【学习笔记】Java 反射机制-2（反射操作泛型、注解）

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问题﹐但是，一旦编译完成,所有和泛型有关的类型全部擦除

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

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

package com.kuang.reflection;
import java.util.List;
import java.util.Map;
//通过反射获取泛型
public class Test11 {
	public void test01(Map map，List list){
		system.out.println( "test01");
    }
    public Map test02(){
        system.out.println( "test02");
    	return null;
    }
	public static void main(string[] args) throws NoSuchMethodException {
		Method method = Test11.class.getMethod(name:"test01",Map.class,List.class);
		Type[] genericParameterTypes = method.getGenericParameterTypes();
		for(Type genericParameterType : genericParameterTypes){
			system.out.println("#"+genericParameterType);
			if (genericParameterType instanceof ParameterizedType){
                //获取真实的参数信息
			Type[] actualTypeArguments = ((ParameterizedType) genericParameterType) .getActualTypeArguments;
            for(Type actualTypeArgument:actualTypeArguments) {
				system.out.println(actualTypeArgument);
            }
            }
        }
        //返回值类型
        method = Test11.class.getMethod( name: "test02"，..parameterTypes: null);
        Type genericReturnType = method.getGenericReturnType() ;
		if (genericReturnType instanceof ParameterizedType){
			Type[] actualTypeArguments = ((ParameterizedType)genericReturnType).getActualTypeArguments();
            for(Type actualTypeArgument:actualTypeArguments){
				system.out.println(actualTypeArgument);
            }
        }
    }
}

getAnnotations

getAnnotation

练习:0RM

了解什么是ORM ?

Object relationship Mapping -->对象关系映射

class Student{
int id;
String name;
int age;
}

类和表结构对应属性和字段对应对象和记录对应

要求:利用注解和反射完成类和表结构的映射关系

package com. kuang.reflection;
//练习反射操作注解
public class Test12 {
    public static void main(String[] args) throws ClassNotFoundException {
	class c1 = class.forName ( "com.kuang.reflection.student2");
	//通过反射获得注解
	Annotation[] annotations = c1.getAnnotations();
    for (Annotation annotation : annotations){
			system.out.println(annotation);
    }
    //获得注解的value的值.
	Tablekuang tablekuang = (Tablekuang)c1.getAnnotation(Tablekuang.class);
    String value = tablekuang.value();
	system.out.println(value);
	//获得类指定的注解
	Field f = ci.getDeclaredField ( name: "name" );//可以换成id  age等
	Fieldkuang annotation = f.getAnnotation(Fieldkuang.class);
    system.out.println(annotation. columnName());
	system.out.println(annotation.type());
	System.out.println(annotation.length());    
        
    }

}
@Tablekuang("db_student")
class student2{
    @Fieldkuang(columnNmae="db_id",type="int",length=10)
	private int id;
    @Fieldkuang(columnNmae="db_age",type="int",length=10)
    private int age;
    @Fieldkuang(columnNmae="db_id",type="varchar",length=10)
    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 void setId(int id) {
		this.id = id;
	}
	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;
    }
    override
	public string tostring() {
	return "student2i" +
			"id=" + id +
			",age=" + age +
			", name='" + name + ''' +
        	'}';
    }
}



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

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