📄 master-ejb-4.htm
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<h3><font face="Verdana, Arial, Helvetica, sans-serif" ><b>精通ejb【四】</b></font></h3>
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<p>状态会话Bean基础 </p>
<p>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。
<br>
javax.ejb.EnterpriseBean接口继承了java.io.Serializable,用以实现写入读出操作。 <br>
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。
</p>
<p>状态bean的钝化过程 <br>
计数bean的远程接口 <br>
远程接口定义了一个业务方法count()，它将在企业bean类中实现。 </p>
<p><br>
激活状态bean <br>
package com.wiley.compBooks.roman.session.count; <br>
import javax.ejb.*; <br>
import java.rmi.RemoteException; <br>
/** <br>
* These are CountBean's business logic methods. <br>
* <br>
* This interface is what clients operate on when they <br>
* interact with EJB objects. The container vendor will <br>
implement this interface; the implemented object is <br>
* the EJB Object, which delegates invocations to the <br>
* actual bean. <br>
*/ <br>
public interface Count extends EJBObject { <br>
/** <br>
* Increments the int stored as conversational state <br>
*/ <br>
public int count() throws RemoteException; <br>
} <br>
Source Count.java </p>
<p>package com.wiley.compBooks.roman.session.count; <br>
import javax.ejb.*; <br>
/** <br>
* Demonstration Stateful Session Bean. This bean is <br>
* initialized to some integer value and has a business <br>
* method that increments the value. <br>
* <br>
* This example shows the basics of how to write a stateful <br>
* session bean and how passivation/activation works. <br>
*/ <br>
public class CountBean implements SessionBean { <br>
private SessionContext ctx; <br>
// The current counter is our conversational state. <br>
public int val; <br>
// <br>
// Business methods <br>
// <br>
/** <br>
* Counts up <br>
*/ <br>
public int count() { <br>
System.out.println("count()"); <br>
return ++val; <br>
} <br>
// <br>
// EJB-required methods <br>
// <br>
public void ejbCreate(int val) throws CreateException { <br>
this.val = val; <br>
System.out.println("ejbCreate()"); <br>
} <br>
public void ejbRemove() { <br>
System.out.println("ejbRemove()"); <br>
} <br>
public void ejbActivate() { <br>
System.out.println("ejbActivate()"); <br>
} <br>
public void ejbPassivate() { <br>
System.out.println("ejbPassivate()"); <br>
} <br>
public void setSessionContext(SessionContext ctx) { <br>
} <br>
} <br>
Source CountBean.java <br>
Bean实现了javax.ejb.SessionBean。所以，它必须定义所有SessionBean定义的方法。 <br>
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。
</p>
<p>计数bean的home接口 <br>
package com.wiley.compBooks.roman.session.count; <br>
import javax.ejb.*; <br>
import java.rmi.RemoteException; <br>
/** <br>
* This is the home interface for CountBean. This interface <br>
* is implemented by the EJB Server's glue-code tools - the <br>
* implemented object is called the Home Object and serves <br>
* as a factory for EJB Objects. <br>
* <br>
* One create() method is in this Home Interface, which <br>
* corresponds to the ejbCreate() method in the CountBean file. <br>
*/ <br>
public interface CountHome extends EJBHome { <br>
/* <br>
* This method creates the EJB Object. <br>
* <br>
* @param val Value to initialize counter to <br>
* <br>
* @return The newly created EJB Object. <br>
*/ <br>
Count create(int val) throws RemoteException, CreateException; <br>
} <br>
Source CountHome.java. <br>
计数bean的配置描述符 </p>
<p>计数bean的配置描述符 <br>
计数bean的环境属性 <br>
生成计数bean的Ejb-jar文件 <br>
计数bean的客户端代码 <br>
package com.wiley.compBooks.roman.session.count; <br>
import javax.ejb.*; <br>
import javax.naming.*; <br>
import java.util.Properties; <br>
/** <br>
* This class is a simple example of client code that invokes <br>
