目錄
Java中的註解
Java中1.5中開始引入註解,我們最熟悉的應該是:@Override, 它的定義如下:
/**
* 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 {
}
從註釋,我們可以看出,@Override的作用是,提示編譯器,使用了@Override註解的方法必須override父類或者java.lang.Object中的一個同名方法。我們看到@Override的定義中使用到了 @Target, @Retention,它們就是所謂的“元註解”
元註解
@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用於提示註解被保留多長時間,有三種取值:
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 保留在源碼級別,被編譯器拋棄(@Override就是此類);
- RetentionPolicy.CLASS被編譯器保留在編譯後的類文件級別,但是被虛擬機丟棄;
- RetentionPolicy.RUNTIME保留至運行時,可以被反射讀取。
@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用於提示該註解使用的地方,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
}
分別表示該註解可以被使用的地方:
1)類,接口,註解,enum;
2)屬性域;
3)方法;
4)參數;
5)構造函數;
6)局部變量;
7)註解類型;
8)包
所以:
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}
表示 @Override 只能使用在方法上,保留在源碼級別,被編譯器處理,然後拋棄掉。
@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 {
}
表示註解是否能被 javadoc 處理並保留在文檔中。
@Inherited(不常用)
元註解是一個標記註解,@Inherited闡述了某個被標註的類型是被繼承的。如果一個使用了@Inherited修飾的annotation類型被用於一個class,則這個annotation將被用於該class的子類。
使用 元註解 來自定義註解 和 處理自定義註解
有了元註解,那麼我就可以使用它來自定義我們需要的註解。結合自定義註解和Spring AOP或者過濾器,是一種十分強大的武器。比如可以使用註解來實現權限的細粒度的控制——在類或者方法上使用權限註解,然後在Spring AOP或者過濾器中進行攔截處理。下面是一個關於登錄的權限的註解的實現:
/**
* 不需要登錄註解
*/
@Target({ ElementType.METHOD, ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface NoLogin {
}
我們自定義了一個註解 @NoLogin, 可以被用於 方法 和 類 上,註解一直保留到運行期,可以被反射讀取到。該註解的含義是:被 @NoLogin 註解的類或者方法,即使用戶沒有登錄,也是可以訪問的。下面就是對註解進行處理了:
/**
* 檢查登錄攔截器
* 如不需要檢查登錄可在方法或者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("當前操作handler不爲HandlerMethod=" + handler.getClass().getName() + ",req="
+ request.getQueryString());
return true;
}
HandlerMethod handlerMethod = (HandlerMethod) handler;
String methodName = handlerMethod.getMethod().getName();
// 判斷是否需要檢查登錄,方法上
NoLogin noLogin = handlerMethod.getMethod().getAnnotation(NoLogin.class);
if (null != noLogin) {
if (logger.isDebugEnabled()) {
logger.debug("當前操作methodName=" + methodName + "不需要檢查登錄情況");
}
return true;
}
// 判斷是否需要檢查登錄,類上
noLogin = handlerMethod.getMethod().getDeclaringClass().getAnnotation(NoLogin.class);
if (null != noLogin) {
if (logger.isDebugEnabled()) {
logger.debug("當前操作methodName=" + methodName + "不需要檢查登錄情況");
}
return true;
}
if (null == request.getSession().getAttribute(CommonConstants.SESSION_KEY_USER)) {
logger.warn("當前操作" + methodName + "用戶未登錄,ip=" + request.getRemoteAddr());
response.getWriter().write(JsonConvertor.convertFailResult(ErrorCodeEnum.NOT_LOGIN).toString()); // 返回錯誤信息
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 {
}
}
上面我們定義了一個登錄攔截器,首先使用反射來判斷方法上是否被 @NoLogin 註解:
NoLogin noLogin = handlerMethod.getMethod().getAnnotation(NoLogin.class);
然後判斷類是否被 @NoLogin 註解:
noLogin = handlerMethod.getMethod().getDeclaringClass().getAnnotation(NoLogin.class);
如果被註解了,就返回 true,如果沒有被註解,就判斷是否已經登錄,沒有登錄則返回錯誤信息給前臺和false. 這是一個簡單的使用 註解 和 過濾器 來進行權限處理的例子。
擴展開來,那麼我們就可以使用註解,來表示某方法或者類,只能被具有某種角色,或者具有某種權限的用戶所訪問,然後在過濾器中進行判斷處理。