Java 中的线程 | 等待（），通知（）和通知所有（）

Question

I have an interface as below(assume it's a Java Interface and methods with void return types)

interface FileAccessOperations{
    fun requestReadAccess()
    fun releaseReadAccess()
    fun requestWriteAccess()
    fun releaseWriteAccess()
}

Now I have to implement these methods where I need to manage single file's read/write permission between large number of threads.

I have implemented as below.

public class FileOperationsImpl implements FileAccessOperations {

    /* When count is
     * (equals 0) -> File is free, no thread is accessing it (Write and read request is possible)
     * (greater than 0) -> File is being read by #count of threads.(Reads are still possible. No write)
     * (lesser than 0) -> File is in write mode.(Write/Read is not possible)
     *
     * So basically, the file can read by multiple threads at a time.
     * But only one thread can write at a given time.
     */
    private int count = 0;

    @Override
    synchronized public void requestReadAccess() {
        while(count < 0){
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        count++;
    }

    @Override
    synchronized public void releaseReadAccess() {
        if(count > 0){
            count--;
        }
        if(count == 0){
            notifyAll();
        }
    }

    @Override
    synchronized public void requestWriteAccess() {
        while(count != 0){
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        count--;
    }

    @Override
    synchronized public void releaseWriteAccess() {
        count = 0;
        notifyAll();
    }
}

Did I do it right?

Answer 1

I see several issues with your implementation:

1. releaseReadAccess does not check if any read access is actually held, and thus can acquire write access (make count go negative).

2. releaseWriteAccess has a similar problem, it can increase count beyond 0, creating false read lock.

3. If requestWriteAccess is called while any lock exists, it will block until lock is released, but then do nothing.
   Since this function does not return any value, the calling thread will never know it didn't actually get the lock.

4. Similar issue with requestReadAccess - it will wait until it can grant the lock, but won't increment count , so no lock will be granted.

Generally speaking, it is a better design practice to have request functions return true or false to indicate weather lock was successfully obtained, even if they block, just in case there are some error conditions.