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Chapter 4. Deploying an Apache Camel Servlet Endpoint

Abstract

This tutorial describes how to deploy a Camel application, which is implemented using the Camel servlet component. The Camel application gets installed into the Web server as a servlet, receiving messages through the servlet endpoint which are then processed in a Camel route.

4.1. Apache Camel Servlet Example
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Overview
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Figure 4.1, “Camel Servlet Example Deployed in a Web Server” gives an overview of the Camel servlet example deployed in a Web server, which lets you see how the servlet's URL is constructed from settings at different configuration layers. The Web server's host and port, the WAR file name, the url-pattern setting from web.xml, and the endpoint URI of the Camel servlet endpoint are combined to give the URL, http://localhost:8080/camel-example-servlet-tomcat-2.15.1.redhat-620133/camel/hello.

Figure 4.1. Camel Servlet Example Deployed in a Web Server

camel-example-servlet-tomcat example
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The code for this example is available from the standard Apache Camel distribution, under the examples/camel-example-servlet-tomcat directory. For details of how to install the Apache Camel distribution, see the section called “Install Apache Camel”.

Camel servlet component
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The Camel servlet component is used to process incoming HTTP requests, where the HTTP endpoint is bound to a published servlet. The servlet component is implemented by the following servlet class: 
org.apache.camel.component.servlet.CamelHttpTransportServlet
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To create a Camel servlet endpoint in a Camel route, define a servlet endpoint URI with the following syntax: 
servlet://RelativePath[?Options]
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Where RelativePath specifies the tail segment of the HTTP URL path for this servlet.

web.xml file
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To deploy the Apache Camel servlet example, you must provide a properly configured web.xml file. In the camel-example-servlet-tomcat project, the web.xml file is stored at the following location: 
camel-example-servlet-tomcat/src/main/webapp/WEB-INF/web.xml
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Example 4.1, “web.xml File for the camel-example-servlet-tomcat Example” shows the contents of the web.xml file.

Example 4.1. web.xml File for the camel-example-servlet-tomcat Example

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

  <display-name>My Web Application</display-name>

  <!-- location of spring xml files -->
  <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:camel-config.xml</param-value>
  </context-param>

  <!-- the listener that kick-starts Spring -->
  <listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>

  <!-- Camel servlet -->
  <servlet>
    <servlet-name>CamelServlet</servlet-name>
    <servlet-class>org.apache.camel.component.servlet.CamelHttpTransportServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
  </servlet>

  <!-- Camel servlet mapping -->
  <servlet-mapping>
    <servlet-name>CamelServlet</servlet-name>
    <url-pattern>/camel/*</url-pattern>
  </servlet-mapping>

</web-app>
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The key settings in the preceding web.xml file are:
servlet/servlet-class
Specifies the org.apache.camel.component.servlet.CamelHttpTransportServlet class, which implements the Camel servlet component.
servlet-mapping/url-pattern
Determines which URLs are routed to this servlet. In general, the servlet URL has the following form: 
http://Host:Port/WARFileName/URLPattern
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Where the base URL, http://Host:Port, is determined by the configuration of the Web server, the WARFileName is the root of the WARFileName.war WAR file, and the URLPattern is specified by the contents of the url-pattern element.
Assuming that the Web server port is set to 8080, the camel-example-servlet-tomcat example servlet will match URLs of the following form: 
http://localhost:8080/camel-example-servlet-tomcat-2.15.1.redhat-620133/camel/*
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listener/listener-class
This element launches the Spring container.
context-param
This element specifies the location of the Spring XML file, camel-config.xml, in the WAR. The Spring container will read this parameter and load the specified Spring XML file, which contains the definition of the Camel route.

Example Camel route
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Example 4.2, “Route Definition for the Camel Servlet Example” shows the Camel route for this example, defined in a Spring XML file, using Camel's XML DSL syntax.

Example 4.2. Route Definition for the Camel Servlet Example

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:camel="http://camel.apache.org/schema/spring"
       xsi:schemaLocation="
         http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
         http://camel.apache.org/schema/spring http://camel.apache.org/schema/spring/camel-spring.xsd">

  <camelContext xmlns="http://camel.apache.org/schema/spring">

    <route>
      <!-- incoming requests from the servlet is routed -->
      <from uri="servlet:///hello"/>
      <choice>
        <when>
          <!-- is there a header with the key name? -->
          <header>name</header>
          <!-- yes so return back a message to the user -->
          <transform>
            <simple>Hello ${header.name} how are you?</simple>
          </transform>
        </when>
        <otherwise>
          <!-- if no name parameter then output a syntax to the user -->
          <transform>
            <constant>Add a name parameter to uri, eg ?name=foo</constant>
          </transform>
        </otherwise>
      </choice>
    </route>

  </camelContext>

</beans>
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Because the servlet URL, servlet:///hello, specifies the relative path, /hello, the complete URL to access this servlet is the following: 
http://localhost:8080/camel-example-servlet-tomcat-2.15.1.redhat-620133/camel/hello
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