Object Pool
Also known as
- Resource Pool
Intent
The Object Pool design pattern manages a pool of reusable objects, optimizing resource use by recycling objects rather than creating and destroying them repeatedly.
Explanation
Real world example
In our war game we need to use oliphaunts, massive and mythic beasts, but the problem is that they are extremely expensive to create. The solution is to create a pool of them, track which ones are in-use, and instead of disposing them re-use the instances.
In plain words
Object Pool manages a set of instances instead of creating and destroying them on demand.
Wikipedia says
The object pool pattern is a software creational design pattern that uses a set of initialized objects kept ready to use – a "pool" – rather than allocating and destroying them on demand.
Programmatic Example
Here's the basic Oliphaunt class. These giants are very expensive to create.
public class Oliphaunt {
private static final AtomicInteger counter = new AtomicInteger(0);
private final int id;
public Oliphaunt() {
id = counter.incrementAndGet();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public int getId() {
return id;
}
@Override
public String toString() {
return String.format("Oliphaunt id=%d", id);
}
}
Next, we present the ObjectPool and more specifically OliphauntPool.
public abstract class ObjectPool<T> {
private final Set<T> available = new HashSet<>();
private final Set<T> inUse = new HashSet<>();
protected abstract T create();
public synchronized T checkOut() {
if (available.isEmpty()) {
available.add(create());
}
var instance = available.iterator().next();
available.remove(instance);
inUse.add(instance);
return instance;
}
public synchronized void checkIn(T instance) {
inUse.remove(instance);
available.add(instance);
}
@Override
public synchronized String toString() {
return String.format("Pool available=%d inUse=%d", available.size(), inUse.size());
}
}
public class OliphauntPool extends ObjectPool<Oliphaunt> {
@Override
protected Oliphaunt create() {
return new Oliphaunt();
}
}
Finally, here's how we utilize the pool.
var pool = new OliphauntPool();
var oliphaunt1 = pool.checkOut();
var oliphaunt2 = pool.checkOut();
var oliphaunt3 = pool.checkOut();
pool.checkIn(oliphaunt1);
pool.checkIn(oliphaunt2);
var oliphaunt4 = pool.checkOut();
var oliphaunt5 = pool.checkOut();
Program output:
Pool available=0 inUse=0
Checked out Oliphaunt id=1
Pool available=0 inUse=1
Checked out Oliphaunt id=2
Checked out Oliphaunt id=3
Pool available=0 inUse=3
Checking in Oliphaunt id=1
Checking in Oliphaunt id=2
Pool available=2 inUse=1
Checked out Oliphaunt id=2
Checked out Oliphaunt id=1
Pool available=0 inUse=3
Class diagram
Applicability
Use the Object Pool pattern when
- You need to frequently create and destroy objects, leading to high resource allocation and deallocation costs.
- The objects are expensive to create and maintain (e.g., database connections, thread pools).
- A fixed number of objects need to be controlled, like in connection pooling.
- Object reuse can significantly improve system performance and resource management.
Known Uses
- Database connection pooling in Java applications.
- Thread pooling in Java concurrent programming.
- Pooling of socket connections in network applications.
- Object pools in game development for frequently created and destroyed game objects.
Consequences
Benefits:
- Improved Performance: Reduces the overhead of object creation and garbage collection.
- Resource Management: Controls the number of instances, reducing resource contention and limiting resource usage.
- Scalability: Allows the application to handle more requests by reusing a fixed number of objects.
Trade-offs:
- Complexity: Adds complexity to the codebase, requiring careful management of the pool.
- Thread Safety: Requires careful handling of concurrent access to the pool, introducing potential synchronization issues.
- Initialization Cost: Initial creation of the pool can be resource-intensive.
Related Patterns
Singleton: Ensures a single instance of the pool is used, providing a global point of access.
Flyweight: Shares fine-grained objects to reduce memory usage, complementing object pooling by managing object state efficiently.
Factory Method: Often used to create objects within the pool, abstracting the instantiation process.