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The NOT NULL constraint is essentially a data integrity mechanism that ensures some table columns always have valid, nonempty values. For example, if the business rules dictate that a Category record must always have a name, we can specify a NOT NULL constraint on the CATEGORY_NAME column, and the database will only allow rows to be inserted where a CATEGORY_NAME is specified. If no CATEGORY_NAME is provided, the database will not allow the row to be inserted.

When using entity beans, one of the common problems you ll encounter is retrieving information about the data that entity beans represent. For example, you want to know how many rows of data exist of a certain type. Using multiple entity bean lookups (either directly or through finder methods) will certainly provide you with your desired information but will cost you dearly in performance as your data set grows. The EJB 2.0 specification details a new solution to this problem. It allows you to declare methods in the home interface of an EJB (called home methods) that return information about the entity data without returning an EJB instance.

Structured Query Language (SQL)

40 41 server.registerMBean(statistics, new ObjectName("OrderProcess:name=stats")); 42 ... 43 server.invoke(serverName, "start", null, null); 44 } 45 46 public static void main(String[] str) throws Exception { 47 JMXExample t = new JMXExample(); 48 t.start(); 49 Injector injection = new Injector(); 50 injection.start(); 51 } 52 } The purpose of this modification is to handle the initialization of the MBeanServer component and the HTTP adapter and to register the manageable resource. In our example, the adapter is accessible through the http://localhost:8080 address, which is defined by the added fields on lines 22 and 23 in Listing 9-27. An instance of MBeanServer is created on line 31, and the adapter is initialized on line 39. The manageable resource is then registered in the MBeanServer component (see line 5). Finally, we define two monitors. (For clarity, the actual code of line 42 is shown in Listing 9-28.) Listing 9-28. The Definitions of the Monitors for Listing 9-27 01 02 03 04 05 06 07 08 09 10 11 12 13 14 16 17 18 19 20 21 22 23 24 25 CounterMonitor ordersCounter = new CounterMonitor(); ObjectName ordersCounterName = new ObjectName("OrderProcess","monitor","ordersCounter"); server.registerMBean(ordersCounter, ordersCounterName); ordersCounter.setThreshold(new Integer(5)); ordersCounter.setOffset(new Integer(5)); ordersCounter.setNotify(true); ordersCounter.setDifferenceMode(false); ordersCounter.setObservedObject(new ObjectName("OrderProcess:name=stats")); ordersCounter.setObservedAttribute("Orders"); ordersCounter.setGranularityPeriod(100L); ordersCounter.addNotificationListener( new NotificationListener() { public void handleNotification(Notification notification, Object handback) { System.out.println( "JMX Notification - Orders : threshold overflow"); } }, null, null); ordersCounter.start(); StringMonitor statusMonitor = new StringMonitor(); ObjectName statusMonitorName = new ObjectName("OrderProcess","monitor", "statusMonitor");

An easy way to ensure uniqueness for surrogate primary keys is to set the key for a new record to a number greater than the last created record. Although you could manage this kind of column yourself, databases provide various mechanisms for managing key sequences. The easiest and most transparent of these mechanisms is an identity column constraint (such as the identity column constraints supported by DB2, Sybase, and SQL Server). When you designate a column as an identity, the database automatically generates a value for you when you create a new record. For example, if the ITEM_ID primary key for the ITEMS table is an identity, when we create a new record we do not specify a primary key value ourselves. Instead, during record creation the database looks at the last row inserted, generates a new value by incrementing the last key, and sets the ITEM_ID value on our behalf. Some other databases like Oracle don t support incrementing keys as an internal function of the column, but help you generate keys using an external mechanism called sequences (DB2 supports sequences in addition to identities). Each time you insert a new record, you can ask the sequence to generate a key that you can use in the INSERT statement. A few databases don t support sequence generation at all, in which case you must implement similar functionality yourself. Fortunately, EJB 3 transparently handles all these situations on your behalf, using the table generator.

To find information about entity data sets, add an EJB home method to the entity bean. For example, the following EquityHome home interface (to the EquityBean) declares a method, getCountOfSymbols(), as a home method:

B.3 Structured Query Language (SQL)