Java RPC通信机制之XML-RPC：Apache XML-RPC 3.0开发简介
XML-RPC是一种简单的，轻量级的通过HTTP协议进行RPC通信的规范。本文以Apache XML-RPC 3.0为基础，对XML-RPC的基本原理及Apache XML-RPC 3.0的主要特性进行了讨论和分析。
XML-RPC是一种简单的，轻量级的通过HTTP协议进行RPC通信的规范。一个XML-RPC消息就是一个请求体为XML的HTTP-POST请求，被调用的方法在服务器端执行并将执行结果以XML格式编码后返回。
以下是通过ethereal抓到的一个典型的XML-RPC调用包（为便于阅读，进行了格式化）：
POST /xmlrpc HTTP/1.1
Content-Type: text/xml
User-Agent: Apache XML RPC 3.0 (Jakarta Commons httpclient Transport)
Host: 135.252.156.147:8080
Content-Length: 260
<?xml?version="1.0"?encoding="UTF-8"?>
<methodCall?xmlns:ex="http://ws.apache.org/xmlrpc/namespaces/extensions">
????? <methodName>Calculator.add</methodName>
????? <params>
????? ????? <param>
??????????? ????? <value>
??????????? ????? ????? <i4>2</i4>
??????????? ????? </value>
????? ????? </param>
????? ????? <param>
??????????? ????? <value>
??????????? ????? ????? <i4>3</i4>
??????????? ????? </value>
????? ????? </param>
????? </params>
</methodCall>
而对应的返回数据包为：
HTTP/1.1 200 OK
Server: Apache XML-RPC 1.0
Connection: close
Content-Type: text/xml
Content-Length: 189
<?xml?version="1.0"?encoding="UTF-8"?>
<methodResponse?xmlns:ex="http://ws.apache.org/xmlrpc/namespaces/extensions">
????? <params>
????? ????? <param>
??????????? ????? <value>
??????????? ????? ????? <i4>5</i4>
??????????? ????? </value>
????? ????? </param>
????? </params>
</methodResponse>
其格式很简单，几乎是不言自明的，分别用methodCall和methodResponse标签标识发送给Server的调用请求和Server的返回结果，请求方法的名称用methodName标识，参数用params和param标识，而参数的类型标签则如下表所示：
Tag
Java Type
说明
<i4> or <int>
Integer/int
4字节带符号整数值
<boolean>
Boolean
0 (false) or 1 (true)
<string>
String
字符串
<double>
Double
双精度带符号浮点值
<dateTime.iso8601>
java.util.Date
日期/时间
<base64>
byte[]
base64编码的二进制数据
<struct>
java.util.Map
键值对，键为String类型，而值为任意有效类型
<array>
Object[]
java.util.List
对象数组
下面举一个实际运用XML-RPC进行RPC调用的例子，XML-RPC规范有多种针对不同语言的实现，这里我们使用的是Apache的XML-RPC3.0RC1。
在开始之前，需到http://jakarta.apache.org/commons/index.html下载如下程序包：
commons-codec-1.3（通用编码/解码算法实现，可参考http://www.devx.com/Java/Article/29795/1954?pf=true或http://jakarta.apache.org/commons/codec/userguide.html来获得该软件包的详细信息）
commons-httpclient-3.0.1（HTTP协议的客户端编程工具包，详细介绍见http://www-128.ibm.com/developerworks/cn/opensource/os-httpclient/）
将上述通用工具包解压后，拷贝其中的jar文件到XML-RPC解压目录的dist目录中。
并添加如下环境变量：
XMLRPC_HOME????? XML-RPC的解压目录
XMLRPC_LIB?????? %XMLRPC_HOME%/dist
XMLRPCCLASSPATH????? %XMLRPC_LIB%/xmlrpc-common-3.0rc1.jar;%XMLRPC_LIB%/xmlrpc-server-3.0rc1.jar;%XMLRPC_LIB%/xmlrpc-client-3.0rc1.jar;%XMLRPC_LIB%/commons-httpclient-3.0.1.jar;%XMLRPC_LIB%/commons-codec-1.3.jar
?
整个应用很简单，通过XML-RPC调用Server端提供的HelloHandler.sayHello方法回显一个字符串信息。下面是HelloHandler接口及其实现类相关代码：
//?HelloHandler.java
package demo.xmlrpc;
?
public?interface?HelloHandler {
??????public?String sayHello(String str);
}
?
