Java ComboPooledDataSource exceeds pool size and does not reuse

Question

I have the following code that gets called by an other application (That I can not change) to read form a Database.
The method is called in a loop very often and DOSes the DB.
At The DB I can see that there are many connecetions opend ... increasing to some hundred ... and than the DB crashed due to the load.

// Called in a loop
private <T> T execute(String query, PostProcessor<T> postProc, PreProcessor preProcs) throws OperationFailedException {
    try (Connection conn 
            = Objects.requireNonNull(dataSourceRef.get(), "No Connection").getConnection();
         PreparedStatement smt = conn.prepareStatement(query)) {
        preProc.process(smt);
        return postProc.process(smt.executeQuery());
    } catch (SQLException e) {
        throw new OperationFailedException(e.getMessage(), e);
    }
}

the date source gets initialised before ...

// class variable
// AtomicReference<PooledDataSource> dataSourceRef = new AtomicReference<PooledDataSource>();

// Init method
ComboPooledDataSource cpds = new ComboPooledDataSource();
cpds.setDriverClass(config.getConnectionDriver());
cpds.setJdbcUrl(config.getConnectionString());
cpds.setUser(config.getConnectionUser());
cpds.setPassword(config.getConnectionPassword());
// cpds.setMaxPoolSize(10);
dataSourceRef.getAndSet(cpds); 

My question why is this happening.
I thought due to the pooling not for each query a new connections should be used. As well by setting the max pool size this is not working.

As well I tried it with try-catch-finally construct and closing the stm and the conn after use. (As I've read somewhere that finally might get called delayed under high load scenarios ... I thouht that might be the case)

But still why is the pool size exceeded? How can I limit that and block the method untill a connection is avalible again before continuing?