Using an Entity to Join Messages from Two MDBs
Java Platform, Enterprise Edition (Java EE) 8
The Java EE Tutorial

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Using an Entity to Join Messages from Two MDBs

This section explains how to write, compile, package, deploy, and run an application that uses the JMS API with an entity. The application uses the following components:

  • An application client that both sends and receives messages

  • Two message-driven beans

  • An entity class

You will find the source files for this section in the tut-install`/examples/jms/clientmdbentity/` directory. Path names in this section are relative to this directory.

The following topics are addressed here:

Overview of the clientmdbentity Example Application

This application simulates, in a simplified way, the work flow of a company’s human resources (HR) department when it processes a new hire. This application also demonstrates how to use the Java EE platform to accomplish a task that many JMS applications need to perform.

A messaging client must often wait for several messages from various sources. It then uses the information in all these messages to assemble a message that it then sends to another destination. The common term for this design pattern (which is not specific to JMS) is joining messages. Such a task must be transactional, with all the receives and the send as a single transaction. If not all the messages are received successfully, the transaction can be rolled back. For an application client example that illustrates this task, see Using Local Transactions.

A message-driven bean can process only one message at a time in a transaction. To provide the ability to join messages, an application can have the message-driven bean store the interim information in a Java Persistence API entity. The entity can then determine whether all the information has been received; when it has, the entity can report this back to one of the message-driven beans, which then creates and sends the message to the other destination. After it has completed its task, the entity can be removed.

The basic steps of the application are as follows.

  1. The HR department’s application client generates an employee ID for each new hire and then publishes a message (M1) containing the new hire’s name, employee ID, and position. It publishes the message to a topic because the message needs to be consumed by two message-driven beans. The client then creates a temporary queue, ReplyQueue, with a message listener that waits for a reply to the message. (See Creating Temporary Destinations for more information.)

  2. Two message-driven beans process each message: One bean, OfficeMDB, assigns the new hire’s office number, and the other bean, EquipmentMDB, assigns the new hire’s equipment. The first bean to process the message creates and persists an entity named SetupOffice, then calls a business method of the entity to store the information it has generated. The second bean locates the existing entity and calls another business method to add its information.

  3. When both the office and the equipment have been assigned, the entity business method returns a value of true to the message-driven bean that called the method. The message-driven bean then sends to the reply queue a message (M2) describing the assignments. Then it removes the entity. The application client’s message listener retrieves the information.

Figure 49-5 illustrates the structure of this application. Of course, an actual HR application would have more components; other beans could set up payroll and benefits records, schedule orientation, and so on.

Figure 49-5 assumes that OfficeMDB is the first message-driven bean to consume the message from the client. OfficeMDB then creates and persists the SetupOffice entity and stores the office information. EquipmentMDB then finds the entity, stores the equipment information, and learns that the entity has completed its work. EquipmentMDB then sends the message to the reply queue and removes the entity.

Figure 49-5 An Enterprise Bean Application: Client to Message-Driven Beans to Entity

Diagram of application showing an application client, two message-driven beans, and an entity, as well as the associated topic and queue

Writing the Application Components for the clientmdbentity Example

Writing the components of the application involves coding the application client, the message-driven beans, and the entity class.

The following topics are addressed here:

Coding the Application Client: HumanResourceClient.java

The application client, HumanResourceClient.java, found under clientmdbentity-app-client, performs the following steps:

  1. Defines a topic for the application, using the java:app namespace because the topic is used in both the application client and the EJB module

  2. Injects ConnectionFactory and Topic resources

  3. Creates a TemporaryQueue to receive notification of processing that occurs, based on new-hire events it has published

  4. Creates a JMSConsumer for the TemporaryQueue, sets the `JMSConsumer’s message listener, and starts the connection

  5. Creates a MapMessage

  6. Creates five new employees with randomly generated names, positions, and ID numbers (in sequence) and publishes five messages containing this information

The message listener, HRListener, waits for messages that contain the assigned office and equipment for each employee. When a message arrives, the message listener displays the information received and determines whether all five messages have arrived. When they have, the message listener notifies the main method, which then exits.

Coding the Message-Driven Beans for the clientmdbentity Example

This example uses two message-driven beans, both under clientmdbentity-ejb:

  • EquipmentMDB.java

  • OfficeMDB.java

The beans take the following steps.

  1. They inject a MessageDrivenContext resource, an EntityManager, and a JMSContext.

  2. The onMessage method retrieves the information in the message. The EquipmentMDB’s `onMessage method chooses equipment, based on the new hire’s position; the OfficeMDB’s `onMessage method randomly generates an office number.

  3. After a slight delay to simulate real world processing hitches, the onMessage method calls a helper method, compose.

  4. The compose method takes the following steps.

  5. It either creates and persists the SetupOffice entity or finds it by primary key.

  6. It uses the entity to store the equipment or the office information in the database, calling either the doEquipmentList or the doOfficeNumber business method.

  7. If the business method returns true, meaning that all of the information has been stored, it retrieves the reply destination information from the message, creates a JMSProducer, and sends a reply message that contains the information stored in the entity.

