Dubbo基于Hessian实现了自己Hessian协议，可以直接通过配置的Dubbo内置的其他协议，在服务消费方进行远程调用，也就是说，服务调用方需要使用Java语言来基于Dubbo调用提供方服务，限制了服务调用方。同时，使用Dubbo的Hessian协议实现提供方服务，而调用方可以使用标准的Hessian接口来调用，原生的Hessian协议已经支持多语言客户端调用，支持语言如下所示：

• Java：http://hessian.caucho.com/#Java
• Flash/Flex：http://hessian.caucho.com/#FlashFlex
• Python：http://hessian.caucho.com/#Python
• C++：http://hessian.caucho.com/#C
• C#：http://hessian.caucho.com/#NETC
• D：http://hessian.caucho.com/#D
• Erlang：http://hessian.caucho.com/#Erlang
• PHP：http://hessian.caucho.com/#PHP
• Ruby：http://hessian.caucho.com/#Ruby
• Objective-C：http://hessian.caucho.com/#ObjectiveC

下面，我们的思路是，先基于Dubbo封装的Hessian协议，实现提供方服务和消费方调用服务，双方必须都使用Dubbo来开发；然后，基于Dubbo封装的Hessian协议实现提供方服务，然后服务消费方使用标准的Hessian接口来进行远程调用，分别使用Java和Python语言来实现。而且，我们实现的提供方服务通过Tomcat发布到服务注册中心。
首先，使用Java语言定义一个搜索服务的接口，代码如下所示：

package org.shirdrn.platform.dubbo.service.rpc.api;

public interface SolrSearchService {
	String search(String collection, String q, String type, int start, int rows);
}

上面接口提供了搜索远程调用功能。

基于Dubbo的Hessian协议实现提供方服务

提供方实现基于Dubbo封装的Hessian协议，实现接口SolrSearchService，实现代码如下所示：

package org.shirdrn.platform.dubbo.service.rpc.server;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.shirdrn.platform.dubbo.service.rpc.api.SolrSearchService;
import org.shirdrn.platform.dubbo.service.rpc.utils.QueryPostClient;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SolrSearchServer implements SolrSearchService {

	private static final Log LOG = LogFactory.getLog(SolrSearchServer.class);
	private String baseUrl;
	private final QueryPostClient postClient;
	private static final Map<String, FormatHandler> handlers = new HashMap<String, FormatHandler>(0);
	static {
		handlers.put("xml", new FormatHandler() {
			public String format() {
				return "&wt=xml";
			}
		});
		handlers.put("json", new FormatHandler() {
			public String format() {
				return "&wt=json";
			}
		});
	}

	public SolrSearchServer() {
		super();
		postClient = QueryPostClient.newIndexingClient(null);
	}

	public void setBaseUrl(String baseUrl) {
		this.baseUrl = baseUrl;
	}

	public String search(String collection, String q, String type, int start, int rows) {
		StringBuffer url = new StringBuffer();
		url.append(baseUrl).append(collection).append("/select?").append(q);
		url.append("&start=").append(start).append("&rows=").append(rows);
		url.append(handlers.get(type.toLowerCase()).format());
		LOG.info("[REQ] " + url.toString());
		return postClient.request(url.toString());
	}

	interface FormatHandler {
		String format();
	}
}

因为考虑到后面要使用标准Hessian接口来调用，这里接口方法参数全部使用内置标准类型。然后，我们使用Dubbo的配置文件进行配置，文件search-provider.xml的内容如下所示：

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:dubbo="http://code.alibabatech.com/schema/dubbo"
	xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
     http://code.alibabatech.com/schema/dubbo http://code.alibabatech.com/schema/dubbo/dubbo.xsd">

	<dubbo:application name="search-provider" />
	<dubbo:registry
		address="zookeeper://slave1:2188?backup=slave3:2188,slave4:2188" />
	<dubbo:protocol name="hessian" port="8080" server="servlet" />
	<bean id="searchService"
		class="org.shirdrn.platform.dubbo.service.rpc.server.SolrSearchServer">
		<property name="baseUrl" value="http://nginx-lbserver/solr-cloud/" />
	</bean>
	<dubbo:service
		interface="org.shirdrn.platform.dubbo.service.rpc.api.SolrSearchService"
		ref="searchService" path="http_dubbo/search" />

