@Repository

迈不过友情╰ 2022-03-10 00:35 143阅读 0赞

> 初学spring boot，遇到的第一个坎就是好多注解都不知道作用，即使照猫画虎把代码敲出来运行成功，也没有些许成就感。原因其实很简单，我觉得自己还没真的学会。接下来就是找资料，理解原理与过程，不求理解的深度，起码也得把自己说服了。本文档是从英语文档翻译过来，外加了一些自己的理解，其中表述错误的地方还望留言指正。

源文档链接：[http://zetcode.com/springboot/repository/][http_zetcode.com_springboot_repository]

--------------------

### @Repository ###

`@Repository`注解修饰哪个类，则表明这个类具有对对象进行CRUD（增删改查）的功能，而且`@Repository`是`@Component`注解的一个派生品，所以被`@Repository`注解的类可以自动的被`@ComponentScan` 通过路径扫描给找到。（这也在一定程度上解释了，为什么被`@Repository`注解的类也能`@Autowired`）

--------------------

### Spring Boot @Repository 例子 ###

下面这个例子将会讲述`@Repository`的用法，这个例子展示的是HTML表中的国家/地区列表。该程序使用的Spring boot 2.1.0版本。

pom.xml
    src
    ├───main
    │   ├───java
    │   │   └───com
    │   │       └───zetcode
    │   │           │   Application.java
    │   │           ├───controller
    │   │           │       MyController.java
    │   │           ├───model
    │   │           │       Country.java
    │   │           ├───repository
    │   │           │       CountryRepository.java
    │   │           └───service
    │   │                   CountryService.java
    │   │                   ICountryService.java
    │   └───resources
    │       │   application.yml
    │       │   import.sql
    │       ├───static
    │       │       index.html
    │       └───templates
    │               showCountries.ftl
    └───test
        └───java

这是工程架构。

> pom.xml

<?xml version="1.0" encoding="UTF-8"?>
    <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
        <modelVersion>4.0.0</modelVersion>
    
        <groupId>com.zetcode</groupId>
        <artifactId>repositoryex</artifactId>
        <version>1.0-SNAPSHOT</version>
        <packaging>jar</packaging>
    
        <properties>
            <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
            <maven.compiler.source>11</maven.compiler.source>
            <maven.compiler.target>11</maven.compiler.target>
        </properties>
    
        <parent>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-parent</artifactId>
            <version>2.1.0.RELEASE</version>
        </parent>
    
        <dependencies>
    
            <dependency>
                <groupId>com.h2database</groupId>
                <artifactId>h2</artifactId>
                <scope>runtime</scope>
            </dependency>
    
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-devtools</artifactId>
                <optional>true</optional>
            </dependency>
    
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-web</artifactId>
            </dependency>
    
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-freemarker</artifactId>
            </dependency>
    
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-data-jpa</artifactId>
            </dependency>
    
        </dependencies>
    
        <build>
            <plugins>
                <plugin>
                    <groupId>org.springframework.boot</groupId>
                    <artifactId>spring-boot-maven-plugin</artifactId>
                </plugin>
            </plugins>
        </build>
    
    </project>

这是Maven build文件，h2依赖包括H2数据库驱动。  
Spring boot starters是一系列非常方便的依赖描述符，它大大简化了maven的配置。`spring-boot-start-parent`包含spring boot application的常用配置；`spring-boot-devtools`包含spring boot的开发工具；`spring-boot-starter-web`支持classic和RESTFul的web应用程序启动；`spring-boot-starter-web-freemarker`是构建基于Freemarker模板引擎的web应用程序的启动器，它采用Tomcat作为默认的嵌入式容器。`spring-boot-starter-data-jpa`是基于Hibernate的Spring Data JPA的启动器。

