How to check whether an OutputStream is closed

Question

Is there anyway to check whether an OutputStream is closed without attempting to write to it and catching the IOException?

For example, consider the fol

Answer 1

No. If you implement your own, you could write an isClosed method, but if you don't know the concrete class, then no. OutputStream is just an abstract class. Here's it's implementation:

   /**
 * Closes this output stream and releases any system resources 
 * associated with this stream. The general contract of <code>close</code> 
 * is that it closes the output stream. A closed stream cannot perform 
 * output operations and cannot be reopened.
 * <p>
 * The <code>close</code> method of <code>OutputStream</code> does nothing.
 *
 * @exception  IOException  if an I/O error occurs.
 */
public void close() throws IOException {
}

Answer 2

The underlying stream may not know it its closed until you attempt to write to it (e.g. if the other end of a socket closes it)

The simplest approach is to use it and handle what happens if it closed then, rather than testing it first as well.

No matter what you test, there is always the chance you will get an IOException, so you cannot avoid the exception handling code. Adding this test is likely to complicate the code.

Answer 3

Unfortunately OutputStream API does not have method like isClosed().

So, I know only one clear way: create your class StatusKnowingOutputStream that wraps any other output stream and implements its close() method as following:

public void close() {
    out.close();
    closed = true;
}

Now add method isClosed()

public boolean isClosed() {
    return closed;
}

Answer 4

by using out.checkError()

while(!System.out.checkError()) {
    System.out.println('hi');
}

found it here: How do I get java to exit when piped to head

Answer 5

If you're doing this in tests, use mockito Spys, then do a verify

I had a test effectively

import static org.mockito.Mockito.spy;
import static org.mockito.Mockito.times;
import static org.mockito.Mockito.verify;

import org.junit.Test;
import java.io.InputStream;

class MyTest {
    @Test
    public void testInputStreamCloseCalled() throws IOException {
        final InputStream spied = spy(...)
    
        // Do something that should call .close() on the spied InputStream
        spied.close()

        verify(spied, times(1)).close();
    }
}

Where ... is the input stream you're acting upon.

Would work with OutputStreams too.

Answer 6

public boolean isStreamClosed(FileOutputStream out){
    try {
        FileChannel fc = out.getChannel();
        return fc.position() >= 0L; // This may throw a ClosedChannelException.
    } catch (java.nio.channels.ClosedChannelException cce) {
        return false;
    } catch (IOException e) {
    }
    return true;
}

This is possible only for a FileOutputStream!