</beans>

因为使用Tomcat发布提供方服务，所以我们需要实现Spring的org.springframework.web.context.ContextLoader来初始化应用上下文（基于Spring的IoC容器来管理服务对象）。实现类SearchContextLoader代码如下所示：

package org.shirdrn.platform.dubbo.context;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

import org.shirdrn.platform.dubbo.service.rpc.server.SolrSearchServer;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.web.context.ContextLoader;

public class SearchContextLoader extends ContextLoader implements ServletContextListener {

	@Override
	public void contextDestroyed(ServletContextEvent arg0) {
		// TODO Auto-generated method stub

	}

	@Override
	public void contextInitialized(ServletContextEvent arg0) {
		String config = arg0.getServletContext().getInitParameter("contextConfigLocation");
		ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(config);
		context.start();
	}

}

最后，配置Web应用部署描述符文件，web.xml内容如下所示：

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4"
	xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
	<display-name>http_dubbo</display-name>

	<listener>
		<listener-class>org.shirdrn.platform.dubbo.context.SearchContextLoader</listener-class>
	</listener>
	<context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>classpath:search-provider.xml</param-value>
	</context-param>

	<servlet>
		<servlet-name>search</servlet-name>
		<servlet-class>com.alibaba.dubbo.remoting.http.servlet.DispatcherServlet</servlet-class>
		<init-param>
			<param-name>home-class</param-name>
			<param-value>org.shirdrn.platform.dubbo.service.rpc.server.SolrSearchServer</param-value>
		</init-param>
		<init-param>
			<param-name>home-api</param-name>
			<param-value>org.shirdrn.platform.dubbo.service.rpc.api.SolrSearchService</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>
	<servlet-mapping>
		<servlet-name>search</servlet-name>
		<url-pattern>/search</url-pattern>
	</servlet-mapping>

	<welcome-file-list>
		<welcome-file>index.html</welcome-file>
		<welcome-file>index.htm</welcome-file>
		<welcome-file>index.jsp</welcome-file>
		<welcome-file>default.html</welcome-file>
		<welcome-file>default.htm</welcome-file>
		<welcome-file>default.jsp</welcome-file>
	</welcome-file-list>
</web-app>

启动Tomcat以后，就可以将提供方服务发布到服务注册中心，这里服务注册中心我们使用的是ZooKeeper集群，可以参考上面Dubbo配置文件search-provider.xml的配置内容。

下面，我们通过两种方式来调用已经注册到服务注册中心的服务。

• 基于Dubbo的Hessian协议远程调用

服务消费方，通过Dubbo配置文件来指定注册到注册中心的服务，配置文件search-consumer.xml的内容，如下所示：

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:dubbo="http://code.alibabatech.com/schema/dubbo"
	xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
     http://code.alibabatech.com/schema/dubbo http://code.alibabatech.com/schema/dubbo/dubbo.xsd">

	<dubbo:application name="search-consumer" />
	<dubbo:registry
		address="zookeeper://slave1:2188?backup=slave3:2188,slave4:2188" />
	<dubbo:reference id="searchService"
		interface="org.shirdrn.platform.dubbo.service.rpc.api.SolrSearchService" />

</beans>

然后，使用Java实现远程调用，实现代码如下所示：

package org.shirdrn.platform.dubbo.service.rpc.client;

import java.util.concurrent.Callable;
import java.util.concurrent.Future;

import org.shirdrn.platform.dubbo.service.rpc.api.SolrSearchService;
import org.springframework.beans.BeansException;
import org.springframework.context.support.AbstractXmlApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.alibaba.dubbo.rpc.RpcContext;

public class SearchConsumer {

	private final String collection;
	private AbstractXmlApplicationContext context;
	private SolrSearchService searchService;

	public SearchConsumer(String collection, Callable<AbstractXmlApplicationContext> call) {
		super();
		this.collection = collection;
		try {
			context = call.call();
			context.start();
			searchService = (SolrSearchService) context.getBean("searchService");
		} catch (BeansException e) {
			e.printStackTrace();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	public Future<String> asyncCall(final String q, final String type, final int start, final int rows) {
		Future<String> future = RpcContext.getContext().asyncCall(new Callable<String>() {
			public String call() throws Exception {
				return search(q, type, start, rows);
			}
		});
		return future;
	}

	public String syncCall(final String q, final String type, final int start, final int rows) {
		return search(q, type, start, rows);
	}

	private String search(final String q, final String type, final int start, final int rows) {
		return searchService.search(collection, q, type, start, rows);
	}

	public static void main(String[] args) throws Exception {
		final String collection = "tinycollection";
		final String beanXML = "search-consumer.xml";
		final String config = SearchConsumer.class.getPackage().getName().replace('.', '/') + "/" + beanXML;
		SearchConsumer consumer = new SearchConsumer(collection, new Callable<AbstractXmlApplicationContext>() {
			public AbstractXmlApplicationContext call() throws Exception {
				final AbstractXmlApplicationContext context = new ClassPathXmlApplicationContext(config);
				return context;
			}
		});

