EJB Stateless Session Begining
參考：http://www.tusc.com.au/tutorial/html/index.html
一、Enterprise Beans
在EJB（Enterprise Java Beans）中定義了兩種不同類別的Enterprise Bean ：
l 會話 Bean (Session Bean)
l 實體Bean (Entity Bean)
1. 會話 Bean (Session Bean)
請把Session Beans看作任務而非持久數據，只有這樣，你才能清晰地理解使用它的場合和原因。比如說，Session Bean可能會對數據庫內的用戶進行檢索操作，但并不把用戶表示為持久的業務對象。Session Beans可以同其他類型的Java beans通信，這種能力已經超出了數據庫事務概念之外。在這種方式下工作的分布式應用程序特別適合于采用Session Beans。
所有的Session Beans都派生于javax.ejb.SessionBean類。就像其名稱那樣， Session Beans只存在于單一客戶會話中。這里的客戶可能是Java Servlet、桌面應用程序、別的EJB乃至采用Java Bean的其他事務信息方式。Session Bean的生存期從客戶程序發起接觸開始，在客戶通過調用EJB宿主接口的remove()方法顯式破壞bean時終止。會話還會在預先設置的時間終止，而這種時間設置則由EJB容器內確定。考慮到服務器的資源，開發者往往會設法避免處理“超時”設置而在編寫客戶程序時做相應的延期處理。此外，開發者還應該在EJB容器重新啟動的時候小心Session Bean的短暫壽命；客戶程序可以重新獲得同類bean的引用，但很有可能卻不是同一bean。
Session Beans可以是無狀態也可以是有狀態的，因而兩者會顯示出不同的行為。Session Bean的狀態概念類似于HTTP設置：客戶程序和服務器之間的交互（在這種情況下就是bean自身）發生在定義的環境之內。有狀態的Session Bean會在多個方法調用的情況下保存客戶和bean的有關信息。有狀態的Session Bean只能同一個客戶程序通訊，而無狀態Session bean的實例卻可以同時和若干個客戶程序通訊。
會話 Bean 是調用它的客戶端代碼要完成的工作。當客戶端與服務器建立聯系，那么一個會話 Bean 就建立起來了。根據會話 Bean 的狀態不同有分為:
A. 狀態會話 Bean (Stateful Session Bean)
B. 無狀態會話 Bean (Stateless Session Bean)
1.1 狀態會話 Bean (Stateful Session Bean)
當客戶機和服務器建立連接之后，狀態會話 Bean (Stateful Session Bean) 將一直在客戶機和服務器之間保持著用戶的某個狀態。例如：用戶使用銀行的ATM時，經過驗證之后，用戶可以連續執行多次操作，在這個過程當中，用戶的合法狀態將一直被保留，直到她將信用卡取出，結束這次操作。這時，狀態會話 Bean (Stateful Session Bean) 也就被銷毀。
1.2無狀態會話 Bean (Stateless Session Bean)
當客戶機和服務器建立連接之后，無狀態會話 Bean (Stateless Session Bean)處理單一的用戶請求或商務過程。無狀態會話 Bean (Stateless Session Bean)不需要從以前的請求中提取任何狀態。例如，用戶的用戶密碼確認。用戶輸入密碼后，發送請求。組件返回真或假來確認用戶，一旦過程完成，無狀態會話 Bean (Stateless Session Bean) 也宣告結束。
2. 實體Bean (Entity Bean)
實體Bean (Entity Bean)只是數據模型，它不包括商務邏輯。實體Bean (Entity Bean)可以將關系/對象數據庫的數據映射到內存中供其它組件使用。實體Bean (Entity Bean)是一直存在的，而且具有很高的容錯性能。實體Bean (Entity Bean)能供允許多用戶同時訪問。
二、會話 Bean (Session Bean)
Ejb的執行過程是被放在一個EJB容器中進行的，所以客戶端不會直接調用我們寫好的Enterprise Bean ，而是調用EJB容器生成的一個EJBObject (EJB對象)來實現。那么，我們在編寫服務器端的Enterprise Bean 時，就要考慮這點。既然客戶端不能直接訪問，就由EJBObject來代勞，所以在編寫服務器端時，就要編寫服務器端的一個接口（Remote）用來與客戶機聯系，實力化EJBObject。要生成EJBObject 就要調有Home 接口，來建立這個實力。
以下是會話 Bean 的代碼分析：
A.Enterprise Bean 類：sailorsy.class
1.setSessionContext(SessionContext ctx)方法
它是EJB容器和Enterprise Bean互相作用的關口。
import java.rmi.*;
import javax.ejb.*;
public class sailorsy implements SessionBean{
private SessionContext ctx=null;
public voic setSessionContext(SessionContext ctx){
this.ctx=ctx;
}//setSessionContext
}//class sailorsy
2.ejbCreate(…)方法
它可以初始化Enterprise Bean ,可以定義不同的ejbCreate(…)方法，每個方法所帶的參數不同。但是，必許要存在至少一種。
import java.rmi.*;
import javax.ejb.*;
public class sailorsy implements SessionBean{
private SessionContext ctx=null;
public voic setSessionContext(SessionContext ctx){
this.ctx=ctx;
}//setSessionContext
public void ejbCreate() {
}//ejbCreate
}//class sailorsy
3.ejbPassivate()方法
