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我正在研究Vaadin框架中的Web應用程序以從MySQL數據庫後端檢索一些數據。應用程序本身是一個調查系統，用於跟蹤軟件顧問的技能（並評估他們）。我的一些同事已經爲此寫了後端代碼。現在我正試圖將Vaadin前端連接到它，我遇到了一個非常煩人的問題。我有兩個DAO類，PersonDAO和SurveyDAO。使用Vaadin Hibernates ThreadLocal <Session> .get（）在第一次調用後返回null
先調用PersonDAO，然後調用SurveyDAO。 PersonDAO從存儲在線程本地會話變量中的HibernateUtil類（不是我自己寫的）中檢索會話。當SurveyDAO完全相同時，ThreadLocal Session.get（）方法返回空引用！經過一番閱讀後，我知道Vaadin與Sessions合作有奇怪的方式。但是我看不出我能如何解決這個問題，或者當時還有另一種與DAO合作的方式。
這裏是通過代碼完整的步行路程：
在我的主界面，在加載web應用程序，我創建一個由由的PersonDAO返回一個列表填充ComboBox組件。如下所示：
private ComboBox createConsultantsComboBox() {
ComboBox comboBox = new ComboBox("Surveyed consultants:");
BeanItemContainer<Person> consultantContainer = new BeanItemContainer<>(Person.class);
consultantContainer.addAll(personDAO.findAll());
comboBox.setContainerDataSource(consultantContainer);
comboBox.addValueChangeListener(new ConsultantSelectedListener());
return comboBox;
}
此DAO無縫工作。但是，當組合框中的值發生變化（選擇了顧問）時，我使用該人員的ID從SurveyDAO中獲取List<Survey>的顧問調查。
private class ConsultantSelectedListener implements ValueChangeListener {
@Override
public void valueChange(ValueChangeEvent event) {
if (event.getProperty().getValue() != null) {
Person selectedConsultant = (Person) event.getProperty().getValue();
leftPanel.addComponent(createSurveysComboBox(selectedConsultant.getId()));
}
}
}
這是出問題：這是使用下面的代碼，這是連接到充滿Person對象的組合框一個ValueChangeListener完成。 createSurveysComboBox（）方法與createConsultantsComboBox（）方法的工作方式相同，但它有一個int id參數傳遞給SurveyDAO，因爲它在查詢中作爲參數需要。然而，當SurveyDAO調用findByPersonId，我得到一個NullPointerException和以下堆棧跟蹤打印：
jun 30, 2016 10:15:27 AM com.vaadin.server.DefaultErrorHandler doDefault
SEVERE:
java.lang.NullPointerException
at be.kapture.util.HibernateUtil.getSession(HibernateUtil.java:55)
at be.kapture.dao.AbstractDAO.getCurrentSession(AbstractDAO.java:16)
at be.kapture.dao.SurveyDAO.findByPersonId(SurveyDAO.java:24)
at be.kapture.web.ConsultantSkillsUI.createSurveysComboBox(ConsultantSkillsUI.java:92)
at be.kapture.web.ConsultantSkillsUI.access$100(ConsultantSkillsUI.java:27)
at be.kapture.web.ConsultantSkillsUI$ConsultantSelectedListener.valueChange(ConsultantSkillsUI.java:82)
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:498)
at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:508)
at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:198)
at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:161)
at com.vaadin.server.AbstractClientConnector.fireEvent(AbstractClientConnector.java:1008)
at com.vaadin.ui.AbstractField.fireValueChange(AbstractField.java:1159)
at com.vaadin.ui.AbstractField.setValue(AbstractField.java:570)
at com.vaadin.ui.AbstractSelect.setValue(AbstractSelect.java:732)
at com.vaadin.ui.AbstractField.setValue(AbstractField.java:468)
at com.vaadin.ui.ComboBox.changeVariables(ComboBox.java:730)
at com.vaadin.server.communication.ServerRpcHandler.changeVariables(ServerRpcHandler.java:603)
at com.vaadin.server.communication.ServerRpcHandler.handleInvocations(ServerRpcHandler.java:422)
at com.vaadin.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:273)
at com.vaadin.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:79)
at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:41)
at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1409)
at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:364)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1099)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:672)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2508)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2497)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:745)
