Hessian的使用以及理解

• Hessian版本：3.1.5

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将包括如下的内容：

1. Hessian的基本使用
2. Hessian的原理
3. Hessian和Spring 的结合使用
4. 扩展

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简单说来，Hessian是一个轻量级的RPC框架(RPC是什么？请参考这里)。

它基于HTTP协议传输，使用Hessian二进制序列化，对于数据包比较大的情况比较友好。

但是它的参数和返回值都需要实现Serializable接口。

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简单实现一个Hessian的例子：

1. 1.创建接口和实现类

public interface Basic {String sayHello(String name);
}

public class BasicImpl implements Basic{public String sayHello(String name) {return "This is Hello words from HESSIAN Server. " + name;}
}

1. 2.配置HessianServlet, web.xml中：

  <servlet><servlet-name>HessianServlet</servlet-name><servlet-class>com.caucho.hessian.server.HessianServlet</servlet-class><init-param><param-name>service-class</param-name><param-value>example.impl.BasicImpl</param-value></init-param></servlet><servlet-mapping><servlet-name>HessianServlet</servlet-name><url-pattern>/hessian</url-pattern></servlet-mapping>

我们将会把Servlet部署在Tomcat上，端口8080。

1. 3.编写客户端代码：

public class BasicClient {public static void main(String[] args) {try {String url = "http://localhost:8080/hessian";HessianProxyFactory factory = new HessianProxyFactory();factory.setOverloadEnabled(true);Basic basic = (Basic) factory.create(Basic.class, url);System.out.println(basic.sayHello("SW"));}catch (Exception e){e.printStackTrace();}}

创建HessianProxyFactory对象，创建Basic “对象”，然后调用sayHello()方法。

整个过程感觉很简单，并没有什么配置。

启动Tomcat，运行Client。

输出如下：

This is Hello words from HESSIAN Server. SW

可见是调用成功了。

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等等，这个过程到底发生了些什么？

我们先从服务端说起，主要是有这几个步骤：

• 编写接口和实现类
• 在web.xml中声明HessianServlet，并且将上一步的实现类设置为Servlet的[service-class]属性值
• 将Servlet部署在Tomcat容器中

可见我们所有的工作都围绕在HessianServlet在展开。该Servlet中有两个比较重要的方法：init()、service();

init方法初始化服务和服务对象，主要分为3步：

通过home-class或者service-class创建服务端的实现类实例；

if (_homeImpl != null) {}else if (getInitParameter("home-class") != null) {String className = getInitParameter("home-class");Class homeClass = loadClass(className);_homeImpl = homeClass.newInstance();init(_homeImpl);}else if (getInitParameter("service-class") != null) {String className = getInitParameter("service-class");Class homeClass = loadClass(className);_homeImpl = homeClass.newInstance();init(_homeImpl);}else {if (getClass().equals(HessianServlet.class))throw new ServletException("server must extend HessianServlet");_homeImpl = this;}

通过home-api或者api-class加载实现类的接口对象；

      if (_homeAPI != null) {}else if (getInitParameter("home-api") != null) {String className = getInitParameter("home-api");_homeAPI = loadClass(className);}else if (getInitParameter("api-class") != null) {String className = getInitParameter("api-class");_homeAPI = loadClass(className);}else if (_homeImpl != null)_homeAPI = _homeImpl.getClass();

init方法还会创建HessianSkeleton对象，这是Hessian服务端的核心功能部分。

HessianSkeleton继承自AbstractSkeleton，其构造方法，将会从实现类中抽取方法和方法的Method对象，并且存储到_methodMap中。

对于一个Servlet来说其service方法是对外提供服务的方法：

/*** Execute a request.  The path-info of the request selects the bean.* Once the bean's selected, it will be applied.*/public void service(ServletRequest request, ServletResponse response)throws IOException, ServletException{HttpServletRequest req = (HttpServletRequest) request;HttpServletResponse res = (HttpServletResponse) response;if (! req.getMethod().equals("POST")) {res.setStatus(500, "Hessian Requires POST");PrintWriter out = res.getWriter();res.setContentType("text/html");out.println("<h1>Hessian Requires POST</h1>");return;}String serviceId = req.getPathInfo();String objectId = req.getParameter("id");if (objectId == null)objectId = req.getParameter("ejbid");ServiceContext.begin(req, serviceId, objectId);try {InputStream is = request.getInputStream();OutputStream os = response.getOutputStream();HessianInput in = new HessianInput(is);HessianOutput out = new HessianOutput(os);if (objectId != null)_objectSkeleton.invoke(in, out);else_homeSkeleton.invoke(in, out);} catch (RuntimeException e) {throw e;} catch (ServletException e) {throw e;} catch (Throwable e) {throw new ServletException(e);} finally {ServiceContext.end();}}