		String q = "q=上海&fl=*&fq=building_type:1";
		int start = 0;
		int rows = 10;
		String type = "xml";
		for (int k = 0; k < 10; k++) {
			for (int i = 0; i < 10; i++) {
				start = 1 * 10 * i;
				if (i % 2 == 0) {
					type = "xml";
				} else {
					type = "json";
				}
				String result = consumer.syncCall(q, type, start, rows);
				System.out.println(result);
				// Future<String> future = consumer.asyncCall(q, type, start,
				// rows);
				// System.out.println(future.get());
			}
		}
	}

}

执行该调用实现，可以远程调用提供方发布的服务。
这种方式限制了服务调用方也必须使用Dubbo来开发调用的代码，也就是限制了编程的语言，而无论是对于内部还是外部，各个团队之间必然存在语言的多样性，如果限制了编程语言，那么开发的服务也只能在内部使用。

• 基于标准Hessian协议接口的远程调用

下面，使用标准Hessian接口来实现远程调用，这时就不需要关心服务提供方的所使用的开发语言，因为最终是通过HTTP的方式来访问。我们需要下载Hessian对应语言的调用实现库，才能更方便地编程。

使用Java语言实现远程调用
使用Java语言实现，代码如下所示：

package org.shirdrn.rpc.hessian;

import org.shirdrn.platform.dubbo.service.rpc.api.SolrSearchService;

import com.caucho.hessian.client.HessianProxyFactory;

public class HessianConsumer {

	public static void main(String[] args) throws Throwable {

		String serviceUrl = "http://10.95.3.74:8080/http_dubbo/search";
		HessianProxyFactory factory = new HessianProxyFactory();

		SolrSearchService searchService = (SolrSearchService) factory.create(SolrSearchService.class, serviceUrl);

		String q = "q=上海&fl=*&fq=building_type:1";
		String collection = "tinycollection";
		int start = 0;
		int rows = 10;
		String type = "xml";
		String result = searchService.search(collection, q, type, start, rows);
		System.out.println(result);
	}
}

我们只需要知道提供服务暴露的URL和服务接口即可，这里URL为http://10.95.3.74:8080/http_dubbo/search，接口为org.shirdrn.platform.dubbo.service.rpc.api.SolrSearchService。运行上面程序，可以调用提供方发布的服务。

使用Python语言实现远程调用
使用Python客户端来进行远程调用，我们可以从https://github.com/bgilmore/mustaine下载，然后安装Hessian的代理客户端Python实现库：

git clone https://github.com/bgilmore/mustaine.git
cd mustaine
sudo python setup.py install

然后就可以使用了，使用Python进行远程调用的实现代码如下所示：

#!/usr/bin/python

# coding=utf-8
from mustaine.client import HessianProxy

serviceUrl = 'http://10.95.3.74:8080/http_dubbo/search'
q = 'q=*:*&fl=*&fq=building_type:1'
start = 0
rows = 10
resType = 'xml'
collection = 'tinycollection'

if __name__ == '__main__':
     proxy = HessianProxy(serviceUrl)
     result = proxy.search(collection, q, resType, start, rows)
     print result

运行上面程序，就可以看到远程调用的结果。

参考链接

• https://github.com/alibaba/dubbo
• http://alibaba.github.io/dubbo-doc-static/Home-zh.htm
• http://alibaba.github.io/dubbo-doc-static/User+Guide-zh.htm
• http://alibaba.github.io/dubbo-doc-static/Developer+Guide-zh.htm
• http://alibaba.github.io/dubbo-doc-static/Administrator+Guide-zh.htm
• http://alibaba.github.io/dubbo-doc-static/FAQ-zh.htm
• http://hessian.caucho.com/#HessianImplementationsDownload
• https://github.com/bgilmore/mustaine