ServletListener Component
Available as of Camel 2.11
This component is used for bootstrapping Camel applications in web applications. For example beforehand people would have to find their own way of bootstrapping Camel, or rely on 3rd party frameworks such as Spring to do it.
SidebarThis component supports Servlet 2.x onwards, which mean it works also in older web containers; which is the goal of this component.
Though Servlet 2.x requires to use a web.xml file as configuration.
For Servlet 3.x containers you can use annotation driven configuration to boostrap Camel using the @WebListener, and implement your own class, where you boostrap Camel. Doing this still puts the challenge how to let end users easily configure Camel, which you get for free with the old school web.xml file.
Maven users will need to add the following dependency to their pom.xml for this component:
xml<dependency>
<groupId>org.apache.camel</groupId>
<artifactId>camel-servletlistener</artifactId>
<version>x.x.x</version>
<!-- use the same version as your Camel core version -->
</dependency>
Using
You would need to chose one of the following implementations of the abstract class org.apache.camel.component.servletlistener.CamelServletContextListener.
JndiCamelServletContextListener which uses the JndiRegistry to leverage JNDI for its registry.SimpleCamelServletContextListener which uses the SimpleRegistry to leverage a java.util.Map as its registry.
To use this you need to configure the org.apache.camel.component.servletlistener.CamelServletContextListener in the WEB-INF/web.xml file as shown below:
{snippet:id=web|lang=xml|url=camel/trunk/components/camel-servletlistener/src/test/resources/myweb.xml}Options
The org.apache.camel.component.servletlistener.CamelServletContextListener supports the following options which can be configured as context-param in the web.xml file.
confluenceTableSmallOption | Type | Description |
|---|
propertyPlaceholder.XXX | | To configure property placeholders in Camel. You should prefix the option with "propertyPlaceholder.", for example to configure the location, use propertyPlaceholder.location as name. You can configure all the options from the Properties component. |
jmx.XXX | | To configure JMX. You should prefix the option with "jmx.", for example to disable JMX, use jmx.disabled as name. You can configure all the options from org.apache.camel.spi.ManagementAgent. As well the options mentioned on the JMX page. |
name | String
| To configure the name of the CamelContext. |
messageHistory | Boolean
| Camel 2.12.2: Whether to enable or disable Message History (enabled by default). |
streamCache | Boolean
| Whether to enable Stream caching. |
trace | Boolean
| Whether to enable Tracer. |
delayer | Long
| To set a delay value for Delay Interceptor. |
handleFault | Boolean
| Whether to enable handle fault. |
errorHandlerRef | String
| Refers to a context scoped Error Handler to be used. |
autoStartup | Boolean
| Whether to start all routes when starting Camel. |
useMDCLogging | Boolean
| Whether to use MDC logging. |
useBreadcrumb | Boolean
| Whether to use breadcrumb. |
managementNamePattern | String
| To set a custom naming pattern for JMX MBeans. |
threadNamePattern | String
| To set a custom naming pattern for threads. |
properties.XXX | | To set custom properties on CamelContext.getProperties. This is seldom in use. |
routebuilder.XXX | | To configure routes to be used. See below for more details. |
CamelContextLifecycle | | Refers to a FQN classname of an implementation of org.apache.camel.component.servletlistener.CamelContextLifecycle. Which allows to execute custom code before and after CamelContext has been started or stopped. See below for further details. |
XXX | | To set any option on CamelContext. |
Examples
See Servlet Tomcat No Spring Example.
Accessing the created CamelContext
Available as of Camel 2.14/2.13.3/2.12.5
The created CamelContext is stored on the ServletContext as an attribute with the key "CamelContext". You can get hold of the CamelContext if you can get hold of the ServletContext as shown below:
ServletContext sc = ...
CamelContext camel = (CamelContext) sc.getAttribute("CamelContext");
Configuring routes
You need to configure which routes to use in the web.xml file. You can do this in a number of ways, though all the parameters must be prefixed with "routeBuilder".
Using a RouteBuilder class
By default Camel will assume the param-value is a FQN classname for a Camel RouteBuilder class, as shown below:
xml <context-param>
<param-name>routeBuilder-MyRoute</param-name>
<param-value>org.apache.camel.component.servletlistener.MyRoute</param-value>
</context-param>
You can specify multiple classes in the same param-value as shown below:
xml <context-param>
<param-name>routeBuilder-routes</param-name>
<!-- we can define multiple values separated by comma -->
<param-value>
org.apache.camel.component.servletlistener.MyRoute,
org.apache.camel.component.servletlistener.routes.BarRouteBuilder
</param-value>
</context-param>
The name of the parameter does not have a meaning at runtime. It just need to be unique and start with "routeBuilder". In the example above we have "routeBuilder-routes". But you could just as well have named it "routeBuilder.foo".
