Enterprise Java in a Nutshell

Enterprise Java in a Nutshell

Kyle BrownKyle BrownSenior Java ConsultantSenior Java ConsultantIBM WebSphere IBM WebSphere ServicesServicesRTP, NCRTP, [email protected]@us.ibm.com

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© Copyright IBM Corporation 1999

Overview

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What is Enterprise Java?

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Enterprise Java and MVC

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Separating Presentation and Content

Business Objects(Model)

Servlets(Interaction Controllers)

Input Page(View)

Display Page(View)

JSP(view)

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3

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5View

Server

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This corresponds to specific pieces of Enterprise Java:

(1) The Model is represented by Java Beans and Enterprise Java beans. It is implemented with Java code that may use IBM’s enterprise connectors.(2) The View is implemented using Java Server Pages (JSP’s)(3) The Control logic is implemented as Java Servlets

Each layer may be implemented by different people, giving a better separation of roles. There are consistent interfaces between the layers.


A Server-side Java ArchitectureWeb Browser

Browser + AppletApplication

DB

Java Servlets

Java Server Pages

Enterprise Java

BeansEJB

Services

IIOPJD

BC

, JM

SIIO

P

HT

TP

Client Tier

2nd Tier Servers

Third Tier Servers

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Java and the Web

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Java Servlets

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Servlet Lifecycle

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init()

service()

destroy()

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One additional thing to remember is that there is only ONE instance of each servlet class. Multiple threads invoke the service() method in the same instance. This is a key point that has some ramifications; the biggest being that servlets should be stateless. The state of your application is stored outside of your servlet (more on that later).

One exception to the above: A servlet can implement javax.servlet.SingleThreadModel. This forces the servlet to only have a single thread active within it at any time. This is usually implemented (by the servlet runtime) by providing a pool of servlet instances with each request acquiring exclusive access to one instance for the duration of the service method.


Simple Servlet

public class MyServlet extends HttpServlet { public void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {

// get stream to output HTML on! res.setContentType("text/html"); PrintWriter out = res.getWriter();

// send out a simple banner out.println("<HTML><BODY>"); out.println("<h1>Hello World!</h1>");

out.println("</BODY></HTML>"); }}

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This is the simplest servlet. It just returns some HTML to the requestor. However, the three parts:(1) Set the content type(2) Get a PrintWriter(3) Output on the PrintWriterare seen in even the most complicated servlets.


HttpSession

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WebSphere Session Storage

Browser id value12345 aSession

aSession

key value"Customer" aCustomer"Name" "Bob"

cookie name value

"sesessionid" 12345

Cookie List

Application Server

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Java Server Pages

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JSP Sample

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Enterprise Java Beans

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OMG’s CORBARMI (Java Remote Message Invocation)X/Open XA Transaction architecture

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RMI Over IIOP

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Client Program

Standard RMI Engine

ORB

IIOP

TCP/IP

Generated RMI Code

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XA Transactions

Application/EJB Server

Transaction Manager

Resource ManagerResource

Manager

SQL, etc.

OTS TX

XA

JDBC 2.0JMS

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So what’s an EJB?

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EJB Server Software

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CORBA Service EJB Equivalent Explanation

Naming JNDI + EJB Name service*

Associate names with objects on the network

Lifecycle EJB Homes + Distributed GC

Manage objects with Object Factories

Security EJB Security Provide secured access to networked objects

Persistence Entity Beans Provide a standard way for objects to save themselves

Transaction EJB Transactions** Propagate transactions across object resources

EJB services and CORBA Services

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EJB Objects

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Client

EJB Server/Container

Home

EJBObject

Bean

HomeStub

EJBObjectStub

Bean Data

EJB Architecture

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This shows all of the pieces of the EJB architecture. All that you get on the clent side is a stub (a proxy) to an actual object on the server side. The Home is what allows you to create and find EJB’s. The Beans themselves exist only on the server. An EJB Object is a container-provided object that handles things like:

*persistence in CMP entity beans*distribution details (CORBA, RMI)

The bean itself only implements business methods -- the framework handles the rest!


EJB Homes

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EJB Clients

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Bean Types

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EJB Types

EnterpriseJava

Beans

Entity Beans

Session Beans

ContainerManaged

BeanManaged

Stateless Stateful

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Stateless vs. Stateful Session Beans

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Entity Bean persistence

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EJB Implementation

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EJB Deployment

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Deploy Beans to EJB Server

Write EJB Client(s)

Test Beans and EJB Client(s)

Write Remote Interface

Write Home

Interface

Write Bean Class

EJB Development Process

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Counter Example

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Home Interfaces

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CounterHome

create()create(int )

javax.ejb.EJBHome<<interface>>

<<interface>>

java.rmi.Remote<<interface>>

33

Only Stateful session beans have more than one (default) create() method. create() methods vary only in the number of parameters they take. As we will see, each new create() method corresponds one-to-one with an ejbCreate() method in the Bean.

The supertype (EJBHome) defines three methods

public void remove (Handle handle) throws java.rmi.RemoteExcpetion, javax.ejb.RemoveException;

public void remove(Object pKey) throws java.rmi.RemoteException, javax.ejb.RemoveException;

(You generally don’t remove() Session beans -- removing by a primary Key will only work for Entity beans)

public EJBMetaData getEJBMetaData() throws java.rmi.RemoteException;

The EJBMetaData allows you to get to information like the EJBHome, the Home interrface class, the Remote interface class, and hte primary key class. It also allows you to ask if a bean is a session or entity bean.


Counter Home Example

package com.ibm.ejb.examples.simple;

/*** This is a Home Interface **/public interface CounterHome extends javax.ejb.EJBHome { public Counter create() throws javax.ejb.CreateException,

java.rmi.RemoteException; public Counter create(int arg1) throws javax.ejb.CreateException,

java.rmi.RemoteException;}

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Remote Interfaces

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java.rmi.Remote<<interface>>

Counter

void increment()void reset()

<<interface>>

int getValue()

35


Counter Remote Interface

package com.ibm.ejb.examples.simple;

/*** This is an Enterprise Java Bean Remote Interface*/public interface Counter extends javax.ejb.EJBObject {

int getValue() throws java.rmi.RemoteException;

void increment() throws java.rmi.RemoteException;

void reset() throws java.rmi.RemoteException;}

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Implementing a Session Bean

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The lifecycle methods are methods defined in the SessionBean interface. You need to implement all of them, but WHAT you do in each method depends on the bean. The next two slides discuss these methods in more detail.


Creating Stateful Beans

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Passivation/Activation

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The problem is with Stateful beans they take up room since you must have MANY instances running around (one per client). You don’t want them all in memory at once!

Entity Beans can also be passivated and activated.


SessionBean lifecycle methods

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ejbActivate() and ejbPassivate() are methods that must be implemented, but the do not need to do anything. When a bean isselected for passivation, the ejbPassivate() method will be called in the bean before the bean is serialized to a file.

Likewise, after a bean is loaded from a serialized file, the ejbActivate() method is called in that bean.

These methods are generally used to setup and tear-down instance variables that cannot be serialized -- either variables that contain transient information (like caches) or non-serializable objects (like AWT objects, or TCP connections).


Contexts

EJBContext

getRollbackOnly () : booleansetRollbackOnly () : voidgetEJBHome () : javax.ejb.EJBHomegetEnvironment () : java.util.PropertiesisCallerInRole ( : java.security.Identity) : booleangetCallerIdentity () : java.security.IdentitygetUserTransaction () : javax.jts.UserTransaction

<<interface>>

EntityContext

getEJBObject () : javax.ejb.EJBObjectgetPrimaryKey () : Object

<<interface>>SessionContext

getEJBObject () : javax.ejb.EJBObject<<interface>>

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Session beans implement setSessionContext(). Entity beans implement setEntityContext().Here is the definition of EJBContext:

public interface javax.ejb.EJBContext{public abstract Identity getCallerIdentity();public abstract EJBHome getEJBHome();public abstract Properties getEnvironment();public abstract boolean getRollbackOnly();public abstract UserTransaction getUserTransaction();public abstract boolean isCallerInRole(Identity role);public abstract void setRollbackOnly();}

getCallerIdentity is used in EJB Security to obtain the identity of the caller. Likewise isCallerInRole() allows you to ask if the caller is acting in a role. These features is not used in WebSphere 2.0. getEJBHome obtains a rference to the EJB Home for this bean. We will look at using getUserTransaction(), setRollbackOnly() and getRollbackOnly() in the section on transactions.


