Java 如何在资源中使用@PersistenceUnit

我正在修补一个Jersey Web服务，它可以访问集成到Glassfish4应用服务器中的默认JavaDB。其思想是在服务器上的应用程序运行时创建新对象，并使用Java持久性API（JPA）访问它们，即使服务器关闭或应用程序从服务器上删除

第一个问题是，这是否可行

第二个问题是关于我在注入EntityManagerFactory时遇到的一些问题，这样我就可以在处理HTTP请求时访问它

我的简单测试用例如下所示：

@Path("jpaResource/")
public class JpaResource {

    @PersistenceUnit
    private EntityManagerFactory emf;

    @GET
    @Path("html")
    @Produces(MediaType.TEXT_HTML)
    public String getHtml() {

        EntityManagerFactory factory = Persistence.createEntityManagerFactory("WebApp");    System.out.println(aThing);     }

        Thing thing = (Thing) factory.createEntityManager().createQuery("select u from Thing u").getResultList().get(1);

        return "<h1>Hey HTML! I'm: " + thing.getDescription() + "</h1>";        
    }

    @GET
    @Path("text")
    @Produces(MediaType.TEXT_PLAIN)
    public String getText() {

        Thing thing = (Thing) emf.createEntityManager().createQuery("select u from Thing u").getResultList().get(0);

        return "Hey text! I'm: " + thing.getDescription();
    }
}

persistence.xml文件存储在我的eclipse项目的…/src/main/resources/META-INF文件夹中，如下所示：

<?xml version="1.0" encoding="UTF-8" ?>
<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"
    version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
    <persistence-unit name="WebApp" transaction-type="RESOURCE_LOCAL">
        <!--provider>org.eclipse.persistence.jpa.PersistenceProvider</provider-->
        <!--provider>org.hibernate.ejb.HibernatePersistence</provider-->
        <jta-data-source>jdbc/__default</jta-data-source>
        <class>com.test.webapp.objects.Thing</class>
        <properties>
            <property name="eclipselink.ddl-generation" value="create-tables"/>
            <property name="eclipselink.ddl-generation.output-mode" value="database"/>
        </properties>
    </persistence-unit>
</persistence>

StandardWrapperValve[Jersey Web Application]: Servlet.service() for servlet Jersey Web Application threw exception
java.lang.NullPointerException
    at com.test.webapp.resources.JpaResource.getText(JpaResource.java:66)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.glassfish.jersey.server.model.internal.ResourceMethodInvocationHandlerFactory$1.invoke(ResourceMethodInvocationHandlerFactory.java:81)
    at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.invoke(AbstractJavaResourceMethodDispatcher.java:125)
    at org.glassfish.jersey.server.model.internal.JavaResourceMethodDispatcherProvider$TypeOutInvoker.doDispatch(JavaResourceMethodDispatcherProvider.java:195)
    at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.dispatch(AbstractJavaResourceMethodDispatcher.java:91)
    at org.glassfish.jersey.server.model.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:346)
    at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:341)
    at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:101)
    at org.glassfish.jersey.server.ServerRuntime$1.run(ServerRuntime.java:224)
    at org.glassfish.jersey.internal.Errors$1.call(Errors.java:271)
    at org.glassfish.jersey.internal.Errors$1.call(Errors.java:267)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:315)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:297)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:267)
    at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:317)
    at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:198)
    at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:946)
    at org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:323)
    at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:372)
    at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:335)
    at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:218)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
    at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:357)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:260)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:188)
    at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:191)
    at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:168)
    at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:189)
    at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:288)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:206)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:136)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:114)
    at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
    at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:838)
    at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:113)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:115)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:55)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:135)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:564)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:544)
    at java.lang.Thread.run(Thread.java:744)

