Java Annotation,Java注解

Annotation

Annotaions (also known as metadata) provide a formalized way to add information to yourcode so that you can easily use that data at some later point.
Defining annotationsHere is the definition of the annotation above. You can see that annotation definitions look a lot like interface definitions.

In fact, they compile to class files like any other Java interface:Apart from the @ symbol, the definition of @Test is much like that of an empty interface.

An annotation definition also requires the meta-annotations @Target and @Retention.

@Target defines where you can apply this annotation(a method or a field, for example).

@Retention defines whether the annotations are availablein the (SOURCE), in the classfiles (CLASS), or at run time(RUNTIME).

e.g.:

//UseCase.java

package annotationtest;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface UseCase {
	public int id();
	public String description() default "没有描述";
}

//PasswordUtils.java

package annotationtest;

public class PasswordUtils {

	@UseCase(id=1, description="validate 密码")
	public boolean validatePassword(String password){
		return true;
	}
	@UseCase(id=2, description="encrypt 密码")
	public boolean encryptPassword(String password){
		return true;
	}
	@UseCase(id=3, description="checkfor 密码")
	public boolean checkforPassword(String password){
		return true;
	}	
	
}
//Test.java

package annotationtest;

import java.lang.reflect.Method;

public class Test {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		
		Method[] methods = PasswordUtils.class.getDeclaredMethods();

		for (Method m : methods) {
			UseCase uc= m.getAnnotation(UseCase.class);
			System.out.println("id=" + uc.id()+",description="+uc.description());
		}
		
	}

}

// from : thinking in java.