Understanding Object Pooling in Java: Commons Pool 2, Jedis, and HikariCP

In typical coding, expensive objects such as threads, database connections, or TCP connections are pooled to avoid costly creation and destruction.

Commons Pool 2

Commons Pool 2 provides a generic object pool implementation. The core class GenericObjectPool creates pools using a factory and configuration.

public GenericObjectPool(final PooledObjectFactory
factory, final GenericObjectPoolConfig
config)

Case Study: Jedis

Jedis, a Redis client, uses Commons Pool to manage connections. The factory creates PooledObject instances wrapped in DefaultPooledObject . The pool stores idle objects in a LinkedBlockingDeque and uses parameters such as maxTotal , maxIdle , and minIdle to control size.

public PooledObject
makeObject() throws Exception {
    Jedis jedis = null;
    try {
        jedis = new Jedis(jedisSocketFactory, clientConfig);
        jedis.connect();
        return new DefaultPooledObject<>(jedis);
    } catch (JedisException je) {
        if (jedis != null) {
            try { jedis.quit(); } catch (RuntimeException e) { logger.warn("Error while QUIT", e); }
            try { jedis.close(); } catch (RuntimeException e) { logger.warn("Error while close", e); }
        }
        throw je;
    }
}

Key Configuration Parameters

maxTotal : maximum number of objects in the pool.

maxIdle : maximum idle objects.

minIdle : minimum idle objects.

maxWaitMillis : maximum wait time for borrowing.

blockWhenExhausted : whether to block when the pool is exhausted.

Various test* flags control validation on create, borrow, return, and idle.

JMH Benchmark

A JMH benchmark comparing pooled Jedis versus direct Jedis shows roughly a 5× throughput improvement when using the pool.

@Fork(2)
@State(Scope.Benchmark)
@Warmup(iterations = 5, time = 1)
@Measurement(iterations = 5, time = 1)
@BenchmarkMode(Mode.Throughput)
public class JedisPoolVSJedisBenchmark {
    JedisPool pool = new JedisPool("localhost", 6379);

    @Benchmark
    public void testPool() {
        Jedis jedis = pool.getResource();
        jedis.set("a", UUID.randomUUID().toString());
        jedis.close();
    }

    @Benchmark
    public void testJedis() {
        Jedis jedis = new Jedis("localhost", 6379);
        jedis.set("a", UUID.randomUUID().toString());
        jedis.close();
    }
}

HikariCP Database Connection Pool

HikariCP is the default Spring Boot connection pool, optimized with FastList instead of ArrayList , byte‑code enhancements via Javassist, and a lock‑free ConcurrentBag , delivering superior performance.

Practical Advice

Set pool sizes according to workload (typically 20‑50 DB connections), configure timeouts, and monitor pool metrics to avoid resource exhaustion; similar principles apply to HTTP client pools, RPC pools, and thread pools.