如果有人知道我做错了什么，请告诉我

问候

[编辑：]好的，我做了更多的测试，我的项目中有一个ServletContextListener接口的实现，它在应用程序第一次启动时处理初始化

public class ServletContextClass implements ServletContextListener {

    @PersistenceUnit(unitName="WebApp")
    private EntityManagerFactory emf;

    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {

        System.out.println("##############################");

        System.out.println("ServletContextListener started");

        System.out.println("##############################");       
    }

    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {

        System.out.println("###############################");

        System.out.println("SevletContextListener destroyed");

        System.out.println("###############################");      
    }
}

在这个类中，注入工作没有任何问题，我能够访问JavaDB来检索对象

我观看的教程使用了Glassfish3服务器。是否有可能该功能已从Java EE7中删除，因此无法在Glassfish4服务器上工作？我发现了一个类似的问题，这表明我的方法应该与JavaEE6（）配合使用。现在我真的很困惑

我将添加我的web.xml，以便有人有兴趣回答我的问题并需要部署描述符

<?xml version="1.0" encoding="UTF-8"?>
<!-- This web.xml file is not required when using Servlet 3.0 container,
     see implementation details http://jersey.java.net/nonav/documentation/latest/jax-rs.html -->
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
     <listener>
        <listener-class>com.test.webapp.startup.ServletContextClass</listener-class>
      </listener>
    <servlet>
        <servlet-name>Jersey Web Application</servlet-name>
        <servlet-class>org.glassfish.jersey.servlet.ServletContainer</servlet-class>

        <init-param>
            <param-name>jersey.config.server.provider.packages</param-name>
            <param-value>com.test.webapp.resources</param-value>
        </init-param>

        <init-param>
            <param-name>com.sun.jersey.api.json.POJOMappingFeature</param-name>
            <param-value>true</param-value>
        </init-param>

        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>Jersey Web Application</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>
</web-app>

com.test.webapp.startup.ServletContextClass
Jersey Web应用程序
org.glassfish.jersey.servlet.ServletContainer
jersey.config.server.provider.packages
com.test.webapp.resources
com.sun.jersey.api.json.POJOMappingFeature
真的
1.
Jersey Web应用程序
/*

我想问题在于，在每个请求上创建一个新的EntityManager工厂和一个新的EntityManager，然后将其更改为在类init上创建的单个实例。我还建议换成JTA。我不确定我是否完全理解这一点。我想使用@PersistenceUnit一次性注入EntityManagerFactory。我的理解是，框架随后将为我初始化EntotyManagerFactory，并通过注入使其在类中可用（可能我对这个概念的想法是错误的）。但是，注入将不起作用，因为它总是空的。我跟随了Arun Gupta（Java EE 6和GlassFish 3与Eclipse一起）的youtube教程，他这样使用它似乎没有问题。为什么不将您的服务设置为无状态EJB并直接注入EntityManager？应用服务器将负责为您创建和管理EMF。是的，我也考虑过这个解决方案。但是，您似乎不能这样做，因为我有一个动态web项目，并且我不能将EJB模块包括在这个项目中。当然可以。我不知道如何在Eclipse中做到这一点，但我相信您可以做到。

<?xml version="1.0" encoding="UTF-8"?>
<!-- This web.xml file is not required when using Servlet 3.0 container,
     see implementation details http://jersey.java.net/nonav/documentation/latest/jax-rs.html -->
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
     <listener>
        <listener-class>com.test.webapp.startup.ServletContextClass</listener-class>
      </listener>
    <servlet>
        <servlet-name>Jersey Web Application</servlet-name>
        <servlet-class>org.glassfish.jersey.servlet.ServletContainer</servlet-class>

        <init-param>
            <param-name>jersey.config.server.provider.packages</param-name>
            <param-value>com.test.webapp.resources</param-value>
        </init-param>

        <init-param>
            <param-name>com.sun.jersey.api.json.POJOMappingFeature</param-name>
            <param-value>true</param-value>
        </init-param>

        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>Jersey Web Application</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>
</web-app>