In this article, I will discuss the Direct Web Remoting (DWR) technology developed by Joe Walker and maintained by the small IT consultancy Getahead in UK. This technology is a unique way of Ajax-enabling existing Java server-side code by exposing it in JavaScript. This sounds complicated but, in reality, the technology is relatively straightforward. It has some limitations, such as exposing overloaded Java methods or marshaling of very complex Java Objects to JavaScript and will work only in a web application server. But overall, DWR is a very powerful concept that works with any existing Java web application and makes the process of web remoting easier.

Introduction to DWR

If you are familiar with the Web Services specification, the DWR can appear to be conceptually very similar. In Web Services, Java code is "exposed" via a Web Services Definition Language (WSDL) XML file, and external clients connect via stub code by using a predefined protocol. The clients query the exposed methods, and objects travel from the server in a SOAP XML format. With the DWR, the Java code is also defined via a custom XML file and "exposed" via a special Servlet. DWR clients connect via AJAX/JavaScript stub code using a predefined custom protocol and query the exposed methods in JavaScript. DWR is a Java-only technology, whereas Web Services are standards based and are not language specific. However, because of its unique approach to using Ajax and ease of use, DWR technology quietly gained popularity among Java developers. DWR is a freely available for download and use.

Setting Up DWR

If you are familiar with the main JEE project structure, setting up DWR is relatively simple on any Java application server. To add Direct Web Remoting to any JEE project, there are three main steps. First, put the dwr.jar in the application path (this is usually the lib folder under the WEB-INF); second, add mapping to the special DWR Servlet in the main web.xml; and third, add a special dwr.xml descriptor file in the same folder as the web.xml file. The dwr.xml is the xml file that describes what objects and methods are "exposed" for the direct web remoting calls.

This is really all needed to set up the DWR on the server. The client-side is where the "exposed" methods are called. As you can see, the server setup does not require any changes to the existing Java code or server configuration code except extra mapping to just one Servlet.

Here is the directory structure in Eclipse: