我想将同一组数据共享给多个客户。我需要使用 Push 自动更新他们在屏幕上的视图。
我已阅读问答,Vaadin 7 应用程序(“@Push”)中推送的最小示例。现在我需要一个更强大的现实例子。一方面,我知道在 Servlet 环境中拥有一个永无止境的线程并不是一个好主意。
而且我不希望每个用户都有自己的线程,每个用户都自己访问数据库。让一个线程单独检查数据库中的新数据似乎更合乎逻辑。找到后,该线程应将新数据发布到等待更新的所有用户的 UI/布局。
我想将同一组数据共享给多个客户。我需要使用 Push 自动更新他们在屏幕上的视图。
我已阅读问答,Vaadin 7 应用程序(“@Push”)中推送的最小示例。现在我需要一个更强大的现实例子。一方面,我知道在 Servlet 环境中拥有一个永无止境的线程并不是一个好主意。
而且我不希望每个用户都有自己的线程,每个用户都自己访问数据库。让一个线程单独检查数据库中的新数据似乎更合乎逻辑。找到后,该线程应将新数据发布到等待更新的所有用户的 UI/布局。
您将在下面找到几个类的代码。他们一起制作了一个完整的 Vaadin 7.3.8 应用程序示例,使用新的内置推送功能将一组数据同时发布给任意数量的用户。我们通过随机生成一组数据值来模拟检查数据库中的新数据。
当您运行此示例应用程序时,会出现一个窗口,显示当前时间和一个按钮。时间每秒更新一次,持续一百次。
这个时间更新不是真实的例子。时间更新器还有另外两个用途:
要查看此应用程序的真实预期示例,请单击/点击“打开数据窗口”按钮。将打开第二个窗口以显示三个文本字段。每个字段都包含一个随机生成的值,我们假装它来自数据库查询。
这样做需要一些工作,需要几件。让我们回顾一下这些部分。
在当前版本的 Vaadin 7.3.8 中,不需要插件或附加组件来启用Push 技术。甚至与 Push 相关的 .jar 文件也与 Vaadin 捆绑在一起。
有关详细信息,请参阅Vaadin 之书。但实际上您需要做的就是将@Push
注释添加到您的UI子类中。
使用最新版本的 Servlet 容器和 Web 服务器。Push 相对较新,实现也在不断发展,尤其是对于WebSocket种类。例如,如果使用 Tomcat,请务必使用 Tomcat 7 或 8 的最新更新。
我们必须有某种方法来重复查询数据库以获取新数据。
永无止境的线程并不是在 Servlet 环境中执行此操作的最佳方式,因为当 Web 应用程序取消部署或 Servlet 包含关闭时,线程不会结束。Thread会继续在JVM中运行,浪费资源,造成内存泄漏等问题。
理想情况下,我们希望在 Web 应用程序启动(部署)和 Web 应用程序关闭(或取消部署)时得到通知。收到通知后,我们可以启动或中断该数据库查询线程。幸运的是,每个 Servlet 容器都提供了这样的钩子。Servlet 规范要求容器支持该接口ServletContextListener
。
我们可以编写一个实现这个接口的类。当我们的 Web 应用程序(我们的 Vaadin 应用程序)被部署时,我们的监听器类会contextInitialized
被调用。取消部署时,将contextDestroyed
调用该方法。
从这个钩子我们可以启动一个线程。但是有一个更好的方法。Java 配备了ScheduledExecutorService
. 这个类有一个线程池供其使用,以避免实例化和启动线程的开销。您可以将一个或多个任务(Runnable)分配给执行程序,以定期运行。
这是我们的 Web 应用程序侦听器类,使用 Java 8 中可用的 Lambda 语法。
package com.example.pushvaadinapp;
import java.time.Instant;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
/**
* Reacts to this web app starting/deploying and shutting down.
*
* @author Basil Bourque
*/
@WebListener
public class WebAppListener implements ServletContextListener
{
ScheduledExecutorService scheduledExecutorService;
ScheduledFuture<?> dataPublishHandle;
// Constructor.
public WebAppListener ()
{
this.scheduledExecutorService = Executors.newScheduledThreadPool( 7 );
}
// Our web app (Vaadin app) is starting up.
public void contextInitialized ( ServletContextEvent servletContextEvent )
{
System.out.println( Instant.now().toString() + " Method WebAppListener::contextInitialized running." ); // DEBUG logging.
// In this example, we do not need the ServletContex. But FYI, you may find it useful.
ServletContext ctx = servletContextEvent.getServletContext();
System.out.println( "Web app context initialized." ); // INFO logging.
