Does this class cause problems in multithreading?

[torpid flare]

public abstract class ObjectCache<T> {
  protected volatile T object;

  protected int expireAt = 0;

  private final ReentrantLock mutex = new ReentrantLock(true);

  public abstract T create();

  protected abstract int getExpireSeconds();

  public T get()
  {
    if(mutex.isLocked() && this.object != null)
    {
      return this.object;
    }

    mutex.lock();
    try
    {
      int unixTimestamp = (int) (System.currentTimeMillis() / 1000L);
      if(expireAt < unixTimestamp)
      {
        this.expireAt = (int) unixTimestamp + getExpireSeconds();
        this.object = create();
      }
    }
    finally
    {
      mutex.unlock();
    }
    return this.object;
  }
}

Does this class cause multithreading problems?

[slim cedar]

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[median wren]

Yes

[torpid flare]

how and why?

and how can i solve it?

[median wren]

Because of if(mutex.isLocked() && this.object != null)

add me fl btw

[torpid flare]

This is okay object != null It's okay even if it's not thread safe.

worst case will enter the mutex again and get the real value.

In other words, I only care about the object being null, as long as it is not null, at most it will enter the mutex again.

I have already calculated it and wrote it like this, 1-2 threads entering the mutex will not cause any problems.

[median wren]

Your class is not thread safe

whatever you calculated

[torpid flare]

So which exception will return here?

[median wren]

Read here: https://www.techtarget.com/searchstorage/definition/race-condition#:~:text=Check-then-act.,read the value as null.

"Check-then-act. " Part is important for you

[upbeat sail]

if(mutex.isLocked() && this.object != null) { return this.object; } is a mistake

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[torpid flare]

I use volatile variable to avoid this

[median wren]

volatile not solve your race condition

You checked the link I posted?

Check-then-act Part of it

If you need more explaination write here