> resources/application.yml

server:
      port: 8086
      servlet:
        context-path: /SpringBootRepository
    
    spring:
      main:
        banner-mode: "off"
      jpa:
        database: h2
        hibernate:
          dialect: org.hibernate.dialect.H2Dialect
          ddl-auto: create-drop
    
    logging:
      level:
        org:
          springframework: ERROR

在`application.yml`文件中，我们编写了spring boot应用程序的各种配置，服务器的端口和上下文的路径。完成这些设置后，我们通过`localhost:8086/SpringBootRepository/`访问应用程序。在该文件配置中关闭了`banner-mode`。

> resources/import.sql

INSERT INTO countries(name, population) VALUES('China', 1382050000);
    INSERT INTO countries(name, population) VALUES('India', 1313210000);
    INSERT INTO countries(name, population) VALUES('USA', 324666000);
    INSERT INTO countries(name, population) VALUES('Indonesia', 260581000);
    INSERT INTO countries(name, population) VALUES('Brazil', 207221000);
    INSERT INTO countries(name, population) VALUES('Pakistan', 196626000);
    INSERT INTO countries(name, population) VALUES('Nigeria', 186988000);
    INSERT INTO countries(name, population) VALUES('Bangladesh', 162099000);
    INSERT INTO countries(name, population) VALUES('Nigeria', 186988000);
    INSERT INTO countries(name, population) VALUES('Russia', 146838000);
    INSERT INTO countries(name, population) VALUES('Japan', 126830000);
    INSERT INTO countries(name, population) VALUES('Mexico', 122273000);
    INSERT INTO countries(name, population) VALUES('Philippines', 103738000);
    INSERT INTO countries(name, population) VALUES('Ethiopia', 101853000);
    INSERT INTO countries(name, population) VALUES('Vietnam', 92700000);
    INSERT INTO countries(name, population) VALUES('Egypt', 92641000);
    INSERT INTO countries(name, population) VALUES('Germany', 82800000);
    INSERT INTO countries(name, population) VALUES('the Congo', 82243000);
    INSERT INTO countries(name, population) VALUES('Iran', 82800000);
    INSERT INTO countries(name, population) VALUES('Turkey', 79814000);
    INSERT INTO countries(name, population) VALUES('Thailand', 68147000);
    INSERT INTO countries(name, population) VALUES('France', 66984000);
    INSERT INTO countries(name, population) VALUES('United Kingdom', 60589000);
    INSERT INTO countries(name, population) VALUES('South Africa', 55908000);
    INSERT INTO countries(name, population) VALUES('Myanmar', 51446000);
    INSERT INTO countries(name, population) VALUES('South Korea', 68147000);
    INSERT INTO countries(name, population) VALUES('Colombia', 49129000);
    INSERT INTO countries(name, population) VALUES('Kenya', 47251000);
    INSERT INTO countries(name, population) VALUES('Spain', 46812000);
    INSERT INTO countries(name, population) VALUES('Argentina', 43850000);
    INSERT INTO countries(name, population) VALUES('Ukraine', 42603000);
    INSERT INTO countries(name, population) VALUES('Sudan', 41176000);
    INSERT INTO countries(name, population) VALUES('Algeria', 40400000);
    INSERT INTO countries(name, population) VALUES('Poland', 38439000);

创建完数据库和表后，执行上这个sql脚本用其中的数据填充表。

> com/zetcode/model/Country.java

package com.zetcode.model;
    
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.GenerationType;
    import javax.persistence.Id;
    import javax.persistence.Table;
    import java.util.Objects;
    
    @Entity
    @Table(name = "countries")
    public class Country { 
    
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long id;
    
        private String name;
        private int population;
    
        public Country() { 
        }
    
        public Country(Long id, String name, int population) { 
            this.id = id;
            this.name = name;
            this.population = population;
        }
    
        public Long getId() { 
            return id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        public int getPopulation() { 
            return population;
        }
    
        public void setPopulation(int population) { 
            this.population = population;
        }
    
        @Override
        public boolean equals(Object o) { 
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Country country = (Country) o;
            return population == country.population &&
                    Objects.equals(id, country.id) &&
                    Objects.equals(name, country.name);
        }
    
        @Override
        public int hashCode() { 
            return Objects.hash(id, name, population);
        }
    
