2D Aiming and Shooting at the mouse

1. You need a position of touch

    function update(){
            if (Input.GetMouseButtonDown(0)){ 
               pos = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x,Input.mousePosition.y,2));
                /some code/
                }
        }
   
   

2. I have cannon i'n middle of screen from where I'm shooting and i need to rotate him

     function rotate (position : Vector3){
        var d:float;   //distance 
        var angle:float;
        var x:float;
        
        x = position.x;  //basic maths pythagor's phrase :D c^2 = b^2 + a^2
        d = Mathf.Sqrt((position.y*position.y)+(x*x));  //this is distance (c)
        angle = Mathf.Acos(position.y/d) * Mathf.Rad2Deg; //here I get angle
        if (x > 0)   //here i determinate to turn left or right
            for (var i = 0; i < uhol; i++){
                shootingPosTransform.eulerAngles = Vector3(0, 0,-i);
            }
        else 
            for (var j = 0; j < uhol; j++){
                shootingPosTransform.eulerAngles = Vector3(0, 0,j);    
            }
        
    } //note: my cannon is at coordintes 0,0 so if u have another use vector touchPosition-shootingPosition
   
   

3. Creating projectile and shooting

      function shoot(poz:Vector3){
        var strelaObject : GameObject = new GameObject("strela"); //strela means projectile
        var renderer : SpriteRenderer = strelaObject.AddComponent.<SpriteRenderer>();
        var st : Transform = strelaObject.GetComponent("Transform");
        var sr : Rigidbody2D = strelaObject.AddComponent.<Rigidbody2D>();
        var vec2smer : Vector2 = new Vector2((8/poz.y)*poz.x,8); //here is magic for constant move :) if you know touch position you are able to calculate it at some another point so every touch will come form y=8 and x form touch will be moved to height of y=8
        var cc : CircleCollider2D = strelaObject.AddComponent(CircleCollider2D);
        
        renderer.sprite = strelaSprite;    //sprite
        
        cc.radius = 0.2f;   //radius of collider
        cc.sharedMaterial = physMat;
        
        sr.AddForce(vec2smer*silaStrely); //now all what moves projectile is rigidbodyAddForce * someForce (i use int)
        sr.mass = 1;
        sr.gravityScale = 1;
        //sr.isKinematic = true;
    
        st.Translate(0,0,-0.5); //here is fix to get position on z = -0.5
        Destroy(strelaObject,1.5); //and destroy
    }
   
   
   

Quite easy :D I hope i helped you :)

sorry for the late reply, just woke up again, was 1 am when i last answered, anyways, i mean, i'm working in C# but i'll try to convert that from java to see if it works, i personally prefer using a prefab solution though, looking at your code though you seem to be creating the bullet with all it's components from scratch, later on once i get the basics of it all working to perfection i was also gonna add object pooling to the whole process as your method of doing the spawning of the projectiles seem to be very cpu intensive compared to having a few objects activated and deactivated, i think changing the spawn part would be a good way to optimize your script, however the aiming part seems to be a little more complex than what i have going on in my script atm, the following is what i got:

 using UnityEngine;
 using System.Collections;
 
 public class Aiming : MonoBehaviour
 { 
     public GameObject shotPrefab;
 
     public GameObject exit_Point;
 
     private Rigidbody2D rb;
 
     public float speed;
 
     void Update()
     {
         Vector2 target = Camera.main.ScreenToWorldPoint(new Vector2(Input.mousePosition.x, Input.mousePosition.y));
         Vector2 exitPoint = new Vector2(exit_Point.transform.position.x, exit_Point.transform.position.y);
         Vector2 direction = target - exitPoint;
         direction.Normalize();
 
         Quaternion rotation = Quaternion.Euler(0, 0, Mathf.Atan2(direction.y, direction.x) *                                            Mathf.Rad2Deg);
         transform.rotation = rotation;
 
         if (Input.GetMouseButtonDown(0))
         {
             GameObject shot = (GameObject)Instantiate(shotPrefab, exitPoint, rotation);
             rb = shot.GetComponent<Rigidbody2D>();
             rb.AddForce(direction * speed);
         }
     }
 }

i have the GameObjects set to public so i can just drag a prefab and an empty object in rather than create them from scratch, later on ill be adding object pooling by doing a standard list of gameobjects that i activate/de-activate, i make an empty rigid body for the bullet to reference it, define a target, exit point(the gameobject) and a direction based of those 2, a normalize to make it a constant speed, rather than have it very on where you click, since i did the direction really simple(since i use the direction in the addforce, so the longer you are from the player the bigger the number it multiplies etc.), then i use quaternions to rotate it and an innstantiation for finally shooting the bullet out, that's all there is so far(except for the object pooling since ill bbe adding that as a finnal touch).