[[WritingComponents-WritingComponents]]
= Writing Components

Apache Camel is designed to make it very easy to drop in new components
whether they be routing components, transformers, transports etc. The
idea of a component is to be a factory and manager of
xref:endpoint.adoc[Endpoints].

Here are the main steps to writing a component:

* Write a POJO which implements the `Component` interface. The simplest approach is just to derive from `DefaultComponent`.
* To support auto-discovery of your component add a file to
`META-INF/services/org/apache/camel/component/FOO` where FOO is the URI
scheme for your component and any related endpoints created on the fly.
The latter file should contain the definition of the component class.
For example if your component is implemented by the
`com.example.CustomComponent` class, the service file should contain the
following line -- `class=com.example.CustomComponent`.

Users can then either explicitly create your component, configure it and
register with a `CamelContext` or users can use a URI which auto-creates your component.

[[WritingComponents-WritingEndpoints]]
== Writing Endpoints

When implementing an xref:endpoint.adoc[Endpoint] you typically may
implement one or more of the following methods:

* `createProducer` will create a producer for sending message exchanges to the endpoint
* `createConsumer` implements the xref:eventDrivenConsumer-eip.adoc[Event Driven Consumer]
pattern for consuming message exchanges from the endpoint.

Typically you just derive from `DefaultEndpoint`

[[WritingComponents-AnnotatingyourEndpoint]]
== Annotating your Endpoint

If you want to benefit from the automatic generation
of HTML documentation for all the parameters on your endpoint as part of
the maven site reports, you need to
xref:endpoint-annotations.adoc[annotate your Endpoint's parameters].

So this means you add a `@UriEndpoint` annotation to your Endpoint class
and then annotate each parameter you wish to be configured via the URI
configuration mechanism with `@UriParam` (or `@UriParams` for nested
configuration objects).

Refer to the xref:endpoint-annotations.adoc[Endpoint Annotations guide
for details].

[[WritingComponents-Dependencyinjectionandauto-discovery]]
== Dependency injection and auto-discovery

When using auto-discovery the CamelContext will default to its
xref:injector.adoc[Injector] implementation to inject any required or
optional dependencies on the component. This allows you to use
auto-discovery of components via xref:uris.adoc[URIs] while still
getting the benefits of dependency injection.

For example your component can depend on a JDBC DataSource or JMS
ConnectionFactory which can be provided in the ApplicationContext in
Spring.

So you can if you prefer configure your Component using an IoC framework
like Spring; then add it to the CamelContext. Or you can let
the Component auto-inject itself as the endpoints are auto-discovered.

[[WritingComponents-Options]]
== Options

If your component has options you can let it have public getters/setters
and Camel will automatically set the properties when the endpoint is
created. If you however want to take the matter in your own hands, then
you must remove the option from the given parameter list as Camel will
validate that all options are used. If not Camel will throw a
ResolveEndpointFailedException stating some of the options are unknown.

The parameters is provided by Camel in the createEndpoint method from
DefaultComponent:

[source,java]
----
protected abstract Endpoint<E> createEndpoint(String uri, String remaining, Map parameters)
----

The code is an example from the xref:components::seda-component.adoc[SEDA] component that removes the size
parameter:

[source,java]
----
    public BlockingQueue<Exchange> createQueue(String uri, Map parameters) {
        int size = 1000;
        Object value = parameters.remove("size");
        if (value != null) {
            Integer i = convertTo(Integer.class, value);
            if (i != null) {
                size = i;
            }
        }
        return new LinkedBlockingQueue<Exchange>(size);
    }
----

