5.1. Active Object

The Active Object pattern is a concurrency design pattern that decouples method execution from method invocation in multi-threaded applications. It is particularly useful for managing concurrency in systems that use asynchronous method calls and require a high level of responsiveness. The pattern allows objects to process requests concurrently, with each request being encapsulated in a separate command object and executed by a dedicated thread.

The Active Object pattern is typically used when:

1. You want to decouple method invocation from execution to improve responsiveness and simplify concurrency management.
2. You need to manage concurrency in a system with asynchronous method calls.
3. You want to provide a simple interface for clients while hiding the complexity of concurrent execution.

To implement the Active Object pattern, follow these steps:

1. Define an interface for the active object that includes the methods to be executed asynchronously.
2. Create a command object for each method in the interface, encapsulating the method call and its arguments.
3. Implement a scheduler that maintains a queue of command objects and manages their execution by a separate thread or a pool of threads.
4. Implement the active object by providing an implementation of the interface, which submits command objects to the scheduler for execution.

Here’s a simple example of the Active Object pattern in Java:

// Active Object interface
interface ActiveObject {
    Future<Integer> calculateSum(int a, int b);
}

// Command object
class SumCommand implements Callable<Integer> {
    private int a;
    private int b;

    public SumCommand(int a, int b) {
        this.a = a;
        this.b = b;
    }

    @Override
    public Integer call() {
        return a + b;
    }
}

// Active Object implementation
class ActiveObjectImpl implements ActiveObject {
    private final ExecutorService executor = Executors.newSingleThreadExecutor();

    @Override
    public Future<Integer> calculateSum(int a, int b) {
        return executor.submit(new SumCommand(a, b));
    }
}

// Client code
public class Client {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        ActiveObject activeObject = new ActiveObjectImpl();
        Future<Integer> result = activeObject.calculateSum(1, 2);
        // Perform other tasks...
        System.out.println("Sum: " + result.get()); // Outputs: Sum: 3
    }
}

In this example, the ActiveObject interface defines the calculateSum method, which is executed asynchronously. The SumCommand class encapsulates the method call and its arguments, while the ActiveObjectImpl class implements the ActiveObject interface and submits command objects to the scheduler for execution.

Advantages of the Active Object pattern:

1. Improved responsiveness: The pattern decouples method invocation from execution, allowing clients to continue processing without waiting for a method call to complete.
2. Simplified concurrency management: The pattern encapsulates concurrency management within the active object and scheduler, making it easier for clients to interact with the system.
3. Scalability: The pattern supports the use of thread pools and can be extended to handle a larger number of requests, improving the overall scalability of the system.

Disadvantages of the Active Object pattern:

1. Increased complexity: The pattern introduces additional components such as command objects and schedulers, which may make the codebase more complex and harder to manage.
2. Potential performance overhead: The pattern may introduce overhead performance due to the use of additional threads and synchronization mechanisms.

When using the Active Object pattern, carefully consider its advantages and disadvantages. Use the pattern when you want to decouple method invocation from execution and simplify concurrency management in multi-threaded applications. Be aware of the potential limitations introduced by the pattern, such as increased complexity and performance overhead, and ensure that it is applied judiciously to maintain a clean and efficient codebase. Provide clear documentation or guidance for developers so they can understand how the pattern is used and how to extend or modify it as needed.

Note: For complete list of design patterns click here