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Building Java Enterprise Applications by Brett McLaughlin

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Coding the Bean

It’s finally time to write some code. If you are new to EJB, you should pick up the aforementioned Enterprise JavaBeans and skim through Chapter 4 before continuing on. That should give you enough of a basic foundation to understand this chapter, as I’m going to move through the basic entity bean code pretty quickly.

I’ll start the entity bean work with the office data structure. Remember the OFFICES table from last chapter? The table structure is shown again in Figure 4-1 for reference.

The office data structure

Figure 4-1. The office data structure

I’ll use this structure for the first entity bean, as it has a very simple structure. It also does not depend on any other objects (i.e., it has no foreign keys), a subject I will address in the next chapter. Dependent objects introduce some additional considerations, but I will look at those when we start to code beans for tables with foreign keys, like the USERS table. For the time being, it’s enough to know that this office structure is as simple as it gets. You need to store the ID (an int), the city (a String), and the state (another String).

As this bean is simple in nature, it is a perfect candidate for container-managed persistence (CMP). There are no special JDBC calls that need to be made, no multiple-table queries, and no explicit data caching that needs to be performed. In the absence of these special cases, CMP is almost always a better choice than bean-managed persistence (BMP). While CMP beans are not truly portable yet, as each container generates specific implementation classes for that product, the basic classes I’ll look at in a CMP bean are portable. In other words, you won’t be able to take a complete JAR file (with implementation classes) from BEA Weblogic and deploy it on Lutris Enhydra, but you will be able to take the user-coded classes from this chapter and use them to generate classes for both BEA Weblogic and Lutris Enhydra. The resultant JARs can then be deployed on the respective application servers. This adds a bit of work, but is as close to complete portability as is possible in today’s EJB containers. It does allow us to write vendor-neutral code and not have to focus on a specific application server in this chapter.

All these reasons add up to a good case for using CMP instead of BMP. Additionally, the EJB 2.0 specification makes CMP an even more attractive solution, offering almost complete portability in all aspects of CMP entity beans.

Note

While the process of creating the “skeleton” classes for a CMP entity bean (the home and remote interfaces, the primary key, and the implementation class) is vendor-neutral, the means of generating container classes and deploying into a specific application server varies widely from product to product. In this book, I focus on BEA Weblogic, the most prevalent application server in use today, as it would be impossible to cover the ever-growing number of application servers. However, it should be fairly trivial to take the instructions for the Weblogic server and relate them to your own application server.

I’ve moved any steps specific to Weblogic to the appendixes, leaving only vendor-neutral code in the chapters’ content. This should make it easier to see what is general code, and what is platform-specific. Appendix D, then, covers installation and setup of the Weblogic application server for this book.

Adapters and Entity Beans

In preparing to code CMP entity beans, it turns out that most of the EJB callbacks that must be implemented are empty in almost all cases; the container will generate the code for these callbacks at deployment time. Instead of coding these methods into each entity bean implementation, wasting time and space in these beans and generally adding a lot of clutter, you can create an adapter class to handle this task for you. This concept, initially presented in Richard Monson-Haefel’s book, is used here for the clarity it provides in later code. Example 4-1 is a slightly modified version of Richard’s EntityAdapter class, which provides default, empty implementations of the required entity bean callbacks. The entity bean implementation classes in this chapter will extend this class, rather than implementing the javax.ejb.EntityBean interface directly.

Example 4-1. The EntityAdapter Helper Class

package com.forethought.ejb.util;

import javax.ejb.EntityBean;
import javax.ejb.EntityContext;

public class EntityAdapter implements EntityBean {

    protected EntityContext entityContext;

    public void ejbActivate(  ) {
    }

    public void ejbPassivate(  ) {
    }

    public void ejbLoad(  ) {
    }

    public void ejbStore(  ) {
    }

    public void ejbRemove(  ) {
    }

    public void setEntityContext(EntityContext entityContext) {
        this.entityContext = entityContext;
    }

    public void unsetEntityContext(  ) {
        entityContext = null;
    }

    public EntityContext getEntityContext(  ) {
        return entityContext;
    }
}

If you have any implementation classes that need to provide special behavior for a callback, you can override the adapter class; overriding still allows you to leave out any callbacks that are not implemented, keeping code clean. In addition, I’ve introduced the com.forethought.ejb package in this class. All the entity beans created for the example can be put into this base package, in a subpackage using the bean’s name (for example, office or user), with the EntityAdapter class in the util subpackage of the same structure. So entity and session beans end up prefaced with com.forethought.ejb.office or com.forethought.ejb.shoppingCart. This is the same naming scheme you’ll see in Sun’s PetStore and most other enterprise applications.

