java method synchronization and read/write mutual exclusion

Question

I have two methods read() and write() as below in a class.

class Store
{

  public void write()
  {
    // write to store;
  }

  public string  read()
  {
    // read from store;
  }
}

1) The Store object is a singleton.

2) I have a Writer class which will write to the store and several Reader classes which will read from the store at the same time.

My requirement is that when the writer is writing to the store, all the readers should wait. ie, when control is in write() , all the calls to read() should be blocked. How do I achieve this? I have tried synchronize(Store.class) in the write() method, but doesn't seem like work for me.

Answer 1

The best option in this case is to use a reader-writer lock: ReadWriteLock . It allows a single writer, but multiple concurrent readers, so it's the most efficient mechanism for this type of scenario.

Some sample code:

class Store
{
    private ReadWriteLock rwlock = new ReentrantReadWriteLock();

    public void write()
    {
       rwlock.writeLock().lock();
       try {
          write to store;
       } finally {
          rwlock.writeLock().unlock();
       }
    }

    public String read()
    {
       rwlock.readLock().lock();
       try {
          read from store;
       } finally {
          rwlock.readLock().unlock();
       }
    }
}

Answer 2

synchronized is the simplest solution so you should explain what you mean by "doesn't seem like work for me" and perhaps show us how you used it.

The better solution is to use ReentrantReadWriteLock because it allows concurrent access to readers as well.

Answer 3

When a method is synchronized on some object, a thread executing this method blocks all the other threads that try to enter another block/method that is synchronized on the same object .

So, if you want the writer threads to forbid any reader thread to read, you must synchronize both the write and the read method. There is no reason to synchronize on the Store class. Synchronizing on the Store object is more natural:

public synchronized void write() {
  write to store;
}

public synchronized String read() {
  read from store;
}

This, however, (maybe) has a drawback: it also forbids two reader threads to read at the same time. If you really need this to happen, you should use a ReadWriteLock . But this will lead to code that is less performant, and harder to understand and maintain. I would only use it if I have measured that this is needed.

Answer 4

使用 java.util.concurrent 包中的 ReadWriteLock

Answer 5

Take care to read this article:

https://blog.takipi.com/java-8-stampedlocks-vs-readwritelocks-and-synchronized/

• RWLocks can be slow especially when there are many readers to writers
• synchronized is on average the most reliably fast
• there are new locks like StampedLock