Server-side XML-RPC

On the server side, you can either embed the XML-RPC library into an existing server framework, or use the built-in special purpose HTTP server. Let's first look at how to register handler objects to tell an XML-RPC server how to map incoming requests to actual methods.

XML-RPC Handler Objects

The org.apache.xmlrpc.XmlRpcServer and org.apache.xmlrpc.WebServer classes provide methods that let your register and unregister Java objects as XML-RPC handlers:

 addHandler (String name, Object handler);
 removeHandler (String name);

Depending on what kind of handler object you give to the server, it will do one of the following things:

  1. If you pass the XmlRpcServer any Java object, the server will try to resolve incoming calls via object introspection, i.e. by looking for public methods in the handler object corresponding to the method name and the parameter types of incoming requests. The input parameters of incoming XML-RPC requests must match the argument types of the Java method (see conversion table), or otherwise the method won't be found. The return value of the Java method must be supported by XML-RPC.
  2. If you pass the XmlRpcServer an object that implements interface org.apache.xmlrpc.XmlRpcHandler or org.apache.xmlrpc.AuthenticatedXmlRpcHandler the execute() method will be called for every incoming request. You are then in full control of how to process the XML-RPC request, enabling you to perform input and output parameter checks and conversion, special error handling etc.
In both cases, incoming requests will be interpreted as handlerName.methodName with handlerName being the String that the handler has been registered with, and methodName being the name of the method to be invoked. You can work around this scheme by registering a handler with the name "$default". In this case you can drop the handlerName. part from the method name.

Using XML-RPC within a Servlet environment

The XML-RPC library can be embedded into any Web server framework that supports reading HTTP POSTs from an InputStream. The typical code for processing an incoming XML-RPC request looks like this:

 XmlRpcServer xmlrpc = new XmlRpcServer ();
 xmlrpc.addHandler ("examples", new ExampleHandler ());
 byte[] result = xmlrpc.execute (request.getInputStream ());
 response.setContentType ("text/xml");
 response.setContentLength (result.length());
 OutputStream out = response.getOutputStream();
 out.write (result);
 out.flush ();

Note that the execute method does not throw any exception, since all errors are encoded into the XML result that will be sent back to the client. A full example servlet is included in the package. There is a sample XML-RPC Servlet included in the library. You can use it as a starting point for your own needs.

Using the Built-in HTTP Server

The XML-RPC library comes with its own built-in HTTP server. This is not a general purpose web server, its only purpose is to handle XML-RPC requests. The HTTP server can be embedded in any Java application with a few simple lines:

 WebServer webserver = new WebServer (port);
 webserver.addHandler ("examples", someHandler);

You can also start the web server from the command line by typing:

 java org.apache.xmlrpc.WebServer

You can specify the server port, but there's no way to manipulate RPC handlers in command line mode, so you'll either have to modify for your purposes or embed it into your own application. A special bonus when using the built in Web server is that you can set the IP addresses of clients from which to accept or deny requests. This is done via the following methods:

 webserver.setParanoid (true);
 // deny all clients
 webserver.acceptClient ("192.168.0.*"); // allow local access
 webserver.denyClient (""); // except for this one
 webserver.setParanoid (false); // disable client filter

If the client filter is activated, entries to the deny list always override those in the accept list. Thus, webserver.denyClient ("*.*.*.*") would completely disable the web server.

Note that the XML-RPC client in Frontier 5 has its requests hard-coded to URI /RPC2. To work with these clients, you have to configure your server environment to respond to /RPC2. This should be fixed in a newer version.