Spring中EntityManager的使用

EntityManagerFactory和EntityManager是JPA所定义的接口，在没有SPRING之前，如果选择由程序管理，则需要自己创建一个EntityManagerFactory的单例，然后再由该单例在需要进行数据库操作的方法中创建EntityManager实例，并在查询完毕之后关闭（注意：如果忘记关闭，会造成数据库链接不能及时释放，很快整个数据库就会因为链接被用尽而导致程序报错）。

在Spring中如果你使用了SPRING JPA框架，则上述的事情由该框架完成，你只需要在Controller中注入EntityManager, 执行相关查询，不需要管理创建，关闭，SPRING替你完成。

POM文件中所加入的依赖：

<?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>demotest</groupId>
<artifactId>demo</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>

<name>demo</name>
<description>Demo project for Spring Boot</description>

<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.5.3.RELEASE</version>
<relativePath/> <!-- lookup parent from repository -->
</parent>

<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
<java.version>1.8</java.version>
</properties>

<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-rest</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>

<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-security</artifactId>
</dependency>

<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<scope>runtime</scope>
</dependency>
<dependency>

</dependencies>

<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>


<repositories>
<repository>
<id>spring-releases</id>
<url>https://repo.spring.io/libs-release</url>
</repository>
</repositories>
<pluginRepositories>
<pluginRepository>
<id>spring-releases</id>
<url>https://repo.spring.io/libs-release</url>
</pluginRepository>
</pluginRepositories>

</project>

对方法的调用：

package demotest.controller;

import demotest.entity.Role;
import demotest.entity.User;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.security.access.prepost.PreAuthorize;
import org.springframework.web.bind.annotation.CrossOrigin;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Query;
import java.util.List;

/**
 * Created by simon on 2017/6/23.
 */
@CrossOrigin(origins = "http://localhost:3000")
@RestController
@PreAuthorize("hasRole('ADMIN')")
public class EmController {
    //注入EntityManager
    @Autowired
    private EntityManager entityManager;


    @RequestMapping("/emtest/listuser")
    public List<User> getUserlist() {
        //直接在方法中调用，不用关闭，Spring会进行处理
        Query query = entityManager.createNativeQuery("select * from user", User.class);
        return query.getResultList();

    }

    @RequestMapping("/emtest/listrole")
    public List<Role> test2() {
        Query query = entityManager.createNativeQuery("select * from role", Role.class);
        return query.getResultList();
    }
}