##### XML-RPC

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## The XmlRpcClient

Before talking to an XML-RPC server, you need an instance of XmlRpcClient.

The XmlRpcClient is a stateless, thread safe object. The clients configuration occurs by setting the following objects:

Name | Description
ClientConfig | This object is an instance of XmlRpcClientConfig. It has a lot of atomic properties, that specify details like server URL, credentials, character set, and the like.
TransportFactory | The task of the transport factory is to create an object, which uses the client configuration for talking to the server. For example, there is a transport factory, which uses the java.net classes. Another example is a transport factory based on the Jakarta Commons Http Client. However, transport factories don't need to use HTTP: An excellent example is the local transport factory, which talks to an embedded server. This last factory is, of course, very useful for debugging.
XmlWriterFactory | The XmlWriter is an object, which creates XML for you. Typically, you do not need to care for this object, because the defaults should be fine. However, it is useful, if you need a special XML syntax.

So, let's have a look at a first example:

        import org.apache.xmlrpc.client.XmlRpcClient;
        import org.apache.xmlrpc.client.XmlRpcClientConfigImpl;

        XmlRpcClientConfigImpl config = new XmlRpcClientConfigImpl();
        config.setServerURL(new URL("http://127.0.0.1:8080/xmlrpc"));
        XmlRpcClient client = new XmlRpcClient();
        client.setConfig(config);
        Object[] params = new Object[]{new Integer(33), new Integer(9)};
        Integer result = (Integer) client.execute("Calculator.add", params);

In other words, we invoke the remote method Calculator.add, passing the arguments 2 and 3. Hopefully, we know the answer. :-)

## The Transport Factory

The above example uses the java.net.URLConnection classes to talk to the server. What, if you'd prefer to use the Jakarta HTTP Client? There's basically just a single line, you'd need to add:

        import org.apache.xmlrpc.client.XmlRpcClient;
        import org.apache.xmlrpc.client.XmlRpcClientConfigImpl;
        import org.apache.xmlrpc.client.XmlRpcCommonsTransportFactory;

        XmlRpcClientConfigImpl config = new XmlRpcClientConfigImpl();
        config.setServerURL(new URL("http://127.0.0.1:8080/XmlRpcServlet"));
        XmlRpcClient client = new XmlRpcClient();
        client.setTransportFactory(new XmlRpcCommonsTransportFactory(client));
        client.setConfig(config);
        Object[] params = new Object[]{new Integer(2), new Integer(3)};
        Integer result = (Integer) client.execute("Calculator.add", params);

In other words, the transport factory determines the way, how the client communicates with the server. The most important transport factories are:

Name | Description
XmlRpcSunHttpTransportFactory | This is the default factory, connecting to an HTTP server using the `java.net.HttpURLConnection`.
XmlRpcCommonsTransportFactory | Another HTTP transport factory, which uses the Jakarta Commons HttpClient. The main advantage over the default factory is, that the Commons HttpClient allows direct access to the result document. This allows a much lower memory profile.
XmlRpcLiteHttpTransportFactory | Yet another HTTP transport factory, which is based on an own and very lightweight HTTP client. It is possibly the fastest of the HTTP transport factories. On the other hand, it doesn't support HTTP/1.1, thus cannot use keepalive connections.
XmlRpcLocalTransportFactory | This transport factory has an embedded XML-RPC server, which is invoked via direct Java calls. This is particularly useful for debugging and development.

## The Client Configuration

The transport factory uses the clients configuration. Obviously, the clients configuration depends on the transport factory. In particular, different transport factories depend on different configuration types:

- The HTTP transport factories need an instance of `org.apache.xmlrpc.client.XmlRpcHttpClientConfig`.
- The local transport factory requires an instance of

  `org.apache.xmlrpc.client.XmlRpcLocalClientConfig`.

For convenience, you can simply use the `org.apache.xmlrpc.client.XmlRpcClientConfigImpl`, which implements both interfaces.

Let's have a look at the various properties, which HTTP client configurations accept:

Property Name | Description
basicUserName basicPassword | The user name and password, if your HTTP server requires basic authentication.
basicEncoding | Specifies the encoding being used to create the base 64 encoded Authorization header, which is being used for basic authentication. By default, the value of the encoding property is used. The encoding property itself defaults to UTF-8.
contentLengthOptional | Enables the faster and memory saving streaming mode: The client will not set the content-length header and the request is directly written to the HTTP requests output stream. The XML-RPC specification requires setting a content-length header. For that reason, the streaming mode is only available, if the property enabledForExtensions is set was well.
enabledForExceptions | Whether the client should request, that the server returns exceptions as serializable objects. If the server does, then the client will deserialize such exceptions and throw them, as if they had been cause within the clients code.
enabledForExtensions | Whether the vendor extensions of Apache XML-RPC should be enabled. By default, Apache XML-RPC is strictly compliant to the XML-RPC specification. Unfortunately, this specification has serious limitations. For example, it requires setting a content-length header. This enforces writing the XML-RPC request and response to byte arrays, before sending them over the net. Vendor extensions include the very fast and memory saving streaming mode (by disabling the content-length header), the compression of request and/or response. In particular, a lot of additional data types may be transmitted, when extensions are enabled: longs, shorts, bytes, floats, DOM nodes, instances of java.io.Serializable, or JAXB objects.
encoding | Sets the encoding, which is used for creating the XML-RPC request. The default encoding is UTF-8. Typically, the encoding is also used for the basic authentications, if any. However, you may specify a different encoding for the credentials using the basicEncoding property.
gzipCompressing | Whether the XML-RPC request should be compressed. Request compression is violating the XML-RPC specification, that's why gzipCompressing is only available, if the enabledForExtension property is also set. For the same reason, you should not assume, that the server is able to handle compressed requests, unless you know, that the server is itself running version 3 of Apache XML-RPC.
gzipRequesting | Requests, that the server will be compressing the response. Response compression is violating the XML-RPC specification. Therefore, this feature is only available, if the enabledForExtension property is set. Also, do not assume, that the server will actually compress the response, unless it is an Apache XML-RPC 3 server.

And these properties are for configuring the local transport factory:

Property Name | Description
xmlRpcServer | This is the embedded XML-RPC server, which is called to execute the clients requests. Obviously, this is an extremely fast transport. However, its main use is for debugging and development.