无法使用 Hibernate 在 Spring MVC 中获取多对多映射数据答案

【问题标题】：Unable to get Many-to-Many mapping data in Spring MVC with Hibernate无法使用 Hibernate 在 Spring MVC 中获取多对多映射数据
【发布时间】：2017-09-26 06:25:17
【问题描述】：

在引用此Hibernate docs 之后。
在我的例子中，将“Person”替换为“Employee”，将“Phone”替换为 Company，将“Person_phone”替换为“EmployeeCompany”。

以上 Hibernate 文档示例和我的项目示例的唯一区别。 在 Hibernate 文档示例中，“Person_Phone”将“phones_Id”作为唯一约束。而在我的“EmployeeCompany”案例中，我将“employeeId”和“companyId”组合为主键。

基本上我有一个包含 m:n 关系的 Employee 和 Company 表。这意味着一个员工可以在多个公司工作，一个公司可以有多个员工。

将这种 m:n 关系解析为 1:n 关系。我创建了第三个表“EmployeeCompany”。

使用下面的 getEmployees() 函数，我想获取与 Employee 关联的所有公司，因为我在 Employee 表中使用了 @OneToMany 注释。

public List<Employee> getEmployees() {
    Session session = this.sessionFactory.getCurrentSession();
    List<Employee> employees = session.createQuery("from Employee").list();
    return employees;
}

它在“Employee”类中抛出与“companies”相关的异常。虽然从“Employee”类中删除“companies”字段工作正常，但我没有获取与 Employee 关联的公司数据。

员工模型类：

@Entity
@Table(name = "employee")
public class Employee implements Serializable{
@Id
@GeneratedValue
@Column(name = "employeeId")
private Long employeeId;

@Column(columnDefinition = "text not null")
private String name;

@OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
private List<Company> companies = new ArrayList<Company>();

public Long getEmployeeId() {
    return employeeId;
}

public void setEmployeeId(Long employeeId) {
    this.articleId = articleId;
}

public String getName() {
    return heading;
}

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

public List<Company> getCompanies() {
    return companies;
}

public void setCompanies(List<Company> companies) {
    this.companies = companies;
}
}

EmployeeCompany 模型类：

@Entity
@Table(name = "employee_company")
public class EmployeeCompany implements Serializable {

    @EmbeddedId
    private EmployeeCompanyKey key;

    public EmployeeCompanyKey getKey() {
        return key;
    }

    public void setKey(EmployeeCompanyKey key) {
        this.key = key;
    }
}

EmployeeCompanyKey 类：

@Embeddable
public class EmployeeCompanyKey implements Serializable{
    @Column(name = "companyId",columnDefinition = "varchar(100) not null")
    private String companyId;

    @Column(name = "employeeId",columnDefinition = "bigint(20) unsigned not null")
    private Long employeeId;


    public String getCompanyId() {
        return companyId;
    }

    public void setCompanyId(String companyId) {
        this.companyId = companyId;
    }

    public Long getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(Long employeeId) {
        this.employeeId = employeeId;
    }


    public int hashCode(){
        final int prime = 31;
        int result = 1;
        result = prime * result
                + ((companyId == null) ? 0 : companyId.hashCode());
        result = prime * result + employeeId.hashCode();
        return result;
    }


    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        EmployeeCompanyKey other = (EmployeeCompanyKey) obj;
        if (companyId == null) {
            if (other.companyId != null)
                return false;
        } else if (!companyId.equals(other.companyId)){
            return false;
        }
        if (employeeId != other.employeeId)
            return false;
        return true;
    }

}

公司模型类：

@Entity
@Table(name = "company")
public class Company implements Serializable{

    @Id
    @Column(name = "companyId",columnDefinition = "varchar(100) not null")
    private String companyId;

    @Column(name = "name",columnDefinition = "text not null")
    private String name;

    public String getCompanyId() {
        return companyId;
    }

    public void setCompanyId(String companyId) {
        this.companyId = companyId;
    }


    public String getName() {
        return name;
    }

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

这是我用来制作数据库的 SQL 查询：

DROP TABLE IF EXISTS `employee`;
CREATE TABLE employee (
`employeeId` bigint(20) unsigned not null auto_increment,
`name` text not null,
primary key(`employeeId`)
)ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;   

DROP TABLE IF EXISTS `employee_company`;
CREATE TABLE employee_company (
`employeeId` bigint(20) unsigned not null,
`companyId` varchar(100) not null,
primary key(`employeeId`,`companyId`),
CONSTRAINT `company_employee_company_fk` FOREIGN KEY (`companyId`) REFERENCES `company`(`companyId`) ON DELETE CASCADE,
CONSTRAINT `employee_employee_company_fk` FOREIGN KEY (`employeeId`) REFERENCES `employee`(`employeeId`) ON DELETE CASCADE
)ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;   


