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问题：

无法在java web应用程序中创建类型为的对象

赵雪峰
2023-03-14

我一直在开发一个java应用程序，我遇到了一个无法找到答案的问题。我的应用程序是一个postgresql学生数据库，应用程序是在maven中使用MCV模式制作的，对于web部件，我使用Spring和hibernate，因为IDE使用Intellij。

我可以毫无问题地启动它，但在访问localhost页面时，我会出现以下错误：

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；SQL[n/a]；嵌套异常为org.hibernate.exception.sqlgrammareexception:无法提取结果集

在intelij中，我得到以下错误：

2019-04-17 20:38:43.283错误14120---[nio-8010-exec-7]o. a. c. c.C.[.[.]Servlet.service（）的servlet[调度Servlet]在上下文路径[]抛出异常[请求处理失败；嵌套异常是org.springframework.dao.InvalidDataAccessResourceUsageExc0019：无法提取ResultSet；SQL[n/a]；嵌套异常org.hibernate.exception.SQLGrammarExc0019：无法提取ResultSet]与根本原因org.postgresql.util.PSQLExc0019： ERROR：关系"教师"不存在"

我不明白为什么这种关系不存在，因为我的表上有@ManyToOne，它与老师有关，而在postgresql中是相同的关系。

我的教师示范班是：

package project.M.Entities;

import javax.persistence.*; import java.io.Serializable;

@Entity @Table(name = "Teacher") public class Teacher implements
Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    @Column(name="name", nullable = false)
    private String name;
    @Column(name="address", nullable = true)
    private String address;
    @Column(name="email",nullable = false)
    private String email;
    @Column(name="age")
    private Integer age;
    @Column(name="passsword",nullable = false)
    private String password;

    public Teacher(Integer idTask, String description, String filepath, String str, String ema, Integer it) { }


        public Teacher(int id, String name, String password, String address, String email, int age) {
            this.id = id;
            this.name = name;
            this.address = address;
            this.email = email;
            this.age = age;
            this.password = password;
        }

        public Teacher( String name,String password, String address, String email, int age) {
            this.name = name;
            this.address = address;
            this.email = email;
            this.age = age;
                this.password = password;
        }


    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getPassword() {
        return password;
    }

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

    @Override
    public String toString() {
        return "Teacher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", address='" + address + '\'' +
                ", email='" + email + '\'' +
                ", age=" + age +
                ", password='" + password + '\'' +
                '}';
    } }

它连接到我的课程，模型是

package project.M.Entities; import javax.persistence.Column;
import javax.persistence.Entity; import
javax.persistence.GeneratedValue; import
javax.persistence.GenerationType; import javax.persistence.Id; import
javax.persistence.JoinColumn; import javax.persistence.ManyToOne;
import javax.persistence.Table; import
javax.validation.constraints.NotNull; import java.io.Serializable;

@Entity @Table(name = "courses") public class Course implements
Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    @Column(name="name",nullable = false)
    private String name;
    //@Column()
    //private Integer TeacherID;
    @ManyToOne
    @JoinColumn(name = "teacherID",referencedColumnName = "id")
    @NotNull
    private project.M.Entities.Teacher teacher;

    public Course(Integer id, String name, Teacher teacher) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
    }

    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 project.M.Entities.Teacher getTeacher() {
        return teacher;
    }

    public void setTeacherID(Teacher teacherID) {
        this.teacher = teacherID;
    }

    @Override
    public String toString() {
        return "Course{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", teacher=" + teacher +
                '}';
    } }

至于我为老师提供的服务：

package project.C;

import project.M.Entities.Teacher;
import project.M.Services.TeacherService;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.*;
import org.springframework.web.servlet.ModelAndView;

import javax.inject.Inject;
import java.util.List;

@Controller
public class TeacherController {

    @Inject
    TeacherService teacherService;

    @RequestMapping(value ="" ,method = RequestMethod.GET)
    public ModelAndView getTeachers()
    {
        List<Teacher> teacherList = teacherService.getAllTeachersWithCourses();


        ModelAndView mav = new ModelAndView("TeacherView");
        mav.addObject("teachersWithCoursesListId", teacherList);

        return mav;

    }
    @RequestMapping(value = "", method = RequestMethod.POST)
    public ModelAndView postTeacher(@ModelAttribute(value = "newTeacher") Teacher newTeacher)
    {
        teacherService.create(newTeacher);
        return new ModelAndView("redirect:teachers");