SessionContext

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The reason you want to use getEJBObject() to send a message to yourself and not this is that the EJBObject is the one that handles issues of transactions, multithreading and persistence. You bypass all of that if you use this.


Counter Example Implementation

package com.ibm.ejb.examples.simple;/** Lots of import statements left out for space **/

public class CounterBean implements javax.ejb.SessionBean { private javax.ejb.SessionContext mySessionCtx = null;public int counterValue =0;

public void ejbActivate() throws java.rmi.RemoteException {}public void ejbCreate() {}public void ejbCreate(int initialValue) {

counterValue = initialValue;}public void ejbPassivate() throws java.rmi.RemoteException {}public void ejbRemove() throws java.rmi.RemoteException {}public int getCounterValue() { return counterValue; }public void increment() {counterValue++;}public void reset() {counterValue = 0;}public void setSessionContext(javax.ejb.SessionContext ctx) throws java.rmi.RemoteException {

mySessionCtx = ctx;}}

43

There’s a lot here to talk through. The bits in RED are the lifecycle methods. These are automatically created by VA-J. If you’re not using VA-J, then you’ll have to code them yourself. Note that most of the lifecycle methods do nothing.

The parts in blue are the business logic methods. You will always code these yourself.

Note that every method can throwjava.rmi.RemoteException. You can also decide to throw an application-defined exception in your methods. Your business method may also throw the java.rmi.RemoteException to indicate a system-level failure.


Deploying EJBs

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44

A Jar file is a standard Java deployment format. It’s really just a ZIP file.

There are tools to take your .class files and create the appropriate JAR files that we’ll see next.


Generated Classes Example

_CounterHomeStub

_CounterStub

CounterHelper

CounterHomeHelper_impl

_CounterHomeSkeleton

valueCounterHomeCounterHomeHolder

_impl

_CounterSkeleton

value CounterCounterHolder

EJSRemoteCounter

EJSCounterHome

CounterBean

ebRef

getBean()

45

The classes can be grouped together as follows:

_XXXStub and _XXXSkeleton classes are RMI Stubs and Skeletons

EJSXXX classes are the Container-generated objects that handle the lifecycle and frameworkparts of the system.

XXXHelpers are CORBA objects used in the narrow() operation with JNDI

XXXHolders are CORBA objects that are used in marshalling objects in a CORBA "Any"


EJB Clients

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JNDI

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JNDI API

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SPI

Other SPIs...

Client Program

47

In our case the names will be matched to a Home. We will, of course, get back a Proxy to that Home.


JNDI Details

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48

There is another package javax.naming.directory that is used for nested contexts. We won’t use it in this course.

You will want to import the javax.naming package into all of your clients.

InitialContext is a concrete class that implements the Context interface. In EJB’s today you most often will only use an InitialContext --you will probably not take advantage of the directory features of Contexts.


Binding

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49

The whole interface is kind of similar to a Hashtable -- it puts together key-value pairs.

Which Homes a container binds to is determined as part of the EJB deployment process, which we will look at later.


InitialContext in EJS

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There 12 or so possible environment variables that can be set in the Hashtable. You can view them in the Javadocs. EJS only uses the two listed above.

The INITIAL_CONEXT_FACTORY environment variable tells JNDI which SPI to use for this particular Context. The PROVIDER_URL tells the Cos Naming SPI where to locate the Persistent naming service (PNS). The value of PROVIDER_URL should be set to the URL of the naming service (localhost would just be "iiop:///"). The value of INITIAL_CONTEXT_FACTORY should always be set to "com.ibm.jndi.CosNaming.CNInitalContextFactory" (the EJS implementation class that builds Contexts in this version of WebSphere).


Getting an InitialContext

javax.naming.InitialContext initContext = null;java.util.Hashtable properties = new java.util.Hashtable(2);properties.put(javax.naming.Context.PROVIDER_URL, "iiop:///");

// local nameserver URLproperties.put(javax.naming.Context.INITIAL_CONTEXT_FACTORY,

"com.ibm.jndi.CosNaming.CNInitialContextFactory"); // IBM name services factory

try {initContext = new javax.naming.InitialContext(properties);

} catch (NamingException e) {System.out.println("Exception: " + e);

}

51

This is the start of a simple example of a Counter.

