Mapping Table Relationships for Bean-Managed Persistence - The J2EE 1.4 Tutorial

Mapping Table Relationships for Bean-Managed Persistence

In a relational database, tables can be related by common columns. The relationships between the tables affect the design of their corresponding entity beans. The entity beans discussed in this section are backed up by tables with the following types of relationships:

One-to-one

One-to-many

Many-to-many

One-to-One Relationships

In a one-to-one relationship, each row in a table is related to a single row in another table. For example, in a warehouse application, a storagebin table might have a one-to-one relationship with a widget table. This application would model a physical warehouse in which each storage bin contains one type of widget and each widget resides in one storage bin.

Figure 26-1 illustrates the storagebin and widget tables. Because the storagebinid uniquely identifies a row in the storagebin table, it is that table's primary key. The widgetid is the primary key of the widget table. The two tables are related because the widgetid is also a column in the storagebin table. By referring to the primary key of the widget table, the widgetid in the storagebin table identifies which widget resides in a particular storage bin in the warehouse. Because the widgetid of the storagebin table refers to the primary key of another table, it is called a foreign key. (The figures in this chapter denote a primary key with PK and a foreign key with FK.)

Figure 26-1 One-to-One Table Relationship

A dependent (child) table includes a foreign key that matches the primary key of the referenced (parent) table. The values of the foreign keys in the storagebin (child) table depend on the primary keys in the widget (parent) table. For example, if the storagebin table has a row with a widgetid of 344, then the widget table should also have a row whose widgetid is 344.

When designing a database application, you can choose to enforce the dependency between the parent and child tables. There are two ways to enforce such a dependency: by defining a referential constraint in the database or by performing checks in the application code. The storagebin table has a referential constraint named fk_widgetid:
CREATE TABLE storagebin
   (storagebinid VARCHAR(3) 
    CONSTRAINT pk_storagebin PRIMARY KEY,
    widgetid VARCHAR(3),
    quantity INTEGER,
    CONSTRAINT fk_widgetid
    FOREIGN KEY (widgetid)
     REFERENCES widget(widgetid)); 
The source code for the following example is in this directory:
<INSTALL>/j2eetutorial14/examples/ejb/storagebin/src/  
The StorageBinBean and WidgetBean classes illustrate the one-to-one relationship of the storagebin and widget tables. The StorageBinBean class contains variables for each column in the storagebin table, including the foreign key, widgetId:
private String storageBinId;
private String widgetId;
private int quantity; 
The ejbFindByWidgetId method of the StorageBinBean class returns the storageBinId that matches a given widgetId:
public String ejbFindByWidgetId(String widgetId)
   throws FinderException {

   String storageBinId;

   try {
      storageBinId = selectByWidgetId(widgetId);
    } catch (Exception ex) {
        throw new EJBException("ejbFindByWidgetId: " + 
           ex.getMessage());
    }

   if (storageBinId == null) {
      throw new ObjectNotFoundException
         ("Row for widgetId " + widgetId + " not found.");
   }
   else {
      return storageBinId;
   }
} 
The ejbFindByWidgetId method locates the widgetId by querying the database in the selectByWidgetId method:
private String selectByWidgetId(String widgetId) 
   throws SQLException {

   String storageBinId;

   makeConnection();
   String selectStatement =
         "select storagebinid " +
         "from storagebin where widgetid = ? ";
   PreparedStatement prepStmt =
         con.prepareStatement(selectStatement);
   prepStmt.setString(1, widgetId);

   ResultSet rs = prepStmt.executeQuery();

   if (rs.next()) {
      storageBinId = rs.getString(1);
   }
   else {
      storageBinId = null;
   }

   prepStmt.close();
   releaseConnection();
   return storageBinId;
} 
To find out in which storage bin a widget resides, the StorageBinClient program calls the findByWidgetId method:
String widgetId = "777";
StorageBin storageBin = 
   storageBinHome.findByWidgetId(widgetId);
String storageBinId = (String)storageBin.getPrimaryKey();
int quantity = storageBin.getQuantity(); 
Running the StorageBinBean Example

Create the storagebin database table.

In a terminal window, go to this directory:

  <INSTALL>/j2eetutorial14/examples/ejb/storagebin/

Type this command:

  asant create-db_common

In deploytool, deploy the StorageBinApp.ear file, which is in this directory:

<INSTALL>/j2eetutorial14/examples/ejb/provided-ears/

Run the client.

