精通EJB（4） 状态会话Bean基础

        Stateful Session Bean可以一对一的维持某个调用客户的状态，并且在不同的方法调用中维持这个状态， 由于对于每一个并发用户，必须有一个对应的Stateful Session Bean，为了提高系统的效率，Stateful Session Bean可以在一定的客户空闲时间后被写入二级存储设备(如硬盘)，在客户发出新的调用请求后，再从二级存储 设备恢复到内存中。但是在多用户下，Stateless Session Bean运行效率高于Stateful Session Bean。
        javax.ejb.EnterpriseBean接口继承了java.io.Serializable,用以实现写入读出操作。
        当EJB容器调用ejbPassivate()方法钝化了bean之后，就可以把它写入二级存储设备，然后容器调用ejbActivate()方法激活bean，把它从二级存储设备中读出。
       
        状态bean的钝化过程
        计数bean的远程接口
        远程接口定义了一个业务方法count()，它将在企业bean类中实现。
       
       
        激活状态bean
        package com.wiley.compBooks.roman.session.count;
        import javax.ejb.*;
        import java.rmi.RemoteException;
        /**
        * These are CountBean′s business logic methods.
        *
        * This interface is what clients operate on when they
        * interact with EJB objects. The container vendor will
        implement this interface; the implemented object is
        * the EJB Object, which delegates invocations to the
        * actual bean.
        */
        public interface Count extends EJBObject {
        /**
        * Increments the int stored as conversational state
        */
        public int count() throws RemoteException;
        }
        Source Count.java
       
        package com.wiley.compBooks.roman.session.count;
        import javax.ejb.*;
        /**
        * Demonstration Stateful Session Bean. This bean is
        * initialized to some integer value and has a business
        * method that increments the value.
        *
        * This example shows the basics of how to write a stateful
        * session bean and how passivation/activation works.
        */
        public class CountBean implements SessionBean {
        private SessionContext ctx;
        // The current counter is our conversational state.
        public int val;
        //
        // Business methods
        //
        /**
        * Counts up
        */
        public int count() {
        System.out.println("count()");
        return ++val;
        }
        //
        // EJB-required methods
        //
        public void ejbCreate(int val) throws CreateException {
        this.val = val;
        System.out.println("ejbCreate()");
        }
        public void ejbRemove() {
        System.out.println("ejbRemove()");
        }
        public void ejbActivate() {
        System.out.println("ejbActivate()");
        }
        public void ejbPassivate() {
        System.out.println("ejbPassivate()");
        }
        public void setSessionContext(SessionContext ctx) {
        }
        }
        Source CountBean.java
        Bean实现了javax.ejb.SessionBean。所以，它必须定义所有SessionBean定义的方法。
        OjbCreate()初始化带了val的参数。它将作为counter的初始状态。在钝化和激活bean的过程中，val变量被保护。
       
        计数bean的home接口
        package com.wiley.compBooks.roman.session.count;
        import javax.ejb.*;
        import java.rmi.RemoteException;
        /**
        * This is the home interface for CountBean. This interface
        * is implemented by the EJB Server′s glue-code tools - the
        * implemented object is called the Home Object and serves
        * as a factory for EJB Objects.
        *
        * One create() method is in this Home Interface, which
        * corresponds to the ejbCreate() method in the CountBean file.
        */
        public interface CountHome extends EJBHome {
        /*
        * This method creates the EJB Object.
        *
        * @param val Value to initialize counter to
        *
        * @return The newly created EJB Object.
        */
        Count create(int val) throws RemoteException, CreateException;
        }
        Source CountHome.java.
        计数bean的配置描述符
       