//?HelloHandlerImpl.java
package demo.xmlrpc;
?
public?class?HelloHandlerImpl implements?HelloHandler {
??????public?String sayHello(String str){
????? ??????return?"Hello, "?+ str +?"!";
????? }
}
以下是对应的Server端源代码：
//?Server1.java
package demo.xmlrpc;
?
import java.io.IOException;
import java.io.OutputStream;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
?
import org.apache.xmlrpc.XmlRpcException;
import org.apache.xmlrpc.server.PropertyHandlerMapping;
import org.apache.xmlrpc.server.XmlRpcServerConfigImpl;
import org.apache.xmlrpc.webserver.XmlRpcServletServer;
?
public?class?Server1?extends HttpServlet {
??????private?XmlRpcServletServer server;
?????
??????public?void?init(ServletConfig pConfig) throws ServletException {
????? ????? super.init(pConfig);
????? ??????try?{
??????????? ??????// create a new XmlRpcServletServer object
??????????? ????? server =?new?XmlRpcServletServer();
??????????? ??????// set up handler mapping of XmlRpcServletServer object
??????????? ????? PropertyHandlerMapping phm =?new?PropertyHandlerMapping();
??????????? ????? phm.addHandler("HelloHandler",?HelloHandlerImpl.class);??????????? ?????
??????????? ????? server.setHandlerMapping(phm);
??????????? ??????// more config of XmlRpcServletServer object?????
??????????? ????? XmlRpcServerConfigImpl serverConfig = (XmlRpcServerConfigImpl)server.getConfig();
??????????? ????? serverConfig.setEnabledForExtensions(true);
??????????? ????? serverConfig.setContentLengthOptional(false);
????? ????? }?catch?(XmlRpcException e) {
??????????? ??????try?{
??????????? ????? ????? log("Failed to create XmlRpcServer: "?+ e.getMessage(), e);
??????????? ????? }?catch?(Throwable ignore) {
??????????? ????? }
??????????? ??????throw?new?ServletException(e);
????? ????? }
????? }
?????
??????public?void?doPost(HttpServletRequest pRequest, HttpServletResponse pResponse)
????? ????? throws IOException, ServletException {
????? ????? server.execute(pRequest, pResponse);
????? }
}
以下是对应的Client端源代码：
//?Client1.java
package demo.xmlrpc;
?
import java.io.IOException;
import java.net.MalformedURLException;
import java.util.Vector;
import java.net.URL;
?
import org.apache.xmlrpc.XmlRpcException;
import org.apache.xmlrpc.client.XmlRpcClient;
import org.apache.xmlrpc.client.XmlRpcClientConfigImpl;
?
public?class?Client1?{
??????public?static?void?main(String[] args) {
????? ??????try?{
??????????? ??????// config client
??????????? ????? XmlRpcClientConfigImpl config =?new?XmlRpcClientConfigImpl();
??????????? ????? config.setServerURL(new?URL("http://localhost:8080/jsp/XmlRpcServer"));?????// should be modified according to your configuration of jsp container
??????????? ??????// create a new XmlRpcClient object and bind above config object with it
??????????? ????? XmlRpcClient client =?new?XmlRpcClient();
??????????? ????? client.setConfig(config);
??????????? ??????// create parameter list
??????????? ????? Vector<String>?params =?new?Vector<String>();
??????????? ????? params.addElement("Tom");
??????????? ??????// execute XML-RPC call
??????????? ????? String result = (String) client.execute("HelloHandler.sayHello", params);??????????? ?????
