如何从jar读取外部文本文件

Question

I have a text file that gets written to a network folder and I want my users to be able to click on a jar file which will read in the text file, sort it, and output a sorted file to the same folder. But I'm having trouble formatting the syntax of the InputStream to read the file in.

When I use a FileReader instead of an InputStreamReader the following code works fine in eclipse, but returns empty when run from the jar. When I change it to InputStream like my research suggests - I get a NullPointerException like it can't find the file.

Where did I go wrong? :)

public class sort {

    /**
     * @param args
     */
    public static void main(String[] args) {
        sort s = new sort();
        ArrayList<String> farmRecords = new ArrayList<String>();
        farmRecords = s.getRecords(); 

        String testString = new String(); 
        if(farmRecords.size() > 0){
            //do some work to sort the file

        }else{
            testString = "it didn't really work"; 
        }
        writeThis(testString); 
    }



public ArrayList<String> getRecords(){
        ArrayList<String> records = new ArrayList(); 
        BufferedReader br; 
        InputStream recordsStream = this.getClass().getResourceAsStream("./input.IDX");
        try {

            String sCurrentLine;
            InputStreamReader recordsStreamReader = new InputStreamReader(recordsStream);
            br = new BufferedReader(recordsStreamReader);
            while ((sCurrentLine = br.readLine()) != null) {
                System.out.println(sCurrentLine); 
            }
            br.close();

        } catch (IOException e) {
            e.printStackTrace();
        } 

        return records; 
}

private static void writeThis(String payload){
    String filename = "./output.IDX"; 
    try {
        BufferedWriter fr = new BufferedWriter(new FileWriter(filename));
        fr.write(payload);
        fr.close();
    } catch (IOException e) {

        e.printStackTrace();
    }
}

}

Answer 1

getResourceAsStream() loads files from the classpath. If you are running this from the command line, you would need the current directory ( . ) on the classpath. If you want to load arbitrary files from the file system, you should use FileInputStream (or FileReader to save having to subsequently wrap the input stream in a reader).

Answer 2

Using a FIS to get a file inside a jar will not work since the file is not on the file system per se. You should use getResrouceAsStream() for that.

Also, to access a file inside a jar, you must add an "!" to the file path. Is the file inside the jar? If not, then try a script to start the jar after passing the classpath:

start.sh

    java -cp .:your.jar com.main.class.example.run

Execute this script (on linux) or modify it as per your platform.

Also, you can use the following code to print out the classpath. This way you can check whether your classpath contains the file?

ClassLoader sysClassLoader = ClassLoader.getSystemClassLoader();

    // Get the URLs
    URL[] urls = ((URLClassLoader) sysClassLoader).getURLs();

    for (int i = 0; i < urls.length; i++) {
        System.out.println(urls[i].getFile());
    }
}