System.out.println( "TRACE Servlet Context Name : " + ctx.getServletContextName() );
System.out.println( "TRACE Server Info : " + ctx.getServerInfo() );
// Schedule the periodic publishing of fresh data. Pass an anonymous Runnable using the Lambda syntax of Java 8.
this.dataPublishHandle = this.scheduledExecutorService.scheduleAtFixedRate( () -> {
System.out.println( Instant.now().toString() + " Method WebAppListener::contextDestroyed->Runnable running. ------------------------------" ); // DEBUG logging.
DataPublisher.instance().publishIfReady();
} , 5 , 5 , TimeUnit.SECONDS );
}
// Our web app (Vaadin app) is shutting down.
public void contextDestroyed ( ServletContextEvent servletContextEvent )
{
System.out.println( Instant.now().toString() + " Method WebAppListener::contextDestroyed running." ); // DEBUG logging.
System.out.println( "Web app context destroyed." ); // INFO logging.
this.scheduledExecutorService.shutdown();
}
}
在该代码中,您会看到定期调用 DataPublisher 实例,要求它检查新数据,如果找到,则将其传递给所有感兴趣的 Vaadin 布局或小部件。
package com.example.pushvaadinapp;
import java.time.Instant;
import net.engio.mbassy.bus.MBassador;
import net.engio.mbassy.bus.common.DeadMessage;
import net.engio.mbassy.bus.config.BusConfiguration;
import net.engio.mbassy.bus.config.Feature;
import net.engio.mbassy.listener.Handler;
/**
* A singleton to register objects (mostly user-interface components) interested
* in being periodically notified with fresh data.
*
* Works in tandem with a DataProvider singleton which interacts with database
* to look for fresh data.
*
* These two singletons, DataPublisher & DataProvider, could be combined into
* one. But for testing, it might be handy to keep them separated.
*
* @author Basil Bourque
*/
public class DataPublisher
{
// Statics
private static final DataPublisher singleton = new DataPublisher();
// Member vars.
private final MBassador<DataEvent> eventBus;
// Constructor. Private, for simple Singleton pattern.
private DataPublisher ()
{
System.out.println( Instant.now().toString() + " Method DataPublisher::constructor running." ); // DEBUG logging.
BusConfiguration busConfig = new BusConfiguration();
busConfig.addFeature( Feature.SyncPubSub.Default() );
busConfig.addFeature( Feature.AsynchronousHandlerInvocation.Default() );
busConfig.addFeature( Feature.AsynchronousMessageDispatch.Default() );
this.eventBus = new MBassador<>( busConfig );
//this.eventBus = new MBassador<>( BusConfiguration.SyncAsync() );
//this.eventBus.subscribe( this );
}
// Singleton accessor.
public static DataPublisher instance ()
{
System.out.println( Instant.now().toString() + " Method DataPublisher::instance running." ); // DEBUG logging.
return singleton;
}
public void register ( Object subscriber )
{
System.out.println( Instant.now().toString() + " Method DataPublisher::register running." ); // DEBUG logging.
this.eventBus.subscribe( subscriber ); // The event bus is thread-safe. So hopefully we need no concurrency managament here.
}
public void deregister ( Object subscriber )
{
System.out.println( Instant.now().toString() + " Method DataPublisher::deregister running." ); // DEBUG logging.
// Would be unnecessary to deregister if the event bus held weak references.
// But it might be a good practice anyways for subscribers to deregister when appropriate.
this.eventBus.unsubscribe( subscriber ); // The event bus is thread-safe. So hopefully we need no concurrency managament here.
}
public void publishIfReady ()
{
System.out.println( Instant.now().toString() + " Method DataPublisher::publishIfReady running." ); // DEBUG logging.
// We expect this method to be called repeatedly by a ScheduledExecutorService.
DataProvider dataProvider = DataProvider.instance();
Boolean isFresh = dataProvider.checkForFreshData();
if ( isFresh ) {
DataEvent dataEvent = dataProvider.data();
if ( dataEvent != null ) {
System.out.println( Instant.now().toString() + " Method DataPublisher::publishIfReady…post running." ); // DEBUG logging.
this.eventBus.publishAsync( dataEvent ); // Ideally this would be an asynchronous dispatching to bus subscribers.
}
}
}
@Handler
public void deadEventHandler ( DeadMessage event )
{
// A dead event is an event posted but had no subscribers.
// You may want to subscribe to DeadEvent as a debugging tool to see if your event is being dispatched successfully.