    }
    @RequestMapping(value ="", method = RequestMethod.DELETE)
    public ModelAndView deleteTeacher(@ModelAttribute(value = "newTeacher") Teacher newTeacher)
    {
        teacherService.delete(newTeacher);

        return new ModelAndView("redirect:teachers");

    }
/*
    @RequestMapping(value = "", method = RequestMethod.POST)
    @ResponseBody
    public void set(@RequestParam("idTeacher") Integer idTask, @RequestParam("name") String description, @RequestParam("password") String filepath, @RequestParam("address") String str, @RequestParam("email") String ema,@RequestParam("age")Integer it){

// assign parameters to taskDocumentEntity by constructor args or setters
        Teacher document = new Teacher(idTask,description,filepath,str,ema,it);
        teacherService.create(document);
    }
    */

}

对于教师服务，我有以下几点

package project.M.Services;

import project.M.Entities.Teacher; import
project.M.Repository.TeacherRepository; import java.util.List; import
java.util.Optional;

import javax.inject.Inject;

import org.springframework.stereotype.Service;

@Service public class TeacherService {
    private int count;

    @Inject
    TeacherRepository teacherRepository;

    public List<Teacher> getAllTeachersWithCourses()
    {
        return teacherRepository.findAll();
    }

    public void create(Teacher newTeacher)
    {
        System.out.println("aaaa"+newTeacher);
         teacherRepository.save(newTeacher);
    }
    public void delete(Teacher newTeacher)
    {
         teacherRepository.delete(newTeacher);
    }

   public void update(Teacher old,Teacher st)    {
        if(old==null)
        {
            System.out.println("eror at update");
        }
        else
        {
            teacherRepository.delete(old);
            teacherRepository.save(st);
        }    }

}

我的老师我试图访问的视图是：

  <!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org" xmlns:th1="http://www.w3.org/1999/xhtml">

<head>
    <title>Teachers</title>
    <link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css" />
    <link href="//maxcdn.bootstrapcdn.com/font-awesome/4.3.0/css/font-awesome.min.css" rel="stylesheet" />

</head>

<body>


<table  border="1">
    <thead>
    <tr>
        <th>teacher.ID</th>
        <th>teacher.NAME</th>
        <th>teacher.PASSWORD</th>
        <th>teacher.EMAIL</th>
        <th>teacher.AGE</th>
        <th>course entire data</th>
    </tr>
    </thead>
    <tbody>
    <tr th:each="teacher : ${ teachersWithCoursesList }">

        <td th:text="${ teacher.id }">ID</td>
        <td th:text="${ teacher.name }"></td>
        <td th:text="${ teacher.password }"></td>
        <td th:text="${ teacher.email }"></td>
        <td th:text="${ teacher.age }"></td>
        <td th1:text="${ teacher.courses }">
            <p th:each="course: ${ teacher.courses  }">
                <span th:text="${course.id}"></span>
                <span th:text="${course.name}"></span>
            </p>
        </td>

    </tr>
    </tbody>
</table>

<br/>
<br/>
<br/>

<form   th:object="${newTeacher}" method="post">
    <div class="col-sm-12">
        <label>Name</label>
        <input placeholder="Teacher Name" name="name"/>
    </div>
    <div class="col-sm-12">
        <label>Password</label>
        <input placeholder="Teacher password" name="password"/>
    </div>
    <div class="col-sm-12">
        <label>Email</label>
        <input placeholder="Teacher email" name="email"/>
    </div>

    <button type="submit" class = "btn btn-primary">ADD Teacher</button>
</form>

</body>

也是DB的sql：课程

 -- Table: public."Courses"

-- DROP TABLE public."Courses";

CREATE TABLE public."Courses"
(
    "idCourses" integer NOT NULL,
    "Name" "char"[] NOT NULL,
    "TeacherID" integer NOT NULL,
    CONSTRAINT "Courses_pkey" PRIMARY KEY ("idCourses"),
    CONSTRAINT "teacherID" FOREIGN KEY ("TeacherID")
        REFERENCES public."Teacher" ("idTeacher") MATCH SIMPLE
        ON UPDATE NO ACTION
        ON DELETE NO ACTION
)
WITH (
    OIDS = FALSE
)
TABLESPACE pg_default;

