Enterprise bean developers are required to provide a bean class, two remote interfaces, and for entity beans, a primary key class. Of these types, the remote interfaces and primary key class are visible to the client, while the bean class is not. The remote interface, home interface, and primary key contribute to the client-side API in EJB. The methods defined in these types as well as the methods of their supertypes provide the mechanisms that clients use to interact with an EJB business system.
The following sections examine in more detail the home interface, the remote interface, and the primary key, as well as other types that make up EJB's client-side API. This will provide you with a better understanding of how the client-side API is used and its relationship with the bean class on the EJB server.
Enterprise JavaBeans 1.0 defines its distributed interfaces in terms of Java RMI. RMI assumes that both the client and server are Java applications, so it takes full advantage of Java types as arguments and return values. Enterprise JavaBeans 1.1 also defines its distributed interfaces in terms of Java RMI, but it enforces compliance with CORBA's interface, reference, and value types by requiring that only Java RMI-IIOP types be used. In other words, the underlying protocol can be anything that the vendor wants as long as it supports the types of interfaces and arguments specified by Java RMI-IIOP. In a future version of EJB, Java RMI-IIOP ( Java RMI over IIOP) will be the required programming model for accessing beans. Requiring partial support for the Java RMI-IIOP standard ensures that early Java RMI-IIOP adopters are supported and makes for a seamless transition for other vendors in the future.
To be CORBA-compliant, Java RMI-IIOP had to restrict the definition of interfaces and arguments to types that map nicely to CORBA. The restrictions are really not all that bad, and you probably won't even notice them while developing your beans, but it's important to know what they are. The next few paragraphs discuss the Java RMI programming model for both EJB 1.0 and EJB 1.1, and point out the additional restrictions placed on RMI-IIOP types after discussing the restrictions shared by traditional Java RMI and Java RMI-IIOP.
The supertypes of the home interface and remote interface, javax.ejb.EJBHome and javax.ejb.EJBObject, both extend java.rmi.Remote. As Remote interface subtypes, they are expected to adhere to the Java RMI specification for Remote interfaces. The Java RMI specification states that every method defined in a Remote interface must throw a java.rmi.RemoteException. The RemoteException is used when problems occur with the distributed object communications, like a network failure or inability to locate the object server. In addition, Remote interface types can throw any application-specific exceptions (exceptions defined by the application developer) that are necessary. The following code shows the remote interface to the TravelAgent bean discussed in Chapter 2, "Architectural Overview". TravelAgent has several remote methods, including bookPassage(). The bookPassage() method can throw a RemoteException (as required), in addition to an application exception, IncompleteConversationalState.
public interface TravelAgent extends javax.ejb.EJBObject {
public void setCruiseID(int cruise) throws RemoteException, FinderException;
public int getCruiseID() throws RemoteException;
public void setCabinID(int cabin) throws RemoteException, FinderException;
public int getCabinID() throws RemoteException;
public int getCustomerID() throws RemoteException;
public Ticket bookPassage(CreditCard card, double price)
throws RemoteException,IncompleteConversationalState;
public String [] listAvailableCabins(int bedCount)
throws RemoteException, IncompleteConversationalState;
}
Java RMI requires that all parameters and return values be either Java primitive types (int, double, byte, etc.) or objects that implement java.io.Serializable. Serializable objects are passed by copy (a.k.a. passed by value), not by reference, which means that changes in a serialized object on one tier are not automatically reflected on the others. Objects that implement Remote, like Customer, Cruise, and Cabin, are passed as remote references--which is a little different. A remote reference is a Remote interface implemented by a distributed object stub. When a remote reference is passed as a parameter or returned from a method, it is the stub that is serialized and passed by value, not the object server remotely referenced by the stub. In the home interface for the TravelAgent bean, the create() method takes a reference to a Customer bean as its only argument.
public interface TravelAgentHome extends javax.ejb.EJBHome {
public TravelAgent create(Customer customer)
throws RemoteException, CreateException;
}
The customer is a remote reference to a Customer bean that is passed into the create() method. When a remote reference is passed or returned in EJB, the EJB object stub is passed by copy. The copy of the EJB object stub points to the same EJB object as the original stub. This results in both the bean instance and the client having remote references to the same EJB object. So changes made on the client using the remote reference will be reflected when the bean instance uses the same remote reference. Figure 5-1 and Figure 5-2 show the difference between a serializable object and a remote reference argument in Java RMI.
In addition to the Java RMI programming model discussed earlier, Java RMI-IIOP imposes additional restrictions on the remote interfaces and value types used in EJB 1.1. These restrictions are born of limitations inherit in the Interface Definition Language (IDL) upon which CORBA IIOP is based. The exact nature of these limitations is outside the scope of this book. I have only listed two restrictions because the others, like IDL name collisions, are so rarely encountered that it wouldn't be constructive to mention them.[1]
[1]To learn more about CORBA IDL and its mapping to the Java language consult The Common Object Request Broker: Architecture and Specification and The Java Language to IDL Mapping available at the OMG site (www.omg.org).
