The Apache Camel threading model is based on the powerful Java concurrency API, java.util.concurrent, that first became available in Sun's JDK 1.5. The key interface in this API is the ExecutorService interface, which represents a thread pool. Using the concurrency API, you can create many different kinds of thread pool, covering a wide range of scenarios.

The Apache Camel thread pool API builds on the Java concurrency API by providing a central factory (of org.apache.camel.spi.ExecutorServiceManager type) for all of the thread pools in your Apache Camel application. Centralising the createion of thread pools in this way provides several advantages, including:

Some Apache Camel components—such as SEDA, JMS, and Jetty—are inherently multi-threaded. These components have all been implemented using the Apache Camel threading model and thread pool API.

If you are planning to implement your own Apache Camel component, it is recommended that you integrate your threading code with the Apache Camel threading model. For example, if your component needs a thread pool, it is recommended that you create it using the CamelContext's ExecutorServiceManager object.

Some of the standard processors in Apache Camel create their own thread pool by default. These threading-aware processors are also integrated with the Apache Camel threading model and they provide various options that enable you to customize customize the thread pools that they use.

Table 9 shows the various options for controlling and setting thread pools on the threading-aware processors built-in to Apache Camel.

parallelProcessing()
executorService()
executorServiceRef()

@parallelProcessing
@executorServiceRef

parallelProcessing()
executorService()
executorServiceRef()

@parallelProcessing
@executorServiceRef

parallelProcessing()
executorService()
executorServiceRef()

@parallelProcessing
@executorServiceRef

parallelProcessing()
executorService()
executorServiceRef()

@parallelProcessing
@executorServiceRef

executorService()
executorServiceRef()
poolSize()
maxPoolSize()
keepAliveTime()
timeUnit()
maxQueueSize()
rejectedPolicy()

@executorServiceRef
@poolSize
@maxPoolSize
@keepAliveTime
@timeUnit
@maxQueueSize
@rejectedPolicy

wireTap(String uri, ExecutorService executorService)
wireTap(String uri, String executorServiceRef)

@executorServiceRef

To create a default thread pool for one of the threading-aware processors, enable the parallelProcessing option, using the parallelProcessing() sub-clause, in the Java DSL, or the parallelProcessing attribute, in the XML DSL.

For example, in the Java DSL, you can invoke the multicast processor with a default thread pool (where the thread pool is used to process the multicast destinations concurrently) as follows:

from("direct:start")
  .multicast().parallelProcessing()
    .to("mock:first")
    .to("mock:second")
    .to("mock:third");

You can define the same route in XML DSL as follows

<camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">
    <route>
        <from uri="direct:start"/>
        <multicast parallelProcessing="true">
            <to uri="mock:first"/>
            <to uri="mock:second"/>
            <to uri="mock:third"/>
        </multicast>
    </route>
</camelContext>

The default thread pools are automatically created by a thread factory that takes its settings from the default thread pool profile. The default thread pool profile has the settings shown in Table 10 (assuming that these settings have not been modified by the application code).

It is possible to change the default thread pool profile settings, so that all subsequent default thread pools will be created with the custom settings. You can change the profile either in Java or in Spring XML.

For example, in the Java DSL, you can customize the poolSize option and the maxQueueSize option in the default thread pool profile, as follows:

// Java
import org.apache.camel.spi.ExecutorServiceManager;
import org.apache.camel.spi.ThreadPoolProfile;
...
ExecutorServiceManager manager = context.getExecutorServiceManager();
ThreadPoolProfile defaultProfile = manager.getDefaultThreadPoolProfile();

// Now, customize the profile settings.
defaultProfile.setPoolSize(3);
defaultProfile.setMaxQueueSize(100);
...

In the XML DSL, you can customize the default thread pool profile, as follows:

<camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">
    <threadPoolProfile
        id="changedProfile"
        defaultProfile="true"
        poolSize="3"
        maxQueueSize="100"/>
    ...
</camelContext>

Note that it is essential to set the defaultProfile attribute to true in the preceding XML DSL example, otherwise the thread pool profile would be treated like a custom thread pool profile (see Creating a custom thread pool profile), instead of replacing the default thread pool profile.

