如何在 POJO 中获取 EntityManager？

【问题标题】：How to get a EntityManager in a POJO?如何在 POJO 中获取 EntityManager？
【发布时间】：2016-04-13 14:11:12
【问题描述】：

我目前正在重构旧的遗留代码，并用 EJB 3.0 / 3.1 替换或删除所有 EJB 2.0 引用。我遇到了一个实用程序类，它提供我们的 Oracle 数据库 (11g / 12c) 的当前日期 (java.util.Date) 或时间戳 (java.sql.Timestamp)。对数据库的访问是通过java.sql.Connection 实现的，该java.sql.Connection 是通过在javax.naming.InitialContext 上查找获得的。

我想通过 JPA 或 Hibernate 解决此问题并使用新的 Java 日期/时间 API。所以我重写了实用程序类，但我无法获得有效的javax.persistence.EntityManager 实例。

实用程序类与其他现有的无状态会话和实体 bean 一起打包在一个 server.jar 中，该文件包含在一个 ear 中。持久性单元“seller-em”已经存在并被会话和实体 bean 使用。当我尝试调用函数 getEm() 时，当 emf.createEntitymanager 被执行时，我得到一个 java.lang.NullPointerException。

我还尝试让 Wildfly (8.2.1 Final) 向实体管理器注入：

@PersistenceContext(unitName = "seller-em")
private EntityManager em;

但是 em 为空。

我已经花了一天时间在谷歌和这里寻找提示。我出错的任何想法或任何人都可以给我一个正确方向的提示吗？

最好的问候，
CB

环境：
Java 8 (1.8.0_77)
日食火星 R2 (4.5.2)
Maven 3.3.3
(JBoss) Wildfly 8.2.1 最终版

附件：

package org.example.server.seller.tools;

import java.sql.Timestamp;
import java.time.Instant;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.ZoneId;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;

public class DateUtility {
    private static final String ORACLE_ISO_8601_FORMAT_STRING = "YYYY-MM-DD\"T\"HH:MI:SS.FF6TZH:TZM";
    // 12/31/2999 00:00:00.000000 +01:00 (ECT / European Central Time / TZ:Europe/Berlin)
    // Using Z time, because +01:00, ECT, ECT, UTC+1, etc. results in a java.time.format.DateTimeParseException.
    private static final Instant MAXIMUM_DATE_TIME = Instant.parse("2999-12-30T23:00:00.000000Z");
    private static final Object TREE_LOCK = new Object();

    private static EntityManager getEm() {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("seller-em");
        return emf.createEntityManager();
    }

    public static Instant getDatabaseSystemTimestamp() {
        // EntityManager is not thread-safe, we need to synchronie it.
        synchronized (TREE_LOCK) {
            Query query = getEm().createNativeQuery("SELECT TO_CHAR( SYSTIMESTAMP, :formatString) FROM DUAL");
            query.setParameter("formatString", ORACLE_ISO_8601_FORMAT_STRING);
            String iso8601String = (String) query.getSingleResult();
            return Instant.parse(iso8601String);
        }
    }

    public static Instant getMaximumDateTime() {
        return MAXIMUM_DATE_TIME;
    }

    public static LocalDateTime getCurrentDateTime() {
        return LocalDateTime.ofInstant(getDatabaseSystemTimestamp(), ZoneId.of("Europe/Berlin"));
    }

    public static LocalDate getCurrentDate() {
        return getCurrentDateTime().toLocalDate();
    }

    public static LocalTime getCurrentTime() {
        return getCurrentDateTime().toLocalTime();
    }

    /*
     * Just for backwards compatibility. Opted for java.sql.Timestamp since it is an extension of java.util.Date and
     * therefore can also be used a Date.
     */
    @Deprecated
    public static Timestamp getCurrentDateTimeAsTimestamp() {
        return Timestamp.from(getDatabaseSystemTimestamp());
    }
}

