JoinTable 注释导致 NullPointerException
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【中文标题】JoinTable 注释导致 NullPointerException【英文标题】：JoinTable annotation causes NullPointerException 【发布时间】：2021-02-07 19:14:11 【问题描述】：我正在编写一个地址簿程序作为理解连接表的练习。当用户使用用户名和密码登录时，他们应该能够看到可能是也可能不是注册用户的人员列表。
每个名称记录都可以与凭据记录相关联，但这不是必需的。每个 Credential 记录必须与 Name 记录相关联。我希望这是一对一的关系。我想使用连接表的原因是为了避免在 Name 表中有一个连接列，该列主要包含空值，同时也是为了理解连接表的实现。
命名实体
package com.entity;
import lombok.*;
import javax.persistence.*;
@Getter @Setter
@ToString
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Table(name = "name")
public class Name
@Id
@Column(name = "name_id")
@GeneratedValue(strategy = GenerationType.AUTO)
@Setter(AccessLevel.NONE)
private int nameId;
@Column(name = "first_name")
private String firstName;
@Column(name = "last_name")
private String lastName;
@OneToOne(cascade = CascadeType.ALL)
@JoinTable(name = "name_credential_jt",
joinColumns = @JoinColumn(
name = "name_id_fk",
referencedColumnName = "name_id"),
inverseJoinColumns = @JoinColumn(
name = "credential_id_fk",
referencedColumnName = "credential_id"))
private Credential credential;
public Name(String firstName, String lastName)
super();
this.firstName = firstName;
this.lastName = lastName;
public Name(int nameId, String firstName, String lastName)
super();
this.nameId = nameId;
this.firstName = firstName;
this.lastName = lastName;
凭据实体
package com.entity;
import lombok.*;
import javax.persistence.*;
@Getter @Setter
@ToString
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Table(name = "credential")
public class Credential
@Id
@Column(name = "credential_id")
@GeneratedValue(strategy = GenerationType.AUTO)
@Setter(AccessLevel.NONE)
private int credentialId;
@Column(name = "username")
private String username;
@Column(name = "password")
private String password;
@OneToOne(mappedBy = "credential")
private Name name;
public Credential(String username, String password, Name name)
super();
this.username = username;
this.password = password;
this.name = name;
凭据控制
@GetMapping("/setup")
public ResponseEntity<String> setup()
try
Name name1 = new Name("Abby", "Martin");
NameService nameService = new NameService();
System.out.println(name1);
nameService.save(name1);
name1 = nameService.findAll().get(0);
Credential credential1 = new Credential("abs", "mar", name1);
credentialService.save(credential1);
return new ResponseEntity<>(HttpStatus.OK);
catch (Exception e)
e.printStackTrace();
throw new RuntimeException();
为什么nameService.save(name1) 会抛出NullPointerException？或者更确切地说，我做错了什么，我该如何解决？我相信 name1 包含值 (0, "Abby", "Martin", null)，所以我相信 NPE 来自 name1 的凭据字段的空值。
最后，作为参考，我使用了来自Baeldung 的页面以及各种 Stack Exchange 帖子来解决这个问题，但我还没有成功。
堆栈跟踪
Name(nameId=0, firstName=Abby, lastName=Martin, credential=null)
java.lang.NullPointerException
at com.service.NameService.save(NameService.java:19)
at com.control.CredentialControl.setup(CredentialControl.java:42)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:197)
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:141)
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:106)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:894)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808)
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1060)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:962)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:626)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:888)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1597)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:748)
2021-02-07 13:34:35.659 ERROR 12200 --- [nio-9055-exec-2] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.RuntimeException] with root cause
java.lang.RuntimeException: null
at com.control.CredentialControl.setup(CredentialControl.java:50) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_271]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_271]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_271]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_271]
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:197) ~[spring-web-5.3.3.jar:5.3.3]
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:141) ~[spring-web-5.3.3.jar:5.3.3]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:106) ~[spring-webmvc-5.3.3.jar:5.3.3]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:894) ~[spring-webmvc-5.3.3.jar:5.3.3]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.3.jar:5.3.3]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.3.jar:5.3.3]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1060) ~[spring-webmvc-5.3.3.jar:5.3.3]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:962) ~[spring-webmvc-5.3.3.jar:5.3.3]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.3.3.jar:5.3.3]
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) ~[spring-webmvc-5.3.3.jar:5.3.3]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:626) ~[tomcat-embed-core-9.0.41.jar:4.0.FR]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.3.3.jar:5.3.3]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:733) ~[tomcat-embed-core-9.0.41.jar:4.0.FR]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.41.jar:9.0.41]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.3.3.jar:5.3.3]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.3.jar:5.3.3]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.3.3.jar:5.3.3]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.3.jar:5.3.3]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.3.3.jar:5.3.3]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.3.jar:5.3.3]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) ~[tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:888) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1597) [tomcat-embed-core-9.0.41.jar:9.0.41]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.41.jar:9.0.41]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_271]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_271]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.41.jar:9.0.41]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_271]
【问题讨论】：
你介意发布一个堆栈跟踪吗？ 我修改了 CredentialControl 以帮助演示正在发生的事情。 所以 NPE 发生在 NameService.java 中：第 19 行。@Boug 可能是正确的，并且您的 Service 实例化不正确。 【参考方案1】：您使用NameService nameService = new NameService(); 创建您的服务
如果您使用new 关键字创建它，则任何依赖项都将不可用，因此会出现空指针异常。您必须自动装配该服务，而不是您自己创建它。
当您使用 spring 自动装配时，spring 不会为您提供该类的实际实例，而是您的类的代理，它将按照您阅读文档的方式运行。如果您自己创建它，那么您将无法获得按照您期望的方式运行的代理。您可以自己获得该类的实际实例。
修复：只需进入您的控制器并声明为字段
@Autowired
NameService nameService
然后在你的方法中使用它，比如nameService.findAll(); 这个解决方案只有在你的服务已经自动装配了 nameRepository 并调用了相应的方法时才有效。
但如果我理解你使用服务的方式就像你可以直接调用存储库一样。如果是这种情况，那么我会说您让您的控制器直接访问该存储库，并且您的存储库应该像
public interface NameRepository extends JpaRepository<Name, Long>
然后在你的控制器中制作
@Autowired
NameRepository nameRepository
然后在你的方法中使用它
nameRepository.findAll() 而不是nameService.findAll()
【讨论】：
您指出了问题所在。谢谢你。不过，我没有遵循您的建议，而是创建了一个名为 SetupControl 的新控件类，并在其中移动了我在上面发布的设置方法。在这里，我自动装配了两个服务类。我现在有一个新问题。我使用 setup 方法的方式，它将保存名称和凭据，但不会更新连接表。我是否还需要为连接表创建 dao/service 类并在 setup 方法中触发所有三个？ 我相信我找到了上述问题的答案。我需要做的是用来自 Name 实体的冗长的@JoinTable 注释替换 Credential 实体中的 @OneToOne(mappedBy = "credential") 以便 @JoinTable 代码存在于两个实体中。 ***.com/questions/5107330/…
OneToOne 在一个表中创建一个指向另一个表的外键列。 JoinTable 将使用外键创建另一个表以连接这两个表。这取决于你
请记住，根据文档，JoinTable 应与 OneToMany、ManyToOne 或 ManyToMany 一起使用以上是关于JoinTable 注释导致 NullPointerException的主要内容，如果未能解决你的问题，请参考以下文章
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