25.2 The Spring TaskExecutor abstraction
Prev 25. Task Execution and Scheduling Next

25.2 The Spring TaskExecutor abstraction

25.2 The Spring TaskExecutor abstraction

Spring 2.0 introduces a new abstraction for dealing with executors. Executors are the Java 5 name for the concept of thread pools. The "executor" naming is due to the fact that there is no guarantee that the underlying implementation is actually a pool; an executor may be single-threaded or even synchronous. Spring's abstraction hides implementation details between Java SE 1.4, Java SE 5 and Java EE environments.

Spring's TaskExecutor interface is identical to the java.util.concurrent.Executor interface. In fact, its primary reason for existence is to abstract away the need for Java 5 when using thread pools. The interface has a single method execute(Runnable task) that accepts a task for execution based on the semantics and configuration of the thread pool.

The TaskExecutor was originally created to give other Spring components an abstraction for thread pooling where needed. Components such as the ApplicationEventMulticaster, JMS's AbstractMessageListenerContainer, and Quartz integration all use the TaskExecutor abstraction to pool threads. However, if your beans need thread pooling behavior, it is possible to use this abstraction for your own needs.

25.2.1 TaskExecutor types

There are a number of pre-built implementations of TaskExecutor included with the Spring distribution. In all likelihood, you shouldn't ever need to implement your own.

  • SimpleAsyncTaskExecutor

    This implementation does not reuse any threads, rather it starts up a new thread for each invocation. However, it does support a concurrency limit which will block any invocations that are over the limit until a slot has been freed up. If you're looking for true pooling, keep scrolling further down the page.

  • SyncTaskExecutor

    This implementation doesn't execute invocations asynchronously. Instead, each invocation takes place in the calling thread. It is primarily used in situations where mutlithreading isn't necessary such as simple test cases.

  • ConcurrentTaskExecutor

    This implementation is a wrapper for a Java 5 java.util.concurrent.Executor. There is an alternative, ThreadPoolTaskExecutor, that exposes the Executor configuration parameters as bean properties. It is rare to need to use the ConcurrentTaskExecutor but if the ThreadPoolTaskExecutor isn't robust enough for your needs, the ConcurrentTaskExecutor is an alternative.

  • SimpleThreadPoolTaskExecutor

    This implementation is actually a subclass of Quartz's SimpleThreadPool which listens to Spring's lifecycle callbacks. This is typically used when you have a threadpool that may need to be shared by both Quartz and non-Quartz components.

  • ThreadPoolTaskExecutor

    It is not possible to use any backport or alternate versions of the java.util.concurrent package with this implementation. Both Doug Lea's and Dawid Kurzyniec's implementations use different package structures which will prevent them from working correctly.

    This implementation can only be used in a Java 5 environment but is also the most commonly used one in that environment. It exposes bean properties for configuring a java.util.concurrent.ThreadPoolExecutor and wraps it in a TaskExecutor. If you need something advanced such as a ScheduledThreadPoolExecutor, it is recommended that you use a ConcurrentTaskExecutor instead.

  • TimerTaskExecutor

    This implementation uses a single TimerTask as its backing implementation. It's different from the SyncTaskExecutor in that the method invocations are executed in a separate thread, although they are synchronous in that thread.

  • WorkManagerTaskExecutor

    CommonJ is a set of specifications jointly developed between BEA and IBM. These specifications are not Java EE standards, but are standard across BEA's and IBM's Application Server implementations.

    This implementation uses the CommonJ WorkManager as its backing implementation and is the central convenience class for setting up a CommonJ WorkManager reference in a Spring context. Similar to the SimpleThreadPoolTaskExecutor, this class implements the WorkManager interface and therefore can be used directly as a WorkManager as well.

25.2.2 Using a TaskExecutor

Spring's TaskExecutor implementations are used as simple JavaBeans. In the example below, we define a bean that uses the ThreadPoolTaskExecutor to asynchronously print out a set of messages.

import org.springframework.core.task.TaskExecutor;

public class TaskExecutorExample {

  private class MessagePrinterTask implements Runnable {

    private String message;

    public MessagePrinterTask(String message) {
      this.message = message;
    }

    public void run() {
      System.out.println(message);
    }

  }

  private TaskExecutor taskExecutor;

  public TaskExecutorExample(TaskExecutor taskExecutor) {
    this.taskExecutor = taskExecutor;
  }

  public void printMessages() {
    for(int i = 0; i < 25; i++) {
      taskExecutor.execute(new MessagePrinterTask("Message" + i));
    }
  }
}

As you can see, rather than retrieving a thread from the pool and executing yourself, you add your Runnable to the queue and the TaskExecutor uses its internal rules to decide when the task gets executed.

To configure the rules that the TaskExecutor will use, simple bean properties have been exposed.

<bean id="taskExecutor" class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor">
  <property name="corePoolSize" value="5" />
  <property name="maxPoolSize" value="10" />
  <property name="queueCapacity" value="25" />
</bean>

<bean id="taskExecutorExample" class="TaskExecutorExample">
  <constructor-arg ref="taskExecutor" />
</bean>
Prev Up Next
25. Task Execution and Scheduling Home 25.3 The Spring TaskScheduler abstraction