It is also possible to specify the thread pool for a threading-aware processor more directly, using either the executorService or executorServiceRef options (where these options are used instead of the parallelProcessing option). There are two approaches you can use to customize a processor's thread pool, as follows:

When you pass a bean ID to the executorServiceRef option, the threading-aware processor first tries to find a custom thread pool with that ID in the registry. If no thread pool is registered with that ID, the processor then attempts to look up a custom thread pool profile in the registry and uses the custom thread pool profile to instantiate a custom thread pool.

A custom thread pool can be any thread pool of java.util.concurrent.ExecutorService type. The following approaches to creating a thread pool instance are recommended in Apache Camel:

Ultimately, there is not much difference between the two approaches, because the ThreadPoolBuilder is actually defined using the ExecutorServiceManager instance. Normally, the ThreadPoolBuilder is preferred, because it offers a simpler approach. But there is at least one kind of thread (the ScheduledExecutorService) that can only be created by accessing the ExecutorServiceManager instance directory.

Table 11 shows the options supported by the ThreadPoolBuilder class, which you can set when defining a new custom thread pool.

In Java DSL, you can define a custom thread pool using the ThreadPoolBuilder, as follows:

// Java
import org.apache.camel.builder.ThreadPoolBuilder;
import java.util.concurrent.ExecutorService;
...
ThreadPoolBuilder poolBuilder = new ThreadPoolBuilder(context);
ExecutorService customPool = poolBuilder.poolSize(5).maxPoolSize(5).maxQueueSize(100).build("customPool");
...

from("direct:start")
  .multicast().executorService(customPool)
    .to("mock:first")
    .to("mock:second")
    .to("mock:third");

Instead of passing the object reference, customPool, directly to the executorService() option, you can look up the thread pool in the registry, by passing its bean ID to the executorServiceRef() option, as follows:

// Java
from("direct:start")
  .multicast().executorServiceRef("customPool")
    .to("mock:first")
    .to("mock:second")
    .to("mock:third");

In XML DSL, you access the ThreadPoolBuilder using the threadPool element. You can then reference the custom thread pool using the executorServiceRef attribute to look up the thread pool by ID in the Spring registry, as follows:

<camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">
    <threadPool id="customPool"
                poolSize="5"
                maxPoolSize="5"
                maxQueueSize="100" />

    <route>
        <from uri="direct:start"/>
        <multicast executorServiceRef="customPool">
            <to uri="mock:first"/>
            <to uri="mock:second"/>
            <to uri="mock:third"/>
        </multicast>
    </route>
</camelContext>

If you have many custom thread pool instances to create, you might find it more convenient to define a custom thread pool profile, which acts as a factory for thread pools. Whenever you reference a thread pool profile from a threading-aware processor, the processor automatically uses the profile to create a new thread pool instance. You can define a custom thread pool profile either in Java DSL or in XML DSL.

For example, in Java DSL you can create a custom thread pool profile with the bean ID, customProfile, and reference it from within a route, as follows:

// Java
import org.apache.camel.spi.ThreadPoolProfile;
import org.apache.camel.impl.ThreadPoolProfileSupport;
...
// Create the custom thread pool profile
ThreadPoolProfile customProfile = new ThreadPoolProfileSupport("customProfile");
customProfile.setPoolSize(5);
customProfile.setMaxPoolSize(5);
customProfile.setMaxQueueSize(100);
context.getExecutorServiceManager().registerThreadPoolProfile(customProfile);
...
// Reference the custom thread pool profile in a route
from("direct:start")
  .multicast().executorServiceRef("customProfile")
    .to("mock:first")
    .to("mock:second")
    .to("mock:third");

In XML DSL, use the threadPoolProfile element to create a custom pool profile (where you let the defaultProfile option default to false, because this is not a default thread pool profile). You can create a custom thread pool profile with the bean ID, customProfile, and reference it from within a route, as follows:

<camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">
    <threadPoolProfile
                id="customProfile"
                poolSize="5"
                maxPoolSize="5"
                maxQueueSize="100" />

    <route>
        <from uri="direct:start"/>
        <multicast executorServiceRef="customProfile">
            <to uri="mock:first"/>
            <to uri="mock:second"/>
            <to uri="mock:third"/>
        </multicast>
    </route>
</camelContext>

Some of the standard poll-based components—such as File and FTP—allow you to specify the thread pool to use. This makes it possible for different components to share the same thread pool, reducing the overall number of threads in the JVM.

For example, the File component in EIP Component Reference and the FTP component in EIP Component Reference both expose the scheduledExecutorService property, which you can use to specify the component's ExecutorService object.