注解是Java对于其他语言对Java造成的语言特性压力做出的一种回应，是众多引入到Java SE5中的重要的语言变化之一。
Java1.6内置了三种标准注解和四种元注解（元注解专职负责注解其他注解）。
-
大多数时候我们都是定义自己的注解，并编写自己的处理器来处理它们。
内置的三种标准注解
元注解 @Target(ElementType.METHOD)
package java.lang.annotation;
/**
* @author Joshua Bloch
* @since 1.5
* @jls 9.6.4.1 @Target
* @jls 4.1 The Kinds of Types and Values
*/
public enum ElementType {
/** Class, interface (including annotation type), or enum declaration */
TYPE,
/** Field declaration (includes enum constants) */
FIELD,
/** Method declaration */
METHOD,
/** Formal parameter declaration */
PARAMETER,
/** Constructor declaration */
CONSTRUCTOR,
/** Local variable declaration */
LOCAL_VARIABLE,
/** Annotation type declaration */
ANNOTATION_TYPE,
/** Package declaration */
PACKAGE,
/**
* Type parameter declaration
*
* @since 1.8
*/
TYPE_PARAMETER,
/**
* Use of a type
*
* @since 1.8
*/
TYPE_USE
}
- @Retention(RetentionPolicy.RUNTIME)
package java.lang.annotation;
/**
* @author Joshua Bloch
* @since 1.5
*/
public enum RetentionPolicy {
/**
* Annotations are to be discarded by the compiler.
*/
SOURCE,
/**
* Annotations are to be recorded in the class file by the compiler
* but need not be retained by the VM at run time. This is the default
* behavior.
*/
CLASS,
/**
* Annotations are to be recorded in the class file by the compiler and
* retained by the VM at run time, so they may be read reflectively.
*
* @see java.lang.reflect.AnnotatedElement
*/
RUNTIME
}
定义注解
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface UseCase {
public int id();
public String description() default "no description";
}
使用注解
public class PasswordUtils {
@UseCase(id = 47, description =
"Passwords must contain at least one numeric")
public boolean validatePassword(String password) {
return (password.matches("\\w*\\d\\w*"));
}
@UseCase(id = 48)
public String encryptPassword(String password) {
return new StringBuilder(password).reverse().toString();
}
@UseCase(id = 49, description =
"New passwords can't equal previously used ones")
public boolean checkForNewPassword(
List<String> prevPasswords, String password) {
return !prevPasswords.contains(password);
}
}
编写注解处理器
public class UseCaseTracker {
public static void trackUseCases(List<Integer> useCases, Class<?> cl) {
for (Method m : cl.getDeclaredMethods()) {
UseCase uc = m.getAnnotation(UseCase.class);
if (uc != null) {
System.out.println("Found Use Case: " + uc.id() + " " + uc.description());
useCases.remove(new Integer(uc.id()));
}
}
for (int i : useCases) {
System.out.println("Warning: Missing use case-" + i);
}
}
public static void main(String[] args) {
List<Integer> useCases = new ArrayList<Integer>();
Collections.addAll(useCases, 47, 48, 49, 50);
trackUseCases(useCases, PasswordUtils.class);
}
}
- 运行结果：
Found Use Case: 47 Passwords must contain at least one numeric
Found Use Case: 48 no description
Found Use Case: 49 New passwords can't equal previously used ones
Warning: Missing use case-50
几个注意点：
标签@UseCase由UseCase.java定义，其中包括int元素id,以及一个String元素description。
-
注解元素可用的类型如下（否则编译器报错）：
- 所有基本类型（int, float, boolean等）
- String
- Class
- enum
- Annotation
- 以上类型的数组
注解元素必须要么具有默认值，要么在使用注解时提供元素的值。
注解不支持继承。（见Thinking in Java Fourth Edition 20.2.4）
参考：
- Thinking in Java Fourth Edition (Bruce Eckel)
