java - Play Framework 应用程序在第 6 行不断抛出空指针

Question

我正在尝试在 Play Framework 中制作一个简单的论坛系统，但我收到了一个奇怪的错误。我有一个 forum.java 类，它是错误的根源，但无论里面的代码如何，它总是在第 6 行抛出一个空指针。我认为，这就是相关的内容。

package models;

import io.ebean.Finder;
import io.ebean.Model;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import java.util.List;

@Entity
public class Forum extends Model {

    @Id
    private String id;

    @ManyToOne(cascade = CascadeType.ALL)
    private List<Thread> threads;

有问题的行是 javax.persistence.CascadeType 导入，但我已经删除了它，将其向下移动，重新创建了类。不管怎样，错误继续在第 6 行抛出。它也没有解释导致空指针的原因。当该行为空时它甚至会抛出异常。

作为引用，我还有很多以相同格式布局的其他类。我尝试在 sbt 控制台中清理该项目，并尝试从头开始重新创建该类。我确信这是某种根本性错误，但我不知道从哪里开始。

编辑:添加了堆栈跟踪

[info] play.api.Play - Application started (Dev)
[error] application -

! @77hm04h99 - Internal server error, for (GET) [/forum/1] ->

play.api.http.HttpErrorHandlerExceptions$$anon$1: Execution exception[[NullPointerException: null]]
        at play.api.http.HttpErrorHandlerExceptions$.throwableToUsefulException(HttpErrorHandler.scala:255)
        at play.api.http.DefaultHttpErrorHandler.onServerError(HttpErrorHandler.scala:182)
        at play.core.server.AkkaHttpServer$$anonfun$2.applyOrElse(AkkaHttpServer.scala:320)
        at play.core.server.AkkaHttpServer$$anonfun$2.applyOrElse(AkkaHttpServer.scala:318)
        at scala.concurrent.Future.$anonfun$recoverWith$1(Future.scala:414)
        at scala.concurrent.impl.Promise.$anonfun$transformWith$1(Promise.scala:37)
        at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:60)
        at akka.dispatch.BatchingExecutor$AbstractBatch.processBatch(BatchingExecutor.scala:55)
        at akka.dispatch.BatchingExecutor$BlockableBatch.$anonfun$run$1(BatchingExecutor.scala:91)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
Caused by: java.lang.NullPointerException: null
        at io.ebeaninternal.server.deploy.BeanPropertyAssocMany.createReference(BeanPropertyAssocMany.java:652)
        at io.ebeaninternal.server.deploy.BeanDescriptor.lazyLoadMany(BeanDescriptor.java:2082)
        at io.ebeaninternal.server.deploy.BeanDescriptor.lazyLoadMany(BeanDescriptor.java:2071)
        at io.ebeaninternal.server.loadcontext.DLoadBeanContext$LoadBuffer.loadBean(DLoadBeanContext.java:172)
        at io.ebean.bean.EntityBeanIntercept.loadBeanInternal(EntityBeanIntercept.java:812)
        at io.ebean.bean.EntityBeanIntercept.loadBean(EntityBeanIntercept.java:785)
        at io.ebean.bean.EntityBeanIntercept.preGetter(EntityBeanIntercept.java:885)
        at models.Forum._ebean_get_threads(Forum.java:6)
        at models.Forum.getThreads(Forum.java:37)
        at views.html.forum$.apply(forum.template.scala:41)

堆栈跟踪 v2:

[error] application -

! @77hpce3hh - Internal server error, for (GET) [/forum/1] ->

play.api.http.HttpErrorHandlerExceptions$$anon$1: Execution exception[[PersistenceException: Query threw SQLException:Column "T1.FORUM_ID" not found; SQL statement:
select t0.id, t1.id, t1.title, t1.replies, t1.last_reply, t1.forum_id from forum t0 left join thread t1 on t1.forum_id = t0.id  where t0.id = ?   order by t0.id [42122-196] Bind values:[null] Query was:select t0.id, t1.id, t1.title, t1.replies, t1.last_reply, t1.forum_id from forum t0 left join thread t1 on t1.forum_id = t0.id  where t0.id = ?   order by t0.id]]
        at play.api.http.HttpErrorHandlerExceptions$.throwableToUsefulException(HttpErrorHandler.scala:255)
        at play.api.http.DefaultHttpErrorHandler.onServerError(HttpErrorHandler.scala:182)
        at play.core.server.AkkaHttpServer$$anonfun$2.applyOrElse(AkkaHttpServer.scala:320)
        at play.core.server.AkkaHttpServer$$anonfun$2.applyOrElse(AkkaHttpServer.scala:318)
        at scala.concurrent.Future.$anonfun$recoverWith$1(Future.scala:414)
        at scala.concurrent.impl.Promise.$anonfun$transformWith$1(Promise.scala:37)
        at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:60)
        at akka.dispatch.BatchingExecutor$AbstractBatch.processBatch(BatchingExecutor.scala:55)
        at akka.dispatch.BatchingExecutor$BlockableBatch.$anonfun$run$1(BatchingExecutor.scala:91)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
Caused by: javax.persistence.PersistenceException: Query threw SQLException:Column "T1.FORUM_ID" not found; SQL statement:
select t0.id, t1.id, t1.title, t1.replies, t1.last_reply, t1.forum_id from forum t0 left join thread t1 on t1.forum_id = t0.id  where t0.id = ?   order by t0.id [42122-196] Bind values:[null] Query was:select t0.id, t1.id, t1.title, t1.replies, t1.last_reply, t1.forum_id from forum t0 left join thread t1 on t1.forum_id = t0.id  where t0.id = ?   order by t0.id
        at io.ebean.config.dbplatform.SqlCodeTranslator.translate(SqlCodeTranslator.java:48)
        at io.ebean.config.dbplatform.DatabasePlatform.translate(DatabasePlatform.java:195)
        at io.ebeaninternal.server.query.CQueryEngine.translate(CQueryEngine.java:139)
        at io.ebeaninternal.server.query.DefaultOrmQueryEngine.translate(DefaultOrmQueryEngine.java:38)
        at io.ebeaninternal.server.core.OrmQueryRequest.translate(OrmQueryRequest.java:81)
        at io.ebeaninternal.server.query.CQuery.createPersistenceException(CQuery.java:666)
        at io.ebeaninternal.server.query.CQueryEngine.find(CQueryEngine.java:422)
        at io.ebeaninternal.server.query.DefaultOrmQueryEngine.findId(DefaultOrmQueryEngine.java:162)
        at io.ebeaninternal.server.core.OrmQueryRequest.findId(OrmQueryRequest.java:327)
        at io.ebeaninternal.server.core.DefaultServer.findId(DefaultServer.java:1175)
Caused by: org.h2.jdbc.JdbcSQLException: Column "T1.FORUM_ID" not found; SQL statement:
select t0.id, t1.id, t1.title, t1.replies, t1.last_reply, t1.forum_id from forum t0 left join thread t1 on t1.forum_id = t0.id  where t0.id = ?   order by t0.id [42122-196]
        at org.h2.message.DbException.getJdbcSQLException(DbException.java:345)
        at org.h2.message.DbException.get(DbException.java:179)
        at org.h2.message.DbException.get(DbException.java:155)
        at org.h2.expression.ExpressionColumn.optimize(ExpressionColumn.java:147)
        at org.h2.command.dml.Select.prepare(Select.java:839)
        at org.h2.command.Parser.prepareCommand(Parser.java:263)
        at org.h2.engine.Session.prepareLocal(Session.java:578)
        at org.h2.engine.Session.prepareCommand(Session.java:519)
        at org.h2.jdbc.JdbcConnection.prepareCommand(JdbcConnection.java:1204)
        at org.h2.jdbc.JdbcPreparedStatement.<init>(JdbcPreparedStatement.java:73)

Answer 1

线路

at io.ebeaninternal.server.deploy.BeanPropertyAssocMany.createReference(BeanPropertyAssocMany.java:652)

可以帮助您了解这里出了什么问题:

您滥用了@ManyToOne注释。

在您的应用程序中，您有一个论坛用于许多线程，因此列表上方的注释论坛模型中的线程应该是

@OneToMany

您还需要将 Thread 模型中 Forum 声明上方的注释切换为

@ManyToOne