JAVA动态性——反射机制(三:通过反射操作注解、泛型)

发布于 2020-9-21 14:47
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反射操作注解


可以通过反射API: getAnnotations, getAnnotation获得相关的注解信息

 

Bean.class

package first.reflect;

 

@table("person")

public class Bean {

    @Myfield(column = "id", type = "int", length = 10)

    private int id;

    @Myfield(column = "name", type = "string", length = 10)

    private String name;

    @Myfield(column = "age", type = "int", length = 10)

    private int age;

 

    public Bean() {

 

    }

 

    public Bean(int age, String name, int id) {

        this.age = age;

        this.name = name;

        this.id = id;

    }

 

    @Myfun("getAge函数")

    public int getAge() {

        return age;

    }

 

    public void setAge(int age) {

        this.age = age;

    }

 

    public String getName() {

        return name;

    }

 

    public void setName(String name) {

        this.name = name;

    }

 

    public int getId() {

        return id;

    }

 

    public void setId(int id) {

        this.id = id;

    }

}

 

测试代码:

package first.reflect;

 

import java.lang.annotation.*;

import java.lang.reflect.Field;

import java.lang.reflect.Method;

 

/**

 * 反射获取注解

 */

@table("person")

public class test06 {

    public static void main(String[] args) throws Exception {

        Bean bean = new Bean();

        Class c = bean.getClass();

 

        //获取类的所有注解

        Annotation annotation[] = c.getAnnotations();

        for (Annotation a : annotation)

            System.out.println(a);

 

        //获得类的指定注解

        table t = (table) c.getAnnotation(table.class);

        System.out.println(t.value());

 

        //获得类的属性的注解

        Field f = c.getDeclaredField("name");

        Myfield myfield = f.getAnnotation(Myfield.class);

        System.out.println(myfield.column() + " " + myfield.type() + " " + myfield.length());

 

        //获得类方法的注解

        Method m = c.getDeclaredMethod("getAge");

        Myfun myfun = m.getAnnotation(Myfun.class);

        System.out.println(myfun.value());

    }

}

 

@Target(value = ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@interface table {

    String value();

}

 

@Target(value = ElementType.FIELD)

@Retention(RetentionPolicy.RUNTIME)

@interface Myfield {

    String column();

 

    String type();

 

    int length();

}

 

@Target(value = ElementType.METHOD)

@Retention(RetentionPolicy.RUNTIME)

@interface Myfun {

    String value();

}

 

反射操作泛型


Java采用泛型擦除的机制来引入泛型。Java中的泛型仅仅是给编译器javac使用的,确保数据的安全性和免去强制类型转换的麻烦。但是,一旦编译完成,所有的和泛型有关的类型全部擦除。

 

为了通过反射操作这些类型以迎合实际开发的需要,Java就新增了ParameterizedType,GenericArrayType,TypeVariable和WildcardType几种类型来代表不能被归一到Class类中的类型但是又和原始类型齐名的类型

package first.reflect;

 

import java.lang.reflect.Method;

import java.lang.reflect.ParameterizedType;

import java.lang.reflect.Type;

import java.util.List;

import java.util.Map;

 

/**

 * 通过反射读取泛型

 */

public class test05 {

    public static void test1(Map<String, Bean> map, List<Bean> list) {

        System.out.println("test1");

    }

 

    public static Map<Integer, Bean> test2() {

        System.out.println("test2");

        return null;

    }

 

    public static void main(String[] args) throws Exception {

        //返回指定方法参数的泛型信息

        Method method = test05.class.getDeclaredMethod("test1", Map.class, List.class);

        Type[] types = method.getGenericParameterTypes();

        for (Type t : types) {

            System.out.println("#" + t);

            if (t instanceof ParameterizedType) {

                Type[] genericTypes = ((ParameterizedType) t).getActualTypeArguments();

                for (Type generictype : genericTypes)

                    System.out.println("泛型类型" + generictype);

            }

        }

        //返回指定方法返回值的泛型信息

        Method method1 = test05.class.getDeclaredMethod("test2", null);

        Type returnType = method1.getGenericReturnType();

        if (returnType instanceof ParameterizedType) {

            Type[] genericTypes1 = ((ParameterizedType) returnType).getActualTypeArguments();

            for (Type generictype : genericTypes1)

                System.out.println("返回值的泛型类型" + generictype);

        }

    }

 

}

 

 

作者:Baret H ~

来源:CSDN

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