  8. It removes the entity.

Coding the Entity Class for the clientmdbentity Example

The SetupOffice.java class, also under clientmdbentity-ejb, is an entity class. The entity and the message-driven beans are packaged together in an EJB JAR file. The entity class is declared as follows:

@Entity
public class SetupOffice implements Serializable {

The class contains a no-argument constructor and a constructor that takes two arguments, the employee ID and name. It also contains getter and setter methods for the employee ID, name, office number, and equipment list. The getter method for the employee ID has the @Id annotation to indicate that this field is the primary key:

@Id
public String getEmployeeId() {
    return id;
}

The class also implements the two business methods, doEquipmentList and doOfficeNumber, and their helper method, checkIfSetupComplete.

The message-driven beans call the business methods and the getter methods.

The persistence.xml file for the entity specifies the most basic settings:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
             xmlns="http://xmlns.jcp.org/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence
               http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
  <persistence-unit name="clientmdbentity-ejbPU" transaction-type="JTA">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
    <jta-data-source>java:comp/DefaultDataSource</jta-data-source>
    <properties>
      <property name="eclipselink.ddl-generation"
                value="drop-and-create-tables"/>
    </properties>
  </persistence-unit>
</persistence>

Running the clientmdbentity Example

You can use either NetBeans IDE or Maven to build, deploy, and run the clientmdbentity example.

Because the example defines its own application-private topic and uses the preconfigured default connection factory java:comp/DefaultJMSConnectionFactory and the preconfigured default JDBC resource java:comp/DefaultDataSource, you do not need to create resources for it.

The following topics are addressed here:

To Run clientmdbentity Using NetBeans IDE

  1. Make sure that GlassFish Server has been started (see Starting and Stopping GlassFish Server), as well as the database server (see Starting and Stopping Apache Derby).

  2. From the File menu, choose Open Project.

  3. In the Open Project dialog box, navigate to:

    tut-install/examples/jms/clientmdbentity
  4. Select the clientmdbentity folder.

  5. Click Open Project.

  6. In the Projects tab, right-click the clientmdbentity project and select Build.

    This command creates the following:

    • An application client JAR file that contains the client class and listener class files, along with a manifest file that specifies the main class

    • An EJB JAR file that contains the message-driven beans and the entity class, along with the persistence.xml file

    • An application EAR file that contains the two JAR files along with an application.xml file

      The clientmdbentity.ear file is created in the clientmdbentity-ear/target/ directory.

      The command then deploys the EAR file, retrieves the client stubs, and runs the application client.

To Run clientmdbentity Using Maven

  1. Make sure that GlassFish Server has been started (see Starting and Stopping GlassFish Server), as well as the database server (see Starting and Stopping Apache Derby).

  2. Go to the following directory:

    tut-install/examples/jms/clientmdbentity/
  3. To compile the source files and package, deploy, and run the application, enter the following command:

    mvn install

    This command creates the following:

    • An application client JAR file that contains the client class and listener class files, along with a manifest file that specifies the main class

    • An EJB JAR file that contains the message-driven beans and the entity class, along with the persistence.xml file

    • An application EAR file that contains the two JAR files along with an application.xml file

      The command then deploys the application, retrieves the client stubs, and runs the application client.

Viewing the Application Output

The output in the NetBeans IDE output window or in the terminal window looks something like this (preceded by application client container output and Maven output):

SENDER: Setting hire ID to 50, name Bill Tudor, position Programmer
SENDER: Setting hire ID to 51, name Carol Jones, position Senior Programmer
SENDER: Setting hire ID to 52, name Mark Wilson, position Manager
SENDER: Setting hire ID to 53, name Polly Wren, position Senior Programmer
SENDER: Setting hire ID to 54, name Joe Lawrence, position Director
Waiting for 5 message(s)
New hire event processed:
  Employee ID: 52
  Name: Mark Wilson
  Equipment: Tablet
  Office number: 294
Waiting for 4 message(s)
New hire event processed:
  Employee ID: 53
  Name: Polly Wren
  Equipment: Laptop
  Office number: 186
Waiting for 3 message(s)
New hire event processed:
  Employee ID: 54
  Name: Joe Lawrence
  Equipment: Mobile Phone
  Office number: 135
Waiting for 2 message(s)
New hire event processed:
  Employee ID: 50
  Name: Bill Tudor
  Equipment: Desktop System
  Office number: 200
Waiting for 1 message(s)
New hire event processed:
  Employee ID: 51
  Name: Carol Jones
  Equipment: Laptop
  Office number: 262

The output from the message-driven beans and the entity class appears in the server log.

For each employee, the application first creates the entity and then finds it. You may see runtime errors in the server log, and transaction rollbacks may occur. The errors occur if both of the message-driven beans discover at the same time that the entity does not yet exist, so they both try to create it. The first attempt succeeds, but the second fails because the bean already exists. After the rollback, the second message-driven bean tries again and succeeds in finding the entity. Container-managed transactions allow the application to run correctly, in spite of these errors, with no special programming.

To undeploy the application after you have finished running it, use the Services tab or issue the mvn cargo:undeploy command.


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