实现了数据层的功能

src/main/kotlin/com/msksbr/bookmgr/BookMgrApplication.kt

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+package com.msksbr.bookmgr
+
+import org.springframework.boot.autoconfigure.SpringBootApplication
+import org.springframework.boot.runApplication
+
+@SpringBootApplication
+class BookMgrApplication
+
+fun main(args: Array<String>) {
+    runApplication<BookMgrApplication>(*args)
+}

src/main/kotlin/com/msksbr/bookmgr/entity/Book.kt

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+package com.msksbr.bookmgr.entity
+
+import com.baomidou.mybatisplus.annotation.IdType
+import com.baomidou.mybatisplus.annotation.TableId
+import com.baomidou.mybatisplus.annotation.TableName
+
+@TableName("book")
+data class Book(
+    @TableId(type = IdType.AUTO)
+    val id: Long?=null,
+    var name: String,
+    var author: String,
+    var stock:Int
+)

src/main/kotlin/com/msksbr/bookmgr/entity/BorrowRecord.kt

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+package com.msksbr.bookmgr.entity
+
+import com.baomidou.mybatisplus.annotation.IdType
+import com.baomidou.mybatisplus.annotation.TableId
+import com.baomidou.mybatisplus.annotation.TableName
+import java.util.Date
+
+@TableName("book_record")
+data class BorrowRecord(
+    @TableId(type = IdType.AUTO)
+    val id:Long?=null,
+    var userId:Long,
+    var bookId:Long,
+    var borrowTime: Date,
+    var returnTime:Date?=null,
+    var status:String
+)

src/main/kotlin/com/msksbr/bookmgr/entity/User.kt

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+package com.msksbr.bookmgr.entity
+
+import com.baomidou.mybatisplus.annotation.IdType
+import com.baomidou.mybatisplus.annotation.TableId
+import com.baomidou.mybatisplus.annotation.TableName
+
+@TableName("user")
+data class User(
+    @TableId(type = IdType.AUTO)  // 设置自增主键
+    val id: Long?=null,
+    var username: String,
+    var password: String,
+    var role: String,
+)

src/main/kotlin/com/msksbr/bookmgr/mapper/BookMapper.kt

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+package com.msksbr.bookmgr.mapper
+
+import com.baomidou.mybatisplus.core.mapper.BaseMapper
+import com.msksbr.bookmgr.entity.Book
+import org.apache.ibatis.annotations.Mapper
+
+@Mapper
+interface BookMapper: BaseMapper<Book>

src/main/kotlin/com/msksbr/bookmgr/mapper/BorrowRecordMapper.kt

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+package com.msksbr.bookmgr.mapper
+
+import com.baomidou.mybatisplus.core.mapper.BaseMapper
+import com.msksbr.bookmgr.entity.Book
+import org.apache.ibatis.annotations.Mapper
+
+@Mapper
+interface BorrowRecordMapper: BaseMapper<Book>

src/main/kotlin/com/msksbr/bookmgr/mapper/UserMapper.kt

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+package com.msksbr.bookmgr.mapper
+
+import com.baomidou.mybatisplus.core.mapper.BaseMapper
+import com.msksbr.bookmgr.entity.User
+import org.apache.ibatis.annotations.Mapper
+
+@Mapper
+interface UserMapper: BaseMapper<User>

src/main/resources/application.yaml

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+spring:
+  profiles:
+    active: dev
+  application:
+    name: bookMgr
+  datasource:
+    driver-class-name: ${DB_DRIVER}
+    url: jdbc:${DB_TYPE}://${DB_URL}:${DB_PORT}/${DB_NAME}
+    username: ${DB_USER}
+    password: ${DB_PASSWORD}
+
+mybatis-plus:
+  configuration:
+    # 开启驼峰命名法
+    map-underscore-to-camel-case: true
+    # 开启日志输出sql语句
+    log-impl: org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl

src/test/kotlin/com/msksbr/bookmgr/BookMgrApplicationTests.kt

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+package com.msksbr.bookmgr
+
+import org.junit.jupiter.api.Test
+import org.springframework.boot.test.context.SpringBootTest
+
+@SpringBootTest
+class BookMgrApplicationTests {
+
+    @Test
+    fun contextLoads() {
+    }
+
+}

src/test/kotlin/com/msksbr/bookmgr/mapper/UserMapperTest.kt

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+package com.msksbr.bookmgr.mapper
+
+import com.msksbr.bookmgr.entity.User
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertNotNull
+import org.junit.jupiter.api.Assertions.assertNull
+import org.junit.jupiter.api.Test
+import org.springframework.beans.factory.annotation.Autowired
+import org.springframework.boot.test.context.SpringBootTest
+import org.springframework.transaction.annotation.Transactional
+
+@SpringBootTest
+@Transactional // 测试完可以自动回滚
+class UserMapperTest {
+    // Spring自动装配userMapper
+    @Autowired
+    private lateinit var userMapper: UserMapper
+
+    @Test
+    // 新增测试
+    fun testInsert() {
+        // 准备用户
+        val user = User(null,"Test", "123456", "admin")
+        // 插入
+        userMapper.insert(user)
+        // 拿到插入后的自增id
+        val userId = user.id ?: throw AssertionError("用户插入失败，ID为空")
+        // 查询插入后的用户
+        val res = userMapper.selectById(userId)
+        // 比对
+        assertNotNull(res)
+        assertEquals(user.username, res.username)
+        assertEquals(user.password, res.password)
+        assertEquals(user.role, res.role)
+    }
+    // 修改测试
+    @Test
+    fun testUpdate() {
+        // 先新增
+        val user = User(null, "OldName", "123456", "admin")
+        userMapper.insert(user)
+        val userId = user.id!!
+        // 修改字段
+        user.username="newName"
+        user.password="654321"
+        userMapper.updateById(user)
+        // 比对
+        val res = userMapper.selectById(userId)
+        assertNotNull(res)
+        assertEquals(user.username, res.username)
+        assertEquals(user.password, res.password)
+        assertEquals(user.role, res.role)
+    }
+    // 删除测试
+    @Test
+    fun testDelete() {
+        // 新增
+        val user = User(null,"DelTest", "123456", "admin")
+        userMapper.insert(user)
+        val userId = user.id!!
+        // 删除
+        userMapper.deleteById(userId)
+        // 确认删除
+        val res = userMapper.selectById(userId)
+        assertNull(res)
+    }
+}