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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.
Using a Hessian service from a Java client is like calling a method.
The HessianProxyFactory creates proxies which act like normal Java
objects, with possibility that the method might throw a protocol exception
if the remote connection fails. Using HessianProxyFactory requires
JDK 1.3.
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.
API for Basic service
package hessian.test;
public interface Basic {
public String hello();
}
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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.
Hessian Client for Basic service
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());
}
}
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That's it! There are no more complications to using the client. The
service can add methods and use any Java type for parameters and results.
While most Hessian services will use Resin-CMP or Resin-EJB, to take
advantage of the benefits of EJB, the Hessian library makes it possible
to write services by extending HessianServlet.
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.
Hello Service
package hessian.test;
import com.caucho.hessian.server.HessianServlet;
public class BasicService extends HessianServlet implements Basic {
public String hello()
{
return "Hello, world";
}
}
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Hessian can be used for even small Java devices. The following classes
from com.caucho.hessian.client can be extracted into a J2ME jar:
- MicroHessianInput
- MicroHessianOutput
- HessianRemote
- HessianServiceException
- HessianProtocolException
The following example shows the code for using a cell phone as a
client. It's a bit more complicated than using the proxy, since the
client is responsible for creating the connection and writing the
data.
Hello, world
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);
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The Hessian classes can be used for serialization and deserialization.
Serialization
Object obj = ...;
OutputStream os = new FileOutputStream("test.xml");
HessianOutput out = new HessianOutput(os);
out.writeObject(obj);
os.close();
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Deserialization
InputStream is = new FileInputStream("test.xml");
HessianInput in = new HessianInput(is);
Object obj = in.readObject(null);
is.close();
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Copyright © 1998-2002 Caucho Technology, Inc. All rights reserved.
Resin® is a registered trademark,
and HardCoretm and Quercustm are trademarks of Caucho Technology, Inc. |  |
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