java 反射获取泛型类型 java反射设置泛型

Java反射操作泛型

1.Java泛型

Java采用泛型擦除机制来引入泛型，Java中的放行仅仅是给编译器javac使用的，确保数据的安全性二号免去强制类型转换问题，但是一旦编译完成，所有和泛型有关的类型会全部擦除

2.通过反射操作泛型

为了通过反射操作这些类型，java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

• ParameterizedType：表示一种参数化类型，比如Collection< String >
• GenericArrayType：表示一种元素类型是参数化类型或者类型变量的数组类型
• TypeVariable：是各种类型变量的公共父接口
• WildcardType：代表一种通配符类型表达式

3. 反射操作泛型代码示例

import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;

//通过反射获取泛型
public class Test11 {
    public void test01(Map<String,User> map, List<User> list){
        System.out.println("test01");

    }
    public Map<String,User> test02(){
        System.out.println("test02");
        return null;
    }

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

        Method method = Test11.class.getMethod("test01", Map.class, List.class);
        Type[] genericParameterTypes = method.getGenericParameterTypes();

        for (Type genericParameterType: genericParameterTypes
             ) {
            System.out.println("#" + genericParameterType);
            if(genericParameterType instanceof ParameterizedType){
                Type[] actualTypeArguments = ((ParameterizedType) genericParameterType).getActualTypeArguments();
                for (Type actualTypeArgument:actualTypeArguments
                     ) {
                    System.out.println(actualTypeArgument);
                }
            }
        }

        method = Test11.class.getMethod("test02", null);
        Type genericReturnType = method.getGenericReturnType();

        if(genericReturnType instanceof ParameterizedType){
            Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments();
            for (Type actualTypeArgument:actualTypeArguments
            ) {
                System.out.println(actualTypeArgument);
            }
        }

    }

}

4.反射操作注解实例

1.了解ORM

• Object Relationship Mapping --> 对象关系映射
• 类和表结构对应
• 属性和字段对应
• 对象和记录对应

2.利用注解和放射完成类和表结构的映射关系

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

//反射操作注解
public class Test12 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1 = Class.forName("Student2");

        //通过反射获得注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation:annotations
             ) {
            System.out.println("annotation" + annotation);
        }

        //获取注解指定的值
        TableMichal tablemichal = (TableMichal)c1.getAnnotation(TableMichal.class);
        String value = tablemichal.value();
        System.out.println("value" + value);
        
        //获取类指定的注解
        Field f = c1.getDeclaredField("name");
        FieldMichael annotation = f.getAnnotation(FieldMichael.class);
        System.out.println(annotation.columnName());
        System.out.println(annotation.length());
        System.out.println(annotation.type());


    }
}

@TableMichal("db_student")
class Student2{
    @FieldMichael(columnName = "db_id",type = "int", length = 10)
    private int id;
    @FieldMichael(columnName = "db_age",type = "int", length = 10)
    private int age;
    @FieldMichael(columnName = "db_name",type = "verchar", length = 3)
    private String name;

    public Student2() {
    }

    public Student2(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public int getAge() {
        return age;
    }

    public String getName() {
        return name;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Student2{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}


//类名的注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableMichal{
    String value();
}

//属性的注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FieldMichael{
    String columnName();
    String type();
    int length();
}