ALTER TABLE public."Courses"
    OWNER to postgres;

-- Index: fki_teacherID

-- DROP INDEX public."fki_teacherID";

CREATE INDEX "fki_teacherID"
    ON public."Courses" USING btree
    ("TeacherID")
    TABLESPACE pg_default;

老师

-- Table: public."Teacher"

-- DROP TABLE public."Teacher";

CREATE TABLE public."Teacher"
(
    "idTeacher" integer NOT NULL,
    name "char"[] NOT NULL,
    password "char"[] NOT NULL,
    address "char"[],
    email "char"[] NOT NULL,
    age integer,
    CONSTRAINT "Teacher_pkey" PRIMARY KEY ("idTeacher")
)
WITH (
    OIDS = FALSE
)
TABLESPACE pg_default;

ALTER TABLE public."Teacher"
    OWNER to postgres;

学生：

-- Table: public."Student"

-- DROP TABLE public."Student";

CREATE TABLE public."Student"
(
    address "char"[],
    email "char"[] NOT NULL,
    "idStudent" integer NOT NULL,
    name "char"[] NOT NULL,
    password "char"[] NOT NULL,
    age integer,
    CONSTRAINT "Student_pkey" PRIMARY KEY ("idStudent")
)
WITH (
    OIDS = FALSE
)
TABLESPACE pg_default;

ALTER TABLE public."Student"
    OWNER to postgres;

分数：

-- Table: public.grades

-- DROP TABLE public.grades;

CREATE TABLE public.grades
(
    "IdCourses" integer NOT NULL,
    "idStudent" integer NOT NULL,
    "Grade" integer,
    "Exam" integer,
    "Midterm" integer,
    "examDate" date,
    "midtermDate" date,
    "Id" bigint NOT NULL,
    CONSTRAINT "IdGrades" PRIMARY KEY ("Id"),
    CONSTRAINT "idCourses" FOREIGN KEY ("IdCourses")
        REFERENCES public."Courses" ("idCourses") MATCH SIMPLE
        ON UPDATE NO ACTION
        ON DELETE NO ACTION,
    CONSTRAINT "idStudent" FOREIGN KEY ("idStudent")
        REFERENCES public."Student" ("idStudent") MATCH SIMPLE
        ON UPDATE NO ACTION
        ON DELETE NO ACTION
)
WITH (
    OIDS = FALSE
)
TABLESPACE pg_default;

ALTER TABLE public.grades
    OWNER to postgres;

-- Index: fki_idCourses

-- DROP INDEX public."fki_idCourses";

CREATE INDEX "fki_idCourses"
    ON public.grades USING btree
    ("IdCourses")
    TABLESPACE pg_default;

-- Index: fki_idStudent

-- DROP INDEX public."fki_idStudent";

CREATE INDEX "fki_idStudent"
    ON public.grades USING btree
    ("idStudent")
    TABLESPACE pg_default;