最主要的是调用HessianSkeleton对象的invoke方法。注意，Servlet实例中有两个HessianSkeleton变量，分别是：_objectSkeleton和 _homeSkeleton，调用谁，是由objectid决定的。此处还有不明白的地方。

invoke方法：

首先从HessianInput对象中获取到Method信息，获取到真正的service对象。

根据反射机制，调用service对象的invoke方法，获取到返回值。

最后调用HessianOutput对象将结果写回到调用方。

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客户端代码

Hessian原生API编写客户端HessianClient：

public class BasicClient {public static void main(String[] args) {try {String url = "http://localhost:8080/hessian";HessianProxyFactory factory = new HessianProxyFactory();factory.setOverloadEnabled(true);Basic basic = (Basic) factory.create(Basic.class, url);System.out.println(basic.sayHello("SW"));}catch (Exception e){e.printStackTrace();}}
}

创建HessianProxyFacotry，创建接口Basic的代理对象，然后调用sayHello()方法。

那么重点就在于创建代理对象，首先创建HessianProxyFacotry对象，构造方法中创建了一个HessianProxyResolver对象，这个对象的lookup方法将用来查找远程服务。此外HessianProxyFacotry还有包括权限验证方面的支持。

创建了factory之后，接下来就是通过Class对象和远程服务的URL创建代理对象了。

HessianProxyFactory使用HessianProxy对象作为代理的Handler，也就是说，我们对代理对象的所有操作，都会由这个handler来处理。handler的invoke方法，在进行一些方法名和参数的确认之后，创建HttpURLConnection对象，调用sendRequest方法，将方法名和参数用HessianOutput对象(设置序列化的方式)的call方法，写入到服务端。

主要代码如下：

protected URLConnection sendRequest(String methodName, Object []args)throws IOException{URLConnection conn = null;conn = _factory.openConnection(_url);// Used chunked mode when available, i.e. JDK 1.5.if (_factory.isChunkedPost() && conn instanceof HttpURLConnection) {try {HttpURLConnection httpConn = (HttpURLConnection) conn;httpConn.setChunkedStreamingMode(8 * 1024);} catch (Throwable e) {}}addRequestHeaders(conn);OutputStream os = null;try {os = conn.getOutputStream();} catch (Exception e) {throw new HessianRuntimeException(e);}try {if (log.isLoggable(Level.FINEST)) {PrintWriter dbg = new PrintWriter(new LogWriter(log));os = new HessianDebugOutputStream(os, dbg);}AbstractHessianOutput out = _factory.getHessianOutput(os);out.call(methodName, args);out.flush();return conn;} catch (IOException e) {if (conn instanceof HttpURLConnection)((HttpURLConnection) conn).disconnect();throw e;} catch (RuntimeException e) {if (conn instanceof HttpURLConnection)((HttpURLConnection) conn).disconnect();throw e;}}

服务端拿到请求，进行反序列化，然后将方法调用，再将结果序列化之后写回到connection。所以，客户端在sendRequest之后，所要做的就是将返回的结果进行解析，看返回的code是不是200：

conn = sendRequest(mangleName, args);if (conn instanceof HttpURLConnection) {httpConn = (HttpURLConnection) conn;int code = 500;try {code = httpConn.getResponseCode();} catch (Exception e) {}parseResponseHeaders(conn);if (code != 200) {StringBuffer sb = new StringBuffer();int ch;.....AbstractHessianInput in = _factory.getHessianInput(is);in.startReply();Object value = in.readObject(method.getReturnType());if (value instanceof InputStream) {value = new ResultInputStream(httpConn, is, in, (InputStream) value);is = null;httpConn = null;}elsein.completeReply();return value;

解析HessianInput对象，并且从中读取到结果返回。

至此，服务端和客户端的交互过程已经简单地介绍完毕。

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Spring也为Hessian提供了很友好的支持，通过使用spring-remoting包，我们可以很方便地发布和调用服务。

这部分提供一个简单的实现例子：

在web.xml中，我们配置SpringMVC的DispatcherServlet：

<servlet><servlet-name>SpringMVC</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation</param-name><param-value>classpath:applicationContext.xml</param-value></init-param><load-on-startup>1</load-on-startup></servlet><servlet-mapping><servlet-name>SpringMVC</servlet-name><url-pattern>/remote/*</url-pattern></servlet-mapping>

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://www.springframework.org/schema/beanshttp://www.springframework.org/schema/beans/spring-beans-4.1.xsd"default-lazy-init="true"><bean id = "basicService" class="example.impl.BasicImpl"/><bean name="/basicHessianService" class="org.springframework.remoting.caucho.HessianServiceExporter"><property name="service" ref="basicService"/><property name="serviceInterface" value="example.Basic"/></bean>
</beans>

