Symfony \ Component \ HttpKernel \ Exception \ NotFoundHttpException：找不到对象

我正在研究symfony2项目，我得到了这个例外 . 谁知道造成它的原因是什么？未捕获的异常'Symfony \ Component \ HttpKernel \ Exception \ NotFoundHttpException'，消息'Gmjob \ ExaminationBundle \ Entity \找不到考试对象 . '在/ data / apache / www / emploipublic-sf / vendor / sensio / framework-extra bundle / Sensio / Bundle / FrameworkExtraBundle / Request / ParamConverter / DoctrineParamConverter.php：55 \ nStack

public function apply(Request $request, ConfigurationInterface $configuration)
{
    $name    = $configuration->getName();
    $class   = $configuration->getClass();
    $options = $this->getOptions($configuration);

    // find by identifier?
    if (false === $object = $this->find($class, $request, $options, $name)) {
        // find by criteria
        if (false === $object = $this->findOneBy($class, $request, $options)) {
            if ($configuration->isOptional()) {
                $object = null;
            } else {
                throw new \LogicException('Unable to guess how to get a Doctrine instance from the request information.');
            }
        }
    }

    if (null === $object && false === $configuration->isOptional()) {
        throw new NotFoundHttpException(sprintf('%s object not found.', $class)); // this is line 55
    }

    $request->attributes->set($name, $object);

    return true;
}

这是抛出异常的堆栈跟踪：

[Tue Sep 09 16:56:03 2014] [error] [client 217.89.107.38] PHP致命错误：未捕获异常'Symfony \ Component \ HttpKernel \ Exception \ NotFoundHttpException'，消息'Gmjob \ ExaminationBundle \ Entity \未找到检查对象“ . 在/data/apache/www/emploipublic-sf/vendor/sensio/framework-extra-bundle/Sensio/Bundle/FrameworkExtraBundle/Request/ParamConverter/DoctrineParamConverter.php:55 \ nStack trace：\ n

＃0 /data/apache/www/emploipublic-sf/vendor/sensio/framework-extra-bundle/Sensio/Bundle/FrameworkExtraBundle/Request/ParamConverter/ParamConverterManager.php(92）：Sensio \ Bundle \ FrameworkExtraBundle \ Request \ ParamConverter \ DoctrineParamConverter-> apply（对象（Symfony \ Component \ HttpFoundation \ Request），Object（Sensio \ Bundle \ FrameworkExtraBundle \ Configuration \ ParamConverter））\ n

＃1 /data/apache/www/emploipublic-sf/vendor/sensio/framework-extra-bundle/Sensio/Bundle/FrameworkExtraBundle/Request/ParamConverter/ParamConverterManager.php(48）：Sensio \ Bundle \ FrameworkExtraBundle \ Request \ ParamConverter \ ParamConverterManager-> applyConverter（Object（Symfony \ Component \ HttpFoundation \ Request），Object（Sens in / data / apache / www / emploipublic-sf / vendor / sensio / framework-extra-bundle / Sensio / Bundle / FrameworkExtraBundle / Request / ParamConverter /DoctrineParamConverter.php在第55行