The Remote interface declares an increment() method, a reset() mtehod and a getValue() method. The Home interface is very simple -- it just has a default create() method, and a create(int) method that allows you to create a counter preset to a particular value.

The value of the PROVIDER_URL property is set to an iiop URL of the naming service. / means go to the default port on localhost. Otherwise use the iiop://hostname:port style.

Note: In a real system you would never want to hard code these values as this example shows. Instead you would use a properties file to look these values up at runtime.


Using Home Interfaces

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52

Remember that the Home interface defines the create() and find() methods that allow you to obtain references (Proxies) to EJB’s that live on an EJB Server.

You define the JNDI name that a Home is known in the deployment descriptor.

In EJS the Helper object is automatically generated by the deployment tool when you deploy your EJB. A "narrow()" operation is kind of like a cast -- it takes a generic org.omg.CORBA.Object and "retrieves" its type information so that it can be used.

By the way -- this implementation (for EJS) is different from the default implementattion mentionede in the EJB spec -- in the spec the class java.rmi.PortableObject.narrow() is used instead of the helper narrow() method we use with EJS.


Obtaining a Home Reference

CounterHome home = null;try {

// The InitialContext was retrieved earlierObject obj = initContext.lookup("Counter"); // "Counter" is the JNDI name of the Homeif (obj instanceof org.omg.CORBA.Object) {

home = CounterHomeHelper.narrow((org.omg.CORBA.Object) obj);} else {

System.out.println("Lookup returned unexpected object type.");}

} catch (javax.naming.NamingException e) {System.out.println("Error retrieving the home interface:" + e);

}

53

Remember the proxy pattern. CounterHome is the interface that things implement -- this is why the home variable is typed to theCounterHome Interface. The actual object you get back is a proxy to a Home object on the server.


Obtaining Beans

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try {Counter counter = counterHome.create();counter.reset();counter.increment();counter.increment();int value = counter.getCounterValue();System.out.println("the value is " + value);

} catch (Exception e) {System.out.println("Exception creating new SimpleCounter:" + e);

}

54

find() methods are only used for Entity beans.


Entity Basics

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The container transparently manages concurrency, transactions, persistence and other services for the beans that live in it. Entity Beans are persistent -- they are stored in some sort of database. They will survive server crashes, etc.


Home Interfaces for Entities

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56

Each create() method in a Home Interface corresponds to a ejbCreate() method with the same parameter set in the Bean class. You can write create() methods that fill in all, some, or none of the fields in an Entity bean.


Finder Methods

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57

he "Collection" that is defined in the Spec must ALWAYS be an Enumeration, e.g., a finder method can only return the Remote Interface type or an Enumeration.


Sample Home Interface

public interface EmployeeHome extends EJBHome {

public Employee create (EmployeeKey primaryKey) throws CreateException, RemoteException;

Employee create(EmployeeKey key, String name, String dept, String phone) throws CreateException, RemoteException;

Enumeration findAll() throws RemoteException, FinderException;

public Employee findByPhoneNumber(String phoneNumber) throws RemoteException, FinderException;

public Employee findByPrimaryKey (EmployeeKey primaryKey) throws RemoteException, FinderException;

}

58

This Home Interface demonstrates most of the concepts discussed on the previous slides:

(1) It has two create() methods -- one that takes the PrimaryKey class (EmployeeKey) and another that takes the PrimaryKey plus all of the fields of the object.(2) It has three finder methods;

(a) The default finder method "findByPrimaryKey()"(b) A finderMethod that takes a singleargument "findByPhoneNumber()" and returns an EJBobject that corresponds to the Remote interface(c) A finderMethod that returns an Enumeration of objects that correspond to theRremote interface.

The key thing to remember is that this only specifies WHAT needs to be done -- not HOW. The HOW is defined by the container or thebean.