System.out.println( Instant.now() + " DeadMessage on MBassador event bus : " + event );
}
}
该 DataPublisher 类使用 DataProvider 类来访问数据库。在我们的例子中,我们简单地生成随机数据值,而不是实际访问数据库。
package com.example.pushvaadinapp;
import java.time.Instant;
import java.util.Random;
import java.util.UUID;
/**
* Access database to check for fresh data. If fresh data is found, package for
* delivery. Actually we generate random data as a way to mock database access.
*
* @author Basil Bourque
*/
public class DataProvider
{
// Statics
private static final DataProvider singleton = new DataProvider();
// Member vars.
private DataEvent cachedDataEvent = null;
private Instant whenLastChecked = null; // When did we last check for fresh data.
// Other vars.
private final Random random = new Random();
private Integer minimum = Integer.valueOf( 1 ); // Pick a random number between 1 and 999.
private Integer maximum = Integer.valueOf( 999 );
// Constructor. Private, for simple Singleton pattern.
private DataProvider ()
{
System.out.println( Instant.now().toString() + " Method DataProvider::constructor running." ); // DEBUG logging.
}
// Singleton accessor.
public static DataProvider instance ()
{
System.out.println( Instant.now().toString() + " Method DataProvider::instance running." ); // DEBUG logging.
return singleton;
}
public Boolean checkForFreshData ()
{
System.out.println( Instant.now().toString() + " Method DataProvider::checkForFreshData running." ); // DEBUG logging.
synchronized ( this ) {
// Record when we last checked for fresh data.
this.whenLastChecked = Instant.now();
// Mock database access by generating random data.
UUID dbUuid = java.util.UUID.randomUUID();
Number dbNumber = this.random.nextInt( ( this.maximum - this.minimum ) + 1 ) + this.minimum;
Instant dbUpdated = Instant.now();
// If we have no previous data (first retrieval from database) OR If the retrieved data is different than previous data --> Fresh.
Boolean isFreshData = ( ( this.cachedDataEvent == null ) || ! this.cachedDataEvent.uuid.equals( dbUuid ) );
if ( isFreshData ) {
DataEvent freshDataEvent = new DataEvent( dbUuid , dbNumber , dbUpdated );
// Post fresh data to event bus.
this.cachedDataEvent = freshDataEvent; // Remember this fresh data for future comparisons.
}
return isFreshData;
}
}
public DataEvent data ()
{
System.out.println( Instant.now().toString() + " Method DataProvider::data running." ); // DEBUG logging.
synchronized ( this ) {
return this.cachedDataEvent;
}
}
}
DataProvider 打包新数据以传递给其他对象。我们将 DataEvent 类定义为该包。或者,如果您需要交付多组数据或对象而不是单个,则在您的 DataHolder 版本中放置一个集合。打包任何对想要显示这些新数据的布局或小部件有意义的东西。
package com.example.pushvaadinapp;
import java.time.Instant;
import java.util.UUID;
/**
* Holds data to be published in the UI. In real life, this could be one object
* or could hold a collection of data objects as might be needed by a chart for
* example. These objects will be dispatched to subscribers of an MBassador
* event bus.
*
* @author Basil Bourque
*/
public class DataEvent
{
// Core data values.
UUID uuid = null;
Number number = null;
Instant updated = null;
// Constructor
public DataEvent ( UUID uuid , Number number , Instant updated )
{
this.uuid = uuid;
this.number = number;
this.updated = updated;
}
@Override
public String toString ()
{
return "DataEvent{ " + "uuid=" + uuid + " | number=" + number + " | updated=" + updated + " }";
}
}
将新数据打包到 DataEvent 中后,DataProvider 将其交给 DataPublisher。因此,下一步是将这些数据获取到感兴趣的 Vaadin 布局或小部件以呈现给用户。但是我们如何知道哪些布局/小部件对此数据感兴趣?我们如何将这些数据传递给他们?
