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《Java基础知识》Java注解"@"详解

Java 注解含义：

Java 注解，顾名思义，注解, 就是对某一事物进行添加注释说明，会存放一些信息，这些信息可能对以后某个时段来说是很有用处的。
Java 注解又叫 java 标注，java 提供了一套机制，使得我们可以对方法、类、参数、包、域以及变量等添加标准 (即附上某些信息)。且在以后某个时段通过反射将标注的信息提取出来以供使用。

 

样例（实现一个自己的注解）：

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

@Target(value = {ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotaion {
String value();
}

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

@Target(value = {ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyFieldAnnotaion {
String value();
String type();
String lengths();
}

import java.io.Serializable;

@MyAnnotaion("student")
public class Student implements Serializable {


</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> <span style="color: rgba(0, 0, 255, 1)">long</span> serialVersionUID = 1L<span style="color: rgba(0, 0, 0, 1)">;

@MyFieldAnnotaion(value </span>= "1",type = "int",lengths = "10"<span style="color: rgba(0, 0, 0, 1)">)
String id;
@MyFieldAnnotaion(value </span>= "蕾蕾",type = "String" ,lengths = "200"<span style="color: rgba(0, 0, 0, 1)">)
String name;

</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getId() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> id;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setId(String id) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.id =<span style="color: rgba(0, 0, 0, 1)"> id;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getName() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> name;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setName(String name) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.name =<span style="color: rgba(0, 0, 0, 1)"> name;
}

@Override
</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String toString() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span> "Student{" +
            "id='" + id + '\'' +
            ", name='" + name + '\'' +
            '}'<span style="color: rgba(0, 0, 0, 1)">;
}



}

import org.apache.poi.ss.formula.functions.T;

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;


/**


    

  • 

    通过反射获取注解
    */
    public class AnnotaionDemo {
    public static void main(String[] args) throws Exception {
    Class    <T> clazz = (Class<T>) Class.forName("demo.knowledgepoints.annotation.Student");
    //判断注解是否存在
    if(clazz.isAnnotationPresent(MyAnnotaion.class)){
    System.out.println(    "存在注解：MyAnnotaion");
    }
    

     </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">获取注解列表</span>
 Annotation[] annotaions =<span style="color: rgba(0, 0, 0, 1)"> clazz.getAnnotations();
 </span><span style="color: rgba(0, 0, 255, 1)">for</span> (<span style="color: rgba(0, 0, 255, 1)">int</span> i = 0; i &lt; annotaions.length; i++<span style="color: rgba(0, 0, 0, 1)">) {
     System.out.println(</span>"注解："+<span style="color: rgba(0, 0, 0, 1)">annotaions[i]);
 }

 </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">获取注解内容</span>
 MyAnnotaion myAnnotaion = clazz.getAnnotation(MyAnnotaion.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">) ;
 System.out.println(</span>"注解的值为："+<span style="color: rgba(0, 0, 0, 1)">myAnnotaion.value());

 </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">获取方法上的注解内容</span>
 Field field = clazz.getDeclaredField("id"<span style="color: rgba(0, 0, 0, 1)">);

 </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">方法上注解是否存在</span>
 field.isAnnotationPresent(MyFieldAnnotaion.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">);
 </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">方法注解列表</span>

    

    • 



        field.getAnnotations();


    </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">获取方法上的注解内容</span>
    MyFieldAnnotaion myFieldAnnotaion = field.getAnnotation(MyFieldAnnotaion.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">);
    System.out.println(</span>"value:"+myFieldAnnotaion.value()+"; type:"+myFieldAnnotaion.type()+"; lengths:"+<span style="color: rgba(0, 0, 0, 1)">myFieldAnnotaion.lengths());
}



}

运行结果：

根据这个案例：

1. 注解实现需要关键字：@interface；

2. 在注解上我们还使用了注解 @Target，@Retention。这些被称为元注解，

   Java 中元注解有四个： @Retention @Target @Document @Inherited；

   @Retention：注解的保留位置

        @Retention(RetentionPolicy.SOURCE)   // 注解仅存在于源码中，在 class 字节码文件中不包含

　    @Retention(RetentionPolicy.CLASS)     // 默认的保留策略，注解会在 class 字节码文件中存在，但运行时无法获得，

　    @Retention(RetentionPolicy.RUNTIME)  // 注解会在 class 字节码文件中存在，在运行时可以通过反射获取到

    @Target: 注解的作用目标

        @Target(ElementType.TYPE)   // 接口、类、枚举

        @Target(ElementType.FIELD) // 字段、枚举的常量

        @Target(ElementType.METHOD) // 方法

        @Target(ElementType.PARAMETER) // 方法参数

        @Target(ElementType.CONSTRUCTOR)  // 构造函数

        @Target(ElementType.LOCAL_VARIABLE)// 局部变量

        @Target(ElementType.ANNOTATION_TYPE)// 注解

        @Target(ElementType.PACKAGE) /// 包

    @Document：说明该注解将被包含在 javadoc 中

    @Inherited：说明子类可以继承父类中的该注解

3. 直接里面可以写方法：String value(); 