In a terminal window, go to this directory:

  <INSTALL>/j2eetutorial14/examples/ejb/storagebin/

Type the following command on a single line:

  appclient -client StorageBinAppClient.jar

The client should display the following:

  777 388 500 1.0 Duct Tape

One-to-Many Relationships

If the primary key in a parent table matches multiple foreign keys in a child table, then the relationship is one-to-many. This relationship is common in database applications. For example, an application for a sports league might access a team table and a player table. Each team has multiple players, and each player belongs to a single team. Every row in the child table (player) has a foreign key identifying the player's team. This foreign key matches the team table's primary key.

The sections that follow describe how you might implement one-to-many relationships in entity beans. When designing such entity beans, you must decide whether both tables are represented by entity beans, or only one.

A Helper Class for the Child Table

Not every database table needs to be mapped to an entity bean. If a database table doesn't represent a business entity, or if it stores information that is contained in another entity, then you should use a helper class to represent the table. In an online shopping application, for example, each order submitted by a customer can have multiple line items. The application stores the information in the database tables shown by Figure 26-2.

Figure 26-2 One-to-Many Relationship: Order and Line Items

Not only does a line item belong to an order, but it also does not exist without the order. Therefore, the lineitems table should be represented with a helper class and not with an entity bean. Using a helper class in this case is not required, but doing so might improve performance because a helper class uses fewer system resources than does an entity bean.

The source code for the following example is in this directory:
<INSTALL>/j2eetutorial14/examples/ejb/order/src/ 
The LineItem and OrderBean classes show how to implement a one-to-many relationship using a helper class (LineItem). The instance variables in the LineItem class correspond to the columns in the lineitems table. The itemNo variable matches the primary key for the lineitems table, and the orderId variable represents the table's foreign key. Here is the source code for the LineItem class:
public class LineItem implements java.io.Serializable {

   String productId;
   int quantity;
   double unitPrice;
   int itemNo;
   String orderId;
   

   public LineItem(String productId, int quantity,
     double unitPrice, int itemNo, String orderId) {

      this.productId = productId;
      this.quantity = quantity;
      this.unitPrice = unitPrice;
      this.itemNo = itemNo;
      this.orderId = orderId;
   }

   public String getProductId() {
      return productId;
   }

   public int getQuantity() {
      return quantity;
   }

   public double getUnitPrice() {
      return unitPrice;
   }

   public int getItemNo() {
      return itemNo;
   }

   public String getOrderId() {
      return orderId;
   }
} 
The OrderBean class contains an ArrayList variable named lineItems. Each element in the lineItems variable is a LineItem object. The lineItems variable is passed to the OrderBean class in the ejbCreate method. For every LineItem object in the lineItems variable, the ejbCreate method inserts a row into the lineitems table. It also inserts a single row into the orders table. The code for the ejbCreate method follows:
public String ejbCreate(String orderId, String customerId,
    String status, double totalPrice, ArrayList lineItems)
    throws CreateException {

    try {
       insertOrder(orderId, customerId, status, totalPrice);
       for (int i = 0; i < lineItems.size(); i++) {
          LineItem item = (LineItem)lineItems.get(i);
          insertItem(item);
       }
    } catch (Exception ex) {
        throw new EJBException("ejbCreate: " + 
           ex.getMessage());
    }

    this.orderId = orderId;
    this.customerId = customerId;
    this.status = status;
    this.totalPrice = totalPrice;
    this.lineItems = lineItems ;

    return orderId;
} 
The OrderClient program creates and loads an ArrayList of LineItem objects. The program passes this ArrayList to the entity bean when it invokes the create method:
ArrayList lineItems = new ArrayList();
lineItems.add(new LineItem("p23", 13, 12.00, 1, "123"));
lineItems.add(new LineItem("p67", 47, 89.00, 2, "123"));
lineItems.add(new LineItem("p11", 28, 41.00, 3, "123"));
...
Order duke = home.create("123", "c44", "open",
   totalItems(lineItems), lineItems); 
Other methods in the OrderBean class also access both database tables. The ejbRemove method, for example, not only deletes a row from the orders table but also deletes all corresponding rows in the lineitems table. The ejbLoad and ejbStore methods synchronize the state of an OrderBean instance, including the lineItems ArrayList, with the orders and lineitems tables.

The ejbFindByProductId method enables clients to locate all orders that have a particular product. This method queries the lineitems table for all rows with a specific productId. The method returns a Collection of Order objects. The OrderClient program iterates through the Collection and prints the primary key of each order:
Collection c = home.findByProductId("p67");
Iterator i=c.iterator();
while (i.hasNext()) {
   Order order = (Order)i.next();
   String id = (String)order.getPrimaryKey();
   System.out.println(id);
} 
Running the OrderBean Example

Create the order database table.

