Getting ready

Define a propertyPlaceholder tag inside the camelContext element:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <propertyPlaceholder
      id="properties"
      location="classpath:placeholder.properties"/>
  <!-- other code here -->
</camelContext>

Properties contained in the file placeholder.properties can then be used directly inside your Camel route using placeholders.

The Java code for this recipe is located in the org.camelcookbook.structuringroutes.propertyplaceholder package. Spring XML files are located under src/main/resources/META-INF/spring and prefixed with propertyPlaceholder.

How to do it...

The placeholder syntax is different from the usual Spring ${..} format, in that properties are delimited by {{ to start and }} to end.

<route>
  <from uri="{{start.endpoint}}"/>
  <transform>
    <simple>{{transform.message}}: ${body}</simple>
  </transform>
  <to uri="{{end.endpoint}}"/>
</route>

Consider the following properties file content:

start.endpoint=direct:in
transform.message=I hear you
end.endpoint=mock:out

Configured with these, the preceding route will consume a message from an in-memory endpoint, prefix the body with I hear you and send the result to a mock endpoint for testing.

How it works...

This bridging functionality is necessary since Spring has some limitations in terms of allowing third-party libraries to use its property placeholder mechanism.

The location URI scheme can take the following forms:

Location URIs can themselves contain placeholders for JVM system properties and environment variables:

file:${karaf.home}/etc/application.properties
file:${env:CATALINA_HOME}/etc/application.properties

Instead of using a property-placeholder tag, you can also define a PropertiesComponent object in the Spring context with id as "properties", and it will be integrated as expected:

<bean id="properties"
      class="org.apache.camel.component.properties.PropertiesComponent">
  <property name="location"
            value="classpath:placeholder.properties"/>
</bean>

This also works when using Camel directly from a Java application without Spring:

PropertiesComponent properties = new PropertiesComponent();
properties.setLocation("classpath:placeholder.properties");
camelContext.addComponent(properties);

The placeholder mechanism just shown also works when defining routes in Java. The following route works as expected inside a Camel context that has a properties placeholder configured:

from("{{start.endpoint}}")
  .transform().simple("{{transform.message}}: ${body}")
  .log("Set message to ${body}")
  .to("{{end.endpoint}}");

There's more...

Camel provides a drop-in replacement for a Spring PropertyPlaceholderConfigurer, that enables Spring beans to be initialized with ${…} configuration while allowing Camel logic to make use of {{…}} placeholders, with the one piece of configuration.

<bean id="bridgePropertyPlaceholder"
      class="org.apache.camel.spring.spi.BridgePropertyPlaceholderConfigurer">
  <property name="location"
            value="classpath:placeholder.properties"/>
</bean>

See also