REST with Apache Camel

There are many ways to expose an HTTP endpoint in Camel: jetty, tomcat, servlet, cxfrs and restlet. Two of these components - cxfrs and restlet also support REST semantics just with few lines of code. This simple example demonstrates how to do CRUD operations with camel-restlet and camel-jdbc.

The four HTTP verbs execute different operations and map to the following single URI template:

•  POST - create a new user: /user
•  GET - request the current state of the user specified by the URI: /user/{userId}
•  PUT - update an user at the given URI with new information: /user/{userId}
•  DELETE - remove the user identified by the given URI: /user/{userId}

• There is also a /users URI which returns all the users regardless of the HTTP method used.

Creating such an application with Camel is straightforward. After adding all the necessary dependencies (restlet, spring, jdbc...) configure web.xml to load Camel context:

<context-param>
  <param-name>contextConfigLocation</param-name>
  <param-value>classpath:camel-config.xml</param-value>
</context-param>
<listener>
  <listener-class>org.springframework.web.context.ContextLoaderListener   </listener-class>
</listener>

  and map the Restlet servlet

<servlet>
  <servlet-name>RestletServlet</servlet-name>
  <servlet-class>org.restlet.ext.spring.SpringServerServlet</servlet-class>
  <init-param>
    <param-name>org.restlet.component</param-name>
    <param-value>RestletComponent</param-value>
  </init-param>
</servlet>
<servlet-mapping>
  <servlet-name>RestletServlet</servlet-name>
  <url-pattern>/rs/*</url-pattern>
</servlet-mapping>

In the Spring context, there is a little more Restlet and an in-memory datasource setup code:

<bean id="RestletComponent" class="org.restlet.Component"/>
<bean id="RestletComponentService" class="org.apache.camel.component.restlet.RestletComponent">
   <constructor-arg index="0">
     <ref bean="RestletComponent"/>
   </constructor-arg>
</bean>
<jdbc:embedded-database id="dataSource" type="HSQL">
   <jdbc:script location="classpath:sql/init.sql"/>
</jdbc:embedded-database>

After all the setup is done, the next step is to create Camel routes that will process the HTTP requests and execute appropriate CRUD operations. The first one is createUser route that executes SQL insert command with the parameters from POST requests only and return the newly created user in the response body:

<route id="createUser">
   <from uri="restlet:/user?restletMethod=POST"/>
   <setBody>
     <simple>insert into user(firstName, lastName) values('${header.firstName}','${header.lastName}');  </simple>
   </setBody>
   <to uri="jdbc:dataSource"/>
   <setBody>
     <simple>select * from user ORDER BY id desc LIMIT 1</simple>
   </setBody>
   <to uri="jdbc:dataSource"/>
</route>

The "manipulateUser" route handles GET, PUT and DELETE HTTP methods, but depending on the method used, it executes different SQL commands:

<route id="manipulateUser">

  <from uri="restlet:/user/{userId}?restletMethods=GET,PUT,DELETE"/>

  <choice>

    <when>

    <simple>${header.CamelHttpMethod} == 'GET'</simple>

    <setBody>

      <simple>select * from user where id = ${header.userId}</simple>

    </setBody>

   </when>

   <when>

     <simple>${header.CamelHttpMethod} == 'PUT'</simple>

       <setBody>

       <simple>update user set firstName='${header.firstName}', lastName='${header.lastName}' where id = ${header.userId}</simple>

       </setBody>

   </when>

   <when>

     <simple>${header.CamelHttpMethod} == 'DELETE'</simple>

     <setBody>

       <simple>delete from user where id = ${header.userId}</simple>

     </setBody>

   </when>

   <otherwise>

     <stop/>

   </otherwise>

  </choice>

  <to uri="jdbc:dataSource"/>

</route>

And the last route for listing all the users is self explanatory:

<route id="listUsers">
  <from uri="restlet:/users"/>
  <setBody>
    <constant>select * from user</constant>
  </setBody>
  <to uri="jdbc:dataSource"/>
</route>

If you want to see the application in action, grab the source code from github and run it with the embedded maven-jetty plugin by typing: mvn jetty:run .You can even try some quick queries if you have curl installed:

To create an user, make a http POST request with firstName and lastName parameters
curl -d "firstName=test&lastName=user" http://localhost:8080/rs/user/

To update an existing user, make a http PUT request with firstName and lastName parameters
curl -X PUT -d "firstName=updated&lastName=user" http://localhost:8080/rs/user/2

To retrieve an existing user, make a http GET request with the userId as part of the url
curl -X GET  http://localhost:8080/rs/user/2

To delete an existing user, make a http DELETE request with the userId as part of the url
curl -X DELETE  http://localhost:8080/rs/user/2

To retrieve all the existing users, make a http GET request to users url
curl -X GET  http://localhost:8080/rs/users