目录
Java中的注解
Java中1.5中开始引入注解,我们最熟悉的应该是:@Override, 它的定义如下:
/**
* Indicates that a method declaration is intended to override a
* method declaration in a supertype. If a method is annotated with
* this annotation type compilers are required to generate an error
* message unless at least one of the following conditions hold:
* The method does override or implement a method declared in a
* supertype.
* The method has a signature that is override-equivalent to that of
* any public method declared in Object.
*
* @author Peter von der Ahé
* @author Joshua Bloch
* @jls 9.6.1.4 @Override
* @since 1.5
*/
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}
从注释,我们可以看出,@Override的作用是,提示编译器,使用了@Override注解的方法必须override父类或者java.lang.Object中的一个同名方法。我们看到@Override的定义中使用到了 @Target, @Retention,它们就是所谓的“元注解”
元注解
@Retention
/**
* Indicates how long annotations with the annotated type are to
* be retained. If no Retention annotation is present on
* an annotation type declaration, the retention policy defaults to
* RetentionPolicy.CLASS.
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
/**
* Returns the retention policy.
* @return the retention policy
*/
RetentionPolicy value();
}
@Retention用于提示注解被保留多长时间,有三种取值:
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
}
- RetentionPolicy.SOURCE 保留在源码级别,被编译器抛弃(@Override就是此类);
- RetentionPolicy.CLASS被编译器保留在编译后的类文件级别,但是被虚拟机丢弃;
- RetentionPolicy.RUNTIME保留至运行时,可以被反射读取。
@Target:
package java.lang.annotation;
/**
* Indicates the contexts in which an annotation type is applicable. The
* declaration contexts and type contexts in which an annotation type may be
* applicable are specified in JLS 9.6.4.1, and denoted in source code by enum
* constants of java.lang.annotation.ElementType
* @since 1.5
* @jls 9.6.4.1 @Target
* @jls 9.7.4 Where Annotations May Appear
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
/**
* Returns an array of the kinds of elements an annotation type
* can be applied to.
* @return an array of the kinds of elements an annotation type
* can be applied to
*/
ElementType[] value();
}
@Target用于提示该注解使用的地方,取值有:
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
}
分别表示该注解可以被使用的地方:
1)类,接口,注解,enum;
2)属性域;
3)方法;
4)参数;
5)构造函数;
6)局部变量;
7)注解类型;
8)包
所以:
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}
表示 @Override 只能使用在方法上,保留在源码级别,被编译器处理,然后抛弃掉。
@Documented :
/**
* Indicates that annotations with a type are to be documented by javadoc
* and similar tools by default. This type should be used to annotate the
* declarations of types whose annotations affect the use of annotated
* elements by their clients. If a type declaration is annotated with
* Documented, its annotations become part of the public API
* of the annotated elements.
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Documented {
}
表示注解是否能被 javadoc 处理并保留在文档中。
@Inherited(不常用)
元注解是一个标记注解,@Inherited阐述了某个被标注的类型是被继承的。如果一个使用了@Inherited修饰的annotation类型被用于一个class,则这个annotation将被用于该class的子类。
使用 元注解 来自定义注解 和 处理自定义注解
有了元注解,那么我就可以使用它来自定义我们需要的注解。结合自定义注解和Spring AOP或者过滤器,是一种十分强大的武器。比如可以使用注解来实现权限的细粒度的控制——在类或者方法上使用权限注解,然后在Spring AOP或者过滤器中进行拦截处理。下面是一个关于登录的权限的注解的实现:
/**
* 不需要登录注解
*/
@Target({ ElementType.METHOD, ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface NoLogin {
}
我们自定义了一个注解 @NoLogin, 可以被用于 方法 和 类 上,注解一直保留到运行期,可以被反射读取到。该注解的含义是:被 @NoLogin 注解的类或者方法,即使用户没有登录,也是可以访问的。下面就是对注解进行处理了:
/**
* 检查登录拦截器
* 如不需要检查登录可在方法或者controller上加上@NoLogin
*/
public class CheckLoginInterceptor implements HandlerInterceptor {
private static final Logger logger = Logger.getLogger(CheckLoginInterceptor.class);
@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response,
Object handler) throws Exception {
if (!(handler instanceof HandlerMethod)) {
logger.warn("当前操作handler不为HandlerMethod=" + handler.getClass().getName() + ",req="
+ request.getQueryString());
return true;
}
HandlerMethod handlerMethod = (HandlerMethod) handler;
String methodName = handlerMethod.getMethod().getName();
// 判断是否需要检查登录,方法上
NoLogin noLogin = handlerMethod.getMethod().getAnnotation(NoLogin.class);
if (null != noLogin) {
if (logger.isDebugEnabled()) {
logger.debug("当前操作methodName=" + methodName + "不需要检查登录情况");
}
return true;
}
// 判断是否需要检查登录,类上
noLogin = handlerMethod.getMethod().getDeclaringClass().getAnnotation(NoLogin.class);
if (null != noLogin) {
if (logger.isDebugEnabled()) {
logger.debug("当前操作methodName=" + methodName + "不需要检查登录情况");
}
return true;
}
if (null == request.getSession().getAttribute(CommonConstants.SESSION_KEY_USER)) {
logger.warn("当前操作" + methodName + "用户未登录,ip=" + request.getRemoteAddr());
response.getWriter().write(JsonConvertor.convertFailResult(ErrorCodeEnum.NOT_LOGIN).toString()); // 返回错误信息
return false;
}
return true;
}
@Override
public void postHandle(HttpServletRequest request, HttpServletResponse response,
Object handler, ModelAndView modelAndView) throws Exception {
}
@Override
public void afterCompletion(HttpServletRequest request, HttpServletResponse response,
Object handler, Exception ex) throws Exception {
}
}
上面我们定义了一个登录拦截器,首先使用反射来判断方法上是否被 @NoLogin 注解:
NoLogin noLogin = handlerMethod.getMethod().getAnnotation(NoLogin.class);
然后判断类是否被 @NoLogin 注解:
noLogin = handlerMethod.getMethod().getDeclaringClass().getAnnotation(NoLogin.class);
如果被注解了,就返回 true,如果没有被注解,就判断是否已经登录,没有登录则返回错误信息给前台和false. 这是一个简单的使用 注解 和 过滤器 来进行权限处理的例子。
扩展开来,那么我们就可以使用注解,来表示某方法或者类,只能被具有某种角色,或者具有某种权限的用户所访问,然后在过滤器中进行判断处理。