The EJS Persistence model

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59

For instance, we will cover the WebSphere-specific way of implementing finder methods. While the spec says that finder methods can exist, it gives no hints as to how they should be implemented, leaving each vendor to do it themselves.


Specifying Fields

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60

This part of the Deployment descriptor is generated and handled by VisualAge for Java if you are using that tool.


Basic EJS Persistence

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61

The table is automatically created by WebSphere during deployment. Note that DB2 has a table-length restriction that all table names must be less than 18 characters. The table is automatically generated to have the following form:

EJB.<JNDINAME>BeanTbl

Where JNDINAME is the JNDI Name of the bean (also the remote interface name). If your EJBRemoteInterface name is more than 11 characters, this generation will fail and you will have to rename your Remote Interface.


Standard Type Mapping

CMP field Java Type SQL TypeString VARCHARShort / short SMALLINTInteger / int INTEGERFloat / float FLOATDouble / double DOUBLEByte / byte SMALLINTLong / long VARCHAR(22)Character / char CHAR(1)Boolean / boolean SMALLINTJava.math.BegDecimal NUMERICJava.sql.Date DATEJava.sql.Time TIMEJava.sql.Timestamp TIMESTAMPOther (serialize) BLOB

62

One thing to point out in this diagram is that types other than the primitive types are mapped to 1 Megabyte BLOBs. This is not the kind of mapping that you want to use.


FinderHelpers

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63


FinderHelper Example

/** * For each finder method defined in the home interface other than the * findByPrimaryKey method, a query String must be defined here.**/

public interface EmployeeBeanFinderHelper {

public static final String findAllQueryString = "select * from EJB.EmplyeeBeanTbl";

public static final String findByPhoneNumberQueryString = "select * from EJB.EmplyeeBeanTbl where phoneNumber = ?";

}

64

To find the table name and column names for the class, refer to the class EJSJDBCPersisterXXXBean class in the method in the text of the method getCreateTableSQLString()

Note that this is the potential here for hard-to-find errors. If your table definition changes (due to the addition of new fields) your finder helper classes will NOT automatically be updated. This will result in your finder methods failing with null pointer exceptions.


PrimaryKeys

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65


Advanced WebSphere Container Support

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66


Bean Managed Entity Beans

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EntityBean InterfaceHME$FWLYDWH��FDOOHG�RQ�DFWLYDWLRQ�

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68


Passivation Sequence

Client EJBObject BMP Bean Instance

DatabaseContainer

business methodbusiness method

ejbStore() write state to db

ejbPassivate()

69

Writing the state to the database is done through an UPDATE. This is the sequence that is called if the business method is a transaction, and the object has been already activated.


Activation Sequence

DatabaseClient EJBObject BMPBean Instance

business method ejbActivate()

ejbLoad()read state from db

70

During the EJB Load you will need to SELECT from a database. This is the sequence that is called if the object has not yet been activated.


Commit Sequence

Client Container BMPClass Instance

Trasaction Service

Database

UserTransaction.commit()

beforeCompletion()

ejbStore()write state to db

afterCompletion()

ejbPassivate()

71


Example ejbLoad()public void ejbLoad() throws java.rmi.RemoteException {

if (connection == null)throw new java.rmi.RemoteException("No database connection");

// Use the connection to store the new bean into the database:ResultSet rs = null;try {

// Execute the SQL:PreparedStatement preparedStatement = connection.prepareStatement(SELECT_SQL);preparedStatement.setInt(1, ((EmployeeKey) entityContext.getPrimaryKey()).primaryKey);rs = preparedStatement.executeQuery();// Pull the data out of the result set:if (rs.next()) {

customerNumber = rs.getInt(1);firstName = rs.getString(2).trim();lastName = rs.getString(3).trim();// address, city, state, zip are the same

} else {throw new RemoteException("Unable to load data");

}} catch (SQLException e) {

throw new RemoteException(e.getMessage(), e);} finally {

try { if (rs != null)rs.close();

} catch (SQLException e) {}}

}

72


Example ejbStore()

public void ejbStore() throws java.rmi.RemoteException {

if (connection == null)throw new java.rmi.RemoteException("No database connection");

try {PreparedStatement preparedStatement = connection.prepareStatement(UPDATE_SQL);preparedStatement.setString(1, firstName);preparedStatement.setString(2, lastName);preparedStatement.setInt(8, customerNumber);preparedStatement.setString(3, address);preparedStatement.setString(4, city);preparedStatement.setString(5, state);preparedStatement.setString(6, zip);preparedStatement.setString(7, age);preparedStatement.executeUpdate();

} catch (SQLException e) {throw new RemoteException("SQL Problem " + e);

}}

73


EJB Transaction Model

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74


Transaction Definition

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75

Related work means things like database updates.


