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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.
package example;
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 example;
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());
}
}
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.
package example;
public class BasicService implements Basic {
private String _greeting = "Hello, world";
public void setGreeting(String greeting)
{
_greeting = greeting;
}
public String hello()
{
return _greeting;
}
}
<web-app xmlns="http://caucho.com/ns/resin">
<servlet servlet-name="hello"
servlet-class="com.caucho.hessian.server.HessianServlet">
<init>
<home resin:type="example.BasicService">
<greeting>Hello, world</greeting>
</home>
<home-api>example.Basic</home-api>
</init>
</servlet>
<servlet-mapping url-pattern="/hello"
servlet-name="hello"/>
</web-app>
Since the HessianServlet is a standard servlet, it can also be
configured in the standard servlet configuration.
<web-app>
<servlet>
<servlet-name>hello</servlet-name>
<servlet-class>com.caucho.hessian.server.HessianServlet</servlet-class>
<init-param>
<param-name>home-class</param-name>
<param-value>example.BasicService</param-value>
</init-param>
<init-param>
<param-name>home-api</param-name>
<param-value>example.Basic</param-value>
</init-param>
</servlet>
<servlet-mapping>
<url-pattern>/hello</url-pattern>
<servlet-name>hello</servlet-name>
</servlet-mapping>
</web-app>
The Hessian classes can be used for serialization and deserialization.
Hessian's serialization forms the basis for the protocol and taking
control of the serialization lets application use Hessian more efficiently
than the proxy interface for specialized application protocols.
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();
When a distributed application needs to send large amounts of
binary data, it can be more efficient to
use InputStream and OutputStream to
avoid allocating large byte arrays. For example, a file
downloading service could be implemented efficiently using Hessian.
In this example, the client needs to take control of the
Hessian protocol directly, because the proxy interface would require
buffering the entire file before the call returns.
OutputStream os = conn.getOutputStream();
HessianOutput out = new HessianOutput(os);
out.startCall("download");
out.writeString("my-file.txt");
out.completeCall();
InputStream is = conn.getInputStream();
HessianInput in = new HessianInput(is);
in.startReply();
InputStream is = in.readInputStream();
... // save the input stream data somewhere
in.completeReply();
An upload servlet would also take control of the Hessian protocol
to avoid buffering the data argument. Like the client, the implementation
is straightforward, although more complicated than the basic service.
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.
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);
Copyright (c) 1998-2009 Caucho Technology, Inc. All rights reserved.
caucho® ,
resin® and
quercus®
are registered trademarks of Caucho Technology, Inc.
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