* methods on a simple Stateless Enterprise Bean. <br>
* <br>
* We create 3 EJB Objects in this example, but we allow <br>
* the container to have only 2 in memory. This illustrates how <br>
* beans are passivated to storage. <br>
*/ <br>
public class CountClient { <br>
public static void main(String[] args) { <br>
try { <br>
/* <br>
* Get System properties for JNDI initialization <br>
*/ <br>
Properties props = System.getProperties(); <br>
/* <br>
* Get a reference to the Home Object - the <br>
* factory for EJB Objects <br>
*/ <br>
Source CountClient.java <br>
1、需要JNDL初始化上下文 <br>
2、使用JNDL定位home接口 <br>
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 <br>
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。
<br>
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。 <br>
6、最后所有的EJB对象被删除。 <br>
package com.wiley.compBooks.roman.session.count; <br>
import javax.ejb.*; <br>
import javax.naming.*; <br>
import java.util.Properties; <br>
/** <br>
* This class is a simple example of client code that invokes <br>
* methods on a simple Stateless Enterprise Bean. <br>
* <br>
* We create 3 EJB Objects in this example, but we allow <br>
* the container to have only 2 in memory. This illustrates how <br>
* beans are passivated to storage. <br>
*/ <br>
public class CountClient { <br>
public static void main(String[] args) { <br>
try { <br>
/* <br>
* Get System properties for JNDI initialization <br>
*/ <br>
Properties props = System.getProperties(); <br>
/* <br>
* Get a reference to the Home Object - the <br>
* factory for EJB Objects <br>
*/ <br>
Context ctx = new InitialContext(props); <br>
CountHome home = (CountHome) ctx.lookup("CountHome");
<br>
/* <br>
* An array to hold 3 Count EJB Objects <br>
*/ <br>
Count count[] = new Count[3]; <br>
int countVal = 0; <br>
/* <br>
* Create and count() on each member of array <br>
*/ <br>
System.out.println("Instantiating beans..."); <br>
for (int i=0; i < 3; i++) { <br>
/* <br>
* Create an EJB Object and initialize <br>
* it to the current count value. <br>
*/ <br>
count[i] = home.create(countVal); <br>
/* <br>
* Add 1 and print <br>
*/ <br>
countVal = count[i].count(); <br>
System.out.println(countVal); <br>
/* <br>
* Sleep for 1/2 second <br>
*/ <br>
Thread.sleep(500); <br>
} <br>
/* <br>
* Let's call count() on each EJB Object to <br>
* make sure the beans were passivated and <br>
* activated properly. <br>
*/ <br>
System.out.println("Calling count() on beans..."); <br>
for (int i=0; i < 3; i++) { <br>
/* <br>
* Add 1 and print <br>
*/ <br>
countVal = count[i].count(); <br>
System.out.println(countVal); <br>
/* <br>
* Sleep for 1/2 second <br>
*/ <br>
Thread.sleep(500); <br>
} <br>
/* <br>
* Done with EJB Objects, so remove them <br>
*/ <br>
for (int i=0; i < 3; i++) { <br>
count[i].remove(); <br>
} <br>
} catch (Exception e) { <br>
e.printStackTrace(); <br>
} <br>
} <br>
} <br>
Source CountClient.java <br>
运行客户端： <br>
对于BEA的WebLogic，执行： <br>
java -Djava.naming.factory.initial= <br>
weblogic.jndi.TengahInitialContextFactory <br>
-Djava.naming.provider.url= <br>
t3://localhost:7001 <br>
com.wiley.compBooks.roman.session.count.CountClient <br>
客户端输出： <br>
Instantiating beans... <br>
1 <br>
2 <br>
3 <br>
Calling count() on beans... <br>
2 <br>
3 <br>
4 <br>
服务端输出： <br>
ejbCreate() <br>
count() <br>
ejbCreate() <br>
count() <br>
ejbCreate() <br>
ejbPassivate() <br>
count() <br>
ejbPassivate() <br>
ejbActivate() <br>
count() <br>
ejbPassivate() <br>
ejbActivate() <br>
count() <br>
ejbPassivate() <br>
ejbActivate() <br>
count() <br>
ejbPassivate() <br>
ejbActivate() <br>
ejbRemove() <br>
ejbActivate() <br>
ejbRemove() <br>
ejbRemove() <br>
</p>
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