稍后编辑：这是用2个关系更新代码后的错误：

 org.hibernate.tool.schema.spi.CommandAcceptanceException: Unable to execute command [alter table courses drop constraint FKkhvrych02e4jav7oqoemsf498]
    at org.hibernate.tool.schema.internal.exec.GenerationTargetToDatabase.accept(GenerationTargetToDatabase.java:63) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.tool.schema.internal.SchemaDropperImpl.applySqlString(SchemaDropperImpl.java:370) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.tool.schema.internal.SchemaDropperImpl.applySqlStrings(SchemaDropperImpl.java:355) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.tool.schema.internal.SchemaDropperImpl.applyConstraintDropping(SchemaDropperImpl.java:327) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.tool.schema.internal.SchemaDropperImpl.dropFromMetadata(SchemaDropperImpl.java:229) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.tool.schema.internal.SchemaDropperImpl.performDrop(SchemaDropperImpl.java:153) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.tool.schema.internal.SchemaDropperImpl.doDrop(SchemaDropperImpl.java:125) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.tool.schema.internal.SchemaDropperImpl.doDrop(SchemaDropperImpl.java:111) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.tool.schema.spi.SchemaManagementToolCoordinator.performDatabaseAction(SchemaManagementToolCoordinator.java:137) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.tool.schema.spi.SchemaManagementToolCoordinator.process(SchemaManagementToolCoordinator.java:65) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:308) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.boot.internal.SessionFactoryBuilderImpl.build(SessionFactoryBuilderImpl.java:483) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:888) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    at org.springframework.orm.jpa.vendor.SpringHibernateJpaPersistenceProvider.createContainerEntityManagerFactory(SpringHibernateJpaPersistenceProvider.java:57) ~[spring-orm-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:365) ~[spring-orm-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.buildNativeEntityManagerFactory(AbstractEntityManagerFactoryBean.java:390) ~[spring-orm-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:377) ~[spring-orm-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.afterPropertiesSet(LocalContainerEntityManagerFactoryBean.java:341) ~[spring-orm-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1837) ~[spring-beans-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1774) ~[spring-beans-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:593) ~[spring-beans-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515) ~[spring-beans-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320) ~[spring-beans-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318) ~[spring-beans-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1105) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:142) ~[spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:775) ~[spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) ~[spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:316) ~[spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260) ~[spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248) ~[spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at project.Assignment2Application.main(Assignment2Application.java:9) ~[classes/:na]
Caused by: org.postgresql.util.PSQLException: ERROR: relation "courses" does not exist
    at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2440) ~[postgresql-42.2.5.jar:42.2.5]
    at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2183) ~[postgresql-42.2.5.jar:42.2.5]
    at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:308) ~[postgresql-42.2.5.jar:42.2.5]
    at org.postgresql.jdbc.PgStatement.executeInternal(PgStatement.java:441) ~[postgresql-42.2.5.jar:42.2.5]
    at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:365) ~[postgresql-42.2.5.jar:42.2.5]
    at org.postgresql.jdbc.PgStatement.executeWithFlags(PgStatement.java:307) ~[postgresql-42.2.5.jar:42.2.5]
    at org.postgresql.jdbc.PgStatement.executeCachedSql(PgStatement.java:293) ~[postgresql-42.2.5.jar:42.2.5]
    at org.postgresql.jdbc.PgStatement.executeWithFlags(PgStatement.java:270) ~[postgresql-42.2.5.jar:42.2.5]
    at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:266) ~[postgresql-42.2.5.jar:42.2.5]
    at com.zaxxer.hikari.pool.ProxyStatement.execute(ProxyStatement.java:95) ~[HikariCP-3.2.0.jar:na]
    at com.zaxxer.hikari.pool.HikariProxyStatement.execute(HikariProxyStatement.java) ~[HikariCP-3.2.0.jar:na]
    at org.hibernate.tool.schema.internal.exec.GenerationTargetToDatabase.accept(GenerationTargetToDatabase.java:51) ~[hibernate-core-5.2.1.Final.jar:5.2.1.Final]
    ... 35 common frames omitted

共有2个答案

穆德海
2023-03-14

1）让JPA为您构建表和DDL。

spring.jpa.generate-ddl
spring.jpa.hibernate.ddl-auto

https://docs.spring.io/spring-boot/docs/current/reference/html/howto-database-initialization.html

2）在页面中，您正在访问teacher.courses，但是在您的教师实体中没有课程的关系对象。

课程

@ManyToOne(fetch = FetchType.LAZY, optional = false)
@JoinColumn(name = "TeacherID", nullable = false)
private Teacher teacher;

老师

@OneToMany(cascade = CascadeType.ALL,
        fetch = FetchType.LAZY,
        mappedBy = "teacher")
private List<Course> courses = new ArrayList<>();

例如，一张完整的支票https://www.callicoder.com/hibernate-spring-boot-jpa-one-to-many-mapping-example/

焦阎宝
2023-03-14

在你的课程中，你没有遵守JavaBeans命名惯例。

你有一个野战教官，能手能手能手，但能手能手能手。

将setter重命名为settacher

使现代化

@同一字段上的JoinColumn不正确。我可以在数据库中看到Teacher_idTeacher，并在代码中看到不同的值

要检查的另一件事是您的db用户是否对您使用的表拥有所有必要的授权。如果用户无法访问表，则可以正确生成查询，但仍会收到sql语法错误

更新2

下列任何一种：

>

默认架构与Spring Boot使用的公共更改数据库架构不匹配，或者在@Table中指定架构

除此之外，您的映射仍然不正确，id列名与db不匹配。

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