Resin 4 Hessian support
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Latest revision as of 00:00, 16 June 2012
Writing a Hessian service as a plain-old Java object (POJO) eliminates protocol dependencies and simplifies service testing.
The <a href="../hessian-add/">addition example</a> built the Hessian service as an extension of HessianService for simplicity. Most services will want to be independent of the Hessian protocol itself.
[edit] Files in this tutorial
| File | Description |
|---|---|
| <viewfile-link file="WEB-INF/classes/example/MathService.java"/> | Interface for the math service. |
| <viewfile-link file="WEB-INF/classes/example/MathServiceImpl.java"/> | The main service implementation. |
| <viewfile-link file="WEB-INF/resin-web.xml"/> | Configures the environment |
| <viewfile-link file="demo.jsp"/> | Client JSP |
| <viewfile-link file="demo.php"/> | Client PHP |
[edit] Service Implementation
The MathService implementation is just a Java class that implements the MatchService API.
[edit] Example: MathServiceImpl.java
package example;
public class MathServiceImpl implements MathService {
public int add(int a, int b)
{
return a + b;
}
}
[edit] Remote Interface
The Java interface describes the remote API. This example has an addition method, add().
Resin's proxy client implementation uses the remote interface to expose the API to the proxy stub. Strictly speaking, though, the Java remote interface is not required for Hessian. A non-Java client will not use the Java interface, except possibly as documentation.
[edit] Example: MathService.java
package example;
public interface MathService {
public int add(int a, int b);
}
[edit] Service configuration
[edit] Example: resin-web.xml
<web-app xmlns="http://caucho.com/ns/resin">
<servlet-mapping url-pattern="/math/*"
servlet-class="example.MathService">
<protocol uri="hessian:"/>
</servlet-mapping>
<remote-client name="math">
<uri>hessian:url=${webApp.url}/math/</uri>
<interface>example.MathService</interface>
</remote-client>
</web-app>
[edit] Java Client
The client is identical to the basic example.
[edit] Example: demo.jsp
<%@ page import="javax.webbeans.In" %> <%@ page import="example.MathService" %> <%! @In MathService math; %> <pre> 3 + 2 = <%= math.add(3, 2) %> 3 - 2 = <%= math.sub(3, 2) %> 3 * 2 = <%= math.mul(3, 2) %> 3 / 2 = <%= math.div(3, 2) %>
</pre>
3 + 2 = 5 3 - 2 = 1 3 * 2 = 6 3 / 2 = 1
[edit] PHP Client
The client is identical to the basic example.
[edit] Example: demo.php
<?php
$math = java_bean("math");
?>
<pre>
3 + 2 = <?= $math->add(3, 2) ?>
3 - 2 = <?= $math->sub(3, 2) ?>
3 * 2 = <?= $math->mul(3, 2) ?>
3 / 2 = <?= $math->div(3, 2) ?>
</pre>
3 + 2 = 5 3 - 2 = 1 3 * 2 = 6 3 / 2 = 1
The addition example creates a Hessian web services with a servlet and uses that web service from a JSP client and a Python client.
Hessian is a lightweight binary RPC protocol. Transferring binary objects like files, images, or mp3s can avoid the protocol overhead that XML-based protocols require. Since it's simple, its performance should be usable for almost all sites. Hessian is designed to be self-describing, eliminating the requirement for external IDLs or WSDL files. Because it is as small as possible and language-independent, non-Java Hessian implementations are can easily develop comprehensive test suites.
This tutorial only requires the open source Java implementation of the Hessian client and server. It can be downloaded from <a href="http://caucho.com/hessian/">http://www.caucho.com/hessian/</a> for non-Resin clients and servers.
Because Resin's EJB implementation uses Hessian as its primary remote procedure call protocol, EJB developers can easily expose services to clients from other languages.
Because EJB clients and servers are written without knowledge of the underlying protocol, even if you intend to deploy with another protocol, like RMI/IIOP, you can develop using Resin's Hessian.
The <a href="doc|hessian-1.0-spec.xtp">Hessian 1.0 spec</a> and <a href="doc|hessian-2.0-spec.xtp">Hessian 2.0 spec</a> describe the full Hessian protocol.
[edit] Files in this tutorial
| File | Description |
|---|---|
| <viewfile-link file="WEB-INF/classes/example/MathService.java"/> | Interface for the math service. |
| <viewfile-link file="WEB-INF/classes/example/HessianMathService.java"/> | The main service implementation. |
| <viewfile-link file="WEB-INF/web.xml"/> | Configures the environment |
| <viewfile-link file="demo.jsp"/> | Client JSP |
[edit] The Hessian Protocol
A Hessian call is just an HTTP POST to a URL. The arguments are serialized into the Hessian binary format and passed to the server.
