spring boot + JPA + mysql，无法检索结果集答案

【问题标题】：spring boot + JPA + mysql, unable to retrieve result setspring boot + JPA + mysql，无法检索结果集
【发布时间】：2019-05-15 17:26:24
【问题描述】：

我是 Spring Boot 的新手，具有 Java 的基本知识。我正在尝试构建一个微服务来从 mySQL 数据库中检索数据。以下是代码详情。

我能够使用独立的 JDBC 类从同一个表中检索数据。请求帮助。

- 实体类

package com.studentProject.entity;
import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;
import org.springframework.stereotype.Component;

@Entity
@Component
@Table(name="students", schema="test_schema")
public class StudentEntity implements Serializable{
private static final long serialVersionUID = 1L;

@Id
@GeneratedValue
@Column(name="id")
private Integer id;

@Column(name="name")
private String name;

@Column(name="course")
private String course;

@Column(name="country")
private String country;

@Column(name="phone")
private String phone;

public StudentEntity(Integer id, String name, String course, String country, String phone) {
    super();
    this.id = id;
    this.name = name;
    this.course = course;
    this.country = country;
    this.phone = phone;
}

public StudentEntity() {
    super();
    // TODO Auto-generated constructor stub
}

public Integer getId() {
    return id;
}
public void setId(Integer id) {
    this.id = id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getCourse() {
    return course;
}

public void setCourse(String course) {
    this.course = course;
}

public String getCountry() {
    return country;
}

public void setCountry(String country) {
    this.country = country;
}

public String getPhone() {
    return phone;
}

public void setPhone(String phone) {
    this.phone = phone;
}
}

-存储库类

package com.studentProject.repository;

import org.springframework.data.repository.CrudRepository;

import org.springframework.stereotype.Repository;

import com.studentProject.entity.StudentEntity;

@Repository 

public interface StudentRepository extends CrudRepository<StudentEntity, Integer> {

StudentEntity findByName(String name); 

StudentEntity findByCourse(String course);

}

-控制器类

package com.studentProject.controller;

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

import com.studentProject.entity.StudentEntity;
import com.studentProject.repository.StudentRepository;

@RestController
public class StudentController {


    @Autowired
    private StudentRepository studentRepository;

    @Autowired
    List<StudentEntity> studentList;

    public StudentController(StudentRepository studentRepository, StudentEntity studentEntity,
            List<StudentEntity> studentList) {
        super();
        this.studentRepository = studentRepository;
        this.studentList = studentList;
    }

    @GetMapping("/getStudentDetails")
    public Iterable<StudentEntity> getAllStudents(){

        return studentRepository.findAll();
    }

    @GetMapping("/getStudentDetails/getByName")
    public StudentEntity findByName(@RequestParam(value="name") String name) {

        return studentRepository.findByName(name);
    }

    @GetMapping("/getStudentDetails/getById/{id}")
    public Optional<StudentEntity> findById(@PathVariable(value="id") Integer id) {

        return studentRepository.findById(id);
    }

    @GetMapping("/getStudentDetails/welcome")
    public String welcome() {

        return "hi brother welcome";
    }
    }

-属性文件

server.port = 8080
spring.datasource.driver-class-name = com.mysql.cj.jdbc.Driver
spring.datasource.url = jdbc:mysql://localhost:3306/
spring.datasource.username = root
spring.datasource.password = root
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQL5InnoDBDialect
spring.data.jpa.repositories.enabled=true
spring.jpa.show-sql = true
spring.jpa.hibernate.ddl-auto = none
spring.jpa.database-platform=org.hibernate.dialect.MySQL5InnoDBDialect

--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>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.1.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.StudentProject</groupId>
    <artifactId>StudentProject-1</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>StudentProject-1</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>
        <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-devtools</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-jdbc</artifactId>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

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

</project>

-主应用程序

package com.studentProject;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class StudentProject1Application {

    public static void main(String[] args) {
        SpringApplication.run(StudentProject1Application.class, args);
    }

}

-mysql表详细信息 schema=test_schema 和 table=students

错误详情

【问题讨论】：

你能发布你的例外吗？
你的错误是什么？
出现意外错误（类型=内部服务器错误，状态=500）。无法提取结果集；嵌套异常是 org.hibernate.exception.GenericJDBCException：无法提取结果集 org.springframework.orm.jpa.JpaSystemException：无法提取结果集；嵌套异常是 org.hibernate.exception.GenericJDBCException: could not extract ResultSet
引起：java.sql.SQLException：在 com.mysql.cj.jdbc 的 com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129) 没有选择数据库。 exceptions.SQLError.createSQLException(SQLError.java:97) 在 com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122) 在 com.mysql.cj.jdbc.ClientPreparedStatement.executeInternal(ClientPreparedStatement.java:第974章）
您忘记在属性中添加数据库名称`"jdbc:mysql://localhost/databasename";`

标签： mysql spring spring-boot jpa microservices

【解决方案1】：

来自错误日志

原因：java.sql.SQLException：未选择数据库

很明显，spring 找不到数据库，所以按照下面的方法选择数据库。

通过添加数据库名称将spring.datasource.url 值更改为以下。

spring.datasource.url = jdbc:mysql://localhost:3306/test_schema

【讨论】：

如果@Component 被删除，则会遇到错误。上下文初始化期间遇到异常 - 取消刷新尝试：org.springframework.beans.factory.UnsatisfiedDependencyException：创建文件 [...controller\StudentController.class] 中定义的名称为 'studentController' 的 bean 时出错：通过构造函数参数 1 表示的依赖关系不满足；嵌套异常是 org.springframework.beans.factory.NoSuchBeanDefinitionException：没有“com.studentProject.entity.StudentEntity”类型的合格 bean 可用：预计至少有 1 个有资格作为自动装配候选者的 bean。
谢谢，添加数据源URL后得到结果。但是，如果我添加@component 注释会有什么影响
酷...如果对您有帮助，请考虑接受并投票赞成答案...:)

【解决方案2】：

Caused by: java.sql.SQLException: No database selected at

这告诉您必须选择数据库名称，因此您必须将其添加到您的配置中。我看到你已经得到了答案！

spring.datasource.url = jdbc:mysql://localhost:3306/YOUR_DB_SCHEMA

【讨论】：