        计数bean的配置描述符
        计数bean的环境属性
        生成计数bean的Ejb-jar文件
        计数bean的客户端代码
        package com.wiley.compBooks.roman.session.count;
        import javax.ejb.*;
        import javax.naming.*;
        import java.util.Properties;
        /**
        * This class is a simple example of client code that invokes
        * methods on a simple Stateless Enterprise Bean.
        *
        * We create 3 EJB Objects in this example, but we allow
        * the container to have only 2 in memory. This illustrates how
        * beans are passivated to storage.
        */
        public class CountClient {
        public static void main(String[] args) {
        try {
        /*
        * Get System properties for JNDI initialization
        */
        Properties props = System.getProperties();
        /*
        * Get a reference to the Home Object - the
        * factory for EJB Objects
        */
        Source CountClient.java
        1、需要JNDL初始化上下文
        2、使用JNDL定位home接口
        3、使用home对象建立3个不同的计数EJB对象，因此也就和三个不同的客户端建立了会话
        4、配置描述符限制了同时只能有两个bean工作，因此3个bean中一定有钝化的。在调用ejbPassivate()时，打印一条信息。
        5、在每个EJB对象上调用count()方法，调用ejbActivate()方法激活bean，该方法打印一条信息。
        6、最后所有的EJB对象被删除。
        package com.wiley.compBooks.roman.session.count;
        import javax.ejb.*;
        import javax.naming.*;
        import java.util.Properties;
        /**
        * This class is a simple example of client code that invokes
        * methods on a simple Stateless Enterprise Bean.
        *
        * We create 3 EJB Objects in this example, but we allow
        * the container to have only 2 in memory. This illustrates how
        * beans are passivated to storage.
        */
        public class CountClient {
        public static void main(String[] args) {
        try {
        /*
        * Get System properties for JNDI initialization
        */
        Properties props = System.getProperties();
        /*
        * Get a reference to the Home Object - the
        * factory for EJB Objects
        */
        Context ctx = new InitialContext(props);
        CountHome home = (CountHome) ctx.lookup("CountHome");
        /*
        * An array to hold 3 Count EJB Objects
        */
        Count count[] = new Count[3];
        int countVal = 0;
        /*
        * Create and count() on each member of array
        */
        System.out.println("Instantiating beans...");
        for (int i=0; i < 3; i++) {
        /*
        * Create an EJB Object and initialize
        * it to the current count value.
        */
        count[i] = home.create(countVal);
        /*
        * Add 1 and print
        */
        countVal = count[i].count();
        System.out.println(countVal);
        /*
        * Sleep for 1/2 second
        */
        Thread.sleep(500);
        }
        /*
        * Let′s call count() on each EJB Object to
        * make sure the beans were passivated and
        * activated properly.
        */
        System.out.println("Calling count() on beans...");
        for (int i=0; i < 3; i++) {
        /*
        * Add 1 and print
        */
        countVal = count[i].count();
        System.out.println(countVal);
        /*
        * Sleep for 1/2 second
        */
        Thread.sleep(500);
        }
        /*
        * Done with EJB Objects, so remove them
        */
        for (int i=0; i < 3; i++) {
        count[i].remove();
        }
        } catch (Exception e) {
        e.printStackTrace();
        }
        }
        }
        Source CountClient.java
        运行客户端：
        对于BEA的WebLogic，执行：
        java -Djava.naming.factory.initial=
        weblogic.jndi.TengahInitialContextFactory
        -Djava.naming.provider.url=
        t3://localhost:7001
        com.wiley.compBooks.roman.session.count.CountClient
        客户端输出：
        Instantiating beans...
        1
        2
        3
        Calling count() on beans...
        2
        3
        4
        服务端输出：
        ejbCreate()
        count()
        ejbCreate()
        count()
        ejbCreate()
        ejbPassivate()
        count()
        ejbPassivate()
        ejbActivate()
        count()
        ejbPassivate()
        ejbActivate()
        count()
        ejbPassivate()
        ejbActivate()
        count()
        ejbPassivate()
        ejbActivate()
        ejbRemove()
        ejbActivate()
        ejbRemove()
        ejbRemove()