反射获取注解，框架的基础

package aaa;


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

public class test{
    public static void main(String[] args) throws ClassNotFoundException {
        Class<?> u1 = Class.forName("aaa.User");
        //获取类的注解
        Annotation[] annotations = u1.getAnnotations();
        for(Annotation annotation:annotations){
            System.out.println(annotation);//@aaa.TableName("user")
        }
        //获得注解的value
        TableName tableName = u1.getAnnotation(TableName.class);
        String value = tableName.value();
        System.out.println(value);//user

        //获得类属性上的注解
        Field[] declaredFields = u1.getDeclaredFields();
        for(Field field:declaredFields){
            TableField annotation = field.getAnnotation(TableField.class);
            System.out.println(annotation+":"+annotation.columnName()+"--"+annotation.type());
            /*
              @aaa.TableField(columnName="id", type="int"):id--int
　　　　　　　  @aaa.TableField(columnName="account", type="varchar"):account--varchar
　　　　　　　　@aaa.TableField(columnName="password", type="varchar"):password--varchar
　　　　　　　　*/
        }
    }
}


@TableName("user")
class User{

    @TableField(columnName = "id", type = "int")
    private Long id;

    @TableField(columnName = "account", type = "varchar")
    private String account;

    @TableField(columnName = "password", type = "varchar")
    private String password;

    public Long getId() {
        return id;
    }

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

    public String getAccount() {
        return account;
    }

    public void setAccount(String account) {
        this.account = account;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", account='" + account + '\'' +
                ", password='" + password + '\'' +
                '}';
    }
}

//类名注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableName{
    String value();
}
//属性注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface TableField{
    String columnName();
    String type();
}

结合 ORM 框架时，一般会使用注解来标识实体类与数据库表之间的映射关系

package aaa;

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

// 模拟 ORM 框架中的实体类
@Entity(tableName = "user_table")
class User {
    @Id
    @Column(name = "id", type = "bigint")
    private Long id;

    @Column(name = "account", type = "varchar(50)")
    private String account;

    @Column(name = "password", type = "varchar(50)")
    private String password;

    // 省略 getter 和 setter 方法

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", account='" + account + '\'' +
                ", password='" + password + '\'' +
                '}';
    }
}

// 注解：实体类
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Entity {
    String tableName();
}

// 注解：主键
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface Id {
}

// 注解：字段
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface Column {
    String name();
    String type();
}

// 测试类
public class Test {
    public static void main(String[] args) throws ClassNotFoundException {
        Class<?> userClass = Class.forName("aaa.User");

        // 获取实体类的注解
        Entity entityAnnotation = userClass.getAnnotation(Entity.class);
        String tableName = entityAnnotation.tableName();
        System.out.println("Table Name: " + tableName);

        // 获取实体类的所有字段
        Field[] fields = userClass.getDeclaredFields();
        for (Field field : fields) {
            if (field.isAnnotationPresent(Column.class)) {
                Column columnAnnotation = field.getAnnotation(Column.class);
                String columnName = columnAnnotation.name();
                String columnType = columnAnnotation.type();
                System.out.println("Column: " + columnName + ", Type: " + columnType);
            } else if (field.isAnnotationPresent(Id.class)) {
                System.out.println("Primary Key: " + field.getName());
            }
        }
    }
}

输出

Table Name: user_table
Column: id, Type: bigint
Column: account, Type: varchar(50)
Column: password, Type: varchar(50)