10
Working with Oracle Object References

This chapter describes Oracle extensions to standard JDBC that let you access and manipulate object references. The following topics are discussed:

Oracle Extensions for Object References

Oracle supports the use of references (pointers) to Oracle database objects. Oracle JDBC provides support for object references as:

columns in a SELECT-list
IN or OUT bind variables
attributes in an Oracle object
elements in a collection (array) type object

In SQL, an object reference (REF) is strongly typed. For example, a reference to an EMPLOYEE object would be defined as an EMPLOYEE REF, not just a REF.

When you select an object reference in Oracle JDBC, be aware that you are retrieving only a pointer to an object, not the object itself. You have the choice of materializing the reference as a weakly typed oracle.sql.REF instance (or a java.sql.Ref instance for portability), or materializing it as an instance of a custom Java class that you have created in advance, which is strongly typed. Custom Java classes used for object references are referred to as custom reference classes in this manual and must implement the oracle.sql.ORAData interface.

The oracle.sql.REF class implements the standard java.sql.Ref interface (oracle.jdbc2.Ref under JDK 1.1.x).

You can retrieve a REF instance through a result set or callable statement object, and pass an updated REF instance back to the database through a prepared statement or callable statement object. The REF class includes functionality to get and set underlying object attribute values, and get the SQL base type name of the underlying object (for example, EMPLOYEE).

Custom reference classes include this same functionality, as well as having the advantage of being strongly typed. This can help you find coding errors during compilation that might not otherwise be discovered until runtime.

For more information about custom reference classes, see "Custom Reference Classes with JPublisher".

Notes:

If you are using the oracle.sql.ORAData interface for custom object classes, you will presumably use ORAData for corresponding custom reference classes as well. If you are using the standard java.sql.SQLData interface for custom object classes, however, you can only use weak Java types for references (java.sql.Ref or oracle.sql.REF). The SQLData interface is for mapping SQL object types only.
You cannot create REF objects in your JDBC application; you can only retrieve existing REF objects from the database.
You cannot have a reference to an array, even though arrays, like objects, are structured types.

Overview of Object Reference Functionality

To access and update object data through an object reference, you must obtain the reference instance through a result set or callable statement and then pass it back as a bind variable in a prepared statement or callable statement. It is the reference instance that contains the functionality to access and update object attributes.

This section summarizes the following:

statement and result set getter and setter methods for passing REF instances from and to the database
REF class functionality to get and set object attributes

Remember that you can use custom reference classes instead of the ARRAY class. See "Custom Reference Classes with JPublisher".

Object Reference Getter and Setter Methods

Use the following result set, callable statement, and prepared statement methods to retrieve and pass object references. Code examples are provided later in the chapter.

Result Set and Callable Statement Getter Methods

The OracleResultSet and OracleCallableStatement classes support getREF() and getRef() methods to retrieve REF objects as output parameters--either as oracle.sql.REF instances or java.sql.Ref instances (oracle.jdbc2.Ref under JDK 1.1.x). You can also use the getObject() method. These methods take as input a String column name or int column index.

Prepared and Callable Statement Setter Methods

The OraclePreparedStatement and OracleCallableStatement classes support setREF() and setRef() methods to take REF objects as bind variables and pass them to the database. You can also use the setObject() method. These methods take as input a String parameter name or int parameter index as well as, respectively, an oracle.sql.REF instance or a java.sql.Ref instance (oracle.jdbc2.Ref under JDK 1.1.x).

Key REF Class Methods

Use the following oracle.sql.REF class methods to retrieve the SQL object type name and retrieve and pass the underlying object data.

getBaseTypeName(): Retrieves the fully-qualified SQL structured type name of the referenced object (for example, EMPLOYEE).

This is a standard method specified by the java.sql.Ref interface.
getValue(): Retrieves the referenced object from the database, allowing you to access its attribute values. It optionally takes a type map object, or else you can use the default type map of the database connection object.

This method is an Oracle extension.
setValue(): Sets the referenced object in the database, allowing you to update its attribute values. It takes an instance of the object type as input (either a STRUCT instance or an instance of a custom object class).

This method is an Oracle extension.

Retrieving and Passing an Object Reference

This section discusses JDBC functionality for retrieving and passing object references.

Retrieving an Object Reference from a Result Set

To demonstrate how to retrieve object references, the following example first defines an Oracle object type ADDRESS, which is then referenced in the PEOPLE table:

create type ADDRESS as object
   (street_name     VARCHAR2(30),
    house_no        NUMBER);

create table PEOPLE 
    (col1 VARCHAR2(30),
     col2 NUMBER,
     col3 REF ADDRESS);

The ADDRESS object type has two attributes: a street name and a house number. The PEOPLE table has three columns: a column for character data, a column for numeric data, and a column containing a reference to an ADDRESS object.

To retrieve an object reference, follow these general steps:

Use a standard SQL SELECT statement to retrieve the reference from a database table REF column.
Use getREF() to get the address reference from the result set into a REF object.
Let Address be the Java custom class corresponding to the SQL object type ADDRESS.
Add the correspondence between the Java class Address and the SQL type ADDRESS to your type map.
Use the getValue() method to retrieve the contents of the Address reference. Cast the output to a Java Address object.