Method overloading is restricted; a remote interface may not directly extend two or more interfaces that have methods with the same name (even if their arguments are different). A remote interface may, however, overload its own methods and extend a remote interface with overloaded method names. Overloading is viewed, here, as including overriding. Figure 5-3 illustrates both of these situations.
Serializable types must not directly or indirectly implement the java.rmi.Remote interface.
A significant difference between EJB 1.0 and EJB 1.1 is that the new specification requires that remote references be explicitly narrowed using the javax.rmi.PortableRemoteObject.narrow() method. The typical practice in Java would be to cast the reference to the more specific type, as follows:
javax.naming.Context jndiContext;
...
CabinHome home = (CabinHome)jndiContext.lookup("CabinHome");
The javax.naming.Context.lookup() method returns an Object. In EJB 1.0, which uses simple Java RMI, we can assume that it is legal to cast the return argument to a legal Java type. However, EJB 1.1 must be compatible with Java RMI-IIOP, which means that clients must adhere to limitations imposed by the IIOP protocol. IIOP is not specific to any one programming language. As part of the CORBA standard, it must accommodate many programming languages including C++, Ada, COBOL, and others. All programming languages do not support casting, and for this reason casting is not native to IIOP. In fact, some languages have no concept of polymorphism or inheritance (COBOL for example), so implicit casting in CORBA is out of the question.
To accommodate all languages, IIOP does not support stubs that implement multiple interfaces. The stub returned in IIOP implements only the interface specified by the return type of the remote method that was invoked. If the return type is Object, as is the remote reference returned by the lookup() method, the stub will only implement methods specific to the Object type.
Of course, some means for converting a remote reference from a more general type to a more specific type is essential in an object-oriented environment. CORBA provides a mechanism for explicitly narrowing references to a specific type. The javax.rmi.PortableRemoteObject.narrow() method abstracts this mechanism to provide narrowing in IIOP as well as other protocols. Remember while Java RMI-IIOP defines the reference and argument types, in EJB 1.1 it does not define the underlying protocol. Other protocols besides IIOP may also require explicit narrowing. The PortableRemoteObject abstracts the narrowing process so that any protocol can be used.
To narrow the return argument of the Context.lookup() method to the appropriate type, we must explicitly ask for a remote reference that implements the interface we want:
import javax.rmi.PortableRemoteObject;
...
javax.naming.Context jndiContext;
...
Object ref = jndiContext.lookup("CabinHome");
CabinHome home = (CabinHome)
PortableRemoteObject.narrow(ref, CabinHome.class);
When the narrow() method has successfully executed, it returns a stub that implements the Remote interface specified. Because the stub is known to implement the correct type, you can now use Java's native casting to narrow the stub to the correct Remote interface. The narrow() method takes two arguments: the remote reference that is to be narrowed and the type it should be narrowed to. The definition of the narrow() method is:[2]
[2]Other methods included in the PortableRemoteObject class are not important to EJB application developers. They are intended for Java RMI developers.
package javax.rmi;
public class PortableRemoteObject extends java.lang.Object {
public static java.lang.Object narrow(java.lang.Object narrowFrom,
java.lang.Class narrowTo)
throws java.lang.ClassCastException;
...
}
The narrow() method only needs to be used when a remote reference to an EJB home or EJB object is returned without a specific Remote interface type. This occurs in six circumstances:
When an EJB home reference is obtained using the javax.naming.Context.lookup() method:
Object ref = jndiContext.lookup("CabinHome");
CabinHome home = (CabinHome) PortableRemoteObject.narrow(ref, CabinHome.class);
Handle handle = .... // get handle Object ref = handle.getEJBObject(); Cabin cabin = (Cabin) PortableRemoteObject.narrow(ref,Cabin.class);
HomeHandle homeHdle = ... // get home handle
EJBHome ref = homeHdle.getEJBHome();
CabinHome home = (CabinHome)
PortableRemoteObject.narrow(ref, CabinHome.class);
EJBMetaData metaData = homeHdle.getEJBMetaData(); EJBHome ref = metaData.getEJBHome(); CabinHome home = (CabinHome) PortableRemoteObject.narrow(ref, CabinHome.class);
When an EJB object reference is obtained from a collection returned by a Home interface finder method:
ShipHome shipHome = ... // get ship home
Enumeration enum = shipHome.findByCapacity(2000);
while(enum.hasMoreElements()){
Object ref = enum.nextElement();
Ship ship = (Ship) PortableRemoteObject.narrow(ref, Ship.class);
// do something with Ship reference
}
// Officer extends Crewman
Ship ship = // get Ship remote reference
Crewman crew = ship.getCrewman("Burns", "John", "1st Lieutenant");
Officer burns = (Officer) PortableRemoteObject.narrow(crew, Officer.class);
The PortableRemoteObject.narrow()method is not required when the remote type is specified in the method signature. This is true of the create() methods and find methods that return a single bean. For example, the create() and findByPrimaryKey() methods defined in the CabinHome interface (Chapter 4, "Developing Your First Enterprise Beans") do not require the use of narrow() because these methods already return the correct EJB object type. Business methods that return the correct type do not need to use the narrow() method either, as the following code illustrates:
/* The CabinHome.create() method specifies
* the Cabin remote interface as the return type
* so explicit narrowing is not needed.*/
Cabin cabin = cabinHome.create(12345);
/* The CabinHome.findByPrimaryKey() method specifies
* the Cabin remote interface as the return type
* so explicit narrowing is not needed.*/
Cabin cabin = cabinHome.findByPrimaryKey(12345);
/* The Ship.getCrewman() business method specifies
* the Crewman remote interface as the return type
* so explicit narrowing is not needed.*/
Crewman crew = ship.getCrewman("Burns", "John", "1st Lieutenant");
The home interface provides life-cycle operations and metadata for the bean. When you use JNDI to access a bean, you obtain a remote reference, or stub, to the bean's EJB home, which implements the home interface. Every bean type has one home interface, which extends the javax.ejb.EJBHome interface.