一种可能的方法是观察者模式。我们在 Java Swing 和 Vaadin 中都看到了这种模式,例如在 VaadinClickListener
中的 a Button
。这种模式意味着观察者和被观察者相互了解。这意味着在定义和实现接口方面需要做更多的工作。
在我们的例子中,我们不需要数据的生产者(DataPublisher)和消费者(Vaadin 布局/小部件)相互了解。所有小部件想要的只是数据,无需与生产者进一步交互。所以我们可以使用不同的方法,事件总线。在事件总线中,当一些有趣的事情发生时,一些对象会发布一个“事件”对象。当事件对象发布到总线时,其他对象会注册它们有兴趣得到通知。发布后,总线通过调用某个方法并传递事件将该事件发布给所有注册的订阅者。在我们的例子中,将传递 DataEvent 对象。
但是注册的订阅对象上的哪个方法会被调用呢?通过 Java 的注解、反射和自省技术的魔力,任何方法都可以被标记为要调用的方法。只需用注解标记所需的方法,然后在发布事件时让总线在运行时找到该方法。
无需自己构建任何此事件总线。在 Java 世界中,我们可以选择事件总线实现。
最著名的可能是 Google Guava EventBus。Google Guava是 Google 内部开发的一系列各种实用项目,然后开源供其他人使用。EventBus 包就是这些项目之一。我们可以使用 Guava EventBus。事实上,我最初确实使用这个库构建了这个例子。但是 Guava EventBus 有一个限制:它拥有强引用。
当对象注册他们有兴趣被通知时,任何事件总线都必须通过持有对注册对象的引用来保存这些订阅的列表。理想情况下,这应该是一个弱引用,这意味着如果订阅对象达到其用途的尽头并成为垃圾收集的候选对象,该对象可能会这样做。如果事件总线持有强引用,则对象无法进行垃圾回收。弱引用告诉 JVM 我们并没有真正关心对象,我们稍微关心但不足以坚持保留对象。使用弱引用,事件总线会在尝试通知订阅者新事件之前检查空引用。如果为 null,则事件总线可以将该插槽放入其对象跟踪集合中。
您可能认为作为解决强引用问题的解决方法,您可以让已注册的 Vaadin 小部件覆盖该detach
方法。当不再使用 Vaadin 小部件时会通知您,然后您的方法将从事件总线中注销。如果订阅对象从事件总线中取出,则不再有强引用,也不再有问题。但正如 Java Object 方法finalize
并不总是被调用一样,Vaadindetach
方法也不总是被调用。有关详细信息,请参阅Vaadin 专家Henri Sara在此线程上的帖子。依赖可能会导致内存泄漏和其他问题。detach
有关事件总线库的各种 Java 实现的讨论,请参阅我的博客文章。其中我选择了MBassador用于此示例应用程序。它存在的理由是使用弱引用。
要真正更新 Vaadin 布局和小部件的值,有一个大问题。这些小部件在它们自己的用户界面处理线程(该用户的主 Servlet 线程)中运行。同时,您的数据库检查、数据发布和事件总线调度都发生在由执行器服务管理的后台线程上。永远不要从单独的线程访问或更新 Vaadin 小部件!这条规则是绝对关键的。更棘手的是,这样做实际上可能在开发过程中起作用。但是,如果您在生产中这样做,您将处于一个受伤的世界。
那么,我们如何从后台线程获取数据并传递到在主 Servlet 线程中运行的小部件中呢?UI类为此提供了一个方法:access
. 您将Runnable传递给该access
方法,Vaadin 安排该 Runnable 在主用户界面线程上执行。十分简单。
为了结束这个示例应用程序,这里是剩余的类。“MyUI”类替换了由Vaadin 7.3.7 的新 Maven 原型创建的默认项目中的同名文件。
package com.example.pushvaadinapp;
import com.vaadin.annotations.Push;
import com.vaadin.annotations.Theme;
import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.annotations.Widgetset;
import com.vaadin.server.BrowserWindowOpener;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinServlet;
import com.vaadin.ui.Button;
import com.vaadin.ui.Label;
import com.vaadin.ui.UI;
import com.vaadin.ui.VerticalLayout;
import java.time.Instant;
import javax.servlet.annotation.WebServlet;
/**
* © 2014 Basil Bourque. This source code may be used freely forever by anyone
* absolving me of any and all responsibility.
*/
@Push
@Theme ( "mytheme" )
@Widgetset ( "com.example.pushvaadinapp.MyAppWidgetset" )
public class MyUI extends UI
{
Label label = new Label( "Now : " );
Button button = null;
@Override
protected void init ( VaadinRequest vaadinRequest )
{
// Prepare widgets.
this.button = this.makeOpenWindowButton();
// Arrange widgets in a layout.
VerticalLayout layout = new VerticalLayout();
layout.setMargin( Boolean.TRUE );
layout.setSpacing( Boolean.TRUE );
layout.addComponent( this.label );
layout.addComponent( this.button );
// Put layout in this UI.
setContent( layout );
// Start the data feed thread
new FeederThread().start();
}
@WebServlet ( urlPatterns = "/*" , name = "MyUIServlet" , asyncSupported = true )
@VaadinServletConfiguration ( ui = MyUI.class , productionMode = false )
public static class MyUIServlet extends VaadinServlet
{
}
public void tellTime ()
{
label.setValue( "Now : " + Instant.now().toString() ); // If before Java 8, use: new java.util.Date(). Or better, Joda-Time.