Here is the code for these steps (other than adding Address to the type map), where stmt is a previously defined statement object. The PEOPLE database table is defined earlier in this section:

ResultSet rs = stmt.executeQuery("SELECT col3 FROM PEOPLE"); 
while (rs.next())
{
   REF ref = ((OracleResultSet)rs).getREF(1);
   Address a = (Address)ref.getValue();
}

As with other SQL types, you could retrieve the reference with the getObject() method of your result set. Note that this would require you to cast the output. For example:

REF ref = (REF)rs.getObject(1);

There are no performance advantages in using getObject() instead of getREF(); however, using getREF() allows you to avoid casting the output.

Retrieving an Object Reference from a Callable Statement

To retrieve an object reference as an OUT parameter in PL/SQL blocks, you must register the bind type for your OUT parameter.

Cast your callable statement to an OracleCallableStatement:

OracleCallableStatement ocs = 
   (OracleCallableStatement)conn.prepareCall("{? = call func()}");

Register the OUT parameter with this form of the registerOutParameter() method:
```
ocs.registerOutParameter
            (int param_index, int sql_type, String sql_type_name);
```
Where param_index is the parameter index and sql_type is the SQL typecode (in this case, OracleTypes.REF). The sql_type_name is the name of the structured object type that this reference is used for. For example, if the OUT parameter is a reference to an ADDRESS object (as in "Retrieving and Passing an Object Reference"), then ADDRESS is the sql_type_name that should be passed in.
Execute the call:
```
ocs.execute();
```

Passing an Object Reference to a Prepared Statement

Pass an object reference to a prepared statement in the same way as you would pass any other SQL type. Use either the setObject() method or the setREF() method of a prepared statement object.

Continuing the example in "Retrieving and Passing an Object Reference", use a prepared statement to update an address reference based on ROWID, as follows:

PreparedStatement pstmt = 
   conn.prepareStatement ("update PEOPLE set ADDR_REF = ? where ROWID = ?"); 
((OraclePreparedStatement)pstmt).setREF (1, addr_ref);
((OraclePreparedStatement)pstmt).setROWID (2, rowid);

Accessing and Updating Object Values through an Object Reference

You can use the REF object setValue() method to update the value of an object in the database through an object reference. To do this, you must first retrieve the reference to the database object and create a Java object (if one does not already exist) that corresponds to the database object.

For example, you can use the code in the section "Retrieving and Passing an Object Reference" to retrieve the reference to a database ADDRESS object:

ResultSet rs = stmt.executeQuery("SELECT col3 FROM PEOPLE"); 
if (rs.next())
{
   REF ref = rs.getREF(1);
   Address a = (Address)ref.getValue();
}

Then, you can create a Java Address object (this example omits the content for the constructor of the Address class) that corresponds to the database ADDRESS object. Use the setValue() method of the REF class to set the value of the database object:

Address addr = new Address(...);
ref.setValue(addr);

Here, the setValue() method updates the database ADDRESS object immediately.

Custom Reference Classes with JPublisher

This chapter primarily describes the functionality of the oracle.sql.REF class, but it is also possible to access Oracle object references through custom Java classes or, more specifically, custom reference classes.

Custom reference classes offer all the functionality described earlier in this chapter, as well as the advantage of being strongly typed. A custom reference class must satisfy three requirements:

It must implement the oracle.sql.ORAData interface described under "Creating and Using Custom Object Classes for Oracle Objects". Note that the standard JDBC SQLData interface, which is an alternative for custom object classes, is not intended for custom reference classes.
It, or a companion class, must implement the oracle.sql.ORADataFactory interface, for creating instances of the custom reference class.
It must provide a way to refer to the object data. JPublisher accomplishes this by using an oracle.sql.REF attribute.

You can create custom reference classes yourself, but the most convenient way to produce them is through the Oracle JPublisher utility. If you use JPublisher to generate a custom object class to map to an Oracle object, and you specify that JPublisher use a ORAData implementation, then JPublisher will also generate a custom reference class that implements ORAData and ORADataFactory and includes an oracle.sql.REF attribute. (The ORAData implementation will be used if JPublisher's -usertypes mapping option is set to oracle, which is the default.)

Custom reference classes are strongly typed. For example, if you define an Oracle object EMPLOYEE, then JPublisher can generate an Employee custom object class and an EmployeeRef custom reference class. Using EmployeeRef instances instead of generic oracle.sql.REF instances makes it easier to catch errors during compilation instead of at runtime--for example, if you accidentally assign some other kind of object reference into an EmployeeRef variable.

Be aware that the standard SQLData interface supports only SQL object mappings. For this reason, if you instruct JPublisher to implement the standard SQLData interface in creating a custom object class, then JPublisher will not generate a custom reference class. In this case your only option is to use standard java.sql.Ref instances (or oracle.sql.REF instances) to map to your object references. (Specifying the SQLData implementation is accomplished by setting JPublisher's UDT attributes mapping option to jdbc.)

For more information about JPublisher, see "Using JPublisher to Create Custom Object Classes", or refer to the Oracle9i JPublisher User's Guide.

10 Working with Oracle Object References