这里，我们使用了org.springframework.remoting.caucho.HessianServiceExporter来发布服务。将程序部署在tomcat中。

客户端，使用org.springframework.remoting.caucho.HessianProxyFactoryBean来代理请求：

client.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://www.springframework.org/schema/beanshttp://www.springframework.org/schema/beans/spring-beans-4.1.xsd"default-lazy-init="true"><bean id="basicService" class="org.springframework.remoting.caucho.HessianProxyFactoryBean"><property name="serviceUrl" value="http://localhost:8080/remote/basicHessianService"/><property name="serviceInterface" value="example.Basic"/></bean>
</beans>

编写客户端：

public class SpringClient {public static void main(String[] args) {ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"classpath:client.xml"});Basic basic = (Basic)context.getBean("basicService");System.out.println(basic.sayHello("SUNWEI"));}
}

这样，服务端/客户端的代码都已经编写完成。

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最原始的实现，我们的服务是通过Servlet来绑定的，而Spring的实现，我们使用了SpringMVC的加载时机，将配置文件加载。HessianServiceExporter

public class HessianServiceExporter extends RemoteExporter implements HttpRequestHandler, InitializingBean {....

这个类实现了InitializingBean接口，这是spring-beans包中很重要的一个扩展接口。

这个接口的说明如下：

Interface to be implemented by beans that need to react once all their
properties have been set by a BeanFactory: for example, to perform custom
initialization, or merely to check that all mandatory properties have been set.

也就是说，它会随着Spring容器(此处为Spring MVC容器)的启动而被加载。看看HessianServiceExporter的实现：

public void prepare() {HessianSkeleton skeleton = null;try {try {Constructor ctor = (class$com$caucho$hessian$server$HessianSkeleton == null?(class$com$caucho$hessian$server$HessianSkeleton = class$("com.caucho.hessian.server.HessianSkeleton")):class$com$caucho$hessian$server$HessianSkeleton).getConstructor(new Class[]{class$java$lang$Object == null?(class$java$lang$Object = class$("java.lang.Object")):class$java$lang$Object, class$java$lang$Class == null?(class$java$lang$Class = class$("java.lang.Class")):class$java$lang$Class});this.checkService();this.checkServiceInterface();skeleton = (HessianSkeleton)ctor.newInstance(new Object[]{this.getProxyForService(), this.getServiceInterface()});} catch (NoSuchMethodException var4) {Constructor ctor = (class$com$caucho$hessian$server$HessianSkeleton == null?(class$com$caucho$hessian$server$HessianSkeleton = class$("com.caucho.hessian.server.HessianSkeleton")):class$com$caucho$hessian$server$HessianSkeleton).getConstructor(new Class[]{class$java$lang$Object == null?(class$java$lang$Object = class$("java.lang.Object")):class$java$lang$Object});skeleton = (HessianSkeleton)ctor.newInstance(new Object[]{this.getProxyForService()});}} catch (Throwable var5) {throw new BeanInitializationException("Hessian skeleton initialization failed", var5);}if(hessian2Available) {this.skeletonInvoker = new Hessian2SkeletonInvoker(skeleton, this.serializerFactory);} else {this.skeletonInvoker = new Hessian1SkeletonInvoker(skeleton, this.serializerFactory);}}public void handleRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {Assert.notNull(this.skeletonInvoker, "HessianServiceExporter has not been initialized");if(!"POST".equals(request.getMethod())) {throw new HttpRequestMethodNotSupportedException("POST", "HessianServiceExporter only supports POST requests");} else {try {this.skeletonInvoker.invoke(request.getInputStream(), response.getOutputStream());} catch (Throwable var4) {throw new NestedServletException("Hessian skeleton invocation failed", var4);}}}

在prepare方法中，获取service和serviceInterface的配置，创建HessianSkeleton对象。
同时，还实现了HttpRequestHandler，spring-web中的接口。
又因为实现了HttpRequestHandler接口，所以在handleRequest方法中，可以像HessianServlet的service方法一样，调用Hessian2SkeletonInvoker的invoke方法进行实际的方法调用。

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最后一点尾巴

定义一个自己的HttpRequestHandler对象，配置在applicationContext.xml中，然后通过页面访问：

public class MyHandler implements HttpRequestHandler, InitializingBean {public void afterPropertiesSet() throws Exception {System.out.println("初始化 MyHandler");}public void handleRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {System.out.println("执行 MyHandler");}
}

配置在applicationContext.xml中：

<bean id = "/myHandler" class="client.MyHandler"/>

通过Spring MVC的上下文加载该Handler，启动Tomcat的时候，可以看到控制台输出：

初始化 MyHandler

在浏览器中访问：http://localhost:8080/remote/myHandler

将触发执行：执行 MyHandler

-EOF-

转自：https://blog.csdn.net/sunwei_pyw/article/details/74002351