CreateNamedQuery中的java.lang.NullPointerException

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问题描述

我有以下代码，导致产生 java.lang.NullPointerException ：

I have the following code which is resulting in a java.lang.NullPointerException:

List results = em.createNamedQuery("User.findByUserName").setParameter("userName", username).getResultList();

用户是实体bean，其中包含：

User is the entity bean, which contains:

@NamedQuery(name = "User.findByUserName", query = "SELECT u FROM User u WHERE u.userName = :userName"),

有人知道为什么吗？

推荐答案

方法 getResultList（）不会抛出 NullPointerException （如果没有匹配项，则返回空列表），所以我我猜应该是以下其中之一：

A method getResultList() doesn't throw NullPointerException (it returns empty list if there's no match), so I'm guessing that it should be one of the following:

您没有注入/初始化 EntityManager em （您是否忘了
@PersistenceContext 批注？）

字符串用户名为空

you didn't inject/initialize EntityManager em (did you forget@PersistenceContext annotation?)
String username is null

如果EntityManager为null，请检查是否具有persistence.xml文件（这是强制性的！）。看起来应该像这样：

In case of EntityManager null, check whether you have persistence.xml file (it's mandatory!). It should look like:

<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
  <persistence-unit name="OEMSPU" transaction-type="JTA">
    <provider>oracle.toplink.essentials.PersistenceProvider</provider>
    <jta-data-source>ADD JNDI NAME OF YOUR DATASOURCE, e.g. jdbc/sample</jta-data-source>
    <exclude-unlisted-classes>false</exclude-unlisted-classes>
    <properties/>
  </persistence-unit>
</persistence>

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