如果初始化的Enterprise Bean 過多，EJB容器將其中的一些掛起（passivate）,釋放他們所占用的空間。
import java.rmi.*;
import javax.ejb.*;
public class sailorsy implements SessionBean{
private SessionContext ctx=null;
public voic setSessionContext(SessionContext ctx){
this.ctx=ctx;
}//setSessionContext
public void ejbCreate() {
}//ejbCreate
public void ejbPassivate() {
}//ejbPassivate
}//class sailorsy
4.ejbActivate()方法
和ejbPassivate正好相反，它將被掛起的Bean從新調回。
import java.rmi.*;
import javax.ejb.*;
public class sailorsy implements SessionBean{
private SessionContext ctx=null;
public voic setSessionContext(SessionContext ctx){
this.ctx=ctx;
}//setSessionContext
Creating a Stateless Session Bean
This chapter covers how to create a stateless session EJB component. This bean will be responsible for authenticating the user by communicating with the database using Data Access Object (DAO) which encapsulates Java Database Connectivity (JDBC) code. A DAO has all attributes (fields) and behavior (methods) corresponding to the bean it is being used for.
All customers, supplier and manager of MyStore have been assigned a unique username and userid to access services of MyStore, but in order to access these services all these entities have to first login into the system (MyStore). The method for authentication is named loginUser, which takes two String parameters, username and password and returns the userID if authentication is successful.
Note : This method loginUser is a business method, normally business methods carry out operations or processing on values EJB components. From clients perspective, clients can see only business methods and invoke them on bean.
Tasks :
- Create a J2EE project named MyStore.
- Create a Stateless Session Bean named StoreAccess.
- Add a business method in bean named loginUser with the following signature
public String loginUser (String username, String password)
- Create a DAO named StoreAccessDAOImpl under package au.com.tusc.dao. Generate the DAO interface.
- Implement the method named loginUser, generated in DAO interface, in StoreAccessDAOImpl. Method signature is
public String loginUser (String username, String password)
- Add callback methods and implement them.
- Deploy StoreAccess bean.
- Create your test client named SessionClient under package au.com.tusc.client.
- Run your client and test the bean.
Create J2EE Project :
Now, lets start to write our first component of this tutorial.
Go to File > New > LombozJ2EE Project, project creation wizard will pop up.
Insert Project Name MyStore > Next .
Under Java Settings Check source, should be MyStore/src , libraries pointing to $JAVA_HOME > Go Next as shown in fig below.
Note: This step is shown in chapter1, as there is a bug in eclipse 2.1, so its important that you check your library settings are right.
