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Messaging and developing MDBs
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as you need them to be. They can include literals, identifiers, whitespace, expressions, standard brackets, logical and comparison operators, arithmetic operators, and null comparisons. Using our JMS message selector example, we could specify in our MDB that we want to handle only fragile shipping requests as follows:
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@ActivationConfigProperty( propertyName="messageSelector", propertyValue="Fragile IS TRUE")
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Table 4.2 summarizes some common message selector types.
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Table 4.2 Commonly used message selector types. The selector syntax is almost identical to the SQL WHERE clause. Type Literals Description Can be strings, exact or approximate numeric values, or booleans. Example BidManagerMDB 100 TRUE RECIPIENT NumOfBids Fragile JMSTimestamp
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Can be a message property or header name; case sensitive.
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Whitespace
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Same as defined in the Java language specification: space, tab, form feed, and line terminator. Comparison operators, such as =, >, >=, <=, <>. All three types of logical operators NOT, AND, OR are supported. IS NULL and IS NOT NULL comparisons. IS [NOT] TRUE and IS [NOT] FALSE comparisons. RECIPIENT='BidManagerMDB' NumOfBids>=100 RECIPIENT='BidManagerMDB' AND NumOfBids>=100 FirstName IS NOT NULL Fragile IS TRUE Fragile IS FALSE
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Comparison operators Logical operators Null comparison True/false comparison
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We re now ready to examine lifecycle callbacks in MDBs.
4.3.7 Using bean lifecycle callbacks
As you recall from chapter 3, similar to stateless session beans, MDBs have a simple lifecycle (see figure 4.8 for a refresher). The container is responsible for the following:
Working with message-driven beans
Creates MDB instances and sets them up Injects resources, including the message-driven context (discussed in the next chapter in detail) Places instances in a managed pool Pulls an idle bean out of the pool when a message arrives (the container may have to increase the pool size at this point) Executes the message listener method; e.g., the onMessage method When the onMessage method finishes executing, pushes the idle bean back into the method-ready pool As needed, retires (or destroys) beans out of the pool
Figure 4.8 The MDB lifecycle has three states: does not exist, idle, and busy. There are only two lifecycle callbacks corresponding to bean creation and destruction; you can use PostConstruct and PreDestroy methods to receive these callbacks.
Messaging and developing MDBs
The MDB s two lifecycle callbacks are (1) PostConstruct, which is called immediately after an MDB is created and set up and all the resources are injected, and (2) PreDestroy, which is called right before the bean instance is retired and removed from the pool. These callbacks are typically used for allocating and releasing injected resources that are used by the onMessage method, which is exactly what we do in our example. The processShippingRequest method saves shipping requests that the onMessage method extracts from the incoming JMS message:
private void processShippingRequest(ShippingRequest request) throws SQLException { Statement statement = connection.createStatement(); statement.execute( "INSERT INTO " + "SHIPPING_REQUEST (" + request.getInsuranceAmount() + " )"); }
The method creates a statement from an open JDBC connection and uses it to save a record into the SHIPPING_REQUEST table containing all the fields from the ShippingRequest object. The JDBC connection object used to create the statement is a classic heavy-duty resource. It is expensive to open and should be shared whenever possible. On the other hand, it can hold a number of native resources, so it is important to close the connection when it is no longer needed. We accomplish both these goals using callback methods as well as resource injection. First, the JDBC data source that the connection is created from is injected using the @Resource annotation:
@Resource(name="jdbc/TurtleDS") public void setDataSource(DataSource dataSource) { this.dataSource = dataSource; }
The @Resource annotation tells the EJB container that it should look up a java.sql.DataSource named jdbc/TurtleDS from JNDI and pass it to the setDataSource method after creating a new instance of the bean. The setDataSource method, in turn, saves the data source in an instance variable. After injecting resources, the container checks whether there are any designated PostConstruct methods that need to be invoked before the MDB is put into the pool. In our case, we mark the initialize method with the @PostConstruct annotation:
@PostConstruct public void initialize() {
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