How to know if a BufferedReader Stream is closed

问题:

I have two threads in Java.

First thread is closing a bufferedreader (br.close())

When the second thread does a read on the same reader I get an IOException (Stream Closed)

I get this exception even if I use br.ready()

Is there a way to know if the stream is already closed?

回答1:

The method ready() will throw an exception if closed. But even if you added a closed check method, so long as the lock is released between calls (which it is for BufferedReader), the reader might be closed by the time you read it.

I see three options:

1. Wrap your read call with a try/catch block to handle the closed case.
2. Create a subclass of BufferedReader that extends close() to set your own variable that can be used to check to see if the reader is closed. This also requires overriding a lot of methods to do whatever the behavior you want with a closed reader if you want it do something beside throw the IOException.
3. Add a lock of your own and use it to both close the reader (one thread) and check that the buffer is ready and read from it. You can either set a variable directly for the check or just group the ready() and read() calls into the same synchronized block.

回答2:

I don't think there's any method you can call directly to tell if a stream is closed.

If you really need to have two threads sharing this reader, your best option might be to have the one thread call back to the other thread or set a flag to notify it that the stream has been closed so the other thread knows not to try to read from it.

回答3:

If you have already started a read operation, then you will notified with an IOException that a stream is closed. Even if you have called br.ready() before, the exception is happening when your code is blocked at the read method.

There is no way to avoid this. On the contrary, you should expect a read operation to throw an exception and be prepared to handle it appropriately.

回答4:

Another way would be to extend a BufferedReader in your own class, overriding close() method to indicate if it was closed.

回答5:

Indeed, looking at ensureOpen() it checks if the in object is not null. Since it is private we can't access it from outside. But if you really need it, you can use reflection.

 BufferedReader myBR  = new BufferedReader(new FileReader("c:/somefile.txt"));
 myBR.close();

 Class c = myBR.getClass();
 Field in = c.getDeclaredField("in");
 in.setAccessible(true);         
 Object inReader = in.get(myBR);


 if(inReader == null){
     System.out.println("is closed");
 }
 else{
     System.out.println("is open");
 }

回答6:

If only things that read from BufferedReader and close it are your threads, I would just make synchronized section and set some flat when closing the Reader. So in each thread you should:

1. Open synchronized section.
2. Check if stream isn't closed already (check the flag).
3. Read from stream.
4. Close stream.
5. Set the flag.
6. End synchronized section.

Just be careful and avoid anything that could hang in synchronized section. Basically put there only what is necessary, only read data, process it later.