多线程的Java问题

Question

I have problem with synchronized reading of my file. The case is simple: to handle data in the file. My problem is synchronization. I try to make it on object reader of class BufferedReader, but I have two problems. If I initialise reader in "try with resourses" of my method read() I will get to different objects of BufReader for the same object when I use it in different theads. The second one I initialise it in class and get the same object for threads, but a great problem with exeptions and unpredictable behavior. I will write code for the second situation. Can you advice me how to solve. I am studying now, that's why I want advices.

class FileReaderClass {
    private File file = new File("src\\exer1\\Transfers.txt");
    private BufferedReader reader = null;
    private FileReader fr = null;

    StringBuilder sb = new StringBuilder();

    void read() throws IOException {
        try {
            fr = new FileReader(file);
            reader = new BufferedReader(fr);
            String buftext;
                while ((buftext = reader.readLine()) != null){
                    synchronized (reader) {
                        System.out.println(Thread.currentThread().getName());//for testing
                        sb.append(buftext).append("\n");
                        //System.out.println(buftext);
                    }
                }
            }
        catch (IOException e) {
            e.printStackTrace();
        }
        finally {
            if (reader != null)
                reader.close();
            if (fr != null)
                fr.close();
        }
    }
}

class Reader1 implements Runnable {

    private FileReaderClass frc;

    public Reader1(FileReaderClass frc) {
        this.frc = frc;
    }

    @Override
    public void run() {
        try {
            frc.read();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}
public class Ex1 {
    public static void main(String[] args) throws InterruptedException {
        FileReaderClass frc = new  FileReaderClass();
        Thread t1 = new Thread(new Reader1(frc));
        Thread t2 = new Thread(new Reader2(frc));

        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println("---------------------");
        System.out.println(frc.sb);

    }
}

Answer 1

You are sharing the same FileReaderClass instance (that is frc ) in two threads. Your FileReaderClass is not thread safe, that causes the problems. There are two alternatives here:

• Create another instance of FileReaderClass , so that both threads will use different instances.
• Mark the method read as snychronized .

You are synchronizing on the wrong level. Your sync guard is useless, as there are actually two reader s. That's because each time read is called, a reader will be created, which will be a separate instance. We can't even know for sure, which reader instance was the subject of syncing.

Answer 2

If you want to parallelize reading of your file you should use the same reader but use proper synchronization for each read by any thread, so that it stays consistent, the easiest way is to make the whole read method synchronized and instantiate your readers in your class constructor.

As you may want to read and write to your string builder in the correct order, the thread that reads a line must only release the lock after it writes to StringBuilder, making your whole method to be pretty much guarded by a lock. It will work but you won't gain any performance, and for small files it may cause too much overhead, but as an exercise it may suffice.

Your code as it is has the following problems

1. You are always creating new readers, thus every thread calling this method will start from the beginning.
2. As previously stated by another answear you are synchronizing at the wrong level, any thread may read and the thread scheduler can suspend it right after, causing your writes to StringBuilder object to be out of order (i think it is not intended).