java - 如何将引用另一个 A 类型对象的 A 类型数据库对象保存到数据库中？

Question

在我的网络应用程序中，我有一些类别。该类别可以嵌套在另一个类别中。所以我的类结构是:

@javax.persistence.Entity
@Table(name = "Category")
@XmlRootElement
public class CategoryEntity implements Serializable {
	private static final long serialVersionUID = 3738106090716428022L;
@Id
@GeneratedValue
private Long id;
private String name;
@OneToOne(cascade = CascadeType.ALL)
@JoinColumn(name = "parent_id", nullable = true)
@Null
private CategoryEntity parent;

	public CategoryEntity(String name) {
		super();
		this.name = name;
	}

	public Long getId() {
		return id;
	}

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

	public CategoryEntity() {
		super();
	}

	public String getName() {
		return name;
	}

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

	public CategoryEntity getParent() {
		return parent;
	}

	public void setParent(CategoryEntity parent) {
		if (parent.equals(this))
			this.parent = null;
		else
			this.parent = parent;
	}
}

上面的解决方案是我在网上找到的。

如您所见，在数据库表中我需要引用另一个相同类型的对象。
我的sql表(我认为在我的情况下这是正确的):

create table category (
    id bigint not null,
    name varchar(50),
    parent_id bigint null default null,

    constraint category_pk primary key (id),
    constraint fk_parent foreign key (parent_id) references category (id)

);

在这种情况下，我总是得到NullPointerException:

    SEVERE: Servlet.service() for servlet [Spring MVC Dispatcher Servlet] in context with path [/Shop] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
java.lang.NullPointerException
	at org.tenebris.fx.service.rest.CategoryResource.createCategory(CategoryResource.java:39)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:213)
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:126)
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:96)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:617)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:578)
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:80)
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:923)
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852)
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
	at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:789)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:644)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:186)
	at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:160)
	at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346)
	at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:259)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:613)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
	at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:537)
	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1085)
	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:658)
	at org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1556)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1513)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Unknown Source)

如何解决？

Answer 1

试试这个:

@ManyToOne(fetch=FetchType.EAGER)
@JoinColumn(name="parent_category_id")