According to Roman Catholic Church is it only via Jesus you can come in Heaven? Normally it has been said that you only enter Heaven by believing in Jesus. But can you say that that is absolutely and exclusive or do you have to leave an opening because we don't always knows the ways of God?
But can you say that that is absolutely and exclusive or do you have
  to leave an opening because we don't always know the ways of God?


Current teaching leans more toward the latter than the former ... but the simplest (and arguably the best) way is through Christ.   (CCC 1987 - 2029 is a discourse to that effect, in detail, as is the treatment in CCC 1023 - 1029).  A reaffirmation of salvation being through Jesus is found in Dominus Iesus. (@AthanasiusofAlex goes into a detailed analysis of that source in his answer).  The takeaway from that is that if you know of Christ and don't accept the invitation for salvation through him, then having a relationship and a friendship with God is at serious risk.    

Discussion 

In current teaching the general case is that through Christ is the path to salvation.  The Roman Catholic Church allows that there may be salvation subject to the Grace, Mercy (and discretion) of God for those in a relationship of friendship with God who through no fault of their own don't know Christ.  But even then, it was Christ who opened the path back to Heaven, so salvation has been through him in the meta sense. 


  CCC 1026 By his death and Resurrection, Jesus Christ has "opened" heaven
  to us. The life of the blessed consists in the full and perfect
  possession of the fruits of the redemption accomplished by Christ.


Grace per CCC 1996:  


  Grace is favor, the free and undeserved help that God gives us to respond to his call to become children of God, adoptive sons, partakers of the divine nature and of eternal life.


God can give his grace to whomever He so chooses.  How someone responds to that gift of grace will inform their relationship with God.    


From the Catechism of the Catholic Church, article 1023, you
will find this statement in a discussion about "Particular
Judgment":


  1023 Those who die in God's grace and friendship and are perfectly
  purified live for ever with Christ. They are like God for ever, for
  they "see him as he is," face to face:
  
  1025 To live in heaven is "to be with Christ." the elect live "in
  Christ," but they retain, or rather find, their true identity, their
  own name.  

From the Vatican II council, Lumen Gentium, 16 we find: 


  
  Finally, those who have not yet received the Gospel are related in various ways to the people of God. In the first place we must recall
  the people to whom the testament and the promises were given and from
  whom Christ was born according to the flesh.  On account of their
  fathers this people remains most dear to God, for God does not repent > of the gifts He makes nor of the calls He issues. But the plan of > salvation also includes those who acknowledge the Creator. In the first place amongst these there are the Muslims, who, professing to hold the faith of Abraham, along with us adore the one and merciful God, who on the last day will judge mankind. Nor is God far distant from those who in shadows and images seek the unknown God, for it is He who gives to all men life and breath and all things, and as Saviour
  wills that all men be saved. Those also can attain to salvation who
  through no fault of their own do not know the Gospel of Christ or His
  Church, yet sincerely seek God and moved by grace strive by their
  deeds to do His will as it is known to them through the dictates of
  conscience. Nor does Divine Providence deny the helps necessary for
  salvation to those who, without blame on their part, have not yet
  arrived at an explicit knowledge of God and with His grace strive to
  live a good life. Whatever good or truth is found amongst them is
  looked upon by the Church as a preparation for the Gospel. She knows
  that it is given by Him who enlightens all men so that they may
  finally have life. But often men, deceived by the Evil One, have
  become vain in their reasonings and have exchanged the truth of God
  for a lie, serving the creature rather than the Creator.  Or some
  there are who, living and dying in this world without God, are exposed
  to final despair. Wherefore to promote the glory of God and procure
  the salvation of all of these, and mindful of the command of the Lord,
  "Preach the Gospel to every creature", the Church fosters the missions
  with care and attention.
  



To put that all together:  


There is a general principle in Christianity (which includes the RCC) that God's grace is required to get into Heaven.
The usual path to God's grace is via Christ.  (The general case for
Christian denominations, not just the RCC as explained in CCC 1987-2029 cited abpve).
The Catechism states that if you die in God's grace and friendship,
you'll meet Christ in Heaven.  (This opens the door for Lumen Gentium's point, or comes from it, as LG was written before the current catechism was published). 
Lumen Gentium allows that some other people are expected to be in friendship with God and even in His grace.  Even so, it was via Christ's agency that the path to heaven is even open.  That makes all salvation through Christ, in the larger sense. 
The Church has a duty to get as many people as can be to receive Christ and take that general path toward God's grace.  


Without turning this into a nine page long post, the Church teaches (as do most Christians I have engaged on this topic) that forming a relationship with Christ is the direct path to being open to receiving God's grace.  I've had that point explained to me by a variety of clergy and lay people in the Roman Catholic faith, as well a Methodists and Episcopals ... just off the top of my head.  There have been others.  This core point underwrites the longer discussion in the Catechism (CCC 1026) about Christ opening the way to Heaven.