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获取Java类中注解的详细信息

前言：这篇博客主要是为了后续的获取 SpringMVC 中的全部请求 URL 做的准备。

public class AnnotationHelper {

</span><span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">static</span> <span style="color: rgba(0, 0, 255, 1)">final</span> AnnotationHelper helper = <span style="color: rgba(0, 0, 255, 1)">new</span><span style="color: rgba(0, 0, 0, 1)"> AnnotationHelper();

</span><span style="color: rgba(0, 0, 255, 1)">protected</span><span style="color: rgba(0, 0, 0, 1)"> AnnotationHelper() {
    
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">static</span><span style="color: rgba(0, 0, 0, 1)"> AnnotationHelper getInstance() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> helper;
}

</span><span style="color: rgba(0, 128, 0, 1)">/**</span><span style="color: rgba(0, 128, 0, 1)">
 * 得到类上面的注解信息
 * </span><span style="color: rgba(128, 128, 128, 1)">@param</span><span style="color: rgba(0, 128, 0, 1)"> scannerClass
 * </span><span style="color: rgba(128, 128, 128, 1)">@param</span><span style="color: rgba(0, 128, 0, 1)"> allowInjectClass
 * </span><span style="color: rgba(128, 128, 128, 1)">@return</span>
 <span style="color: rgba(0, 128, 0, 1)">*/</span>
<span style="color: rgba(0, 0, 255, 1)">public</span> Annotation getClassAnnotation(Class&lt;?&gt; scannerClass , Class&lt;? <span style="color: rgba(0, 0, 255, 1)">extends</span> Annotation&gt;<span style="color: rgba(0, 0, 0, 1)"> allowInjectClass) {
    
    </span><span style="color: rgba(0, 0, 255, 1)">if</span>(!<span style="color: rgba(0, 0, 0, 1)">scannerClass.isAnnotationPresent(allowInjectClass)) {
        </span><span style="color: rgba(0, 0, 255, 1)">return</span> <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">;
    }
    
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> scannerClass.getAnnotation(allowInjectClass);
}

</span><span style="color: rgba(0, 128, 0, 1)">/**</span><span style="color: rgba(0, 128, 0, 1)">
 * 等到方法级别注解的信息
 * </span><span style="color: rgba(128, 128, 128, 1)">@param</span><span style="color: rgba(0, 128, 0, 1)"> scannerClass：需要被扫描的class文件
 * </span><span style="color: rgba(128, 128, 128, 1)">@param</span><span style="color: rgba(0, 128, 0, 1)"> allowInjectClass：注解的文件
 * </span><span style="color: rgba(128, 128, 128, 1)">@return</span>
 <span style="color: rgba(0, 128, 0, 1)">*/</span>
<span style="color: rgba(0, 0, 255, 1)">public</span> List&lt;Annotation&gt; getMethodAnnotation(Class&lt;?&gt; scannerClass , Class&lt;? <span style="color: rgba(0, 0, 255, 1)">extends</span> Annotation&gt;<span style="color: rgba(0, 0, 0, 1)"> allowInjectClass) {
    List</span>&lt;Annotation&gt; annotations = <span style="color: rgba(0, 0, 255, 1)">new</span> ArrayList&lt;Annotation&gt;<span style="color: rgba(0, 0, 0, 1)">();
    
    </span><span style="color: rgba(0, 0, 255, 1)">for</span><span style="color: rgba(0, 0, 0, 1)">(Method method : scannerClass.getDeclaredMethods()) {
        </span><span style="color: rgba(0, 0, 255, 1)">if</span>(!<span style="color: rgba(0, 0, 0, 1)">method.isAnnotationPresent(allowInjectClass)) {
            </span><span style="color: rgba(0, 0, 255, 1)">continue</span><span style="color: rgba(0, 0, 0, 1)">;
        }
        
        annotations.add(method.getAnnotation(allowInjectClass));
    }
    
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> annotations;
}

</span><span style="color: rgba(0, 128, 0, 1)">/**</span><span style="color: rgba(0, 128, 0, 1)">
 * 使用Java反射得到注解的信息
 * </span><span style="color: rgba(128, 128, 128, 1)">@param</span><span style="color: rgba(0, 128, 0, 1)"> annotation
 * </span><span style="color: rgba(128, 128, 128, 1)">@param</span><span style="color: rgba(0, 128, 0, 1)"> methodName
 * </span><span style="color: rgba(128, 128, 128, 1)">@return</span><span style="color: rgba(0, 128, 0, 1)">
 * </span><span style="color: rgba(128, 128, 128, 1)">@throws</span><span style="color: rgba(0, 128, 0, 1)"> NoSuchMethodException
 * </span><span style="color: rgba(128, 128, 128, 1)">@throws</span><span style="color: rgba(0, 128, 0, 1)"> SecurityException
 * </span><span style="color: rgba(128, 128, 128, 1)">@throws</span><span style="color: rgba(0, 128, 0, 1)"> IllegalAccessException
 * </span><span style="color: rgba(128, 128, 128, 1)">@throws</span><span style="color: rgba(0, 128, 0, 1)"> IllegalArgumentException
 * </span><span style="color: rgba(128, 128, 128, 1)">@throws</span><span style="color: rgba(0, 128, 0, 1)"> InvocationTargetException
 </span><span style="color: rgba(0, 128, 0, 1)">*/</span>
<span style="color: rgba(0, 0, 255, 1)">public</span> Object getAnnotationInfo(Annotation annotation , String methodName) <span style="color: rgba(0, 0, 255, 1)">throws</span><span style="color: rgba(0, 0, 0, 1)"> NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
    </span><span style="color: rgba(0, 0, 255, 1)">if</span>(annotation == <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">) {
        </span><span style="color: rgba(0, 0, 255, 1)">return</span> <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">;
    }
    
    Method method </span>= annotation.getClass().getDeclaredMethod(methodName, <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">);
    </span><span style="color: rgba(0, 0, 255, 1)">return</span> method.invoke(annotation, <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">);
}



}

要判断是否存在指定的 Java 注解，只需要调用isAnnotationPresent 方法，就能够实现是否存在制定的注解。那么，方法以及类上的注解判断，就能够轻松搞定。那么，接下来，就是要获取注解的详细信息。通过调用getAnnotation 方法，就能够获取得到该注解，然后通过 Java 的反射，就能够获取得到该注解中指定方法的结果值。