Under Create J2EE Module, select Web Modules tab > Add.., enter Module name as OnlineStore > OK as shown in figure below.
Under Create J2EE Module, select EJB Modules tab > Add.., enter Module name as MyStoreMgr > OK .
Under Create J2EE Module, select Targeted Servers tab > Select JBOSS 3.2.1 ALL > Add.. > Finish.
Create Stateless Bean :
Go To Package Explorer > Expand Mystore (project) node > select src, right click and menu will pop up.
On pop up menu > New > Lomboz EJB Creation Wizard.
Enter package name au.com.tusc.session, bean name StoreAccess and select bean type as stateless > Finish.
This will create a package named au.com.tusc.session under src and StoreAccessBean under that package as shown in the figure below.
As we can see from the figure below it has created a class level tag @ejb.bean, which has assigned the bean type, name and its JNDI name which will be generated in Home interface. This tag will also generate deployment descriptors in ejb-jar.xml and jboss.xml file as well once you generate your EJB classes, which is covered later on in this chapter.
Note: It will generate the bean name, jndi-name and type of bean in the file. Also, the name of file is appended with word 'Bean' as you gave the name of the bean as StoreAccess only. Again, be careful with naming conventions, specifying the bean name only in the wizard without adding the word 'Bean' to the name as the wizard appends that for you.
Expand MyStoreMgr/META-INF node under Package Explorer. You will find there are seven files which are generated by Lomboz using Xdoclet as shown in the figure below.
Now we are going to generate all the interfaces including Home, Remote, DAO and other helper classes. We will explain why later on, but for the time being just follow the steps.
But before we get too excited, there are a few concepts to cover here.
Go to MyStoreMgr/META-INF > select and open ejbGenerate.xml.
Note: Lomboz uses this file to generate required interfaces and helper classes, so in the event that you have special needs then you will have to customize this file. See ejbdoclet under the Xdoclet documentation.
'ejbGenerate.xml' file is generated only once when you create your EJB module. So any changes made in this file will be reflected even if you modify your bean class and generate your classes again and again.
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As we can see from the code snippet of file shown in figure at right, there are following tags defined.
<dataobject/> is defined for generating data Objects for holding values of EJB component's persistent fields, which correspond to columns in the associated table in the database.
Note: <dataobject/> has been deprecated in favour of Value Object which is more powerful in terms of relationships (1-1, 1-n and n-m).
<utilobject/> Creates method for generating GUID and for accessing Remote and Local Home objects.
<remoteinterface/> Generates remote interfaces for EJBs.
<localinterface/> Generates local interfaces for EJBs.
<homeinterface /> Generates remote home interfaces for EJBs.
<localhomeinterface/>Generates local home interfaces for EJBs.
<entitypk/>Generates primary key classes for entity EJBs.
<entitybmp/>Creates entity bean classes for BMP entity EJBs.
<entitycmp/>
<session/> Generates session bean class.
Note : There is no tag for generating a DAO.
So, we have to include this <dao/> tag.
For details, please refer ejbdoclet under Xdoclet documentation.
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As we can see from the code snippet from this file the following tags are defined.
<jboss/> is a JBOSS specific tag required for JBOSS. You have to specify datasource, datasourcemapping and preferredrelationmapping. As it differs for different databases, so you may have to specify values appropriate to your environment. If these tags are commented out in JBOSS they default to the correct values for the built-in Hypersonic SQL database, but for the moment we'll set them anyway.
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The other two files which are of importance to us are ejb-jar.xml and jboss.xml. The file ejb-jar.xml has all the deployment descriptors for beans and jboss.xml has the JBOSS specific deployment descriptors required by JBOSS.
Note : ejb-jar.xml file is generated every time you generate interface and helper classes for your bean. For the first time, it is empty, and jboss.xml will be generated every time when you will generate your classes for your bean.
Setup DAO :
Now, let's customize ejbGenerate.xml for setting up a DAO.