In a terminal window, go to this directory:

  <INSTALL>/j2eetutorial14/examples/ejb/order/

Type this command:

  asant create-db_common

In deploytool, deploy the OrderBean.ear file, which is in this directory:

<INSTALL>/j2eetutorial14/examples/ejb/provided-ears/

Run the client.

In a terminal window, go to this directory:

  <INSTALL>/j2eetutorial14/examples/ejb/order/

Type the following command on a single line:

  appclient -client OrderBeanClient.jar

The client should display the following lines:

  123 1 p23 12.0   123 2 p67 89.0   123 3 p11 41.0   123   456

An Entity Bean for the Child Table

You should consider building an entity bean for a child table under the following conditions:

The information in the child table is not dependent on the parent table.

The business entity of the child table could exist without that of the parent table.

The child table might be accessed by another application that does not access the parent table.

These conditions exist in the following scenario. Suppose that each sales representative in a company has multiple customers and that each customer has only one sales representative. The company tracks its sales force using a database application. In the database, each row in the salesrep table (parent) matches multiple rows in the customer table (child). Figure 26-3 illustrates this relationship.

Figure 26-3 One-to-Many Relationship: Sales Representative and Customers

The SalesRepBean and CustomerBean entity bean classes implement the one-to-many relationship of the sales and customer tables.

The source code for this example is in this directory:
<INSTALL>/j2eetutorial14/examples/ejb/salesrep/src/ 
The SalesRepBean class contains a variable named customerIds, which is an ArrayList of String elements. These String elements identify which customers belong to the sales representative. Because the customerIds variable reflects this relationship, the SalesRepBean class must keep the variable up-to-date.

The SalesRepBean class instantiates the customerIds variable in the setEntityContext method and not in ejbCreate. The container invokes setEntityContext only once--when it creates the bean instance--thereby ensuring that customerIds is instantiated only once. Because the same bean instance can assume different identities during its life cycle, instantiating customerIds in ejbCreate might cause multiple and unnecessary instantiations. Therefore, the SalesRepBean class instantiates the customerIds variable in setEntityContext:
public void setEntityContext(EntityContext context) {

  this.context = context;
  customerIds = new ArrayList();

  try {
    Context initial = new InitialContext();
    Object objref =
      initial.lookup("java:comp/env/ejb/Customer");

    customerHome = 
      (CustomerHome)PortableRemoteObject.narrow(objref,
        CustomerHome.class);
  } catch (Exception ex) {
    throw new EJBException("setEntityContext: " +
      ex.getMessage());
  }
} 
Invoked by the ejbLoad method, loadCustomerIds is a private method that refreshes the customerIds variable. There are two approaches to coding a method such as loadCustomerIds: fetch the identifiers from the customer database table, or get them from the CustomerBean entity bean. Fetching the identifiers from the database might be faster, but it exposes the code in the SalesRepBean class to the CustomerBean bean's underlying database table. In the future, if you were to change the CustomerBean bean's table (or move the bean to a different Application Server), you might need to change the SalesRepBean code. But if the SalesRepBean class gets the identifiers from the CustomerBean entity bean, no coding changes would be required. The two approaches present a trade-off: performance versus flexibility. The SalesRepBean example opts for flexibility, loading the customerIds variable by calling the findBySalesRep and getPrimaryKey methods of CustomerBean. Here is the code for the loadCustomerIds method:
private void loadCustomerIds() {

   customerIds.clear();

   try {
      Collection c = customerHome.findBySalesRep(salesRepId);
      Iterator i=c.iterator();

      while (i.hasNext()) {
         Customer customer = (Customer)i.next();
         String id = (String)customer.getPrimaryKey();
         customerIds.add(id);
      }
   
