Active object

The active object design pattern decouples method execution from method invocation for objects that each reside in their own thread of control.^[1] The goal is to introduce concurrency, by using asynchronous method invocation and a scheduler for handling requests.^[2]

The pattern consists of six elements:^[3]

• A proxy, which provides an interface towards clients with publicly accessible methods.
• An interface which defines the method request on an active object.
• A list of pending requests from clients.
• A scheduler, which decides which request to execute next.
• The implementation of the active object method.
• A callback or variable for the client to receive the result.

Example

Java

An example of active object pattern in Java.^[4]

class OriginalClass {

    private double val = 0.0;
    
    void doSomething() {
        val = 1.0;
    }

    void doSomethingElse() {
        val = 2.0;
    }
}

class BecomeActiveObject {

    private double val = 0.0;

    private BlockingQueue<Runnable> dispatchQueue = new LinkedBlockingQueue<Runnable>();

    public BecomeActiveObject() {
        new Thread (new Runnable() {
                    
                @Override
                public void run() {
                    while(true) {
                        try {
                            dispatchQueue.take().run();
                        } catch (InterruptedException e) {   
                            // okay, just terminate the dispatcher
                        }
                    }
                }
            }
        ).start();
    }

    //
    void doSomething() throws InterruptedException {
        dispatchQueue.put(new Runnable() {
                @Override
                public void run() { 
                    val = 1.0; 
                }
            }
        );
    }

    //
    void doSomethingElse() throws InterruptedException {
        dispatchQueue.put(new Runnable() {
                @Override
                public void run() { 
                    val = 2.0; 
                }
            }
        );
    }
}

Java 8 (alternative)

Another example of active object pattern in Java.

public class AnotherActiveObject {
    private double val; 
    
    // container for tasks
    // decides which request to execute next 
    // asyncMode=true means our worker thread processes its local task queue in the FIFO order 
    // only single thread may modify internal state
    private final ForkJoinPool fj = new ForkJoinPool(1, ForkJoinPool.defaultForkJoinWorkerThreadFactory, null, true);
    
    // implementation of active object method
    public void doSomething() throws InterruptedException {
        fj.execute(() -> {val = 1.0;});
    }
 
    // implementation of active object method
    public void doSomethingElse() throws InterruptedException {
        fj.execute(() -> {val = 2.0;});
    }
}

See also

• Concurrent object-oriented programming
• Actor model
• Futures and promises
• Live distributed object

References

1. Douglas C. Schmidt; Michael Stal; Hans Rohnert; Frank Buschmann (2000). Pattern-Oriented Software Architecture, Volume 2: Patterns for Concurrent and Networked Objects. John Wiley & Sons. ISBN 0-471-60695-2.
2. Bass, L., Clements, P., Kazman, R. Software Architecture in Practice. Addison Wesley, 2003
3. Lavender, R. Greg; Schmidt, Douglas C. "Active Object" (PDF). Retrieved 2007-02-02.
4. Holub, Allen. "Java Active Objects - A Proposal". Retrieved 2014-06-16.

External links

• Active Object implementation in C++11