Here is the EJBHome interface for EJB 1.1:
// EJB 1.1
public interface javax.ejb.EJBHome extends java.rmi.Remote {
public abstract EJBMetaData getEJBMetaData()
throws RemoteException;
public HomeHandle getHomeHandle() // new in 1.1
throws RemoteException;
public abstract void remove(Handle handle)
throws RemoteException, RemoveException;
public abstract void remove(Object primaryKey)
throws RemoteException, RemoveException;
}
EJB 1.1 adds the getHomeHandle() method for accessing the HomeHandle, which doesn't exist in EJB 1.0:
// EJB 1.0
public interface javax.ejb.EJBHome extends java.rmi.Remote {
public abstract EJBMetaData getEJBMetaData()
throws RemoteException;
public abstract void remove(Handle handle)
throws RemoteException, RemoveException;
public abstract void remove(Object primaryKey)
throws RemoteException, RemoveException;
}
The EJBHome.remove() methods are responsible for deleting a bean. The argument is either the javax.ejb.Handle of the bean or, if it's an entity bean, its primary key. The Handle will be discussed in more detail later, but it is essentially a serializable pointer to a specific bean. When either of the EJBHome.remove() methods are invoked, the remote reference to the bean on the client becomes invalid: the stub to the bean that was removed no longer works. If for some reason the bean can't be removed, a RemoveException is thrown.
The impact of the EJBHome.remove() on the bean itself depends on the type of bean. For session beans, the EJBHome.remove() methods end the session's service to the client. When EJBHome.remove() is invoked, the remote reference to the session beans becomes invalid, and any conversational state maintained by the bean is lost. The TravelAgent bean is stateless, so no conversational state exists (more about this in Chapter 7, "Session Beans").
When a remove() method is invoked on an entity bean, the remote reference becomes invalid, and any data that it represents is actually deleted from the database. This is a far more destructive activity because once an entity bean is removed, the data that it represents no longer exists. The difference between using a remove() method on a session bean and using remove() on an entity bean is similar to the difference between hanging up on a telephone conversation and actually killing the caller on the other end. Both end the conversation, but the end results are a little different.
The following code fragment is taken from the main() method of a client application that is similar to the clients we created to exercise the Cabin and TravelAgent beans. It shows that you can remove beans using a primary key (entity only) or a handle. Removing an entity bean deletes the entity from the database; removing a session bean results in the remote reference becoming invalid.
Context jndiContext = getInitialContext();
// Obtain a list of all the cabins for ship 1 with bed count of 3.
// EJB 1.0: Use native cast instead of narrow()
Object ref = jndiContext.lookup("TravelAgentHome");
TravelAgentHome agentHome = (TravelAgentHome)
PortableRemoteObject.narrow(ref,TravelAgentHome.class);
TravelAgent agent = agentHome.create();
String list [] = agent.listCabins(1,3);
System.out.println("1st List: Before deleting cabin number 30");
for(int i = 0; i < list.length; i++){
System.out.println(list[i]);
}
// Obtain the CabinHome and remove cabin 30. Rerun the same cabin list.
// EJB 1.0: Use native cast instead of narrow()
ref = jndiContext.lookup("CabinHome");
CabinHome c_home = (CabinHome)
PortableRemoteObject.narrow(ref, CabinHome.class);
CabinPK pk = new CabinPK();
pk.id = 30;
c_home.remove(pk);
list = agent.listCabins(1,3);
System.out.println("2nd List: After deleting cabin number 30");
for (int i = 0; i < list.length; i++) {
System.out.println(list[i]);
}
Your output should look something like the following:
1st List: Before deleting cabin number 30 1,Master Suite ,1 3,Suite 101 ,1 5,Suite 103 ,1 7,Suite 105 ,1 9,Suite 107 ,1 12,Suite 201 ,2 14,Suite 203 ,2 16,Suite 205 ,2 18,Suite 207 ,2 20,Suite 209 ,2 22,Suite 301 ,3 24,Suite 303 ,3 26,Suite 305 ,3 28,Suite 307 ,3 30,Suite 309 ,3 2nd List: After deleting cabin number 30 1,Master Suite ,1 3,Suite 101 ,1 5,Suite 103 ,1 7,Suite 105 ,1 9,Suite 107 ,1 12,Suite 201 ,2 14,Suite 203 ,2 16,Suite 205 ,2 18,Suite 207 ,2 20,Suite 209 ,2 22,Suite 301 ,3 24,Suite 303 ,3 26,Suite 305 ,3 28,Suite 307 ,3
First, we create a list of cabins, including the cabin with the primary key 30. Then we remove the Cabin bean with this primary key and create the list again. The second time through, cabin 30 is not listed. Because it was removed, the listCabin() method was unable to find a cabin with a CabinPK.id equal to 30, so it stopped making the list. The bean, including its data, is no longer in the database.
EJBHome.getEJBMetaData() returns an instance of javax.ejb.EJBMetaData that describes the home interface, remote interface, and primary key classes, plus whether the bean is a session or entity bean. This type of metadata is valuable to Java tools like IDEs that have wizards or other mechanisms for interacting with a bean from a client's perspective. A tool could, for example, use the class definitions provided by the EJBMetaData with Java reflection to create an environment where deployed beans can be "wired" together by developers. Of course, information such as the JNDI names and URLs of the beans is also needed.
Most application developers rarely use the EJBMetaData. Knowing that it's there, however, is valuable when you need to create automatic code generators or some other automatic facility. In those cases, familiarity with the Reflection API is necessary.[3] The following code shows the interface definition for EJBMetaData. Any class that implements the EJBMetaData interface must be serializable; it cannot be a stub to a distributed object. This allows IDEs and other tools to save the EJBMetaData for later use.
[3] The Reflection API is outside the scope of this book, but it is covered in Java™ in a Nutshell, by David Flanagan (O'Reilly).
public interface javax.ejb.EJBMetaData {
public abstract EJBHome getEJBHome();
public abstract Class getHomeInterfaceClass();
public abstract Class getPrimaryKeyClass();
public abstract Class getRemoteInterfaceClass();
public abstract boolean isSession();
}
The following code shows how the EJBMetaData for the Cabin bean could be used to get more information about the bean. Notice that there is no way to get the bean class using the EJBMetaData; the bean class is not part of the client API and therefore doesn't belong to the metadata.
Context jndiContext = getInitialContext();
// EJB 1.0: Use native cast instead of narrow()
Object ref = jndiContext.lookup("CabinHome");
CabinHome c_home = (CabinHome)
PortableRemoteObject.narrow(ref, CabinHome.class);
EJBMetaData meta = c_home.getEJBMetaData();
System.out.println(meta.getHomeInterfaceClass().getName());
System.out.println(meta.getRemoteInterfaceClass().getName());
System.out.println(meta.getPrimaryKeyClass().getName());
System.out.println(meta.isSession());
This creates output like the following:
com.titan.cabin.CabinHome com.titan.cabin.Cabin com.titan.cabin.CabinPK false
In addition to providing the class types of the bean, the EJBMetaData also makes available the EJB home for the bean. By obtaining the EJB home from the EJBMetaData, we can obtain references to the EJB object and perform other functions. In the following code, we use the EJBMetaData to get the primary key class, create a key instance, obtain the EJB home, and from it, get a remote reference to the EJB object for a specific cabin entity:
CabinPK pk = (CabinPK)meta.getPrimaryKeyClass().newInstance();
pk.id = 1;
// EJB 1.0: Use native cast instead of narrow()
Object ref = meta.getEJBHome();
CabinHome c_home2 = (CabinHome)
PortableRemoteObject.narrow(ref,CabinHome.class);
Cabin cabin = c_home2.findByPrimaryKey(pk);
System.out.println(cabin.getName());
EJB 1.1 provides a new object called a HomeHandle, which is accessed by calling the EJBObject.getHomeHandle() method. This method returns a javax.ejb.HomeHandle object, which provides a serializable reference to a bean home. The HomeHandle allows a remote home reference to be stored and used later. It is similar to the javax.ejb.Handle and is discussed in more detail near the end of the chapter.
In addition to the standard javax.ejb.EJBHome methods that all home interfaces inherit, home interfaces also include special create and find methods for the bean. We have already talked about create and find methods, but a little review will solidify your understanding of the home interface's role in the client-side API. The following code shows the home interface defined for the Cabin bean:
public interface CabinHome extends javax.ejb.EJBHome {
public Cabin create(int id)
throws CreateException, RemoteException;
public Cabin findByPrimaryKey(CabinPK pk)
throws FinderException, RemoteException;
}
Create methods throw a CreateException if something goes wrong during the creation process; find methods throw a FinderException if the requested bean can't be located. Since these methods are defined in an interface that subclasses Remote, they must also declare that they throw RemoteException.
The create and find methods are specific to the bean, so it is up to the bean developer to define the appropriate create and find methods in the home interface. CabinHome currently has only one create method that creates a cabin with a specified ID and a find method that looks up a bean given its primary key, but it's easy to imagine methods that would create and find a cabin with particular properties--for example, a cabin with three beds, or a deluxe cabin with blue wallpaper. Unlike entity beans, the home interfaces for session beans do not have find methods. Entity beans represent unique identifiable data within a database and therefore can be found. Session beans, on the other hand, do not represent data: they are created to serve a client application and are not persistent, so there is nothing to find. A find method for a session bean would be meaningless.