   外部使用注解可以将值写入，后续可以拿到该值进行使用。

可以提供默认值： String value() default "花花";

 

修改 AnnotaionDemo 的方法实现一个常用功能，通过注解将值注入实体类中： 

import org.apache.poi.ss.formula.functions.T;

import java.lang.reflect.Field;
import java.lang.reflect.Method;


/**


    

  • 

    通过反射获取注解
    */
    public class AnnotaionDemo {
    public static void main(String[] args) throws Exception {
    Student student     = (Student) getbean("demo.knowledgepoints.annotation.Student");
    System.out.println(student);
    }
    

    public static Object getbean(String className) throws Exception{
    Class    <T> clazz = (Class<T>)Class.forName(className);
    //判断注解是否存在 (该注解被当成标记使用)
    if(!clazz.isAnnotationPresent(MyAnnotaion.class)){
    throw new Exception("该类缺失注解“MyAnnotaion”，不能通过反射获取 bean");
    }
    

     </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">创建实体类</span>
 Object object =<span style="color: rgba(0, 0, 0, 1)"> clazz.newInstance();

 Field[] fields </span>=<span style="color: rgba(0, 0, 0, 1)"> clazz.getDeclaredFields();
 </span><span style="color: rgba(0, 0, 255, 1)">for</span> (<span style="color: rgba(0, 0, 255, 1)">int</span> i = 0; i &lt; fields.length; i++<span style="color: rgba(0, 0, 0, 1)">) {
     </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">获取字段名</span>
     String name =<span style="color: rgba(0, 0, 0, 1)"> fields[i].getName();

     </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">过滤序列</span>
     <span style="color: rgba(0, 0, 255, 1)">if</span> (name.equals("serialVersionUID"<span style="color: rgba(0, 0, 0, 1)">)) {
         </span><span style="color: rgba(0, 0, 255, 1)">continue</span><span style="color: rgba(0, 0, 0, 1)">;
     }

     </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">组装SET方法</span>
     Class&lt;?&gt; type =<span style="color: rgba(0, 0, 0, 1)"> clazz.getDeclaredField(name).getType();

     </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 首字母大写</span>
     String replace = name.substring(0, 1<span style="color: rgba(0, 0, 0, 1)">).toUpperCase()
             </span>+ name.substring(1<span style="color: rgba(0, 0, 0, 1)">);
     Method setMethod </span>= clazz.getMethod("set" +<span style="color: rgba(0, 0, 0, 1)"> replace, type);
     </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">获取字段值</span>
     Field field =<span style="color: rgba(0, 0, 0, 1)"> clazz.getDeclaredField(name);
     MyFieldAnnotaion myFieldAnnotaion </span>= field.getAnnotation(MyFieldAnnotaion.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">);
     String value </span>=<span style="color: rgba(0, 0, 0, 1)"> myFieldAnnotaion.value();
     </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">执行set方法</span>
     <span style="color: rgba(0, 0, 255, 1)">if</span> (value != <span style="color: rgba(0, 0, 255, 1)">null</span> &amp;&amp; !""<span style="color: rgba(0, 0, 0, 1)">.equals(value)) {
         </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> ---判断读取数据的类型（判断部分类型）</span>
         <span style="color: rgba(0, 0, 255, 1)">if</span> (type.isAssignableFrom(String.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">)) {
             setMethod.invoke(object, value);
         } </span><span style="color: rgba(0, 0, 255, 1)">else</span> <span style="color: rgba(0, 0, 255, 1)">if</span> (type.isAssignableFrom(<span style="color: rgba(0, 0, 255, 1)">int</span>.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">)
                 </span>|| type.isAssignableFrom(Integer.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">)) {
             setMethod.invoke(object, Integer.parseInt(value));
         } </span><span style="color: rgba(0, 0, 255, 1)">else</span> <span style="color: rgba(0, 0, 255, 1)">if</span> (type.isAssignableFrom(Double.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">)
                 </span>|| type.isAssignableFrom(<span style="color: rgba(0, 0, 255, 1)">double</span>.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">)) {
             setMethod.invoke(object, Double.parseDouble(value));
         } </span><span style="color: rgba(0, 0, 255, 1)">else</span> <span style="color: rgba(0, 0, 255, 1)">if</span> (type.isAssignableFrom(Boolean.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">)
                 </span>|| type.isAssignableFrom(<span style="color: rgba(0, 0, 255, 1)">boolean</span>.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">)) {
             setMethod.invoke(object, Boolean.parseBoolean(value));
         }
     }
 }
 </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> object;

    

    }
    }

运行结果：

案例中使用了反射：反射详解：https://www.cnblogs.com/jssj/p/11723910.html

总结语：注解功能在框架开发中被广泛使用：例如 Spring，MyBatis 等。注解的出现大大的减少了开发工作中的开发工作量。