Most applications will never need to look at the Hessian protocol, but it's simple enough that a basic example can help show what's happening underneath the API.
[edit] Hessian call
c x01 x00 m x00 x03 add I x00 x00 x00 x02 I x00 x00 x00 x03 z
[edit] Hessian reply
r x01 x00 I x00 x00 x00 x05 z
The call does not need to specify the service name because the service is uniquely specified by the URL.
The following Addition example shows how to create a basic server so you can test Hessian.
[edit] A Hessian Example
Using Hessian generally uses three components:
- A remote interface
- The server implementation
- The client (JSP or servlet)
The remote interface is used by the Hessian proxy factory to create a proxy stub implementing the service's interface.
[edit] Service Implementation
Resin's Hessian provides a simple way of creating a server. Just extend
HessianServlet with your remote methods. The Hessian call will just
be a POST to that servlet. HessianServlet will introspect the
service and expose the methods.
[edit] HessianMathService.java
package example;
import com.caucho.hessian.server.HessianServlet;
public class HessianMathService extends HessianServlet {
public int add(int a, int b)
{
return a + b;
}
}
[edit] Remote Interface
The Java interface describes the remote API. This example has an addition method, add().
Resin's proxy client implementation uses the remote interface to expose the API to the proxy stub. Strictly speaking, though, the Java remote interface is not required for Hessian. A non-Java client will not use the Java interface, except possibly as documentation.
[edit] MathService.java
package example;
public interface MathService {
public int add(int a, int b);
}
[edit] Java Client
RPC clients follow the following steps in using a remote object:
- Determine the URL of the remote object.
- Obtain a proxy stub from a proxy factory.
- Call methods on the proxy stub.
[edit] client.jsp
<%@ page import="com.caucho.hessian.client.HessianProxyFactory" %>
<%@ page import="example.MathService" %>
<%
HessianProxyFactory factory = new HessianProxyFactory();
// http://localhost:8080/resin-doc/protocols/tutorial/hessian-add/hessian/math
String url = ("http://" +
request.getServerName() + ":" + request.getServerPort() +
request.getContextPath() + "/hessian/math");
MathService math = (MathService) factory.create(MathService.class, url);
out.println("3 + 2 = " + math.add(3, 2));
%>
3 + 2 = 5
[edit] Python Client
The <a href="http://www.caucho.com/hessian">Hessian site</a> has a basic Python library for Hessian.
[edit] client.py
from hessianlib import Hessian site = "http://localhost:8080/resin-doc/protocols/tutorial/hessian-add" url = site + "/hessian/math" proxy = Hessian(url); print "3 + 2 =", proxy.add(2, 3)
3 + 2 = 5
Using Hessian with Dependency Injection pattern creates services which are simpler, protocol-independent and more easily tested. The Dependency Injection pattern (aka inversion of control) gives Resin the responsibility of configuring and assembling the service, protocol, and client.
[edit] Files in this tutorial
| File | Description |
|---|---|
| <viewfile-link file="WEB-INF/classes/example/GreetingAPI.java"/> | Interface for the greeting service. |
| <viewfile-link file="WEB-INF/classes/example/GreetingImpl.java"/> | The service implementation as a Java object |
| <viewfile-link file="WEB-INF/resin-web.xml"/> | Configures the environment |
| <viewfile-link file="WEB-INF/classes/example/GreetingClientServlet.java"/> | Client Servlet |
[edit] Service Implementation
[edit] GreetingAPI.java
package example;
public interface GreetingAPI {
public String hello();
}
[edit] Service Implementation
The Greeting implementation is a plain Java class that implements the MatchService API. Making the service a plain class offers a number of advantages:
- Simplicity: It can concentrate on its business logic because it doesn't need to implement any protocol-dependent code.
- Independence: It can be reused more easily because it isn't tied to a distributed framework (e.g. in contrast to EJB).
- Testability: It is more easily tested since the test harness doesn't need to implement the protocol or its stubs. The harness can just test the service as a plain old Java object.
<example title="GreetingImpl.java"> package example;
public class GreetingImpl implements GreetingAPI {
private String _greeting = "Hello, world";
public void setGreeting(String greeting)
{
_greeting = greeting;
}
public String greeting()
{
return _greeting;
}
} </example>
[edit] Configuring the Service
URL-based services can use the servlet configuration to define the service. The service class can use <a href="../../doc/resin-ioc.xtp">Resin IoC</a> to inject its own dependencies.
