hessian

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 specification itself is a short and interesting description.

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>

<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");
Hessian2Output out = new Hessian2Output(os);

out.writeObject(obj);
os.close();

InputStream is = new FileInputStream("test.xml");
Hessian2Input in = new Hessian2Input(is);

Object obj = in.readObject(null);
is.close();

When serializing Java objects more complex than primitives or Strings, make sure the classes for those objects implement java.io.Serializable.

When a distributed application needs to send large amounts of binary data, it can be more efficient to use InputStream to avoid allocating large byte arrays. Only the final argument of the method may be an InputStream, since the data is read during invocation. 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.

package example;

public interface Upload {
  public void upload(String filename, InputStream data);
}

If the result is an InputStream, it is very important that the InputStream.close() be put in an finally block, because Hessian will not close the underlying HTTP stream until all the data is read and the input stream is closed.

package example;

public interface Download {
  public InputStream download(String filename, InputStream data);
}

InputStream is = fileProxy.download("test.xml");

try {
  ... // read data here
} finally {
  is.close();
}

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);