We have included a <dao> tag specifying the destination directory for the generated DAO interface and what pattern to be used.
Note : For details, please refer ejbdoclet under Xdoclet documentation.
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We have included the datasource, datasoucremapping and preferredrelationmapping as shown in code snippet of ejbGenerate.xml file on right.
datasource="java:/DefaultDS" is a local JNDI name for data source to be used.
datsourcemapping="Hypersonic SQL" maps data object/value objects and their types to columns and data types associated with these columns.
preferredrelationmapping="foreign-key" defines type of database to be used.
Note : For more details, please refer JBOSS documentation.
Since we are using the Hypersonic database, these parameters are appropriate to that. These parameters relate to the configuration file standardjbosscmp-jdbc.xml which controls the CMP-to-JDBC mappings for JBOSS. This resides in $JBOSS_HOME/server/conf/ , e.g. /opt/jboss/jboss-3.2.1/server/default/conf/.
Code snippet from standardjbosscmp-jdbc.xml is shown in figure at right.
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Note : The way Xdoclet works is bit different from some conventional styles of programming, as the Xdoclet tags will generate these (home and remote) interfaces along with necessary helper classes, which then will used in Bean and DAO Implementation class. However, until these are generated, we cannot write any business methods in Bean and JDBC wrappers in the DAO Implementation class. If this seems confusing then just follow the steps, and hopefully it will become more clear.
Create DAO Interface :
Since we are going to use a DAO to access the database for this Stateless Bean, we have to create a DAOImpl class which will implement that generated DAO interface.
Go to src > add package named au.com.tusc.dao > Add a class StoreAccessDAOImpl in that package.
Now go to your Bean class and declare this tag at the class level (that is at the top) as shown below to generate the DAO interface.
@ejb.dao class="au.com.tusc.session.StoreAccessDAO"
impl-class="au.com.tusc.dao.StoreAccessDAOImpl"
Expand StoreAccessBean node under Package Explorer. Right click and a pop up menu will appear.
On that menu go to Lomboz J2EE > Add EJB to module. Select EJB '[MyStoreMgr]' > OK.
Expand MyStoreMgr node under MyStore Project in Package Explorer. Right click and a menu will pop up.
Go to Lomboz J2EE > Generate EJB Classes as shown in the figure below.
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EJB interfaces and helper classes are generated under ejbsrc/au.com.tusc.session directory as shown in the figure at the right.
Seven files are generated.
StoreAccess is the remote object interface.
StoreAccessLocal is the local object interface.
StoreAccessSession extends our bean class named StoreAccesBean.
StoreAccessHome is the remote home interface.
StoreAccessLocalHome is the local home interface.
StoreAccessUtil is a helper class which has methods for accessing Home and LocalHome interface along with generating GUID.
StoreAccesDAO is the DAO interface which we will use to implement our StoreAccessDAOImpl under au.com.tusc.dao.
StoreAccessDAO is generated by this tag declared in StoreAccesBean shown below. If you don't declare this tag in that file it won't generate this interface.
@ejb.dao class=au.com.tusc.session.StoreAccessDAO
impl-class=au.com.tusc.dao.StoreAccessDAOImpl
Other files of interest which are generated are ejb-jar.xml and jboss.xml under MyStoreMgr/META-INF.
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As shown in the figure on the right, a few descriptors are generated in the ejb-jar.xml file.
These descriptors are generated by the following tag declared in the StoreAccesBean file.
@ejb.bean name ="StoreAccess"
jndi-name="StoreAccessBean"
type="Stateless"
This tag is added by Lomboz's bean creation wizard.
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This tag also generates the following descriptors in jboss.xml as shown in the code snippet below.
So now we know which tags are responsible for generating classes, interfaces and descriptors.
Add Business Method :
Next step is to add a business method in the bean.
Go to StoreAccesBean > Right click > Select New on pop up menu > Select Lomboz Ejb Method Wizard.