<example title="resin-web.xml"> <web-app xmlns="http://caucho.com/ns/resin"
xmlns:resin="urn:java:com.caucho.resin"
xmlns:example="urn:java:example">
<example:GreetingImpl> <resin:Unbound/> <resin:HessianService urlPattern="/hessian/greeting"/> <greeting>Hello from resin-web.xml</greeting> </example:GreetingImpl>
</web-app> </example>
[edit] Client
Configuring the client servlet with Dependency Injection allows for a simpler and more general client. The following client can use any proxy/stub which implements the GreetingAPI without change, for example:
- Hessian proxy
- Burlap proxy
- EJB local object stub
- JMX proxy
- Java bean instance
Using the Dependency Injection pattern, the servlet doesn't care how the proxy is implemented, or how the greeting service is discovered.
[edit] GreetingClientServlet.java
import javax.inject.Inject;
public class GreetingClientServlet extends GenericServlet {
@Inject private GreetingAPI _greeting;
public void service(ServletRequest req, ServletResponse res)
throws IOException, ServletException
{
PrintWriter out = res.getWriter();
out.println("Hello: " + _greeting.greeting());
}
}
[edit] Hessian Client using Dependency Injection
The following code defines the client in the resin.web.xml.
The servlet defined above will inject the GreetingAPI
directly with the WebBeans @In annotation. Because the
GreetingAPI is unique, there's no need to give it a name.
<example title="resin-web.xml"> <web-app xmlns="http://caucho.com/ns/resin"
xmlns:resin="urn:java:com.caucho.resin"
xmlns:example="urn:java:example">
<example:GreetingAPI>
<resin:HessianClient url="http:${webApp.url}/hessian/greeting"/>
</example:GreetingAPI>
<servlet-mapping url-pattern="/client/greeting"
servlet-class="example.GreetingClientServlet"/>
</web-app> </example>
Writing a Hessian service as a plain-old Java object (POJO) eliminates protocol dependencies and simplifies service testing.
[edit] Files in this tutorial
| File | Description |
|---|---|
| <viewfile-link file="WEB-INF/classes/example/LogService.java"/> | Interface for the logging service. |
| <viewfile-link file="WEB-INF/classes/example/LogServiceImpl.java"/> | The main service implementation. |
| <viewfile-link file="WEB-INF/web.xml"/> | Configures the environment |
| <viewfile-link file="demo.jsp"/> | Client JSP |
[edit] Service Implementation
The HelloService implementation is just a Java class that implements the HelloService API. This service responds by placing the results of the request on a JMS queue.
[edit] LogService.java" language="java
package example;
public interface LogService {
public void log(String message);
public String getLog();
}
[edit] LogServiceImpl.java" language="java
package example;
public class LogServiceImpl implements LogService {
private int _sequenceNumber = 1;
private StringBuilder _log = new StringBuilder();
public void log(String message)
{
_log.append(_sequenceNumber + ": " + message + "\n");
_sequenceNumber++;
}
public String getLog()
{
return _log.toString();
}
}
The configuration for this example is the following:
[edit] web.xml" language="xml
<web-app xmlns="http://caucho.com/ns/resin">
<resource var="logService"
jndi-name="example/LogService"
type="example.LogServiceImpl" />
<resource var="serviceQueue"
jndi-name="jms/ServiceQueue"
type="com.caucho.jms.memory.MemoryQueue" />
<resource var="jmsFactory"
jndi-name="jms/ConnectionFactory"
type="com.caucho.jms.ConnectionFactoryImpl" />
<resource type="com.caucho.hessian.server.HessianListener">
<init>
<connection-factory>${jmsFactory}</connection-factory>
<destination>${serviceQueue}</destination>
<service>${logService}</service>
</init>
</resource>
</web-app>
Finally, the service is called from a JSP page:
[edit] demo.jsp
<%@ page import="javax.naming.Context" %>
<%@ page import="javax.naming.InitialContext" %>
<%@ page import="com.caucho.hessian.client.HessianProxyFactory" %>
<%@ page import="example.LogService" %>
<%
// Check the log
Context context = (Context) new InitialContext().lookup("java:comp/env");
LogService logService = (LogService) context.lookup("example/LogService");
out.println("<a href=\"\">Refresh</a><br/>");
out.println("Logged messages:<br/>");
out.println("<pre>");
out.println(logService.getLog());
out.println("");
// Make a request
HessianProxyFactory factory = new HessianProxyFactory();
String url = "jms:jms/ServiceQueue";
LogService log = (LogService) factory.create(LogService.class, url);
log.log("Hello, World"); %> </pre>
<a href="demo.jsp">Try the example</a>
Hessian 2.0 provides cross-language binary object serialization with efficiencies better than java.io serialization. The compaction encodings added to Hessian 2.0 have improved an already-popular cross-platform binary web services protocol. With these changes, Hessian 2.0 now directly competes with java.io serialization in efficiency.
