java annotation（如何创建新的注解）小结

“注解”有三种

1：无实际参数，只有声明

2：只存在单一参数，有些像方法调用

3：有多个参数

标准的“注解”就先不总结了。

想总结一下《如何创建自己的注解》。有很多流行的框架都会用到，所以对以后的学习也会有帮助。

1.无实际参数，只有声明(表达某种含义)

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

/*
 * 独自开发的注解
 * @auther Z,wk
 * @version 1.0
 *
 */

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface SampleRequired{
}

2.有单一参数或多个参数

package sample;

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

//ElementType.TYPE 类的定义

@Target({ 
　　ElementType.TYPE, 
   ElementType.FIELD, 
   ElementType.CONSTRUCTOR, 
   ElementType.METHOD 
}) 
@Retention(RetentionPolicy.RUNTIME) 
public @interface Info { 
　　　String value(); 
     //此处还可以继续添加 
     //String value2(); 
}

3.使用方法

package sample;

import java.util.ArrayList;
import java.util.List;

// ElementType.TYPE :添加到类或接口的定义上
@Info("SampleClass1 Info")
public class SampleClass1 {

    private List list;

    public SampleClass1(){

    }

    @Override
    public boolean equals(Object obj){
        return list.equals(obj);
    }
　　// ElmentType.Method:添加到方法声明上
    @Info("hogehoge")
    public void initList(){
        list = new ArrayList();
        list.add(10);
    }

}

package sample;

@Info("Sample2 class")
public class SampleClass2 {
    
    //ElementType.FIELD : 添加到成员变量上
    @Info("foo field")
    private Foo foo;
    
    //ElementType.CONSTRUCTOR : 添加到构造方法上
    @Info("default constract")
    public SampleClass2(){
        foo = new Foo();
    }
    
    // ElementType.METHOD : 添加到方法声明上
    @Info("bar method")
    public void bar() {
        foo.bar();
    }

}

4. 取得调用“注解”的对象（笔者理解为：很多框架使用注解的意义所在）

package sample;

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

public class Client {
    public static void main(String[] args){
        System.out.println("start annotation sample");
        SampleClass1 sc1 = new SampleClass1();
        SampleClass2 sc2 = new SampleClass2();

        //取得指定的“注解”
        Annotation annotList[] = Info.class.getAnnotations();
        System.out.println("annotation size is [" + annotList.length + "]");
        for(int i=0; i<annotList.length; i++){
            Annotation anno = annotList[i];
            System.out.println("    annotation class is [" + anno.annotationType().getName() + "]");
        }

        /****取得带有注解的CLASS****/
        //SampleClass1
        Annotation annotList1[] = SampleClass1.class.getAnnotations();
        System.out.println("SampleClass1's annotation size is [" + annotList1.length + "]");
        for(int i=0; i<annotList1.length; i++){
            Annotation anno1 = annotList1[i];
            System.out.println("    annotation class is [" + anno1.annotationType().getName() + "]");
        }
        //SampleClass2
        Annotation annotList2[] = SampleClass2.class.getAnnotations();
        System.out.println("SampleClass2's annotation size is [" + annotList2.length + "]");
        for(int i=0; i<annotList2.length; i++){
            Annotation anno2 = annotList2[i];
            System.out.println("    annotation class is [" + anno2.annotationType().getName() + "]");
        }
        System.out.println();
        System.out.println();

        /****取得带有注解的METHOD****/
        //SampleClass1
        Method methodList1[] = SampleClass1.class.getMethods();
        System.out.println("SampleClass1's method count is [" + methodList1.length + "]");
        for(Method method : methodList1){
            System.out.println("    method name is [" + method.getName() + "]");
            for(Annotation annot : method.getAnnotations()){
                System.out.println("        method annotation is [" + annot.annotationType().getName() + "]");
            }
        }
        //SampleClass2
        Method methodList2[] = SampleClass1.class.getMethods();
        System.out.println("SampleClass2's method count is [" + methodList2.length + "]");
        for(Method method : methodList2){
            System.out.println("    method name is [" + method.getName() + "]");
            for(Annotation annot : method.getAnnotations()){
                System.out.println("        method annotation is [" + annot.annotationType().getName() + "]");
            }
        }
        System.out.println();
        System.out.println();

        /****取得带有注解的FIELD（成员变量）****/
        //SampleClass1
        Field fieldList1[] = SampleClass1.class.getFields();
        System.out.println("SampleClass1 has  [" + fieldList1.length + "] fields");
        for(Field field : fieldList1){
            System.out.println("     field name is [" + field.getName() +"]");
            for(Annotation annot : field.getAnnotations()){
                System.out.println("        field annotation is [" + annot.annotationType().getName() + "]");
            }
        }
        //SampleClass1
        Field fieldList2[] = SampleClass2.class.getDeclaredFields();//getFields();
        System.out.println("SampleClass2 has  [" + fieldList2.length + "] fields");
        for(Field field : fieldList2){
            System.out.println("     field name is [" + field.getName() +"]");
            for(Annotation annot : field.getAnnotations()){
                System.out.println("        field annotation is [" + annot.annotationType().getName() + "]");
            }
        }
        System.out.println();
        System.out.println();

    }

}

本人属于初学者，有不对的地方希望过路的大神批评指正。