??????????? ????? System.out.println(result);
????? ????? }?catch?(MalformedURLException e) {
??????????? ????? System.out.println(e.toString());
????? ????? }?catch?(XmlRpcException e) {
??????????? ????? System.out.println(e.toString());
????? ????? }?catch?(IOException e) {
??????????? ????? e.printStackTrace();
????? ????? }
????? }
}
程序源码中已包含了详细的注释，这里就不作过多解释了。但需注意XmlRpcDemo_Client中的ServerURL信息应根据自己的的jsp容器的配置作相应调整，并需设置相应的servlet-mapping信息，在我的jsp目录（Tomcat5.5的Context之一）下的WEB_INF/web.xml文件中存在如下的servlet-mapping信息：
<servlet>
????? <servlet-name>XmlRpcServer</servlet-name>
????? <servlet-class>demo.xmlrpc.Server1</servlet-class>
</servlet>
<servlet-mapping>
????? <servlet-name>XmlRpcServer</servlet-name>
????? <url-pattern>/XmlRpcServer</url-pattern>
</servlet-mapping>
并且，上述Server1.class及其他相关类文件已被拷贝到jsp/WEB-INF/classes/demo/xmlrpc目录下。
在启动Tomcat并执行
java -classpath %CLASSPATH%;%XMLRPCCLASSPATH% demo.xmlrpc.Client1.java
前，你应该将%XMLRPC_HOME%/dist、%XMLRPC_HOME%/lib下的几个jar文件（source就不用拷了）及前面下载的commons-codec-1.3.jar拷贝到%TOMCAT_HOME%/common/lib或jsp/WEB-INF/lib下。
Note：除了上面这种方式，你可以无需编写任何Server端代码，仅通过简单配置完成上述功能，具体可参考：http://ws.apache.org/xmlrpc/server.html
接下来，作为比较，我们来看看XML-RPC2.0中应该如何实现上述功能。
以下是2.0版的Server程序：
//?Server2.java
package demo.xmlrpc;
?
import java.io.IOException;
import java.io.OutputStream;
?
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
?
import org.apache.xmlrpc.XmlRpcServer;
?
public?class?Server2 extends HttpServlet {
??????public?void?doPost(HttpServletRequest request, HttpServletResponse response)
??????????? ????? throws ServletException, IOException {
????? ????? XmlRpcServer xmlrpc =?new?XmlRpcServer();
????? ????? xmlrpc.addHandler("HelloHandler",?new?HelloHandlerImpl());
????? ????? byte[] result = xmlrpc.execute(request.getInputStream());
????? ????? response.setContentType("text/xml");
????? ????? response.setContentLength(result.length);
????? ????? OutputStream out = response.getOutputStream();
????? ????? out.write(result);
????? ????? out.flush();
????? }
}
以下是2.0版的Client程序：
//?Client2.java
package demo.xmlrpc;
?
import java.io.IOException;
import java.net.MalformedURLException;
import java.util.Vector;
?
import org.apache.xmlrpc.XmlRpcClient;
import org.apache.xmlrpc.XmlRpcException;
?
public?class?Client2 {
??????public?static?void?main(String[] args) {
????? ??????try?{
??????????? ????? XmlRpcClient xmlrpc =?newXmlRpcClient("http://localhost:8080/jsp/XmlRpcServer");
??????????? ????? Vector<String>?params =?new?Vector<String>();
??????????? ????? params.addElement("Tom");
??????????? ????? String result = (String) xmlrpc.execute("HelloHandler.sayHello", params);
??????????? ????? System.out.println(result);
????? ????? }?catch?(MalformedURLException e) {
??????????? ????? System.out.println(e.toString());
????? ????? }?catch?(XmlRpcException e) {
??????????? ????? System.out.println(e.toString());
????? ????? }?catch?(IOException e) {
??????????? ??????e.printStackTrace();
?????????? }
????? }
}
总体上看，3.0比2.0在可配置性方面有了一些改进，其它方面则没有太大变化，但由于功能模块的分离，使得3.0较2.0显得更为复杂，已经习惯了2.0单一模块风格的开发者可能需要一些时间适应这种变化。
除了上面的基本功能，XML-RPC3还支持动态代理/工厂和异步通信等特性。
通过运用动态代理特性，我们可以在Server端及Client端共享接口信息，从而在编译期间进行必要的类型检查，在XML-RPC内部，所有的调用仍然是被动态转发给XmlRpcClient对象来完成的。但要使用XML-RPC3的动态代理功能，相应的服务器端的处理器类名称必须是Client端接口类的全名（含包名，该名称一般应该与Server端接口类全名一致），否则将会导致调用失败。以上面的HelloHandler接口为例，其对应的处理器类名称应该为：demo.xmlrpc.HelloHandler。
Note:?动态代理（JDK1.3引入）是Proxy模式、依赖注入（Dependency Injection）及动态代码生成等技术相结合的一种应用，在各新型Web应用框架及容器中被广泛采用。
而要使用XML-RPC的异步通信功能，只需实现org.apache.xmlrpc.client.AsyncCallback接口，该接口包括两个方法：
public?void?handleResult(XmlRpcRequest pRequest, Object pResult);
public?void?handleError(XmlRpcRequest pRequest, Throwable pError);
此外，为了便于在普通应用中使用XML-RPC，XML-RPC还提供了一个WebServer类，以便在应用中内嵌一个HTTP服务器，为Client程序提供HTTP服务。
下面的范例演示了上面提到的几种特性，以下是Server端代码：
//?Server3.java
package demo.xmlrpc;
?
