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Apache Commons VFS is a Virtual File System library.
There is a newer version: 2.9.0
/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.vfs2.util;

import java.io.BufferedOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.util.concurrent.atomic.AtomicBoolean;

import org.apache.commons.vfs2.FileSystemException;

/**
 * An OutputStream that provides buffering and end-of-stream monitoring.
 */
public class MonitorOutputStream extends BufferedOutputStream {
    private final AtomicBoolean finished = new AtomicBoolean(false);

    public MonitorOutputStream(final OutputStream out) {
        super(out);
    }

    /**
     * Closes this output stream.
     * 
     * This makes sure the buffers are flushed, close the output stream and it will call {@link #onClose()} and re-throw
     * last exception from any of the three.
     * 

     * This does nothing if the stream is closed already.
     *
     * @throws IOException if an error occurs.
     */
    @Override
    public void close() throws IOException {
        // do not use super.close()
        // on Java 8 it might throw self suppression, see JDK-8042377
        // in older Java it silently ignores flush() errors
        if (finished.getAndSet(true)) {
            return;
        }

        IOException exc = null;

        // flush the buffer and out stream
        try {
            super.flush();
        } catch (final IOException ioe) {
            exc = ioe;
        }

        // close the out stream without using super.close()
        try {
            super.out.close();
        } catch (final IOException ioe) {
            exc = ioe;
        }

        // Notify of end of output
        try {
            onClose();
        } catch (final IOException ioe) {
            exc = ioe;
        }

        if (exc != null) {
            throw exc;
        }
    }

    /**
     * @param b The character to write.
     * @throws IOException if an error occurs.
     * @since 2.0
     */
    @Override
    public synchronized void write(final int b) throws IOException {
        assertOpen();
        super.write(b);
    }

    /**
     * @param b The byte array.
     * @param off The offset into the array.
     * @param len The number of bytes to write.
     * @throws IOException if an error occurs.
     * @since 2.0
     */
    @Override
    public synchronized void write(final byte[] b, final int off, final int len) throws IOException {
        assertOpen();
        super.write(b, off, len);
    }

    /**
     * @throws IOException if an error occurs.
     * @since 2.0
     */
    @Override
    public synchronized void flush() throws IOException {
        assertOpen();
        super.flush();
    }

    /**
     * @param b The byte array.
     * @throws IOException if an error occurs.
     * @since 2.0
     */
    @Override
    public void write(final byte[] b) throws IOException {
        assertOpen();
        super.write(b);
    }

    /**
     * Check if file is still open.
     * 

     * This is a workaround for an oddity with Java's BufferedOutputStream where you can write to even if the stream has
     * been closed.
     *
     * @throws FileSystemException if already closed.
     * @since 2.0
     */
    protected void assertOpen() throws FileSystemException {
        if (finished.get()) {
            throw new FileSystemException("vfs.provider/closed.error");
        }
    }

    /**
     * Called after this stream is closed.
     * 
     * This implementation does nothing.
     *
     * @throws IOException if an error occurs.
     */
    // IOException is needed because subclasses may need to throw it
    protected void onClose() throws IOException {
    }
}