In addition to package naming, I follow standard practices for the actual bean class names. This entails using the actual name of the data object as the name of an entity bean’s remote interface. In the case of an office, the interface name simply becomes Office. The home interface has “Home” appended to it, resulting in OfficeHome, and the bean implementation itself gets the word “Bean” added to it, resulting in OfficeBean. And with this last detail covered, you’re ready to move to the bean code for the office classes.

The Remote Interface

Once the naming has been determined, it is simple to code the remote interface, which is always a good starting point in EJB coding. In this case, coding the remote interface is a matter of simply providing a few accessors and mutators[19] for the data fields in the office structure. Additionally, you should notice that no methods are provided to modify the ID of an office. That data is intrinsic to the database and is used in indexing, but has no business meaning; as a result, it should never be modified by an application. The only time this information is ever fed to an entity bean is in the finder methods, where an office is located by its primary key (findByPrimaryKey( ) in the home interface), and in the creation of an office, where it is required for row creation (the create( ) method in the remote interface). I’ll look at this in Chapter 5 and discuss how you can avoid even these situations of directly dealing with a database-specific value.

Additionally, you will notice that the ID of the office is returned as an Integer , instead of the Java primitive int type. An Integer is returned for two important reasons. First, CMP 2.0 introduces container-managed relationships (sometimes called CMR, or CMP relationships). This is a way of letting an EJB container manage relationships between entity beans (like the Office bean here, and the User bean in Appendix D). When these relationships are used, the container is responsible for generating additional classes to handle them, similar to a container generating implementation classes for your CMP beans. When these classes are generated, though, most containers make several assumptions; the first is that the primary key value on an entity bean is stored as a Java object (java.lang.Integer), and not as a primitive type (int). While this is not true in all EJB containers, it is in most. For this reason alone, it is better to use Integer instead of int when dealing with primary key types.

Using an Integer with primary keys also has a nice side effect. Because Java programmers are almost always more accustomed to working with the int data type, using Integer makes the primary key value stand out. The result is that developers think a little bit more about working with the value, resulting in primary keys being handled with care, as they should be. Therefore, you will note that the getId( ) method in the remote interface of the Office bean returns an Integer, not an int, and the create( ) method in the bean’s home interface requires an Integer as well.

Something else to note is the apparent naming discrepancy between the database columns and the entity bean. You can see from Figure 4-1 that the primary key column in the database is OFFICE_ID, and the field name, as well as related methods, in the Java class is simply ID (or id as a method variable). This discrepancy may seem a little odd, but turns out to be perfectly natural. In the database layer, simply using ID as the column name can result in some very unclear SQL statements. For example, consider this SQL selecting all users and their offices:

SELECT FIRST_NAME, LAST_NAME, CITY, STATE
  FROM USERS u, OFFICES o
 WHERE u.OFFICE_ID = o.OFFICE_ID

There is no ambiguity; the join occurs between the OFFICE_ID columns of each table. However, consider the following SQL, which would produce the equivalent results when the OFFICES table’s primary key column was named ID:

SELECT FIRST_NAME, LAST_NAME, CITY, STATE
  FROM USERS u, OFFICES o
 WHERE u.OFFICE_ID = o.ID

This is certainly not as clear; add to this statement joins with five, ten, or even more additional tables (something quite common even in medium-size systems), and the joins between columns in different tables can become a nightmare. In the example’s naming system, columns in one table are always joined to columns in other tables with the same name; there is no room left for mistakes.

However, using this same naming in Java results in some odd code. Consider that it is common to use the lowercased name of a class as the name of an object class. For example, an instance of the class Office is often called office . If the ID method variable is named officeId, this practice can result in the rather strange code fragment shown here:

// Get an instance of the Office class
Integer keyValue = office.getOfficeId(  );

It seems a bit redundant to call getOfficeID( ) on an office; while this might be a meaningful method on an instance of the User class, it doesn’t make a lot of sense on the Office class. Here, this is only a minor annoyance, but it could occur hundreds of times in hundreds of classes in a complete application, becoming quite a nuisance. There are enough annoyances in programming without adding to the list, so you should stick to using database conventions in the database, and Java conventions in the application. It takes a little extra concentration during implementation, but is well worth it in the long run.

So, with no further talk, Example 4-2 is the remote interface for the Office bean.

