memoryadd(double x) { this.memorycell += x; } public void memorysubtract(double x) { this.memorycell -= x; } public double memoryread() { return this.memorycell; } public void memoryclear() { this.memorycell = 0.0; } // Method for releasing the public void close() { // Nothing here. } } // end of CalculatorBean April 14, 2011 MTA with Java EE / Stateful Session Beans 7

8 Remarks annotation tells the EJB container that CalculatorBean is a stateful session bean. annotation designates the remote interface of the bean. Must be added if corresponding interface annotation. The value element must be specified if the annotation is applied to the bean class. Must be used if bean class implements more than on remote interface. The instance field memorycell denotes the conversational state of the bean. Since EJB 3.1, JNDI name space syntax is standardized [EJB3.1, 4.4]: portable session bean global JNDI name: java:global[/<app-name>]/<module-name>/<bean-name> [!<fully-qualified-interface-name>] application-specific name space within a Java EE application: java:app/<module-name>/<bean-name> [!<fully-qualified-interface-name>] module-specific name space within a Java EE application: java:module/<bean-name>[!<fully-qualified-interface-name> April 14, 2011 MTA with Java EE / Stateful Session Beans 8

9 Stateful Session Bean Life Cycle [EJB3.1, 4.6] April 14, 2011 MTA with Java EE / Stateful Session Beans 9

10 Conversational State Definition "The conversational state of a stateful session object is defined as the session bean instance s field values, its associated interceptors and their instance field values, plus the transitive closure of the objects from these instances fields reached by following Java object references." [EJB3.1, 4.2] April 14, 2011 MTA with Java EE / Stateful Session Beans 10

11 Passivation/Activation Definition "To efficiently manage the size of its working set, a session bean container may need to temporarily transfer the state of an idle stateful session bean instance to some form of secondary storage. The transfer from the working set to secondary storage is called instance passivation. The transfer back is called activation." [EJB3.1, 4.2] April 14, 2011 MTA with Java EE / Stateful Session Beans 11

12 Bean Provider Responsibilities Regarding Conversational State Ensure that method leaves the instance fields ready to be serialized by the container. The objects that are assigned to the instance s non-transient fields after method completes must be one of the following: A serializable object. A null. A reference to an enterprise bean s business interface. A reference to an enterprise bean s remote interface, even if the stub class is not serializable. A reference to an enterprise bean s remote home interface, even if the stub class is not serializable. A reference to an entity bean s local interface, even if it is not serializable. A reference to an entity bean s local home interface, even if it is not serializable. April 14, 2011 MTA with Java EE / Stateful Session Beans 12

13 Non-Transient Fields Continued A reference to object, even if it is not serializable. A reference to the environment naming context (that is, the java:comp/env JNDI context) or any of its sub contexts. A reference to the UserTransaction interface. A reference to a resource manager connection factory. A reference to an EntityManager object, even if it is not serializable. A reference to an EntityManagerFactory object, even if it is not serializable. A reference to a javax.ejb.timer object. An object that is not directly serializable, but becomes serializable by replacing the references to an enterprise bean s business interface, an enterprise bean s home and component interfaces, the references to the SessionContext object, the references to the java:comp/env JNDI context and its sub contexts, the references to the UserTransaction interface, and the references to the EntityManager and/or EntityManagerFactory by serializable objects during the object s serialization. April 14, 2011 MTA with Java EE / Stateful Session Beans 13

14 Non-Transient Fields Continued II This means, for example, that the Bean Provider must close all JDBC connections in method and assign the instance s fields storing the connections to null. Note: Instance fields can be private. April 14, 2011 MTA with Java EE / Stateful Session Beans 14

15 Callback for Stateful Session Beans Supported life cycle event callback methods for stateful session A method being executed after any dependency injection (not discussed in this session) was made... before the first business method is A method being executed after any method annotated has A method being executed before the container is about to passivate the A method being executed after the instance it has just been reactivated. All callback methods occur in a unspecified transaction and security context. April 14, 2011 MTA with Java EE / Stateful Session Beans 15

16 Client View Communication in a distributed environment «interface» Calculator In the context of EJB, this part is more complicated than shown Client Calculator Calculator «bean» CalculatorBean Stub network Skeleton "The communication stubs used on the client side are artifacts generated at the enterprise bean s deployment time by the Container Provider s tools. The stubs used on the client are specific to the wire protocol used for the remote invocation." [EJB3.1 Core, ] April 14, 2011 MTA with Java EE / Stateful Session Beans 16

17 Getting a Remote Reference to the Calculator Bean: import javax.naming.context; import javax.naming.initialcontext;... // in some method: { // Get the initial JNDI context. Context ctx = new InitialContext(); // Get a Calculator session bean. // Alternative: Use a Service Locator. Calculator calculator =(Calculator) ctx.lookup( java:global/calculator1/calculator1-ejb/calculatorbean ); // Use the bean. calculator.add(12, 25)); calculator.memoryadd(5); } // Remove the bean when no longer used: calculator.close(); // on bean close() meth. April 14, 2011 MTA with Java EE / Stateful Session Beans 17

18 Alternative: Getting Remote Reference by Dependency Injection To get a remote reference in a client running in an client application container : (Note: Can be used in the client's main class only) import javax.ejb.ejb; public class Main private static Calculator calculator; // for example, in the main() method: public static void main(string[] args) { } } // Just use the bean. calculator.add(12, 25); calculator.memoryadd(5); // Remove the bean when no longer used: calculator.close(); April 14, 2011 MTA with Java EE / Stateless Session Beans 18

19 Provoking the Execution Method In a client: Removal of a stateful session bean "A client may remove a stateful session bean by invoking a method of its business interface designated as method." [EJB3.1, 3.4.5] Recommendation: "A client should remove a stateful session bean by invoking a method of its business interface designated as method." April 14, 2011 MTA with Java EE / Stateful Session Beans 19

20 Session Object Identity For Stateful Session Beans For stateful session beans, the following code fragment is valid [EJB3.1 Core, Cart cart1; // alternative: lookup via Cart cart2;... if (cart1.equals(cart1)) { // this test returns true... }... if (cart1.equals(cart2)) { // this test returns false... } April 14, 2011 MTA with Java EE / Stateful Session Beans 20

21 Compare with Session Object Identity for Stateless S. B. For stateless session beans, the following code fragment is valid [EJB3.1, Cart Cart cart2;... if (cart1.equals(cart1)) { // this test returns true... }... if (cart1.equals(cart2)) { // this test returns true... } April 14, 2011 MTA with Java EE / Stateless Session Beans 21

22 References [EJB3.1] JSR 318: Enterprise JavaBeans, Version 3.1 EJB Core Contracts and Requirements April 14, 2011 MTA with Java EE / Stateful Session Beans 22