Why does HikariCP default set keepaliveTime as zero as default

HikariCP keepaliveTime

keepaliveTime

This property controls how frequently HikariCP will attempt to keep a connection alive, in order to prevent it from being timed out by the database or network infrastructure. This value must be less than the maxLifetime value. A "keepalive" will only occur on an idle connection. When the time arrives for a "keepalive" against a given connection, that connection will be removed from the pool, "pinged", and then returned to the pool. The 'ping' is one of either: invocation of the JDBC4 isValid() method, or execution of the connectionTestQuery. Typically, the duration out-of-the-pool should be measured in single digit milliseconds or even sub-millisecond, and therefore should have little or no noticeable performance impact. The minimum allowed value is 30000ms (30 seconds), but a value in the range of minutes is most desirable. Default: 0 (disabled)

Always check the connection availability when calling getConnection()

// com.zaxxer.hikari.pool.HikariPool
...
   private final long aliveBypassWindowMs = Long.getLong("com.zaxxer.hikari.aliveBypassWindowMs", MILLISECONDS.toMillis(500));
...
   public Connection getConnection(final long hardTimeout) throws SQLException
   {
...
         do {
            PoolEntry poolEntry = connectionBag.borrow(timeout, MILLISECONDS);
            if (poolEntry == null) {
               break;
            }

            final long now = currentTime();
            if (poolEntry.isMarkedEvicted() || (elapsedMillis(poolEntry.lastAccessed, now) > aliveBypassWindowMs && !isConnectionAlive(poolEntry.connection))) {
               closeConnection(poolEntry, poolEntry.isMarkedEvicted() ? EVICTED_CONNECTION_MESSAGE : DEAD_CONNECTION_MESSAGE);
               timeout = hardTimeout - elapsedMillis(startTime);
            }
            else {
               metricsTracker.recordBorrowStats(poolEntry, startTime);
               return poolEntry.createProxyConnection(leakTaskFactory.schedule(poolEntry), now);
            }
         } while (timeout > 0L);
...
   }
...