Add a business method with the following signature: public String loginUser (String username, String password).
Select Method Type as Business and Interface as Remote as shown in the figure below..
This wizard generates a loginUser method in our bean class, with the method level tag '@ejb.interface' shown below.
This tag is responsible for generating this method in the Remote Interface (in this case it is StoreAccess which will be created once you generate your classes). This tag is covered later on in this chapter.
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Now, This business method needs to invoke a method on the DAO, which will communicate with the database.
Therefore we add another tag on this method, so that a method with this signature is generated in DAO interface which we can implement in the DAOImpl class. Then this business method can invoke the method in DAOImpl class to get the desired result.
@dao.call name="loginUser"
So add this tag at the method level as shown in the figure at right.
Now generate your EJB classes again as shown in the steps we went through earlier.
Note: OK, OK! For reference these are the steps you have to follow.
Expand 'MyStoreMgr' node under 'MyStore' Project in Package Explorer.
Right click and a pop up menu will appear.
Go to Lomboz J2EE > Generate EJB Classes.
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After generating the classes, we look at first the generated DAO interface and then the generated Session Class.
In StoreAcessDAO two methods are generated.
1. init() by default.
2. loginUser(), generated by tag shown below.
@dao.call name="loginUser"
Note: Please do not edit any class generated by Xdoclect.
In StoreAcessSession two methods of our interest are
1. getDAO() creates instance of DAOImpl calss.
2. loginUser(), calls loginUser method in DAOImpl class, which we have to implement.
Code snippet from 'StoreAccessSession'.
Implement DAO Interface :
Now, we will implement methods in the StoreAccessDAOImpl class.
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First import the following packages.
javax.naming.InitialContext;
javax.sql.DataSource;
java.sql.Connection;
java.sql.PreparedStatement;
java.sql.ResultSet;
java.sql.SQLException;
Change your class declaration so that StoreAccessDAOImpl implements StoreAccessDAO.
Add a field to store the JDBC resource factory reference.
private DataSource jdbcFactory;
In init() method, locate the reference "jdbc/DefaultDS" using the JNDI API, and store the reference in variable jdbcFactory.
Lookup string is "java:comp/env/jdbc/DefaultDS".
Code Snippet is shown in the figure on the right.
Now add the required code in loginUser().
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In method loginUser(), first get the connection to the database using the jdbcFactory.
Create a SQL statement which searches for userid in the table StoreAccess where userid and password is provided for each user.
Return userid if successful, else raise SQLException.
Code snippet is shown in the figure on the right.
Go back to your loginUser method in StoreAccessBean class.
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In StoreAcessBean class under the loginUser method just add some debug statements, as shown below in this code snippet.
Note : We don't have to call the loginUser method in StoreAccessDAOImpl, as it being invoked by the loginUser method in StoreAccessSession class which inherits StoreAccessBean class, that is the StoreAccessSession class has overridden this method.
Code snippet from StoreAccessSession shown below.
Add Callback Methods :
Now, add callback methods to complete this bean as shown below.
- setSessionContext.
- UnsetSessionContext.
Note : These callback methods are invoked by the EJB container.
Add a field to store sessionContext.
protected SessionContext ctx;
Add method setSessionContext with sessionContext as parameter and assign that to the sessionContext variable as shown below in the code snippet.
Similarly add method unsetSessionContext, assign context variable to null as shown above.
Note : StoreAccessSession class inherits the StoreAccessBean abstract class and implements SessionBean, which will override all methods of interface SessionBean. So after finishing the methods in the bean class, generate your EJB classes again. SessionContext methods will be overridden as shown in figure below. Code snippet from StoreAccessSession shown below.
Now let's look at the generated Home and Remote interfaces.
In the case of the Remote interface all business methods declared in the bean are also generated with the same signature. This is due to the class level tag declared in the StoreAccess Bean as discussed above after adding business methods. Code snippet for tag is shown below.