The create and find methods defined in the home interfaces are straightforward and can be easily employed by the client. The create methods on the home interface have to match the ejbCreate() methods on the bean class. A create() and ejbCreate() method match when they have the same parameters, when the arguments are of same type and in the same order. This way, when a client calls the create method on the home interface, the call can be delegated to the corresponding ejbCreate() method on the bean instance. The find methods in the home interface work similarly for bean-managed entities. Every find method in the home interface must correspond to an ejbFind() method in the bean itself. Container-managed entities do not implement ejbFind() methods in the bean class; the EJB container supports find methods automatically. You will discover more about how to implement the ebjCreate() and ejbFind() methods in the bean in Chapters Chapter 6, "Entity Beans" and Chapter 7, "Session Beans".
The business methods of an enterprise bean are defined by the remote interface provided by the bean developer. The javax.ejb.EJBObject interface, which extends the java.rmi.Remote interface, is the base class for all remote interfaces.
The following code is the remote interface for the TravelAgent bean that we developed in Chapter 4, "Developing Your First Enterprise Beans":
public interface TravelAgent extends javax.ejb.EJBObject {
public String [] listCabins(int shipID, int bedCount)
throws RemoteException;
}
Figure 5-4 shows the TravelAgent interface's inheritance hierarchy.
Remote interfaces are focused on the business problem and do not include methods for system-level operations such as persistence, security, concurrency, or transactions. System-level operations are handled by the EJB server, which relieves the client developer of many responsibilities. All remote interface methods for beans must throw, at the very least, a java.rmi.RemoteException, which identifies problems with distributed communications. In addition, methods in the remote interface can throw as many custom exceptions as needed to indicate abnormal business-related conditions or errors in executing the business method. You will learn more about defining custom exceptions in Chapter 6, "Entity Beans".
All remote interfaces extend the javax.ejb.EJBObject interface, which provides a set of utility methods and return types. These methods and return types are valuable in managing the client's interactions with beans. Here is the definition for the EJBObject interface:
public interface javax.ejb.EJBObject extends java.rmi.Remote {
public abstract EJBHome getEJBHome()
throws RemoteException;
public abstract Handle getHandle()
throws RemoteException;
public abstract Object getPrimaryKey()
throws RemoteException;
public abstract boolean isIdentical(EJBObject obj)
throws RemoteException;
public abstract void remove()
throws RemoteException, RemoveException;
}
When the client obtains a reference to the remote interface, it is actually obtaining a remote reference to an EJB object. The EJB object implements the remote interface by delegating business method calls to the bean class; it provides its own implementations for the EJBObject methods. These methods return information about the corresponding bean instance on the server. As discussed in Chapter 2, "Architectural Overview", the EJB object is automatically generated when deploying the bean in the EJB server, so the bean developer doesn't need to write an EJBObject implementation.
The getEJBHome() method returns a remote reference to the EJB home for the bean. The remote reference is returned as a javax.ejb.EJBHome object, which can be narrowed or cast to the specific bean's home interface. This method is useful when an EJB object has left the scope of the EJB home that manufactured it. Because remote references can be passed as references and returned from methods, like any other Java object on the client, a remote reference can quickly find itself in a completely different part of the application from its home. The following code is contrived, but it illustrates how a remote reference can move out of the scope of its home and how getEJBHome() can be used to get a new reference to the EJB home at any time:
public static void main(String [] args) {
try {
Context jndiContext = getInitialContext();
// EJB 1.0: Use native cast instead of narrow()
Object ref = jndiContext.lookup("TravelAgentHome");
TravelAgentHome home = (TravelAgentHome)
PortableRemoteObject.narrow(ref,TravelAgentHome.class);
// Get a remote reference to the bean (EJB object).
TravelAgent agent = home.create();
// Pass the remote reference to some method.
getTheEJBHome(agent);
} catch (java.rmi.RemoteException re){re.printStackTrace();}
catch (Throwable t){t.printStackTrace();}
}
public static void getTheEJBHome(TravelAgent agent)
throws RemoteException {
// The home interface is out of scope in this method,
// so it must be obtained from the EJB object.
// EJB 1.0: Use native cast instead of narrow()
Object ref = agent.getEJBHome();
TravelAgentHome home = (TravelAgentHome)
PortableRemoteObject.narrow(ref,TravelAgentHome.class);
// Do something useful with the home interface.
}
EJBObject.getPrimaryKey() returns the primary key for a bean. This method is only supported by EJB objects that represent entity beans. Entity beans represent specific data that can be identified using this primary key. Session beans represent tasks or processes, not data, so a primary key would be meaningless. To better understand the nature of a primary key, we need to look beyond the boundaries of the client's view into the EJB container's layer, which was introduced in Chapters Chapter 2, "Architectural Overview" and Chapter 3, "Resource Management and the Primary Services".
The EJB container is responsible for persistence of the entity beans, but the exact mechanism for persistence is up to the vendor. In order to locate an instance of a bean in a persistent store, the data that makes up the entity must be mapped to some kind of unique key. In relational databases, data is uniquely identified by one or more column values that can be combined to form a primary key. In an object-oriented database, the key wraps an object ID (OID) or some kind of database pointer. Regardless of the mechanism--which isn't really relevant from the client's perspective--the unique key for an entity bean's data is encapsulated by the primary key, which is returned by the EJBObject.getPrimaryKey() method.