}
class FeederThread extends Thread
{
// This Thread class is merely a simple test to verify that Push works.
// This Thread class is not the intended example.
// A ScheduledExecutorService is in WebAppListener class is the intended example.
int count = 0;
@Override
public void run ()
{
try {
// Update the data for a while
while ( count < 100 ) {
Thread.sleep( 1000 );
access( new Runnable() // Special 'access' method on UI object, for inter-thread communication.
{
@Override
public void run ()
{
count ++;
tellTime();
}
} );
}
// Inform that we have stopped running
access( new Runnable()
{
@Override
public void run ()
{
label.setValue( "Done. No more telling time." );
}
} );
} catch ( InterruptedException e ) {
e.printStackTrace();
}
}
}
Button makeOpenWindowButton ()
{
// Create a button that opens a new browser window.
BrowserWindowOpener opener = new BrowserWindowOpener( DataUI.class );
opener.setFeatures( "height=300,width=440,resizable=yes,scrollbars=no" );
// Attach it to a button
Button button = new Button( "Open data window" );
opener.extend( button );
return button;
}
}
“DataUI”和“DataLayout”完成了这个示例 Vaadin 应用程序中的 7 个 .java 文件。
package com.example.pushvaadinapp;
import com.vaadin.annotations.Push;
import com.vaadin.annotations.Theme;
import com.vaadin.annotations.Widgetset;
import com.vaadin.server.VaadinRequest;
import com.vaadin.ui.UI;
import java.time.Instant;
import net.engio.mbassy.listener.Handler;
@Push
@Theme ( "mytheme" )
@Widgetset ( "com.example.pushvaadinapp.MyAppWidgetset" )
public class DataUI extends UI
{
// Member vars.
DataLayout layout;
@Override
protected void init ( VaadinRequest request )
{
System.out.println( Instant.now().toString() + " Method DataUI::init running." ); // DEBUG logging.
// Initialize window.
this.getPage().setTitle( "Database Display" );
// Content.
this.layout = new DataLayout();
this.setContent( this.layout );
DataPublisher.instance().register( this ); // Sign-up for notification of fresh data delivery.
}
@Handler
public void update ( DataEvent event )
{
System.out.println( Instant.now().toString() + " Method DataUI::update (@Subscribe) running." ); // DEBUG logging.
// We expect to be given a DataEvent item.
// In a real app, we might need to retrieve data (such as a Collection) from within this event object.
this.access( () -> {
this.layout.update( event ); // Crucial that go through the UI:access method when updating the user interface (widgets) from another thread.
} );
}
}
……和……</p>
/*
* To change this license header, choose License Headers in Project Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package com.example.pushvaadinapp;
import com.vaadin.ui.TextField;
import com.vaadin.ui.VerticalLayout;
import java.time.Instant;
/**
*
* @author brainydeveloper
*/
public class DataLayout extends VerticalLayout
{
TextField uuidField;
TextField numericField;
TextField updatedField;
TextField whenCheckedField;
// Constructor
public DataLayout ()
{
System.out.println( Instant.now().toString() + " Method DataLayout::constructor running." ); // DEBUG logging.
// Configure layout.
this.setMargin( Boolean.TRUE );
this.setSpacing( Boolean.TRUE );
// Prepare widgets.
this.uuidField = new TextField( "UUID : " );
this.uuidField.setWidth( 22 , Unit.EM );
this.uuidField.setReadOnly( true );
this.numericField = new TextField( "Number : " );
this.numericField.setWidth( 22 , Unit.EM );
this.numericField.setReadOnly( true );
this.updatedField = new TextField( "Updated : " );
this.updatedField.setValue( "<Content will update automatically>" );
this.updatedField.setWidth( 22 , Unit.EM );
this.updatedField.setReadOnly( true );
// Arrange widgets.
this.addComponent( this.uuidField );
this.addComponent( this.numericField );
this.addComponent( this.updatedField );
}
public void update ( DataEvent dataHolder )
{
System.out.println( Instant.now().toString() + " Method DataLayout::update (via @Subscribe on UI) running." ); // DEBUG logging.
// Stuff data values into fields. For simplicity in this example app, using String directly rather than Vaadin converters.
this.uuidField.setReadOnly( false );
this.uuidField.setValue( dataHolder.uuid.toString() );
this.uuidField.setReadOnly( true );
this.numericField.setReadOnly( false );
this.numericField.setValue( dataHolder.number.toString() );
this.numericField.setReadOnly( true );
this.updatedField.setReadOnly( false );
this.updatedField.setValue( dataHolder.updated.toString() );
this.updatedField.setReadOnly( true );
}
}