findByPersonId代碼可以發現以上。我現在給你提供的代碼片段的AbstractDAO.getCurrentSession（）和HibernateUtil.getSession（）方法（同樣，我沒有寫我自己 - 所以爲什麼我無法弄清楚什麼是錯的）
AbstractDao的
package be.kapture.dao;
import org.hibernate.Session;
import be.kapture.util.HibernateUtil;
public abstract class AbstractDAO<T> {
final Class<T> typeParameterClass;
public AbstractDAO(Class<T> typeParameterClass) {
this.typeParameterClass = typeParameterClass;
}
public Session getCurrentSession() {
return HibernateUtil.getSession();
}
public void create(T t) {
getCurrentSession().save(t);
}
public void update(T t) {
getCurrentSession().update(t);
}
public void delete(T t) {
getCurrentSession().delete(t);
}
public T read(int id) {
return (T) getCurrentSession().get(typeParameterClass, id);
}
}
的HibernateUtil
package be.kapture.util;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.model.naming.ImplicitNamingStrategyJpaCompliantImpl;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import java.util.Objects;
public class HibernateUtil {
private static final SessionFactory sessionFactory = buildSessionFactory();
private static final ThreadLocal<Session> sessionManagers = buildSessionManagers();
private static SessionFactory buildSessionFactory() {
try {
StandardServiceRegistry standardRegistry = new StandardServiceRegistryBuilder()
.configure("/hibernate.cfg.xml").build();
Metadata metadata = new MetadataSources(standardRegistry).addResource("be/kapture/entities/Person.hbm.xml")
.addResource("be/kapture/entities/Skill.hbm.xml")
.addResource("be/kapture/entities/SkillGroup.hbm.xml")
.addResource("be/kapture/entities/SkillNature.hbm.xml")
.addResource("be/kapture/entities/Survey.hbm.xml")
.addResource("be/kapture/entities/SurveyDetail.hbm.xml").getMetadataBuilder()
.applyImplicitNamingStrategy(ImplicitNamingStrategyJpaCompliantImpl.INSTANCE).build();
SessionFactory sessionFactory = metadata.getSessionFactoryBuilder().build();
return sessionFactory;
} catch (Throwable ex) {
System.err.println("initial SessionFactory creation failed " + ex);
throw new ExceptionInInitializerError(ex);
}
}
private static ThreadLocal<Session> buildSessionManagers() {
ThreadLocal<Session> sessionManagers = new ThreadLocal<>();
// System.out.println("sessionFactory.getCurrentSession == null ??");
// System.out.println(sessionFactory.getCurrentSession() == null);
sessionManagers.set(sessionFactory.getCurrentSession());
return sessionManagers;
}
public static Session getSession() {
// Objects.requireNonNull(sessionFactory, "sessionFactory was null");
// Objects.requireNonNull(sessionManagers, "sessionManagers was null");
// System.out.println(sessionManagers.get());
Session session = sessionManagers.get();
// Objects.requireNonNull(session, "session was null");
if(!session.isOpen()){
sessionManagers.remove();
session = sessionFactory.getCurrentSession();
sessionManagers.set(session);
}
session.beginTransaction();
return session;
}
}
在會話檢索方法，你可以看到一些調試輸出。當取消註釋時，這是會話變量HibernateUtil.getSession()將觸發的方法，因爲ThreadLocal返回空引用。
這是爲什麼？
讀入org.Hibernate.Session JavaDoc： 「實現者並不打算使用線程安全，而是每個線程/事務都應該從SessionFactory獲取它自己的實例。」這是否意味着HibernateUtil.getSession方法應該被重寫，以便在每次調用時從Factory中獲取一個新的Session？ –
您是否閱讀過'ThreadLocal'概念？當您正確使用ThreadLocal時，Hibernate會話不需要是線程安全的。但是你需要確保每個線程初始化一次。你應該關閉它，例如當請求結束時。此外，請參閱[此處]（https://docs.jboss.org/hibernate/orm/4.2/devguide/en-US/html/ch02.html#d5e737）瞭解會話中的模式。 –
當你寫下你的評論時，我正在爲自己的問題寫一個答案。在這樣做的時候，我明白你剛纔所說的是什麼。我如何在Vaadin環境中應用這個？我讀過Vaadin並沒有真正使用傳統的請求系統，而是使用多個併發線程異步同步客戶端內容與服務器。我錯了嗎？ –