The primary key can be used to obtain remote references to entity beans using the findByPrimaryKey() method on the home interface. From the client's perspective, the primary key object can be used to identify a unique entity bean. Understanding the context of a primary key's uniqueness is important, as the following code shows:
Context jndiContext = getInitialContext()
// EJB 1.0: Use native cast instead of narrow()
Object ref = jndiContext.lookup("CabinHome");
CabinHome home = (CabinHome)
PortableRemoteObject.narrow(ref,CabinHome.class);
Cabin cabin_1 = home.create(101);
CabinPK pk = (CabinPK)cabin_1.getPrimaryKey();
Cabin cabin_2 = home.findByPrimaryKey(pk);
In this code, the client creates a Cabin, retrieves the primary key of that Cabin, and then uses the key to get a new reference to the Cabin. Thus, we have two local variables, cabin_1 and cabin_2, which are remote references to EJB objects. These both reference the same Cabin bean, with the same underlying data, because they have the same primary key.
The primary key must be used for the correct bean in the correct container. If, for example, you were to obtain a primary key from a Cabin EJB object and then try to use that key in the findByPrimaryKey() method of a different bean type, like a Ship bean, it wouldn't work; either it wouldn't compile or you would get a runtime error or FinderException . While this seems fairly obvious, the primary key's relationship to a specific container and home interface is important. The primary key can only be guaranteed to return the same entity if it is used within the container that produced the key. As an example, imagine that a third-party vendor sells the Cabin bean as a product. The vendor sells the Cabin bean to both Titan and to a competitor. Both companies deploy the bean using their own relational databases with their own data. A CabinPK primary key with an id value of 20 in Titan's EJB system will not map to the same Cabin entity in the competitor's EJB system. Both cruise companies have a Cabin bean with an id equal to 20, but they represent different cabins for different ships. The Cabin beans come from different EJB containers, so their primary keys are not equivalent.
Sun Microsystems' Enterprise JavaBeans™ Specification, Versions 1.0 and 1.1, describes the primary key and object identity in the following way:
Every entity EJB object has a unique identity with its home....If two EJB objects have the same home and same primary key, they are considered identical.
A primary key must implement the java.io.Serializable interface. This means that the primary key, regardless of its form, can be obtained from an EJB object, stored on the client using the Java serialization mechanism, and deserialized when needed. When a primary key is deserialized, it can be used to obtain a remote reference to that entity using findByPrimaryKey(), provided that the key is used on the right home interface and container. Preserving the primary key using serialization might be useful if the client application needs to access specific entity beans at a later date. In EJB 1.0, preserving the primary keys is also useful for beans that maintain relationships to other beans. Bean relationships are discussed in more detail in Chapter 9, "Design Strategies".
The following code shows a primary key that is serialized and then deserialized to reobtain a remote reference to the same bean:
// Obtain cabin 101 and set its name.
Context jndiContext = getInitialContext();
// EJB 1.0: Use native cast instead of narrow()
Object ref = jndiContext.lookup("CabinHome");
CabinHome home = (CabinHome)
PortableRemoteObject.narrow(ref, CabinHome.class);
CabinPK pk_1 = new CabinPK();
pk_1.id = 101;
Cabin cabin_1 = home.findByPrimaryKey(pk_1);
cabin_1.setName("Presidential Suite");
// Serialize the primary key for cabin 101 to a file.
FileOutputStream fos = new FileOutputStream("pk101.ser");
ObjectOutputStream outStream = new ObjectOutputStream(fos);
outStream.writeObject(pk_1);
outStream.flush();
outStream.close();
pk_1 = null;
// Deserialize the primary key for cabin 101.
FileInputStream fis = new FileInputStream("pk101.ser");
ObjectInputStream inStream = new ObjectInputStream(fis);
CabinPK pk_2 = (CabinPK)inStream.readObject();
inStream.close();
// Re-obtain a remote reference to cabin 101 and read its name.
Cabin cabin_2 = home.findByPrimaryKey(pk_2);
System.out.println(cabin_2.getName());
The EJBObject.isIdentical() method compares two EJB object remote references. It's worth considering why Object.equals() isn't sufficient for comparing EJB objects. An EJB object is a distributed object stub and therefore contains a lot of networking and other state. As a result, references to two EJB objects may be unequal, even if they both represent the same unique bean. The EJBObject.isIdentical() method returns true if two EJB object references represent the same bean, even if the EJB object stubs are different object instances.
The following code shows how this might work. It starts by creating two remote references to the TravelAgent bean. These EJB objects both refer to the same type of bean; comparing them with isIdentical() returns true. The two TravelAgent beans were created separately, but because they are stateless they are considered to be equivalent. If TravelAgent had been a stateful bean (which it becomes in Chapter 7, "Session Beans") the outcome would have been very different. Comparing two stateful beans in this manner will result in false because stateful beans have conversational state, which makes them unique. When we use CabinHome.findByPrimaryKey() to locate two EJB objects that refer to the same Cabin entity bean, we know the beans are identical, because we used the same primary key. In this case, isIdentical() also returns true because both EJB object references point to the same entity.
