項(xiàng)目里遇到一個(gè)問(wèn)題：由于安全性的需要，必須由一個(gè)程序?qū)ｉT(mén)負(fù)責(zé)查詢(xún)某一數(shù)據(jù)庫(kù)，然后通過(guò)安全的信道（例如HTTP）將查詢(xún)結(jié)果傳到安全區(qū)域之外的application。為了解決這個(gè)小小的remoting問(wèn)題，我們差點(diǎn)要?jiǎng)佑肊JB。不過(guò)幸虧朋友推薦，找到了Hessian這個(gè)輕量級(jí)的remoting on http工具。

Hessian其實(shí)很像web service，只不過(guò)它的協(xié)議不是SOAP，而是它自己規(guī)定的binary協(xié)議。Hessian的server端提供一個(gè)servlet基類(lèi)，client端獲得一個(gè)service接口（也就是stub）之后調(diào)用上面的方法，stub將方法調(diào)用marshal之后通過(guò)HTTP傳到server，server借助reflection調(diào)用service方法。

簡(jiǎn)單、輕量。對(duì)付普通的remoting問(wèn)題，Hessian足夠了。我們又可以暫時(shí)忘掉EJB和RMI了。

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Hessian is a simple binary protocol for connecting web services. The com.caucho.hessian.client and com.caucho.hessian.server packages do not require any other Resin classes, so can be used in smaller clients, like applets.

Because Hessian is a small protocol, J2ME devices like cell-phones can use it to connect to Resin servers. Because it's powerful, it can be used for EJB services.

The Hessian home page contains the latest information about Hessian including the Hessian specification .

Each service will have a normal Java interface describing the service. The trivial hello, world example just returns a string. Because the Hessian services support Java serialization, any Java type can be used.

              package hessian.test;public interface Basic {  public String hello();}
            

The following is an example of a standalone Hessian client. The client creates a HessianProxyFactory. The client uses the factory to create client stubs with the given target URL and a Java interface for the API. The returned object is a stub implementing the API.

              package hessian.test;import com.caucho.hessian.client.HessianProxyFactory;public class BasicClient {  public static void main(String []args)    throws Exception  {    String url = "http://www.caucho.com/hessian/test/basic";    HessianProxyFactory factory = new HessianProxyFactory();    Basic basic = (Basic) factory.create(Basic.class, url);    System.out.println("Hello: " + basic.hello());  }}
            

Any public method is treated as a service method. So adding new methods is as easy as writing a normal Java class.

Because the service is implemented as a Servlet, it can use all the familiar servlet data in the ServletContext, just like a normal servlet.

              package hessian.test;import com.caucho.hessian.server.HessianServlet;public class BasicService extends HessianServlet implements Basic {  public String hello()  {    return "Hello, world";  }}
            

• MicroHessianInput
• MicroHessianOutput
• HessianRemote
• HessianServiceException
• HessianProtocolException

              import javax.microedition.io.Connector;import javax.microedition.io.HttpConnection;...MicroHessianInput in = new MicroHessianInput();String url = "http://www.caucho.com/hessian/test/basic";HttpConnection c = (HttpConnection) Connector.open(url);c.setRequestMethod(HttpConnection.POST);OutputStream os = c.openOutputStream();MicroHessianOutput out = new MicroHessianOutput(os);out.call("hello", null);os.flush();is = c.openInputStream();MicroHessianInput in = new MicroHessianInput(is);Object value = in.readReply(null);
            

              Object obj = ...;OutputStream os = new FileOutputStream("test.xml");HessianOutput out = new HessianOutput(os);out.writeObject(obj);os.close();
            

              InputStream is = new FileInputStream("test.xml");HessianInput in = new HessianInput(is);Object obj = in.readObject(null);is.close();
            

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