import org.apache.xmlrpc.server.PropertyHandlerMapping;
import org.apache.xmlrpc.server.XmlRpcServer;
import org.apache.xmlrpc.server.XmlRpcServerConfigImpl;
import org.apache.xmlrpc.webserver.WebServer;
?
public?class?Server3?{
??????private?static?final?int?port = 8080;
?
??????public?static?void?main(String [] args) throws Exception {
????? ????? WebServer webServer =?new?WebServer(port);
?
????? ????? XmlRpcServer xmlRpcServer = webServer.getXmlRpcServer();
?
????? ????? PropertyHandlerMapping phm =?new?PropertyHandlerMapping();
????? ????? phm.addHandler("demo.xmlrpc.HelloHandler",?HelloHandlerImpl.class);
??????????? ?
????? ????? xmlRpcServer.setHandlerMapping(phm);
?
????? ????? XmlRpcServerConfigImpl serverConfig = (XmlRpcServerConfigImpl)xmlRpcServer.getConfig();
????? ????? serverConfig.setEnabledForExtensions(true);
????? ????? serverConfig.setContentLengthOptional(false);
?
????? ????? webServer.start();
????? }
}
下面是Client端代码：
//?Client3.java
package demo.xmlrpc;
?
import java.net.URL;
import java.util.List;
import java.util.Vector;
?
import org.apache.xmlrpc.XmlRpcRequest;
import org.apache.xmlrpc.XmlRpcException;
import org.apache.xmlrpc.client.XmlRpcClient;
import org.apache.xmlrpc.client.XmlRpcClientConfigImpl;
import org.apache.xmlrpc.client.AsyncCallback;
import org.apache.xmlrpc.client.util.ClientFactory;
?
class?EchoCallback implements AsyncCallback {
??????public?void?handleResult(XmlRpcRequest pRequest, Object pResult) {
????? ????? System.out.println("Server returns: "?+ (String)pResult);
????? }
?????
??????public?void?handleError(XmlRpcRequest pRequest, Throwable pError) {
????? ????? System.out.println("Error occurs: "?+ pError.getMessage());
????? }
}
?
public?class?Client3?{
??????public?static?void?main(String [] args) throws Exception {
????? ??????// create configuration
????? ????? XmlRpcClientConfigImpl config =?new?XmlRpcClientConfigImpl();
????? ????? config.setServerURL(new?URL("http://localhost:8080/xmlrpc"));
????? ????? config.setEnabledForExtensions(true);
????? ????? config.setConnectionTimeout(60 * 1000);
????? ????? config.setReplyTimeout(60 * 1000);
?
????? ????? XmlRpcClient client =?new?XmlRpcClient();
????? ??????// set configuration
????? ????? client.setConfig(config);
???????????
????? ??????// make a call using dynamic proxy
????? ????? ClientFactory factory =?new?ClientFactory(client);
????? ??????HelloHandler handler = (HelloHandler)factory.newInstance(HelloHandler.class);
????? ????? String str = handler.sayHello("Bill David");
????? ????? System.out.println(str);
???????????
????? ??????// make an asynchronous call
????? ????? List<String> params =?new?Vector<String>();?// for JDK before 1.5, use 'List params = new Vector();'
????? ????? params.add("Tom");
????? ????? client.executeAsync("demo.xmlrpc.HelloHandler.sayHello", params,?newEchoCallback());
????? }
}
Note:由于Server3使用了8080端口，注意不要在Tomcat运行时启动Server3（除非你的Tomcat运行在其他端口）。
1.????XML-RPC，http://ws.apache.org/xmlrpc/
2.????XML-RPC协议，http://hedong.3322.org/archives/000470.html
3.????Dynamic Proxy Classes，http://java.sun.com/j2se/1.3/docs/guide/reflection/proxy.html
4.????透明，动态代理的前世今生，《程序员》2005年第1期。