Context ctx = getInitialContext();
// EJB 1.0: Use native cast instead of narrow()
Object ref = ctx.lookup("TravelAgentHome");
TravelAgentHome agentHome =(TravelAgentHome)
PortableRemoteObject.narrow(ref, TravelAgentHome.class);
TravelAgent agent_1 = agentHome.create();
TravelAgent agent_2 = agentHome.create();
boolean x = agent_1.isIdentical(agent_2);
// x will equal true; the two EJB objects are equal.
// EJB 1.0: Use native cast instead of narrow()
ref = ctx.lookup("CabinHome");
CabinHome c_home = (CabinHome)
PortableRemoteObject.narrow(ref, CabinHome.class);
CabinPK pk = new CabinPK();
pk.id = 101;
Cabin cabin_1 = c_home.findByPrimaryKey(pk);
Cabin cabin_2 = c_home.findByPrimaryKey(pk);
x = cabin_1.isIdentical(cabin_2);
// x will equal true; the two EJB objects are equal.
The primary key used in the Cabin bean is simple. More complex primary keys require us to override Object.equals() and Object.hashCode() in order for the EJBObject.isIdentical() method to work. Chapter 6, "Entity Beans" discusses this in more detail.
The EJBObject.remove() method is used to remove the session or entity bean. The impact of this method is the same as the EJBHome.remove() method discussed previously. For session beans, remove() causes the session to be released and the EJB object reference to become invalid. For entity beans, the actual entity data is deleted from the database and the remote reference becomes invalid. The following code shows the EJBObject.remove() method in use:
Context jndiContext = getInitialContext();
// EJB 1.0: Use native cast instead of narrow()
Object ref = jndiContext.lookup("CabinHome");
CabinHome c_home = (CabinHome)
PortableRemoteObject.narrow(ref,CabinHome.class);
CabinPK pk = new CabinPK();
pk.id = 101;
Cabin cabin = c_home.findByPrimaryKey(pk);
cabin.remove();
The remove() method throws a RemoveException if for some reason the reference can't be deleted.
The EJBObject.getHandle() method returns a javax.ejb.Handle object. The Handle is a serializable reference to the EJB object. This means that the client can save the Handle object using Java serialization and then deserialize it to reobtain a reference to the same EJB object. This is similar to serializing and reusing the primary key. The Handle allows us to recreate an EJB object remote reference that points to the same type of session bean or the same unique entity bean that the handle came from.
Here is the interface definition of the Handle:
public interface javax.ejb.Handle {
public abstract EJBObject getEJBObject()
throws RemoteException;
}
The Handle interface specifies only one method, getEJBObject(). Calling this method returns the EJB object from which the handle was created. Once you've gotten the object back, you can narrow or cast it to the appropriate remote-interface type. The following code shows how to serialize and deserialize the EJB Handle on a client:
// Obtain cabin 100.
Context jndiContext = getInitialContext();
// EJB 1.0: Use native cast instead of narrow()
Object ref = jndiContext.lookup("CabinHome");
CabinHome home = (CabinHome)
PortableRemoteObject.narrow(ref,CabinHome.class);
CabinPK pk_1 = new CabinPK();
pk_1.id = 100;
Cabin cabin_1 = home.findByPrimaryKey(pk_1);
// Serialize the Handle for cabin 100 to a file.
Handle handle = cabin_1.getHandle();
FileOutputStream fos = new FileOutputStream("handle100.ser");
ObjectOutputStream outStream = new ObjectOutputStream(fos);
outStream.writeObject(handle);
outStream.flush();
fos.close();
handle = null;
// Deserialize the Handle for cabin 100.
FileInputStream fis = new FileInputStream("handle100.ser");
ObjectInputStream inStream = new ObjectInputStream(fis);
handle = (Handle)inStream.readObject();
fis.close();
// Reobtain a remote reference to cabin 100 and read its name.
// EJB 1.0: Use native cast instead of narrow()
ref = handle.getEJBObject();
Cabin cabin_2 = (Cabin)
PortableRemoteObject.narrow(ref, Cabin.class);
System.out.println(cabin_2.getName());
At first glance, the Handle and the primary key appear to do the same thing, but in truth they are very different. Using the primary key requires you to have the correct EJB home--if you no longer have a reference to the EJB home, you must look up the container using JNDI and get a new home. Only then can you call findByPrimaryKey() to locate the actual bean. The following code shows how this might work:
// Obtain the primary key from an input stream.
CabinPK primaryKey = (CabinPK)inStream.readObject();
// The JNDI API is used to get a root directory or initial context.
javax.naming.Context ctx = new javax.naming.InitialContext();
// Using the initial context, obtain the EJBHome for the Cabin bean.
// EJB 1.0: Use native cast instead of narrow()
Object ref = ctx.lookup("CabinHome");
CabinHome home = (CabinHome)
PortableRemoteObject.narrow(ref,CabinHome.class);
// Obtain a reference to an EJB object that represents the entity instance.
Cabin cabin_2 = CabinHome.findByPrimaryKey(primaryKey);
The Handle object is easier to use because it encapsulates the details of doing a JNDI lookup on the container. With a Handle, the correct EJB object can be obtained in one method call, Handle.getEJBObject(), rather than using the three method calls required to look up the context, get the home, and find the actual bean.
