如何使用PrintWriter和flush（）在文本文件上打印内容？

Question

I am using Multi-threading to calculate an image. Every Thread calculates a line, when a thread is already calculating a line , should the next thread calculate the line after that one . But I want to be sure that every line get calculated JUST one time, and to make that, I could make a System.out.println(CalculatedLineNumber) and make the output in a text file, so that when i open it with a text editor, i will directly see if the number of lines printed is the same as the one on the textfile. But how should I do that? Here is my code fragment for the run() method where the calculating is done:

public void run() {

                int myRow;
                while ( (myRow = getNextRow()) < getHeight() ) {
                    image.setRGB(0, myRow, getWidth(), 1, renderLine(myRow), 0, 0);
                }
            }

Someone told me that I should use a PrintWriter and flush() or something like that, but i dont know how to use that.. Could anyone help me with that? ("myRow" is the line number that i want to be writed on the text file, and everyone in a different line)

Thankyou so much!!

Answer 1

I want to be sure that every line get calculated JUST one time,

I would suggest that you use a ExecutorService and submit each row as a image job to a pool of threads. See the bottom for a code sample. If you do this right then you don't need to worry about how many output lines there are going to be.

I could make a System.out.println(CalculatedLineNumber)

I don't quite understand the need for this. Is this some sort of accounting file to help you ensure that all of the images have been processed?

Someone told me that I should use a PrintWriter and flush()

You don't need to flush a PrintWriter since it is already synchronized underneath. Just print out the results at the end of each job and if you submitted X row jobs to your threadPool then you will have X lines of output.

All you need to do to use PrintWriter is:

PrintWriter printWriter = new PrintWriter(new File("/tmp/outputFile.txt"));
// each thread can do:
writer.println("Some sort of output: " + myRow);

---

Here's some sample code to show how to use ExecutorService thread pools.

PrintWriter outputWriter = ...;
// create a thread pool with 10 workers
ExecutorService threadPool = Executors.newFixedThreadPool(10);
// i'm not sure exactly how to build the parameter for each of your rows
for (int myRow : rows) {
    // something like this, not sure what input you need to your jobs
    threadPool.submit(new ImageJob(outputWriter, myRow, getHeight(), getWidth()));
}
// once we have submitted all jobs to the thread pool, it should be shutdown
threadPool.shutdown();
...
public class ImageJob implements Runnable {
    private PrintWriter outputWriter;
    private int myRow;
    private int height;
    private int width;
    public MyJobProcessor(PrintWriter outputWriter, int myRow, int height,
            int width, ...) {
        this.outputWriter = outputWriter;
        this.myRow = myRow;
        this.height = height;
        this.width = width;
    }
    public void run() {
        image.setRGB(0, myRow, width, 1, renderLine(myRow), 0, 0);
        outputWriter.print(...);
    }
}