[edit] Files in this tutorial
| File | Description |
|---|---|
| <viewfile-link file="WEB-INF/classes/example/HessianSerializeServlet.java"/> | Serialization Servlet |
| <viewfile-link file="WEB-INF/classes/example/Car.java"/> | Serialized class |
| <viewfile-link file="WEB-INF/classes/example/Color.java"/> | Enumeration for the car color |
| <viewfile-link file="WEB-INF/classes/example/Model.java"/> | Enumeration for the car model |
[edit] Overview
In this simple example, we'll use Hessian 2.0 to serialize three Car objects to a byte array. The serialized data could be saved in a persistent store, or sent as a message in a SOA or JMS application. Because Hessian 2.0 is cross-language, the message could be deserialized by a .NET or even a PHP application.
The efficiency of Hessian 2.0 is about twice that of java.io serialization. This is a tiny example, of course, but does show that you can send compact, cross-language messages without having to use bloated XML solutions like SOAP.
| Service | Size |
|---|---|
| Hessian 2.0 | 139 bytes |
| java.io | 287 bytes |
| Hessian 2.0 with Deflation | 164 bytes |
[edit] Model
The example's model is a Car object with three fields: year, model, and color. The model and color are enumeration types.
[edit] Car.java
package example;
public class Car {
private int year;
private Model model;
private Color color;
}
[edit] Car.java
package example;
public enum Model {
CIVIC,
EDSEL,
MODEL_T,
}
[edit] Color.java
package example;
public enum Model {
BLACK,
GREEN,
BLUE,
}
[edit] Hessian Serialization
The Hessian serialization API resembles
java.io ObjectOutputStream serialization. The general steps
are to create a Hessian2Output
around any OutputStream and write data to the stream.
In this example, we've encapsulated the object in a Hessian 2.0 message
using startMessage and completeMessage to
show how you would create a message for an SOA or JMS application.
[edit] Serialization
ByteArrayOutputStream bos = new ByteArrayOutputStream();
HessianFactory factory = new HessianFactory();
Hessian2Output out = factory.createHessian2Output(bos);
out.startMessage();
out.writeInt(2);
Car car1 = new Car(Model.EDSEL, Color.GREEN, 1954);
out.writeObject(car1);
Car car2 = new Car(Model.MODEL_T, Color.BLACK, 1937);
out.writeObject(car2);
out.completeMessage();
out.close();
byte []data = bos.toByteArray();
The deserialization is the same as serialization.
Create an Hessian2Input around any InputStream
and read data from the stream.
[edit] Deserialization
ByteArrayInputStream bin = new ByteArrayInputStream(data);
HessianFactory factory = new HessianFactory();
Hessian2Input in = factory.createHessianHessian2Input(bin);
in.startMessage();
ArrayList list = new ArrayList();
int length = in.readInt();
for (int i = 0; i < length; i++) {
list.add(in.readObject());
}
in.completeMessage();
in.close();
bin.close();
[edit] Hessian Compression
The <a href="http://caucho.com/resin-3.1/doc/hessian-2.0-spec.xtp">Hessian 2.0 draft specification</a> has added support for envelopes around Hessian messages. These envelopes can provide additional capabilities like compression, encryption, and message signatures. The envelope can also be used to attach routing and reliability information to a message. Since envelopes are nestable, each envelope can be simple and provide powerful capabilities when combined. For example, a secure messaging system might compress, encrypt and then securely sign a message.
The API for using envelopes is wrap() for writing a message
and unwrap() for reading a message. The application
serialization code itself is identical, since the envelope just creates a
Hessian2Input or Hessian2Output wrapper around
the original stream.
[edit] Deflation
Deflation envelope = new Deflation(); ByteArrayOutputStream bos = new ByteArrayOutputStream(); HessianFactory factory = new HessianFactory(); Hessian2Output out = facotyr.createHessian2Output(bos); out = out.wrap(out); out.startMessage(); Car car1 = new Car(Model.EDSEL, Color.GREEN, 1954); out.writeObject(car1); out.completeMessage(); out.close(); byte []data = bos.toByteArray();
[edit] Inflation
Deflation envelope = new Deflation(); ByteArrayInputStream bin = new ByteArrayInputStream(data); HessianFactory factory = new HessianFactory(); Hessian2Input in = factory.createHessian2Input(bin); in = envelope.unwrap(in); in.startMessage(); Object value = in.readObject(); in.completeMessage();