Using package scanning
You can also tell Camel to use package scanning, which mean it will look in the given package for all classes of RouteBuilder types and automatic adding them as Camel routes. To do that you need to prefix the value with "packagescan:" as shown below:
xml <context-param>
<param-name>routeBuilder-MyRoute</param-name>
<!-- define the routes using package scanning by prefixing with packagescan: -->
<param-value>packagescan:org.apache.camel.component.servletlistener.routes</param-value>
</context-param>
Using a XML file
You can also define Camel routes using XML DSL, though as we are not using Spring or Blueprint the XML file can only contain Camel route(s).
In the web.xml you refer to the XML file which can be from "classpath", "file" or a "http" url, as shown below:
xml <context-param>
<param-name>routeBuilder-MyRoute</param-name>
<param-value>classpath:routes/myRoutes.xml</param-value>
</context-param>
And the XML file is:
routes/myRoutes.xmlxmlroutes/myRoutes.xml<?xml version="1.0" encoding="UTF-8"?>
<!-- the xmlns="http://camel.apache.org/schema/spring" is needed -->
<routes xmlns="http://camel.apache.org/schema/spring">
<route id="foo">
<from uri="direct:foo"/>
<to uri="mock:foo"/>
</route>
<route id="bar">
<from uri="direct:bar"/>
<to uri="mock:bar"/>
</route>
</routes>
Notice that in the XML file the root tag is <routes> which must use the namespace "http://camel.apache.org/schema/spring". This namespace is having the spring in the name, but that is because of historical reasons, as Spring was the first and only XML DSL back in the time. At runtime no Spring JARs is needed. Maybe in Camel 3.0 the namespace can be renamed to a generic name.
Configuring propert placeholders
Here is a snippet of a web.xml configuration for setting up property placeholders to load myproperties.properties from the classpath
xml <!-- setup property placeholder to load properties from classpath -->
<!-- we do this by setting the param-name with propertyPlaceholder. as prefix and then any options such as location, cache etc -->
<context-param>
<param-name>propertyPlaceholder.location</param-name>
<param-value>classpath:myproperties.properties</param-value>
</context-param>
<!-- for example to disable cache on properties component, you do -->
<context-param>
<param-name>propertyPlaceholder.cache</param-name>
<param-value>false</param-value>
</context-param>
Configuring JMX
Here is a snippet of a web.xml configuration for configuring JMX, such as disabling JMX.
xml <!-- configure JMX by using names that is prefixed with jmx. -->
<!-- in this example we disable JMX -->
<context-param>
<param-name>jmx.disabled</param-name>
<param-value>true</param-value>
</context-param>
JNDI or Simple as Camel Registry
This component uses either JNDI or Simple as the Registry.
This allows you to lookup Beans and other services in JNDI, and as well to bind and unbind your own Beans.
This is done from Java code by implementing the org.apache.camel.component.servletlistener.CamelContextLifecycle.
Using custom CamelContextLifecycle
In the code below we use the callbacks beforeStart and afterStop to enlist our custom bean in the Simple Registry, and as well to cleanup when we stop.
{snippet:id=e1|lang=java|url=camel/trunk/components/camel-servletlistener/src/test/java/org/apache/camel/component/servletlistener/MyLifecycle.java}Then we need to register this class in the web.xml file as shown below, using the parameter name "CamelContextLifecycle". The value must be a FQN which refers to the class implementing the org.apache.camel.component.servletlistener.CamelContextLifecycle interface.
xml <context-param>
<param-name>CamelContextLifecycle</param-name>
<param-value>org.apache.camel.component.servletlistener.MyLifecycle</param-value>
</context-param>
As we enlisted our HelloBean Bean using the name "myBean" we can refer to this Bean in the Camel routes as shown below:
javapublic class MyBeanRoute extends RouteBuilder {
@Override
public void configure() throws Exception {
from("seda:foo").routeId("foo")
.to("bean:myBean")
.to("mock:foo");
}
}
Important: If you use org.apache.camel.component.servletlistener.JndiCamelServletContextListener then the CamelContextLifecycle must use the JndiRegistry as well. And likewise if the servlet is org.apache.camel.component.servletlistener.SimpleCamelServletContextListener then the CamelContextLifecycle must use the SimpleRegistry
See Also