Sun Transaction Standards

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76

The parties involved in a transaction system are:

ApplicationResource Managerand Application Server

The EJB spec requires a partial implementation of JTA. This hides most system complexity from the programmer.


Transaction Standards

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This pyramid makes several assumptions:

(1) That the EJB Vendor provides a Transaction Manager that implements the JTA and JTS (this is true of WebSphere).(2) That other vendors (or the same vendor) provide XA compliant resource managers. The 3.0 version of IBM WebSphere will contain XA compliant database drivers for Oracle, Sybase and DB2 that will also comply with the JDBC 2.0 standard. The JDBC 2.0 Standard allows for XA compliance, but does not mandate it.


Transaction Demarcation

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78


Transaction DemarcationClients TXN TXN Associated

with EJB Method

TX_NOT_SUPPORTED - -T1 -

TX_REQUIRED - T2

T1 T1

TX_SUPPORTS - -

T1 T1

TX_REQUIRES_NEW - T2

T1 T2

TX_MANDATORY - ERROR

T1 T1

79

TX_REQUIRED is default in VAJ.


Bean Managed Transactions

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80

The bean can obtain the user transaction by going to the Entity or Session Context.


Java Message Service

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81


Distributed EJB Transactions

Application/EJB Server

Transaction Manager

Resource ManagerResource

Manager

SQL, etc.

OTS TX

XA

JDBC 2.0JMS

82


EJB Program Architectures

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83

Facade is a design pattern from Gamma, et. al. A Facade is an object that hides the complex relationships between other objects.


Session Bean Facades

Clients see only the Facade -- they don’t see the underlying

relationships

Session BeanSubsystem

84

In cases like this the methods to the facade are usually in terms of simple, primitve operations on the objects involved. In more complex cases, you need a layer of helper objects that are used to transfer information between the client and the facade.

This approach is most useful when you want to use an alternate means of persisting data rather than using CMP. If you want to store information in CICS or IMS, or any other form of data storage, this is the only approach that is valid. Bean Managed persistence isnot the best approach for these kinds of data representations at this time due to the insufficient support of these systems for XA transactions.


Helper Objects

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85

Any object that is serializable can be returned as the value of a Session bean’s method. This object will be serialized and copied across the network to the requesting client. Likewise modified helper objects can be passed back to the session bean in the same way.

Remember that all accesses to an EJB are network calls. This can adversely affect performance when you have a large number of fine-grained objects that you access from the client. Using helper objects can reduce this network traffic. The client can make several changes at once to a single helper object and then send that helper object to the facade as one network call instead of several -- one for each change.


Facades and Helpers

helper objects

serialized andsent over IIOP

EJB ServerEJB Client

helper objects

Fac

ade EJBs and

other data sources

ViewCode

ControlCode

Control code modifies the helpersaccording to the user's actions.

The facade creates the helpers fromthe EJB's (or other data sources) andlikewise updates them from the changedhelper objects received.

86


Command Architectures

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Command is one of the design patterns from Gamma et. al. A Command is an object that represents an action on another object. It usually has a method (like "run") that carries out that action on the target object. Many different classes of commands may represent different actions, or commands may be parameterized by what they do (and may use reflection to execute their actions).


Commands in Action

helper objects

serialized andsent over IIOP

EJB ServerEJB Client

commands

Fac

ade EJBs and

other data sources

ViewCode

ControlCode

Control code modifies the helpersaccording to the user's actions.

The facade creates the helpers fromthe EJB's (or other data sources) andlikewise updates them from the commands that are received.

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This architecture is used in the JStore example that is available from the IBM website.


Pros and Cons of Session Facades

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Enterprise Java Future directions

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Summary

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Documents

Enterprise Java in a Nutshell