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Pushlets: Send events from servlets to DHTML client browsers

Discover how pushlets, a servlet-based notification mechanism, enables server-side Java objects to call back JavaScript code within a client browser.

By Just Van Den Broecke, JavaWorld.com, 03/01/00

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These days, developers increasingly turn to servlets and JavaServer Pages (JSPs) as Web-based frontends that integrate backend databases, Enterprise JavaBeans, or legacy applications. It is, however, difficult to keep client Web browsers up-to-date when the state of backend application data changes after the client has loaded the HTML page. For example, how do you keep one user up-to-date when she views the contents of a database table in her browser while another user simultaneously updates that same table on the server?

Indeed, applications such as live stock feeds, flight-arrival information, or weather-condition updates, in which the server streams or pushes live data, are difficult to create with servlets or JSPs.

How can we ensure these applications will be capable of notifying the browser after HTML page loading? Or, how can we selectively update parts of a page, such as updating only the stock whose price has changed? As seasoned Java developers, we often reflexively think of applets that use sockets or RMI/CORBA as the only server-to-client notification solution.

In this article we'll look at a solution that may better serve our purpose. First, I describe the current solutions for server-to-Web client notification. Next, I introduce pushlets as an alternative -- and possibly unconventional -- notification mechanism and then present a simple pushlet framework with some sample applications. Finally, I discuss some of the advantages and disadvantages of applying pushlets.

Note: This article's examples and complete source code can be viewed and downloaded from http://www.fluidiom.com:8080

Server-to-Web client notification: Current solutions

Before we delve into the pushlet concept, let's review the existing server-to-Web client solutions. Assume we have a Java Web or application server from which we want to notify client browsers. Possible solutions can be categorized as HTML refresh, server-side callbacks, and messaging.

HTML refresh

The simplest notification solution is the HTML refresh. By using <meta> tags in the header of the HTML document, the page automatically reloads every n seconds. If something has changed on the server since the last reload, we get the new content; otherwise, we get the original page. The following simple line, placed between <head> and </head> in an HTML page, does the trick: 

   <META HTTP-EQUIV="Refresh" CONTENT="4;URL=http://www.justobjects.nl">


While this solution is simple, we must still ask, how long should we make the refresh interval?

Server-side callbacks

In server-side callbacks, a server object calls back a Java-applet client using RMI (Remote Method Invocation) or CORBA (Common Object Request Broker Architecture). Usually the client first passes a remote reference of an RMI or CORBA object to the server. The server keeps a list of those references and calls them sequentially at the time of notification. This solution has been discussed in detail in other JavaWorld articles (see Resources).

Messaging

In messaging, an applet is a client of a messaging server that pushes messages over a TCP/IP connection (java.net.Socket) or sends connectionless UDP messages (java.net.DatagramSocket), the latter possibly even with multicast (java.net.MulticastSocket). You can use messaging middleware (MOMs), such as SoftWired's iBus, IBM's MQSeries, and BEA Systems' WebLogic Events, or develop your own custom messaging with java.io.ObjectStream on top of TCP or UDP sockets. The Java Messaging Service (JMS) is an important standard for messaging.

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Scaling up with PushLetsBy Anonymous on April 18, 2009, 11:08 amHi, Can Pushlet mechanism be scaled up to 1,000 users?. Regards, Raj S

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