Example 4-2. The Remote Interface for the Office Bean

package com.forethought.ejb.office;

import java.rmi.RemoteException;
import javax.ejb.EJBObject;

public interface Office extends EJBObject {

    public Integer getId(  ) throws RemoteException;

    public String getCity(  ) throws RemoteException;
    public void setCity(String city) throws RemoteException;

    public String getState(  ) throws RemoteException;
    public void setState(String state) throws RemoteException;
}

Warning

Lest you fall into an ugly trap, be sure not to use a capital “D” in the getId( ) method (calling it getID( ) is incorrect). This rule holds true when looking at the bean implementation class, as well. While you may prefer this style (as I do), it will cause problems in your container’s CMP process. The container converts the first letter of the variable (the “I” in “Id”) to lowercase, takes the resultant name (“id”), and matches that to a member variable. If you use getID( ), you’ll then be forced to use a member variable called “iD”, which is obviously not what you want. So stick with the uppercase-lowercase convention, and save yourself some trouble.

There’s also a growing trend to name remote interfaces <Bean-Name>Remote, so that the remote interface for our office entity bean would be called OfficeRemote. This convention is a response to the local interfaces introduced in EJB 2.0 (which I’ll discuss in the next chapter). However, I’m not a big fan of this, for a couple of reasons. First and foremost, I like to make the most common case the simplest; since beans most commonly have a remote interface, I make the naming of the remote interface the simplest for a client to work with. Why type “OfficeRemote” when 99 out of 100 cases, you can just type “Office”? Then, if a local interface is needed, the name of that class can be OfficeLocal. The one time this name is used instead of the remote interface, the name change is a clear indication of the use of a local interface. So stick with the bean name for your remote interfaces; programmers writing bean clients will thank you for the simplicity later.

The Local Interface

At this point, you need to stop a minute and think about how your bean is going to be used. It’s clear that any application clients that need to work with offices will require the remote interface you just coded. However, because offices are related to users (refer back to Figure 3-9 if you’re unsure of why this is so), you will also have some entity bean-to-entity bean communication. In this case, the overhead of RMI communication becomes unnecessary, and a local interface can improve performance drastically. It’s important to understand that there is nothing to prevent a bean from providing both local interfaces (for inter-bean communication) and remote interfaces (for client-to-bean communication).

It’s also trivial to code the local interface of a bean once you have the remote interface. Example 4-3 shows this interface, and it’s remarkably similar to the remote interface from the previous section. You’ll use this local interface later, in the User bean, which will have a persistence relationship with the Office bean.

Example 4-3. The Office Bean Local Interface

package com.forethought.ejb.office;

import javax.ejb.EJBException;
import javax.ejb.EJBLocalObject;

public interface OfficeLocal extends EJBLocalObject {

    public Integer getId(  ) throws EJBException;

    public String getCity(  ) throws EJBException;
    public void setCity(String city) throws EJBException;

    public String getState(  ) throws EJBException;
    public void setState(String state) throws EJBException;
}

The Primary Key

Primary keys in beans where only one value is used are a piece of cake. In the case of the Office bean, the primary key is the OFFICE_ID column, named simply id in the Java code you’ve seen so far. All you need to do is identify the field used for the primary key in the ejb-jar.xml deployment descriptor (I’ll detail this more fully in a moment). Your entry will look something like this:

<ejb-name>OfficeBean</ejb-name>
<home>com.forethought.ejb.office.OfficeHome</home>
<remote>com.forethought.ejb.office.Office</remote>
<local-home>com.forethought.ejb.office.OfficeLocalHome</local-home>
<local>com.forethought.ejb.office.OfficeLocal</local>
<ejb-class>com.forethought.ejb.office.OfficeBean</ejb-class>
<persistence-type>Container</persistence-type>
<prim-key-class>java.lang.Integer</prim-key-class>
<reentrant>False</reentrant>

<abstract-schema-name>OFFICES</abstract-schema-name>
<cmp-field><field-name>id</field-name></cmp-field>
<cmp-field><field-name>city</field-name></cmp-field>
<cmp-field><field-name>state</field-name></cmp-field>
<primkey-field>id</primkey-field>

If you do come across a case where more than one value is used for a primary key, you can code an actual Java class. However, this situation is fairly rare, so I won’t cover it here. The majority of cases require you to simply add to your deployment descriptor for handling primary keys. You’ll also notice (again) that the java.lang.Integer type is used; as already discussed, EJB containers generally must work in Java object types, rather than in primitives.

The Home Interface

The home interface is also simple to code. For now, the ID of the office to create is passed directly to the create( ) method. Later, you’ll remove that dependency, and the ID will be determined independently of the application client. You also can add the basic finder, findByPrimaryKey( ), which takes in the Integer primary key type. Example 4-4 shows this code listing.

