active-object/README.md
The Active Object pattern provides a reliable method for asynchronous processing in Java, ensuring responsive applications and efficient thread management. It achieves this by encapsulating tasks within objects that have their own thread and message queue. This separation keeps the main thread responsive and avoids issues like direct thread manipulation or shared state access.
Real-world example
Imagine a busy restaurant where customers place orders with waiters. Instead of the waiters going to the kitchen to prepare the food themselves, they write the orders on slips and hand them to a dispatcher. The dispatcher manages a pool of chefs who prepare the meals asynchronously. Once a chef is free, they pick up the next order from the queue, prepare the dish, and notify the waiter when it's ready for serving.
In this analogy, the waiters represent the client threads, the dispatcher represents the scheduler, and the chefs represent the method execution in separate threads. This setup allows the waiters to continue taking orders without being blocked by the food preparation process, much like the Active Object pattern decouples method invocation from execution to enhance concurrency.
In plain words
The Active Object pattern decouples method execution from method invocation to improve concurrency and responsiveness in multithreaded applications.
Wikipedia says
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.
The pattern consists of six elements:
- 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.
Sequence diagram
This section explains how the Active Object design pattern works in Java, highlighting its use in asynchronous task management and concurrency control.
The Orcs are known for their wildness and untameable soul. It seems like they have their own thread of control based on previous behavior. To implement a creature that has its own thread of control mechanism and expose its API only and not the execution itself, we can use the Active Object pattern.
public abstract class ActiveCreature {
private final Logger logger = LoggerFactory.getLogger(ActiveCreature.class.getName());
private BlockingQueue<Runnable> requests;
private String name;
private Thread thread;
public ActiveCreature(String name) {
this.name = name;
this.requests = new LinkedBlockingQueue<Runnable>();
thread = new Thread(new Runnable() {
@Override
public void run() {
while (true) {
try {
requests.take().run();
} catch (InterruptedException e) {
logger.error(e.getMessage());
}
}
}
}
);
thread.start();
}
public void eat() throws InterruptedException {
requests.put(new Runnable() {
@Override
public void run() {
logger.info("{} is eating!", name());
logger.info("{} has finished eating!", name());
}
}
);
}
public void roam() throws InterruptedException {
requests.put(new Runnable() {
@Override
public void run() {
logger.info("{} has started to roam the wastelands.", name());
}
}
);
}
public String name() {
return this.name;
}
}
We can see that any class that will extend the ActiveCreature class will have its own thread of control to invoke and execute methods.
For example, the Orc class:
public class Orc extends ActiveCreature {
public Orc(String name) {
super(name);
}
}
Now, we can create multiple creatures such as orcs, tell them to eat and roam, and they will execute it on their own thread of control:
public class App implements Runnable {
private static final Logger logger = LoggerFactory.getLogger(App.class.getName());
private static final int NUM_CREATURES = 3;
public static void main(String[] args) {
var app = new App();
app.run();
}
@Override
public void run() {
List<ActiveCreature> creatures = new ArrayList<>();
try {
for (int i = 0; i < NUM_CREATURES; i++) {
creatures.add(new Orc(Orc.class.getSimpleName() + i));
creatures.get(i).eat();
creatures.get(i).roam();
}
Thread.sleep(1000);
} catch (InterruptedException e) {
logger.error(e.getMessage());
Thread.currentThread().interrupt();
} finally {
for (int i = 0; i < NUM_CREATURES; i++) {
creatures.get(i).kill(0);
}
}
}
}
Program output:
09:00:02.501 [Thread-0] INFO com.iluwatar.activeobject.ActiveCreature -- Orc0 is eating!
09:00:02.501 [Thread-2] INFO com.iluwatar.activeobject.ActiveCreature -- Orc2 is eating!
09:00:02.501 [Thread-1] INFO com.iluwatar.activeobject.ActiveCreature -- Orc1 is eating!
09:00:02.504 [Thread-0] INFO com.iluwatar.activeobject.ActiveCreature -- Orc0 has finished eating!
09:00:02.504 [Thread-1] INFO com.iluwatar.activeobject.ActiveCreature -- Orc1 has finished eating!
09:00:02.504 [Thread-0] INFO com.iluwatar.activeobject.ActiveCreature -- Orc0 has started to roam in the wastelands.
09:00:02.504 [Thread-2] INFO com.iluwatar.activeobject.ActiveCreature -- Orc2 has finished eating!
09:00:02.504 [Thread-1] INFO com.iluwatar.activeobject.ActiveCreature -- Orc1 has started to roam in the wastelands.
09:00:02.504 [Thread-2] INFO com.iluwatar.activeobject.ActiveCreature -- Orc2 has started to roam in the wastelands.
Use the Active Object pattern in Java when:
Discover the benefits and trade-offs of using the Active Object pattern in Java, including improved thread safety and potential overhead concerns.
Benefits:
Trade-offs: