Question

I have several threads which need to write to two different text files. So far I have this code:

public class Logger {

    public static void printToGameLog(String value){
        Writer writer = null;
        try {
            writer = new BufferedWriter(new OutputStreamWriter(
                new FileOutputStream("GameLog.txt", true), "utf-8"));
            synchronized(writer){
                writer.write(outputString + "\r\n");
            }
        } catch (IOException ex){
            System.out.println("cannot create log file");
        } 
    }


    public static void printToServerLog(String value){
        Writer writer = null;
        try {
            writer = new BufferedWriter(new OutputStreamWriter(
                new FileOutputStream("serverLog.txt", true), "utf-8"));
            synchronized(writer){
                writer.write(outputString + "\r\n");
            }
        } catch (IOException ex){
            System.out.println("cannot create log file");
        }
    }
}

Is this an acceptable way of ensuring no more than one thread is writing to the same file at the same time?

If a thread calls one of these methods and enters the sync block, then what happens if another thread comes along and tries to execute the same method. When it tries to use the local variable writer, will it try and obtain the same object that has been locked by the other thread and therefore block? I would have thought that it would simply create its own separate variable, which would mean I should make writer a static class variable instead?

Answer 1

Since there are separate log files, I don’t see why you need to have class-level synchronization. Seems like a needless bottleneck. I’d provide sync for each method separately (since it’s fine for them to hit separate files simultaneously):

public class Logger
{
    private static final Object GAME_LOG_LOCK = new Object();
    private static final Object SERVER_LOG_LOCK = new Object();

    public static void printToGameLog(String value){
        synchronized (GAME_LOG_LOCK) {
            Writer writer = null;
            try {
                writer = new BufferedWriter(new OutputStreamWriter(
                    new FileOutputStream("GameLog.txt", true), "utf-8"));
                writer.write(outputString + "\r\n");
            } catch (IOException ex){
                System.out.println("cannot create log file");
            } 
        }
    }

    public static void printToServerLog(String value){
        synchronized (SERVER_LOG_LOCK) {   
            Writer writer = null;
            try {
                writer = new BufferedWriter(new OutputStreamWriter(
                    new FileOutputStream("serverLog.txt", true), "utf-8"));
                writer.write(outputString + "\r\n");
            } catch (IOException ex){
                  System.out.println("cannot create log file");
            }
        }
    }
}