Example 4-4. The Home Interface for the Office Bean

package com.forethought.ejb.office;

import java.rmi.RemoteException;
import javax.ejb.CreateException;
import javax.ejb.EJBHome;
import javax.ejb.FinderException;

public interface OfficeHome extends EJBHome {

    public Office create(Integer id, String city, String state)
        throws CreateException, RemoteException;

    public Office findByPrimaryKey(Integer officeID)
        throws FinderException, RemoteException;
}

Like the remote interface, many folks have taken to calling the remote home interface <Bean-Name>HomeRemote (in this case OfficeHomeRemote), again in deference to local interfaces. And in the same vein, I recommend against it for the same reasons as the remote interface. It’s best to leave the remote home interface as-is, and use OfficeLocalHome as needed.

The Local Home Interface

Just as you coded up a local interface for persistence relationships and bean-to-bean communication, you should create a corresponding local home interface. This is extremely similar to the remote home interface, and bears little discussion. Example 4-5 is the Office bean’s local home interface.

Example 4-5. The Local Home Interface for the Office Bean

package com.forethought.ejb.office;

import javax.ejb.CreateException;
import javax.ejb.EJBException;
import javax.ejb.EJBLocalHome;
import javax.ejb.FinderException;

public interface OfficeLocalHome extends EJBLocalHome {

    public OfficeLocal create(Integer id, String city, String state)
        throws CreateException, EJBException;

    public OfficeLocal findByPrimaryKey(Integer officeID)
        throws FinderException, EJBException;
}

The Bean Implementation

Last, but not least, Example 4-6 is the bean implementation class. Notice that it extends the EntityAdapter class instead of directly implementing EntityBean, like other examples you may find. Because the bean’s persistence is container-managed, the accessor and mutator methods are declared abstract. The container will handle the method implementations that make these updates affect the underlying data store.

Example 4-6. The Implementation for the Office Bean

package com.forethought.ejb.office;

// EJB imports
import javax.ejb.CreateException;

import com.forethought.ejb.util.EntityAdapter;

public abstract class OfficeBean extends EntityAdapter {

    public Integer ejbCreate(Integer id, String city, String state) 
        throws CreateException {

        setId(id);
        setCity(city);
        setState(state);

        return null;
    }

    public void ejbPostCreate(int id, String city, String state) 
        throws CreateException {

        // Empty implementation
    }

    public abstract Integer getId(  );
    public abstract void setId(Integer id);

    public abstract String getCity(  );
    public abstract void setCity(String city);

    public abstract String getState(  );
    public abstract void setState(String state);
}

Take special note of the throws CreateException clause on the ejbCreate( ) and ejbPostCreate( ) methods. I have several books on EJB 2.0 on my desk right now that omit this clause; however, leaving it out causes several application servers, including the J2EE reference implementation, to fail on deployment. Therefore, be sure to have your bean creation methods throw this exception. It also makes sense in that the subclasses of the Office class that the container creates need to be able to report errors during bean creation, and a CreateException gives them that ability. Since a subclass can’t add new exceptions to the method declaration, the throws clause must exist in your bean class.

Also, be sure that your creation methods use the other methods in the class for assignment. A common mistake is to code the ejbCreate( ) method like this:

public Integer ejbCreate(Integer id, String city, String state) 
    throws CreateException {

    this.id = id;
    this.city = city;
    this.state = state;

    return null;
}

This was common in EJB 1.1, but doesn’t work so well in EJB 2.0. You want to be sure that you invoke the container-generated methods, which will handle database access. Invoking the container-generated methods also means you don’t have to explicitly define member variables for the class, so that’s one less detail to worry about. Also note that the creation method invokes setId( ) , which I earlier said wouldn’t be made available to clients. That remains true, because even though it’s in the bean’s implementation class, the remote interface does not expose the method, keeping it hidden from the client.

One final note before moving on: you should notice in this book’s source code (downloadable from http://www.newInstance.com) that the methods in the bean implementation are not commented, as they are in the remote interface. This is a fairly standard practice; methods are commented (and therefore available in Javadoc) in interfaces, but these comments are not duplicated in the implementation, which generally makes implementation classes simpler to move through. If there are details specific to the implementation that need to be documented, they are suitable for commenting; however, such comments usually are made in the code and are preceded with the double-slash (//), rather than being Javadoc-style comments. Such practices are followed in all the EJBs in this chapter and the rest of the book.



[19] Throughout this book, the terms accessor and mutator are used; you may be more familiar with the terms getter and setter. However, as I’m a dog person, and my wife is a veterinary technician, we both realize that a setter is an animal, not a Java term. So an accessor provides access to a variable (the getXXX( ) style methods), and a mutator modifies a variable (the setXXX( ) style methods).

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