概述

ORM（Object Relationship Mapping）物件關係對映

• 類和表結構對應
• 屬性和欄位對應
• 物件和記錄對應

例項

/**
 * 練習反射操作註解
 */
public class Demo07 {
    public static void main(String[] args) throws Exception {
        Class c1 = Class.forName("com.gbhh.reflections.Student2");
        //通過反射獲得註解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation:annotations
             ) {
            System.out.println(annotation);
        }
        //獲取註釋的value值
        TableGang tableGang = (TableGang)c1.getAnnotation(TableGang.class);
        String value = tableGang.value();
        System.out.println(value);
//        獲取類指定的註解
        Field f = c1.getDeclaredField("name");
        FieldGang annotation = f.getAnnotation(FieldGang.class);
        System.out.println(annotation.columnName());
        System.out.println(annotation.type());
        System.out.println(annotation.length());

    }
}
@TableGang("db_student2")
class Student2{
    @FieldGang(columnName = "db_id",type = "int",length = 20)
    private int id;
    @FieldGang(columnName = "db_age",type = "int",length = 21)
    private int age;
    @FieldGang(columnName = "db_name",type = "varchar",length = 22)
    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;
    }
}
//類名的註解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableGang{
    String value();
}
//屬性的註解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FieldGang{
    String columnName();
    String type();
    int length();
}