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社区首页 >问答首页 >JoinTable注解导致NullPointerException

问JoinTable注解导致NullPointerException
EN

Stack Overflow用户
提问于 2021-02-08 03:14:11
回答 1查看 38关注 0票数 1

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的列表。

每个姓名记录可以与一个凭证记录相关联，但这不是必需的。每个凭证记录必须与一个姓名记录相关联。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，因为它会保存大部分空值，同时也是为了理解连接表的实现。

​

​

名称实体

代码语言：javascript
复制
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;
    }
}

凭证实体

代码语言：javascript
复制
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;
    }
}

CredentialControl

代码语言：javascript
复制
@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，我该如何纠正它？我认为name1包含值(0, "Abby", "Martin", null)，因此我认为NPE来自name1的credential字段的空值。

最后，作为参考，我使用了Baeldung的一个页面以及各种Stack Exchange帖子来解决这个问题，但我还没有成功。

堆栈跟踪

代码语言：javascript
复制
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]
spring-data-jpa
java
hibernate
jpa
join
EN

回答 1

Stack Overflow用户

回答已采纳

发布于 2021-02-08 03:54:27

您可以使用NameService nameService = new NameService();创建服务

如果你用new关键字创建它，那么任何依赖项都将不可用，因此空指针异常。您必须自动创建该服务，而不是自己创建它。

当你自动连接spring时，spring不会给你一个该类的实际实例，而是你的类的一个代理，它将按照你已经阅读的文档的方式来表现它应该表现的行为。如果你自己创建它，那么你不会得到一个按照你期望的方式运行的代理。您可以自己获取该类的一个实际实例。

Fix:只需进入控制器并声明为一个字段

代码语言：javascript
复制
@Autowired
NameService nameService

然后只需在你的方法中使用它，比如nameService.findAll();，只有当你的服务已经自动连接了nameRepository并调用了相应的方法时，这个解决方案才会起作用。

但是，如果我很好地理解您使用服务的方式，您可以直接调用存储库。如果是这种情况，那么我会说让您的控制器直接访问该存储库，您的存储库应该如下所示

代码语言：javascript
复制
public interface NameRepository extends JpaRepository<Name, Long> { }

然后在你的控制器中设置

代码语言：javascript
复制
    @Autowired
    NameRepository nameRepository

然后在你的方法中使用它，如下所示

用nameRepository.findAll()代替nameService.findAll()

票数 1
EN
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：

https://stackoverflow.com/questions/66092059

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