Java PersistenceContext未注入EntityManager

试图通过注释

PersistenceContext

访问

EntityManager

时，我收到一个NullPointerException

import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;

@Stateless
public class UserService {

    @PersistenceContext(unitName = "jcrdb")
    private EntityManager em;

    ...
}

路径

src/main/webapp/META-INF中的My
persistence.xml
：

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0"
    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_1_0.xsd">
    <persistence-unit name="jcrdb" transaction-type="RESOURCE_LOCAL">
        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
        <!-- <jta-data-source>jdbc/jcrdb</jta-data-source> -->
        <exclude-unlisted-classes>false</exclude-unlisted-classes>
        <properties>
            <property name="hibernate.show_sql" value="true" />
            <property name="hibernate.format_sql" value="true" />
            <!-- this tell Hibernate to update the DDL when it starts, very useful 
                for development, dangerous in production -->
            <property name="hibernate.hbm2ddl.auto" value="update" />
            <property name="javax.persistence.logging.level" value="INFO" />
            <property name="javax.persistence.jdbc.driver" value="org.postgresql.Driver" />
            <property name="javax.persistence.jdbc.url" value="jdbc:postgresql://127.0.0.1:5432/jcrdb" />
            <property name="javax.persistence.jdbc.user" value="jcr_admin" />
            <property name="javax.persistence.jdbc.password" value="q1w2e3r4" />
            <!-- <property name="tomee.jpa.factory.lazy" value="true" />-->
        </properties>
    </persistence-unit>
</persistence>

像这样直接获取EntityManager没有问题：

EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("jcrdb");
em = entityManagerFactory.createEntityManager();

我试着在jetty和tomEE运行这个

在我看来，ejb的东西似乎不起作用。我在这里遗漏了什么（所有这些都是新的）？
如果持久化单元是资源本地的，则必须使用EntityManagerFactory

您可以参考问题和答案了解完整的细节。
您从哪里获取
persistence.xml
文件？我知道如果您使用java配置（而不是XML配置），您可以如何选择它

您还需要设置数据源：

@Bean
public DataSource springJpaDataSource()
{
DriverManagerDataSource dataSource = new DriverManagerDataSource();
dataSource.setUrl("jdbc:mysql://localhost/ABCXYZ");
dataSource.setUsername("tomcatUser");
dataSource.setPassword("password1234");
return dataSource;
}

此外，在配置文件上还需要@EnableTransactionManagement注释

如果您想摆脱persistence.xml并需要完全用java进行持久性配置，则需要将上面返回LocalContainerEntityManagerFactoryBean的内容替换为：

@Bean
public LocalContainerEntityManagerFactoryBean entityManagerFactoryBean()
{
Map<String, Object> properties = new Hashtable<>();
properties.put("javax.persistence.schema-generation.database.action",
"none");
HibernateJpaVendorAdapter adapter = new HibernateJpaVendorAdapter();
adapter.setDatabasePlatform("org.hibernate.dialect.MySQL5InnoDBDialect");
LocalContainerEntityManagerFactoryBean factory =
new LocalContainerEntityManagerFactoryBean();
factory.setJpaVendorAdapter(adapter);
factory.setDataSource(this.springJpaDataSource());
factory.setPackagesToScan("com.wrox.site.entities");
factory.setSharedCacheMode(SharedCacheMode.ENABLE_SELECTIVE);
factory.setValidationMode(ValidationMode.NONE);
Configuring Persistence in Spring Framework ❘ 605
factory.setJpaPropertyMap(properties);
return factory;
}

@Bean
public LocalContainerEntityManagerFactoryBean entityManagerFactoryBean（）
{
映射属性=新哈希表（）；
properties.put（“javax.persistence.schema generation.database.action”，
“无”）；
HibernateJavaEndorapter适配器=新的HibernateJavaEndorapter（）；
setDatabasePlatform（“org.hibernate.dialent.mysql5innodbdialent”）；
LocalContainerEntityManagerFactoryBean工厂=
新的LocalContainerEntityManagerFactoryBean（）；
factory.setJpaVendorAdapter（适配器）；
setDataSource（this.springJpaDataSource（））；
setPackagesToScan（“com.wrox.site.entities”）；
factory.setSharedCacheMode（SharedCacheMode.ENABLE_SELECTIVE）；
setValidationMode（ValidationMode.NONE）；
在Spring框架中配置持久性❘ 605
factory.SetJPapPropertyMap（属性）；
返回工厂；
}
有时会发生这种情况，因为Spring没有注册。
要解决此问题，可以按如下方式手动定义：

<bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"/>



默认值将由
和
XML标记注册。
您使用的是spring吗？？如果没有我还没有使用spring，请打印完整的堆栈跟踪。堆栈跟踪仅显示我的应用程序和服务器的初始化方法。NPE是根本原因。请查看此链接…我认为使用“@”无状态bean的配置存在问题。请查看谢谢此链接非常有用！我添加了两行，但是现在在访问emf时得到了一个NullPointerException。因此，我将坚持使用获取emf的编程版本。这样，我也可以保证只有一个EntityManager（就像在链接中建议的那样）。
@Bean
public DataSource springJpaDataSource()
{
DriverManagerDataSource dataSource = new DriverManagerDataSource();
dataSource.setUrl("jdbc:mysql://localhost/ABCXYZ");
dataSource.setUsername("tomcatUser");
dataSource.setPassword("password1234");
return dataSource;
}

@Bean
public LocalContainerEntityManagerFactoryBean entityManagerFactoryBean()
{
Map<String, Object> properties = new Hashtable<>();
properties.put("javax.persistence.schema-generation.database.action",
"none");
HibernateJpaVendorAdapter adapter = new HibernateJpaVendorAdapter();
adapter.setDatabasePlatform("org.hibernate.dialect.MySQL5InnoDBDialect");
LocalContainerEntityManagerFactoryBean factory =
new LocalContainerEntityManagerFactoryBean();
factory.setJpaVendorAdapter(adapter);
factory.setDataSource(this.springJpaDataSource());
factory.setPackagesToScan("com.wrox.site.entities");
factory.setSharedCacheMode(SharedCacheMode.ENABLE_SELECTIVE);
factory.setValidationMode(ValidationMode.NONE);
Configuring Persistence in Spring Framework ❘ 605
factory.setJpaPropertyMap(properties);
return factory;
}

<bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"/>