  } catch (Exception ex) {
       throw new EJBException("Exception in loadCustomerIds: " +
          ex.getMessage());
  }
} 
If a customer's sales representative changes, the client program updates the database by calling the setSalesRepId method of the CustomerBean class. The next time a business method of the SalesRepBean class is called, the ejbLoad method invokes loadCustomerIds, which refreshes the customerIds variable. (To ensure that ejbLoad is invoked before each business method, set the transaction attributes of the business methods to Required.) For example, the SalesRepClient program changes the salesRepId for a customer named Mary Jackson as follows:
Customer mary = customerHome.findByPrimaryKey("987");
mary.setSalesRepId("543"); 
The salesRepId value 543 identifies a sales representative named Janice Martin. To list all of Janice's customers, the SalesRepClient program invokes the getCustomerIds method, iterates through the ArrayList of identifiers, and locates each CustomerBean entity bean by calling its findByPrimaryKey method:
SalesRep janice = salesHome.findByPrimaryKey("543");
ArrayList a = janice.getCustomerIds();
i = a.iterator();

while (i.hasNext()) {
   String customerId = (String)i.next();
   Customer customer = 
customerHome.findByPrimaryKey(customerId);
   String name = customer.getName();
   System.out.println(customerId + ": " + name);
} 
Running the SalesRepBean Example

Create the salesrep database table.

In a terminal window, go to this directory:

  <INSTALL>/j2eetutorial14/examples/ejb/salesrep/

Type this command:

  asant create-db_common

In deploytool, deploy the SalesRepApp.ear file, which is in this directory:

<INSTALL>/j2eetutorial14/examples/ejb/provided-ears/

Run the client.

In a terminal window, go to this directory:

  <INSTALL>/j2eetutorial14/examples/ejb/salesrep/

Type the following command on a single line:

  appclient -client SalesRepAppClient.jar

The client should display the following lines:

  customerId = 221   customerId = 388   customerId = 456   customerId = 844   987: Mary Jackson   221: Alice Smith   388: Bill Williamson   456: Joe Smith   844: Buzz Murphy

Many-to-Many Relationships

In a many-to-many relationship, each entity can be related to multiple occurrences of the other entity. For example, a college course has many students and each student may take several courses. In a database, this relationship is represented by a cross-reference table containing the foreign keys. In Figure 26-4, the cross-reference table is the enrollment table. These tables are accessed by the StudentBean, CourseBean, and EnrollerBean classes.

Figure 26-4 Many-to-Many Relationship: Students and Courses

The source code for this example is in this directory:
<INSTALL>/j2eetutorial14/examples/ejb/enroller/src/ 
The StudentBean and CourseBean classes are complementary. Each class contains an ArrayList of foreign keys. The StudentBean class contains an ArrayList named courseIds, which identifies the courses the student is enrolled in. Similarly, the CourseBean class contains an ArrayList named studentIds.

The ejbLoad method of the StudentBean class adds elements to the courseIds ArrayList by calling loadCourseIds, a private method. The loadCourseIds method gets the course identifiers from the EnrollerBean session bean. The source code for the loadCourseIds method follows:
private void loadCourseIds() {

   courseIds.clear();

   try {
      Enroller enroller = enrollerHome.create();
      ArrayList a = enroller.getCourseIds(studentId);
      courseIds.addAll(a);

  } catch (Exception ex) {
       throw new EJBException("Exception in loadCourseIds: " +
          ex.getMessage());
  }
} 
Invoked by the loadCourseIds method, the getCourseIds method of the EnrollerBean class queries the enrollment table:
select courseid from enrollment
where studentid = ?  
Only the EnrollerBean class accesses the enrollment table. Therefore, the EnrollerBean class manages the student-course relationship represented in the enrollment table. If a student enrolls in a course, for example, the client calls the enroll business method, which inserts a row:
insert into enrollment
values (studentid, courseid) 
If a student drops a course, the unEnroll method deletes a row:
delete from enrollment
where studentid = ? and courseid = ? 
And if a student leaves the school, the deleteStudent method deletes all rows in the table for that student:
delete from enrollment
where student = ? 
The EnrollerBean class does not delete the matching row from the student table. That action is performed by the ejbRemove method of the StudentBean class. To ensure that both deletes are executed as a single operation, you must ensure that they belong to the same transaction. See Chapter 30 for more information.

Running the EnrollerBean Example

Create the enroller database table.

In a terminal window, go to this directory:

  <INSTALL>/j2eetutorial14/examples/ejb/enroller/

Type this command:

  asant create-db_common

In deploytool, deploy the EnrollerApp.ear file, which is in this directory:

<INSTALL>/j2eetutorial14/examples/ejb/provided-ears/

Run the client.

In a terminal window, go to this directory:

  <INSTALL>/j2eetutorial14/examples/ejb/enroller/

Type the following command on a single line:

  appclient -client EnrollerAppClient.jar

The client should display the following lines:

  Denise Smith:   220 Power J2EE Programming   333 XML Made Easy   777 An Introduction to Java Programming   An Introduction to Java Programming:   823 Denise Smith   456 Joe Smith   388 Elizabeth Willis