As opposed to Burlap and Hessian, which are both lightweight protocols using their own slim serialization mechanisms, Spring Http invokers use the standard Java serialization mechanism to expose services through HTTP. This has a huge advantage if your arguments and return types are complex types that cannot be serialized using the serialization mechanisms Hessian and Burlap use (refer to the next section for more considerations when choosing a remoting technology).
Under the hood, Spring uses either the standard facilities provided
by J2SE to perform HTTP calls or Commons
HttpClient
. Use the latter if you need more
advanced and easy-to-use functionality. Refer to jakarta.apache.org/commons/httpclient
for more info.
Setting up the HTTP invoker infrastructure for a service objects
much resembles the way you would do using Hessian or Burlap. Just as
Hessian support provides the
HessianServiceExporter
, Spring's HttpInvoker
support provides the
org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter
.
To expose the AccountService
(mentioned above)
within a Spring Web MVC DispatcherServlet
, the
following configuration needs to be in place in the dispatcher's
application context:
<bean name="/AccountService" class="org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter"> <property name="service" ref="accountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
Such an exporter definition will be exposed through the
DispatcherServlet
's standard mapping facilities,
as explained in the section on Hessian.
Alternatively, create an
HttpInvokerServiceExporter
in your root
application context (e.g. in
'WEB-INF/applicationContext.xml'
):
<bean name="accountExporter" class="org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter"> <property name="service" ref="accountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
In addition, define a corresponding servlet for this exporter in
'web.xml'
, with the servlet name matching the bean
name of the target exporter:
<servlet> <servlet-name>accountExporter</servlet-name> <servlet-class>org.springframework.web.context.support.HttpRequestHandlerServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>accountExporter</servlet-name> <url-pattern>/remoting/AccountService</url-pattern> </servlet-mapping>
f you are running outside of a servlet container and are using Sun's Java 6, then you can use the built-in HTTP server implementation. You can configure the SimpleHttpServerFactoryBean together with a SimpleHttpInvokerServiceExporter as is shown in this example:
<bean name="accountExporter" class="org.springframework.remoting.httpinvoker.SimpleHttpInvokerServiceExporter"> <property name="service" ref="accountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean> <bean id="httpServer" class="org.springframework.remoting.support.SimpleHttpServerFactoryBean"> <property name="contexts"> <util:map> <entry key="/remoting/AccountService" value-ref="accountExporter"/> </util:map> </property> <property name="port" value="8080" /> </bean>
Again, linking in the service from the client much resembles the way you would do it when using Hessian or Burlap. Using a proxy, Spring will be able to translate your calls to HTTP POST requests to the URL pointing to the exported service.
<bean id="httpInvokerProxy" class="org.springframework.remoting.httpinvoker.HttpInvokerProxyFactoryBean"> <property name="serviceUrl" value="http://remotehost:8080/remoting/AccountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
As mentioned before, you can choose what HTTP client you want to
use. By default, the HttpInvokerProxy
uses the
J2SE HTTP functionality, but you can also use the Commons
HttpClient
by setting the
httpInvokerRequestExecutor
property:
<property name="httpInvokerRequestExecutor"> <bean class="org.springframework.remoting.httpinvoker.CommonsHttpInvokerRequestExecutor"/> </property>