        @Override
        public String toString() { 
            final StringBuilder sb = new StringBuilder("Country{");
            sb.append("id=").append(id);
            sb.append(", name='").append(name).append('\'');
            sb.append(", population=").append(population);
            sb.append('}');
            return sb.toString();
        }
    }

`Country`是一个实例，每一个实力必须包含这两个注解`@Entity`和`@Id`。以前，我们必须将`ddl-auto`选项设置成`create-drop`表明Hibernate将从这个实例中构建表。

@Entity
    @Table(name = "countries")
    public class Country {

`@Entity`注解指明该类是一个实例并且将会映射成一个数据库表。`@Table`注解指定了这个数据库表的名字。

@Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

@Id指定了这个实例的主键，`@GeneratedValue`规定了这个主键的值得生成策略。

> com/zetcode/repository/CountryRepository.java

package com.zetcode.repository;
    
    import com.zetcode.bean.Country;
    import org.springframework.data.repository.CrudRepository;
    import org.springframework.stereotype.Repository;
    
    @Repository
    public interface CountryRepository extends CrudRepository<Country, Long> { 
    
    }

`CountryRepository`被`@Repository`注解，通过继承spring`CrudRepository`，CountryRepository拥有了对数据进行CRUD的方法，包括`findAll()`。

> com/zetcode/service/ICountryService.java

package com.zetcode.service;
    
    import com.zetcode.bean.Country;
    import java.util.List;
    
    public interface ICountryService { 
    
        public List<Country> findAll();
    }

`ICountryService`接口中定义了一个`findall()`方法。

> com/zetcode/service/CountryService.java

package com.zetcode.service;
    
    import com.zetcode.model.Country;
    import com.zetcode.repository.CountryRepository;
    import java.util.List;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Service;
    
    @Service
    public class CountryService implements ICountryService { 
    
        @Autowired
        private CountryRepository repository;
    
        @Override
        public List<Country> findAll() { 
    
            var countries = (List<Country>) repository.findAll();
            return countries;
        }
    }

`CountryService`继承`ICountryService`并实现`findAll()`方法。

@Autowired
    private CountryRepository repository;

用`@Autowired`注解`CountryRepository`。

@Override
    public List<Country> findAll() {
        var countries = (List<Country>) repository.findAll();
        return countries;
    }

`findAll()`方法的返回值是数据库表中所有国家的列表。

> com/zetcode/controller/MyController.java

package com.zetcode.controller;
    
    import com.zetcode.model.Country;
    import com.zetcode.service.ICountryService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Controller;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.servlet.ModelAndView;
    
    import java.util.HashMap;
    import java.util.List;
    
    @Controller
    public class MyController { 
    
        @Autowired
        ICountryService countryService;
    
        @RequestMapping("/showCountries")
        public ModelAndView findCities() { 
    
            var countries = (List<Country>) countryService.findAll();
    
            var params = new HashMap<String, Object>();
            params.put("countries", countries);
    
            return new ModelAndView("showCountries", params);
        }
    }

`MyController`接收一个来自客户端的请求。

@Controller
    public class MyController {

`MyController`这个controller被`@controller`注解。

@Autowired
    ICountryService countryService;

`ICountryService`被注入成员变量`countryService`中。

var countries = (List<Country>) countryService.findAll();

我们用`findAll()`方法取回了所有的国家。

> Application.java

package com.zetcode;
    
    import org.springframework.boot.SpringApplication;
    import org.springframework.boot.autoconfigure.SpringBootApplication;
    
    @SpringBootApplication
    public class Application  { 
        
        public static void main(String[] args) { 
            SpringApplication.run(Application.class, args);
        }
    }

`Application`是启动Spring boot应用程序的入口。`@SpringBootApplication`注解支持自动配置和组件扫描。它可以代替`@Configuration`，`@ EnableAutoConfiguration`和`@ComponentScan`三个注解的功能。

[http_zetcode.com_springboot_repository]: http://zetcode.com/springboot/repository/