So, loginUser is generated in a Remote Interface called StoreAccess as shown below because of this tag.
In the case of the Home Interface only one method is created named 'create', which is generated by default because of the <homeinterface/> tag in ejbGenerate.xml as shown below.
Also, other then that, it has JNDI_NAME and COMP_NAME (which is the logical name to lookup the component) is also generated, these are generated because of this tag declared at class level in 'StoreAccessBean' class shown below in figure.
Note : For further options associated with these tags please refer to the 'ejbdoclet' documentation in Xdoclet.
Now, all the aspects are pretty much covered, and our bean's functionality is complete. Now for the deployment descriptors..
Deploy Bean :
In order to deploy our bean we have to declare a few tags in the StoreAccessBean class as shown below in the code snippet.
Add the tag shown below in at the class level (at the top).
@ejb.resource-ref res-ref-name="jdbc/DefaultDS"
res-type="javax.sql.Datasource"
res-auth="Container"
This tag will generate deployment descriptors in ejb-jar.xml, as the bean has to know which datasource you are going to connect to, what is its type, etc. This will generate these descriptors as shown in code snippet below.
Add the tag shown below in StoreAccessBean at the class level (at the top).
@jboss.resource-ref res-ref-name="jdbc/DefaultDS" jndi-name="java:/DefaultDS"
This tag will generate deployment descriptors in jboss.xml, as the application server has to know with what jndi-name datasource it has been registered with. This will generate these descriptors as shown in the code snippet below.
Now, everything is complete, and it's time to deploy the bean.
First, regenerate your EJB classes as shown in the steps above for the final time.
Note : We have regenerated the classes again and again, in order to explain every step and its result. Once you are familiar with these steps you will need much fewer of these iterations. Either way, it doesn't matter, as your implementation always remains untouched by this process.
Go to Lomboz J2EE View > expand node MyStore > expand MyStoreMgr > select 'Jboss 3.2.1 ALL' .
Right click > select Debug Sever on the pop up menu as shown in figure below.
Go to MyStoreMgr node in LombozJ2EE view > right click > select Deploy on the pop up menu as shown in the figure below.
And now wait for your deployment result.
If everything goes fine, you will have this message under your console as shown in the figure below.
So, now our bean is deployed successfully, let's create our test client, which will invoke the loginUser method on 'StoreAccessBean'.
Create your Test Client :
Go to Project MytStore node > select src node > right click.
Select New on pop up menu > select Lomboz EJB Test Client Wizard as shown in the figure below.
Select package name au.com.tusc.client, name as SessionClient and select Ejb Home as au.com.tusc.session.StoreAccessHome and Ejb Interface as au.com.tusc.session.StoreAccess as shown in the figure below.
This will generate the required methods for you in your SessionClient class and you have to just invoke the loginUser method on the bean as shown below.
Now the last step is to write code in your client.
So add these lines under the testBean method as shown in figure below.
System.out.println("Request from client : ");
System.out.println("Reply from Server: Your userid is " +
myBean.loginUser("ANDY","PASSWD"));
Test your Client :
Now, in order to test your client, Select SessionClient node > Go at top level menu and select the icon with the 'Running Man'.
On that select 'Run as' > select 'Java Application', as shown below.
Now under your console, if you get your reply for 'ANDY' as 'U2', then your call is successful as shown below.
Note : So our Stateless Session Bean is deployed and tested successfully and from now onwards you should be comfortable using Lomboz. In future we will not go into the detail of the steps for using Lomboz and will concentrate more on other aspects of beans.
小結
通過表示J2EE應用程序中的任務或者行動。Session Beans可以因此而處理大量的狀態。尤其是在同其他EJB合用的情況下。理解Session Beans的瞬時現象對開發者的工作會帶來莫大的幫助。有狀態和無狀態Beans的細微差別就如同充分利用其行為一樣重要。在掌握以上的概念之后，正確的設計和實現Session Beans必然會顯著改進EJB應用程序的功能和效率。