Declarative Programming in Java
What makes EJB components special is the declarative programming model through which we can specify the services such as security, persistence, transaction etc., that the container should provide. An EJB only implements
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Approaches to Programming
There are two approaches to programming called imperative programming and declarative programming. Imperative programming gives a list of instructions to execute in a particular order -- Java program that counts the number of words in a text file is an example of the imperative approach. Declarative programming describes a set of conditions, and lets the system figure out how to fulfill them. The SQL statement SELECT COUNT(*) FROM XYZ is an example for the declarative approach. In other words, \"specifying how\" describes imperative programming and \"specifying what is to be done, not how\" describes declarative programming.
Annotations
The Tiger release of Java (JDK 1.5) adds a new language construct called annotation (proposed by JSR-175). Annotation is a generic mechanism for associating metadata (declarative information) with program elements such as classes, methods, fields, parameters, local variables, and packages. The compiler can store the metadata in the class files. Later, the VM or other programs can look for the metadata to determine how to interact with the program elements or change their behavior.
Declaring an Annotation
Declaring an annotation is very simple -- it takes the form of an interface declaration with an @ preceding it and optionally marked with meta-annotations, as shown below:
package njunit.annotation;
import java.lang.annotation.*;
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface UnitTest {
String value();
}
The Retention meta-annotation declares that the @UnitTest annotation should be stored in the class file and retained by the VM so it may be read reflectively. The Target meta-annotation declares that the @UnitTest annotation can be used to annotate methods in a Java class. @interface declares the @UnitTest annotation with one member called value, which returns a String.
Using an Annotation
Here is an example that shows how to use the @UnitTest annotation declared in the previous section:
import njunit.annotation.*;
public class Example {
@UnitTest(value=\"Test 1. This test will pass.\")
public void pass() {
assert 10 > 5;
}
@UnitTest(\"Test 2. This test will fail.\")
public void fail() {
assert 10 < 5;
}
}
An annotation is applied to the code element by placing an annotation statement (@AnnotationType(...)) before the program element. Annotation values take the form \"name=value\"; for example, @UnitTest(value=\"some text\"). Single-member annotations with a member named value are treated specially and can use the shorthand @UnitTest(\"some text\"). In the example, the @UnitTest annotation is associated with the pass and fail methods.
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