How HikariCP’s FastList Outperforms ArrayList for Statement Management

HikariCP is known as the fastest database connection pool middleware, partly because it replaces ArrayList with a custom FastList.

FastList is built on top of ArrayList but removes range checking and overrides many methods to improve performance for managing Statements.

/**
 * Fast list without range checking.
 *
 * @author Brett Wooldridge
 */
public final class FastList<T> extends ArrayList<T> {
    ...
}

Key differences include:

add : overridden to assign directly and only expand when necessary, using left‑shift doubling instead of ArrayList’s 1.5× growth.

get, set, remove(int) : overridden without range checks, assuming valid indices.

removeLast : a new method not present in ArrayList, used to close Statements in LIFO order.

Methods such as toArray, containsAll, addAll, removeAll throw UnsupportedOperationException because they are unnecessary for HikariCP’s use case.

size and isEmpty behave the same as in ArrayList.

Example of the overridden add method:

public boolean add(T element) {
    try {
        elementData[size++] = element;
    } catch (ArrayIndexOutOfBoundsException e) {
        final int oldCapacity = elementData.length;
        final int newCapacity = oldCapacity << 1;
        @SuppressWarnings("unchecked")
        final T[] newElementData = (T[]) Array.newInstance(clazz, newCapacity);
        System.arraycopy(elementData, 0, newElementData, 0, oldCapacity);
        newElementData[size - 1] = element;
        elementData = newElementData;
    }
    return true;
}

The get method simply returns elementData[index] without a range check, and removeLast retrieves and clears the last element:

public T removeLast() {
    T element = elementData[--size];
    elementData[size] = null;
    return element;
}

In summary, FastList is a specialized, high‑performance list tailored for HikariCP’s internal Statement management, sacrificing general‑purpose safety checks to achieve faster execution in its specific scenario.