还有堆栈跟踪：

java.lang.NullPointerException
    at org.hibernate.engine.transaction.internal.jta.JtaStatusHelper.getStatus(JtaStatusHelper.java:76)
    at org.hibernate.engine.transaction.internal.jta.JtaStatusHelper.isActive(JtaStatusHelper.java:118)
    at org.hibernate.engine.transaction.internal.jta.CMTTransaction.join(CMTTransaction.java:149)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.joinTransaction(AbstractEntityManagerImpl.java:1602)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.postInit(AbstractEntityManagerImpl.java:210)
    at org.hibernate.jpa.internal.EntityManagerImpl.<init>(EntityManagerImpl.java:91)
    at org.hibernate.jpa.internal.EntityManagerFactoryImpl.internalCreateEntityManager(EntityManagerFactoryImpl.java:345)
    at org.hibernate.jpa.internal.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:313)
    at org.example.server.seller.tools.DateUtility.getEm(DateUtility.java:26)
    at org.example.server.seller.tools.DateUtility.getDatabaseSystemTimestamp(DateUtility.java:32)
    at org.example.server.seller.tools.DateUtilityTest.testDateUtility(DateUtilityTest.java:94)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
    at org.jboss.arquillian.junit.Arquillian$6$1.invoke(Arquillian.java:270)
    at org.jboss.arquillian.container.test.impl.execution.LocalTestExecuter.execute(LocalTestExecuter.java:60)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.invokeObservers(EventContextImpl.java:99)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:81)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:135)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:115)
    at org.jboss.arquillian.core.impl.EventImpl.fire(EventImpl.java:67)
    at org.jboss.arquillian.container.test.impl.execution.ContainerTestExecuter.execute(ContainerTestExecuter.java:38)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.invokeObservers(EventContextImpl.java:99)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:81)
    at org.jboss.arquillian.test.impl.TestContextHandler.createTestContext(TestContextHandler.java:102)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.test.impl.TestContextHandler.createClassContext(TestContextHandler.java:84)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.test.impl.TestContextHandler.createSuiteContext(TestContextHandler.java:65)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.jboss.arquillian.core.impl.ObserverImpl.invoke(ObserverImpl.java:94)
    at org.jboss.arquillian.core.impl.EventContextImpl.proceed(EventContextImpl.java:88)
    at org.jboss.arquillian.core.impl.ManagerImpl.fire(ManagerImpl.java:135)
    at org.jboss.arquillian.test.impl.EventTestRunnerAdaptor.test(EventTestRunnerAdaptor.java:111)
    at org.jboss.arquillian.junit.Arquillian$6.evaluate(Arquillian.java:263)
    at org.jboss.arquillian.junit.Arquillian$4.evaluate(Arquillian.java:226)
    at org.jboss.arquillian.junit.Arquillian.multiExecute(Arquillian.java:314)
    at org.jboss.arquillian.junit.Arquillian.access$100(Arquillian.java:46)
    at org.jboss.arquillian.junit.Arquillian$5.evaluate(Arquillian.java:240)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
    at org.jboss.arquillian.junit.Arquillian$2.evaluate(Arquillian.java:185)
    at org.jboss.arquillian.junit.Arquillian.multiExecute(Arquillian.java:314)
    at org.jboss.arquillian.junit.Arquillian.access$100(Arquillian.java:46)
    at org.jboss.arquillian.junit.Arquillian$3.evaluate(Arquillian.java:199)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
    at org.jboss.arquillian.junit.Arquillian.run(Arquillian.java:147)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:160)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:138)
    at org.jboss.arquillian.junit.container.JUnitTestRunner.execute(JUnitTestRunner.java:65)
    at org.jboss.arquillian.protocol.servlet.runner.ServletTestRunner.executeTest(ServletTestRunner.java:160)
    at org.jboss.arquillian.protocol.servlet.runner.ServletTestRunner.execute(ServletTestRunner.java:126)
    at org.jboss.arquillian.protocol.servlet.runner.ServletTestRunner.doGet(ServletTestRunner.java:90)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
    at io.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:86)
    at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:62)
    at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36)
    at org.wildfly.extension.undertow.security.SecurityContextAssociationHandler.handleRequest(SecurityContextAssociationHandler.java:78)
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
    at io.undertow.servlet.handlers.security.SSLInformationAssociationHandler.handleRequest(SSLInformationAssociationHandler.java:131)
    at io.undertow.servlet.handlers.security.ServletAuthenticationCallHandler.handleRequest(ServletAuthenticationCallHandler.java:57)
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
    at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:46)
    at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:64)
    at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest(AuthenticationMechanismsHandler.java:58)
    at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:70)
    at io.undertow.security.handlers.SecurityInitialHandler.handleRequest(SecurityInitialHandler.java:76)
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
    at org.wildfly.extension.undertow.security.jacc.JACCContextIdHandler.handleRequest(JACCContextIdHandler.java:61)
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
    at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:261)
    at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:248)
    at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:77)
    at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:167)
    at io.undertow.server.Connectors.executeRootHandler(Connectors.java:199)
    at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:761)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

persistence.xml：

<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"
             version="2.1">

    <persistence-unit name="seller-em">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <jta-data-source>java:/DBUNIT</jta-data-source>
        <properties>
            <property name="hibernate.show_sql" value="true" />
            <property name="hibernate.dialect" value="org.hibernate.dialect.Oracle10gDialect" />
        </properties>
    </persistence-unit>

</persistence>

【问题讨论】：

  • 如果你得到一些 NPE，那么会期望它是与堆栈跟踪和什么对象为空的声明一起发布的第一件事！
  • 您好尼尔斯托克顿，感谢您的回复。添加堆栈跟踪是常识，我只是忘了添加它。对不起。我已经编辑了我的原始帖子并附加了堆栈跟踪。
  • 因为事务 (JTA) 存在问题，请发布您的 persistence.xml 并检查您提供的数据源
  • 我附加了 persistence.xml。但这应该不是问题，因为现有的 bean 已经在使用这个持久性单元和为其配置的数据源。我仔细检查了 Wildfly 服务器的standalone.xml。一切看起来都应该如此。

标签： jpa persistence entitymanager


【解决方案1】：

我自己找到了解决方案。发布堆栈跟踪后，我在 JtaStatusHelper 类上查找了 NPE，并在 stackoverflow 上获得了成功。在this thread我找到了决定性的提示。

我在我的 persistence.xml 中添加了以下行并且它起作用了：

<property name="hibernate.transaction.jta.platform" value="org.hibernate.service.jta.platform.internal.JBossAppServerJtaPlatform" />

感谢您的帮助。
最好的补课，
CB

【讨论】：

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