Furthermore, while the primary key can be used to obtain remote references to unique entity beans, it is not available for session beans; a handle can be used with either type of enterprise bean. This makes using a handle more consistent across bean types. Consistency is, of course, good in its own right, but it isn't the whole story. Normally, we think of session beans as not having identifiable instances because they exist for only the life of the client session, but this is not exactly true. We have mentioned (but not yet shown) stateful session beans, which retain state information between method invocations. With stateful session beans, two instances are not equivalent. A handle allows you to work with a stateful session bean, deactivate the bean, and then reactivate it at a later time using the handle.
A client could, for example, be using a stateful session bean to process an order when the process needs to be interrupted for some reason. Instead of losing all the work performed in the session, a handle can be obtained from the EJB object and the client application can be closed down. When the user is ready to continue the order, the handle can be used to obtain a reference to the stateful session EJB object. Note that this process is not as fault tolerant as using the handle or primary key of an entity object. If the EJB server goes down or crashes, the stateful session bean will be lost and the handle will be useless. It's also possible for the session bean to time out, which would cause the container to remove it from service so that it is no longer available to the client.
Changes to the container technology can invalidate both handles and primary keys. If you think your container technology might change, be careful to take this limitation into account. Primary keys obtain EJB objects by providing unique identification of instances in persistent data stores. A change in the persistence mechanism, however, can impact the integrity of the key.
The javax.ejb.HomeHandle is similar in purpose to javax.ejb.Handle. Just as the Handle is used to store and retrieve references to EJB objects, the HomeHandle is used to store and retrieve remote references to EJB homes. In other words, the HomeHandle can be stored and later used to access an EJB home's remote reference the same way that a Handle can be serialized and later used to access an EJB object's remote reference. The HomeHandle and the method EJBHome.getHomeHandle() are new to EJB 1.1. The following code shows how the HomeHandle can be obtained, serialized, and used.
// Obtain cabin 100.
Context jndiContext = getInitialContext();
// EJB 1.0: Use native cast instead of narrow()
Object ref = jndiContext.lookup("CabinHome");
CabinHome home = (CabinHome)
PortableRemoteObject.narrow(ref,CabinHome.class);
// Serialize the HomeHandle for the cabin bean.
HomeHandle homeHandle = home.getHomeHandle();
FileOutputStream fos = new FileOutputStream("handle.ser");
ObjectOutputStream outStream = new ObjectOutputStream(fos);
outStream.writeObject(homeHandle);
outStream.flush();
fos.close();
homeHandle = null;
// Deserialize the HomeHandle for the cabin bean.
FileInputStream fis = new FileInputStream("handle.ser");
ObjectInputStream inStream = new ObjectInputStream(fis);
homeHandle = (HomeHandle)inStream.readObject();
fis.close();
// EJB 1.0: Use native cast instead of narrow()
EJBHome home = homeHandle.getEJBHome();
CabinHome home2 = (CabinHome)
PortableRemoteObject.narrow(home,CabinHome.class);
Different vendors define their concrete implementations of the EJB handle differently. However, thinking about a hypothetical implementation of handles will give you a better understanding of how they work. In this example, we define the implementation of a handle for an entity bean. Our implementation encapsulates the JNDI lookup and use of the home's findByPrimaryKey() method so that any change that invalidates the key invalidates preserved handles that depend on that key. Here's the code for our hypothetical implementation of a Handle:
package com.titan.cabin;
import javax.naming.InitialContext;
import javax.naming.Context;
import javax.naming.NamingException;
import javax.ejb.EJBObject;
import javax.ejb.Handle;
import java.rmi.RemoteException;
import java.util.Properties;
public class VendorX_CabinHandle
implements javax.ejb.Handle, java.io.Serializable {
private CabinPK primary_key;
private String home_name;
private Properties jndi_properties;
public VendorX_CabinHandle(CabinPK pk, String hn, Properties p) {
primary_key = pk;
home_name = hn;
jndi_properties = p;
}
public EJBObject getEJBObject() throws RemoteException {
try {
Context ctx = new InitialContext(jndi_properties);
// EJB 1.0: Use native cast instead of narrow()
Object ref = ctx.lookup(home_name);
CabinHome home =(CabinHome)
PortableRemoteObject.narrow(ref,CabinHome.class);
return home.findByPrimaryKey(primary_key);
} catch (javax.ejb.FinderException fe) {
throw new RemoteException("Cannot locate EJB object",fe);
} catch (javax.naming.NamingException ne) {
throw new RemoteException("Cannot locate EJB object",ne);
}
}
}
The Handle is less stable than the primary key because it relies on the networking configuration and naming--the IP address of the EJB server and the JNDI name of the bean's home--to remain stable. If the EJB server's network address changes or the name used to identify the home changes, the handle becomes useless.
In addition, some vendors choose to implement a security mechanism in the handle that prevents its use outside the scope of the client application that originally requested it. How this mechanism would work is unclear, but the security limitation it implies should be considered before attempting to use a handle outside the client's scope.
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| 5.1. Locating Beans with JNDI |  | 6. Entity Beans |
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