Java 注解和反射（六）获取泛型，注解信息

反射操作泛型

**Java采用泛型擦出的机制来引入泛型，Java中的泛型仅仅是给编译器javac使用的，确保数据

　的安全性和免除强制类型转换问题，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

**ParmeterizedType：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

**TypeVarible：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

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public class Test10 {     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 = Test10.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 = Test10.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);             }         }     } }

 

 

反射操作注解

getAnnotations

getAnnotation

 

练习ORM

了解什么是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

要求：利用注解和反射完成类和表结构的映射关系

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//练习反射操作注解 public class Test11 {     public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {         Class c1 = Class.forName("refection.Student");         //通过反射获得注解         Annotation[] annotations = c1.getAnnotations();         for (Annotation annotation : annotations) {             System.out.println(annotation);         }         //获得注解Value的值         TableLuo tableLuo = (TableLuo) c1.getAnnotation(TableLuo.class);         String value = tableLuo.value();         System.out.println(value);           //获得属性的注解         Field name = c1.getDeclaredField("name");         FieldLuo annotations1 = name.getAnnotation(FieldLuo.class);         System.out.println(annotations1.columnName());         System.out.println(annotations1.length());         System.out.println(annotations1.type());     } }   @TableLuo("db_student") class Student{     @FieldLuo(columnName = "db_id",type = "int",length = 10)     private int id;     @FieldLuo(columnName = "db_age",type = "int",length = 10)     private int age;     @FieldLuo(columnName = "db_name",type = "varchar",length = 3)     private String name;       public Student() {     }       public Student(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 "student{" +                 "id=" + id +                 ", age=" + age +                 ", name='" + name + '\'' +                 '}';     } }   //类名注解 @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @interface TableLuo{     String value(); }   //属性注解 @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) @interface FieldLuo{     String columnName();     String type();     int length(); }