Understanding the Expression Framework

The TopLink expression framework provides methods through the following classes:

The Expression class provides most general functions, such as toUpperCase.
The ExpressionMath class supplies mathematical methods.

Example 97-1 illustrates how to use the Expression class.

Example 97-1 Using the Expression Class

expressionBuilder.get("lastName").equal("Smith");

Example 97-2 illustrates how to use the ExpressionMath class.

Example 97-2 Using the ExpressionMath Class

ExpressionMath.abs(ExpressionMath.subtract(emp.get("salary"),
emp.get("spouse").get("salary")).greaterThan(10000)

This division of functionality enables TopLink expressions to provide similar mathematical functionality to the java.lang.Math class, but keeps both the Expression and ExpressionMath classes from becoming unnecessarily complex.

Expressions Compared to SQL

Expressions offer the following advantages over SQL when you access a database:

Expressions are easier to maintain because the database is abstracted.
Changes to descriptors or database tables do not affect the querying structures in the application.
Expressions enhance readability by standardizing the Query interface so that it looks similar to traditional Java calling conventions. For example, the Java code required to get the street name from the Address object of the Employee class looks like this:
```
emp.getAddress().getStreet().equals("Meadowlands");
```
The expression to get the same information is similar:
```
emp.get("address").get("street").equal("Meadowlands");
```
Expressions allow read queries to transparently query between two classes that share a relationship. If these classes are stored in multiple tables in the database, TopLink automatically generates the appropriate join statements to return information from both tables.

Expressions simplify complex operations. For example, the following Java code retrieves all employees that live on "Meadowlands" whose salary is greater than 10,000:

ExpressionBuilder emp = new ExpressionBuilder();
Expression exp = emp.get("address").get("street").equal("Meadowlands");
Vector employees = session.readAllObjects(Employee.class,
  exp.and(emp.get("salary").greaterThan(10000)));

TopLink automatically generates the appropriate SQL from that code:

SELECT t0.VERSION, t0.ADDR_ID, t0.F_NAME, t0.EMP_ID, t0.L_NAME, t0.MANAGER_ID, t0.END_DATE, t0.START_DATE, t0.GENDER, t0.START_TIME, t0.END_TIME,t0.SALARY FROM EMPLOYEE t0, ADDRESS t1 WHERE (((t1.STREET = 'Meadowlands')AND (t0.SALARY > 10000)) AND (t1.ADDRESS_ID = t0.ADDR_ID))

WARNING:

Allowing an unverified SQL string to be passed into methods (for example: readAllObjects(Class class, String sql) method) makes your application vulnerable to SQL injection attacks.