項目里遇到一個問題：由于安全性的需要，必須由一個程序專門負責查詢某一數據庫，然后通過安全的信道（例如HTTP）將查詢結果傳到安全區域之外的application。為了解決這個小小的remoting問題，我們差點要動用EJB。不過幸虧朋友推薦，找到了Hessian這個輕量級的remoting on http工具。

Hessian其實很像web service，只不過它的協議不是SOAP，而是它自己規定的binary協議。Hessian的server端提供一個servlet基類，client端獲得一個service接口（也就是stub）之后調用上面的方法，stub將方法調用marshal之后通過HTTP傳到server，server借助reflection調用service方法。

簡單、輕量。對付普通的remoting問題，Hessian足夠了。我們又可以暫時忘掉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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