How can I create a Global FileInputStream object, which will be available to access from other class within in my project?

Question

What I wanted to do is create a global FileInputStream object and use it a long of my application.

I have the follow class which create my FileInputStream objec and return the result of my query over the XML file:

package Engine;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.ArrayList; 
import com.ximpleware.AutoPilot;
import com.ximpleware.VTDException;
import com.ximpleware.VTDGen;
import com.ximpleware.VTDNav;

public class BACENGQueryXMLbyVTD {
  public String query;
   public String path;
    public BACENGQueryXMLbyVTD(String query, String path) {
     this.query = query;
     this.path = path;
}
public ArrayList query() throws IOException, VTDException {
    System.out.println(path);
    File f = new File(path);
    //System.out.println(f);
    FileInputStream fis = new FileInputStream(f);
    //System.out.println(fis);
    byte[] xmlContent = new byte[(int) f.length()];
    fis.read(xmlContent);
    VTDGen vg = new VTDGen();
    vg.setDoc(xmlContent);
    vg.parse(false);
    VTDNav vn = vg.getNav();
    AutoPilot ap = new AutoPilot(vn);
    int node = 0;
    ap.selectXPath(query);
    int i;
    ArrayList<String> interfaces = new ArrayList<String>();
    while ((i = ap.evalXPath()) != -1) {
        vn.push();
        interfaces.add(vn.toString(i + 1));
    }
    ap.resetXPath();
    return interfaces;
}
}

This class is called from my main class which has a loop.

 for (int c = 0; c < nodeNames.size(); c++) {
 String Queries2 = "/import_data/group/node[@name='" +nodeNames.get(c) + "']/interface/@network_value";
 BACENGQueryXMLbyVTD getNodesInterfaces = new BACENGQueryXMLbyVTD(Queries2, TransitPath);
 listOfNodeInterfaces = getNodesInterfaces.query();
}

It's working fine, however in order to reduce the consume of IO resource over my server HD. I would like to create an unique FileInputStream object and use it for each query whcih has to be executed.

Could somebody point out the way to do it?

Answer 1

Separate your concerns - BACENGQueryXMLbyVTD is both loading the data and executing the query.

First load the file into a byte[] outside your loop, then pass it to BACENGQueryXMLbyVTD. You might also want to pass the query as an argument to the query method.

You'll end up with a BACENGQueryXMLbyVTD that looks like this (with the disclaimer that I'm not familiar with VTD-XML, so this might the creation of objects from that API might not work exactly like this):

public class BACENGQueryXMLbyVTD 
{
  private byte[] doc;


  public BACENGQueryXMLbyVTD(byte[] doc) 
  {
    this.doc = doc;
  }

  public List<String> query(String query) throws IOException, VTDException 
  {
    VTDGen generator = new VTDGen();
    generator.setDoc(doc);
    generator.parse(false);

    VTDNav navigator = generator.getNav();

    AutoPilot autoPilot = new AutoPilot(navigator);
    autoPilot.selectXPath(query);

    List<String> nodeInterfaces = new ArrayList<String>();

    int i;
    while ((i = autoPilot.evalXPath()) != -1) 
    {
      navigator.push();
      nodeInterfaces.add(navigator.toString(i + 1));
    }

    return nodeInterfaces;
  }
}

That you can then call like:

byte[] xmlContent = ... //load the xml from anywhere you like, not just a file as previously.

BACENGQueryXMLbyVTD getNodesInterfaces = new BACENGQueryXMLbyVTD(xmlContent);

for (String nodeName : nodeNames) 
{
  String query = "/import_data/group/node[@name='" + nodeName + "']/interface/@network_value";
  nodeInterfaces = getNodesInterfaces.query(query);
  ...
}

You might also want to switch to the standard Java XPATH APIs - they're a lot clearer and better documented than VTD-XML.