DROP TABLE IF EXISTS `company`;
CREATE TABLE company (
`companyId` varchar(100) not null,
`name` text not null,
primary key(`companyId`)
)ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

请解释我在哪里做错了？另外，如果我想要公司数据和所有员工数据，我该怎么办？

编辑 1： 这是我的邮递员响应输出：

    <body>
        <h1>HTTP Status 500 - Could not write content: failed to lazily initialize a collection of role: com.argnv.myproject.Models.Employee.companies, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]-&gt;com.argnv.myproject.Models.Employee[&quot;companies&quot;]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.argnv.myproject.Models.Employee.companies, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]-&gt;com.argnv.myproject.Models.Employee[&quot;companies&quot;])</h1>
        <HR size="1" noshade="noshade">
        <p>
            <b>type</b> Exception report
        </p>
        <p>
            <b>message</b>
            <u>Could not write content: failed to lazily initialize a collection of role: com.argnv.myproject.Models.Employee.companies, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]-&gt;com.argnv.myproject.Models.Employee[&quot;companies&quot;]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.argnv.myproject.Models.Employee.companies, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]-&gt;com.argnv.myproject.Models.Employee[&quot;companies&quot;])</u>
        </p>
        <p>
            <b>description</b>
            <u>The server encountered an internal error that prevented it from fulfilling this request.</u>
        </p>
        <p>
            <b>exception</b>
            <pre>org.springframework.http.converter.HttpMessageNotWritableException: Could not write content: failed to lazily initialize a collection of role: com.argnv.myproject.Models.Employee.companies, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]-&gt;com.argnv.myproject.Models.Employee[&quot;companies&quot;]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.argnv.myproject.Models.Employee.companies, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]-&gt;com.argnv.myproject.Models.Employee[&quot;companies&quot;])
    org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:238)
    org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:208)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:161)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:101)
    org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:167)
    org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:71)
    org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:126)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:776)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:705)
    org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
</pre>
        </p>
        <p>
            <b>root cause</b>
            <pre>com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.argnv.myproject.Models.Employee.companies, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]-&gt;com.argnv.myproject.Models.Employee[&quot;companies&quot;])
    com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:232)
    com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:197)
    com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:184)
    com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:605)
    com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:142)
    com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:100)
    com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:21)
    com.fasterxml.jackson.databind.ser.std.AsArraySerializerBase.serialize(AsArraySerializerBase.java:186)
    com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:118)
    com.fasterxml.jackson.databind.ObjectMapper.writeValue(ObjectMapper.java:1819)
    org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:231)
    org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:208)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:161)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:101)
    org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:167)
    org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:71)
    org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:126)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:776)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:705)
    org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
</pre>
        </p>
        <p>
            <b>root cause</b>
            <pre>org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.argnv.myproject.Models.Employee.companies, could not initialize proxy - no Session
    org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:566)
    org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:186)
    org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:545)
    org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:124)
    org.hibernate.collection.internal.PersistentBag.iterator(PersistentBag.java:266)
    com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:90)
    com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:23)
    com.fasterxml.jackson.databind.ser.std.AsArraySerializerBase.serialize(AsArraySerializerBase.java:186)
    com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:569)
    com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:597)
    com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:142)
    com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:100)
    com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:21)
    com.fasterxml.jackson.databind.ser.std.AsArraySerializerBase.serialize(AsArraySerializerBase.java:186)
    com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:118)
    com.fasterxml.jackson.databind.ObjectMapper.writeValue(ObjectMapper.java:1819)
    org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:231)
    org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:208)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:161)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:101)
    org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:167)
    org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:71)
    org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:126)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:776)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:705)
    org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
</pre>
        </p>
        <p>
            <b>note</b>
            <u>The full stack trace of the root cause is available in the Apache Tomcat/7.0.53 logs.</u>
        </p>
        <HR size="1" noshade="noshade">
        <h3>Apache Tomcat/7.0.53</h3>
    </body>

【问题讨论】：

如果您收到错误消息，请在此处发布原始消息，而不是related with "companies" in "Employee" Class.
Mapping @ManyToMany association table with extra columns的可能重复

标签： java hibernate spring-mvc

【解决方案1】：

EmployCompanyKey.class

@Embeddable
public class EmployeeCompanyKey implements Serializable{
@Column(name = "companyId",columnDefinition = "varchar(100) not null")
private String companyId;

@Column(name = "companyId",columnDefinition = "bigint(20) unsigned not null")
private Long employeeId;

改变

 @Column(name = "companyId",columnDefinition = "bigint(20) unsigned not null")
private Long employeeId;

到

@Column(name = "employeeId",columnDefinition = "bigint(20) unsigned not null") private Long employeeId;

然后再试一次？

【讨论】：

这是拼写错误。但仍然无法正常工作。我更新了描述错误响应的问题