commons-io之ThresholdingOutputStream 和 DeferredFileOutputStream

1. ThresholdingOutputStream
这个类的意图主要是可以处理设置了临界值的OutputStream做出处理，
它当然是复写了三个write方法。
有三个步骤：
             1. 检测是否到达临界值
             2. 调用底层的Stream写数据
             3. 改变已写数据的游标written
里面有两个抽象的方法：
protected abstract OutputStream getStream()
protected abstract void thresholdReached()
意图分别是得到底层的Stream，一个是可以让子类在threshold到达后可以做出响应。


2. DeferredFileOutputStream
是ThresholdingOutputStream的子类
它的意图也很明确，主要是实现延迟写。
首先是数据会写到基于内存的OutStream, 当到达threshold后，会将内存中的数据又
写到指定的文件里面。这个类里面比较有亮点的是thresholdReached函数的实现，
首先将memoryOutputStream里面的数据写到目标文件的FileOutputStream里面，
将memoryOutputStream设置成null，然后将currentOutputStream更新为目标文件的
FileOutputStream，这样后续的数据就会写入文件中。

package org.apache.commons.io.output;

import java.io.IOException;
import java.io.OutputStream;


/**
 * An output stream which triggers an event when a specified number of bytes of
 * data have been written to it. The event can be used, for example, to throw
 * an exception if a maximum has been reached, or to switch the underlying
 * stream type when the threshold is exceeded.
 * <p>
 * This class overrides all <code>OutputStream</code> methods. However, these
 * overrides ultimately call the corresponding methods in the underlying output
 * stream implementation.
 * <p>
 * NOTE: This implementation may trigger the event <em>before</em> the threshold
 * is actually reached, since it triggers when a pending write operation would
 * cause the threshold to be exceeded.
 *
 * @author <a href="mailto:[e-mail]">Martin Cooper</a>
 *
 * @version $Id: ThresholdingOutputStream.java 540714 2007-05-22 19:39:44Z niallp $
 */
public abstract class ThresholdingOutputStream
    extends OutputStream
{

    // ----------------------------------------------------------- Data members


    /**
     * The threshold at which the event will be triggered.
     */
    private int threshold;


    /**
     * The number of bytes written to the output stream.
     */
    private long written;


    /**
     * Whether or not the configured threshold has been exceeded.
     */
    private boolean thresholdExceeded;


    // ----------------------------------------------------------- Constructors


    /**
     * Constructs an instance of this class which will trigger an event at the
     * specified threshold.
     *
     * @param threshold The number of bytes at which to trigger an event.
     */
    public ThresholdingOutputStream(int threshold)
    {
        this.threshold = threshold;
    }


    // --------------------------------------------------- OutputStream methods


    /**
     * Writes the specified byte to this output stream.
     *
     * @param b The byte to be written.
     *
     * @exception IOException if an error occurs.
     */
    public void write(int b) throws IOException
    {
        checkThreshold(1);
        getStream().write(b);
        written++;
    }


    /**
     * Writes <code>b.length</code> bytes from the specified byte array to this
     * output stream.
     *
     * @param b The array of bytes to be written.
     *
     * @exception IOException if an error occurs.
     */
    public void write(byte b[]) throws IOException
    {
        checkThreshold(b.length);
        getStream().write(b);
        written += b.length;
    }


    /**
     * Writes <code>len</code> bytes from the specified byte array starting at
     * offset <code>off</code> to this output stream.
     *
     * @param b   The byte array from which the data will be written.
     * @param off The start offset in the byte array.
     * @param len The number of bytes to write.
     *
     * @exception IOException if an error occurs.
     */
    public void write(byte b[], int off, int len) throws IOException
    {
        checkThreshold(len);
        getStream().write(b, off, len);
        written += len;
    }


    /**
     * Flushes this output stream and forces any buffered output bytes to be
     * written out.
     *
     * @exception IOException if an error occurs.
     */
    public void flush() throws IOException
    {
        getStream().flush();
    }


    /**
     * Closes this output stream and releases any system resources associated
     * with this stream.
     *
     * @exception IOException if an error occurs.
     */
    public void close() throws IOException
    {
        try
        {
            flush();
        }
        catch (IOException ignored)
        {
            // ignore
        }
        getStream().close();
    }


    // --------------------------------------------------------- Public methods


    /**
     * Returns the threshold, in bytes, at which an event will be triggered.
     *
     * @return The threshold point, in bytes.
     */
    public int getThreshold()
    {
        return threshold;
    }


    /**
     * Returns the number of bytes that have been written to this output stream.
     *
     * @return The number of bytes written.
     */
    public long getByteCount()
    {
        return written;
    }


    /**
     * Determines whether or not the configured threshold has been exceeded for
     * this output stream.
     *
     * @return <code>true</code> if the threshold has been reached;
     *         <code>false</code> otherwise.
     */
    public boolean isThresholdExceeded()
    {
        return (written > threshold);
    }


    // ------------------------------------------------------ Protected methods


    /**
     * Checks to see if writing the specified number of bytes would cause the
     * configured threshold to be exceeded. If so, triggers an event to allow
     * a concrete implementation to take action on this.
     *
     * @param count The number of bytes about to be written to the underlying
     *              output stream.
     *
     * @exception IOException if an error occurs.
     */
    protected void checkThreshold(int count) throws IOException
    {
        if (!thresholdExceeded && (written + count > threshold))
        {
            thresholdExceeded = true;
            thresholdReached();
        }
    }

    /**
     * Resets the byteCount to zero.  You can call this from 
     * {@link #thresholdReached()} if you want the event to be triggered again. 
     */
    protected void resetByteCount() 
    {
        this.thresholdExceeded = false;
        this.written = 0;
    }

    // ------------------------------------------------------- Abstract methods


    /**
     * Returns the underlying output stream, to which the corresponding
     * <code>OutputStream</code> methods in this class will ultimately delegate.
     *
     * @return The underlying output stream.
     *
     * @exception IOException if an error occurs.
     */
    protected abstract OutputStream getStream() throws IOException;


    /**
     * Indicates that the configured threshold has been reached, and that a
     * subclass should take whatever action necessary on this event. This may
     * include changing the underlying output stream.
     *
     * @exception IOException if an error occurs.
     */
    protected abstract void thresholdReached() throws IOException;
}

package org.apache.commons.io.output;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;

import org.apache.commons.io.IOUtils;


/**
 * An output stream which will retain data in memory until a specified
 * threshold is reached, and only then commit it to disk. If the stream is
 * closed before the threshold is reached, the data will not be written to
 * disk at all.
 * <p>
 * This class originated in FileUpload processing. In this use case, you do
 * not know in advance the size of the file being uploaded. If the file is small
 * you want to store it in memory (for speed), but if the file is large you want
 * to store it to file (to avoid memory issues).
 *
 * @author <a href="mailto:[e-mail]">Martin Cooper</a>
 * @author gaxzerow
 *
 * @version $Id: DeferredFileOutputStream.java 606381 2007-12-22 02:03:16Z ggregory $
 */
public class DeferredFileOutputStream
    extends ThresholdingOutputStream
{

    // ----------------------------------------------------------- Data members


    /**
     * The output stream to which data will be written prior to the theshold
     * being reached.
     */
    private ByteArrayOutputStream memoryOutputStream;


    /**
     * The output stream to which data will be written at any given time. This
     * will always be one of <code>memoryOutputStream</code> or
     * <code>diskOutputStream</code>.
     */
    private OutputStream currentOutputStream;


    /**
     * The file to which output will be directed if the threshold is exceeded.
     */
    private File outputFile;

    /**
     * The temporary file prefix.
     */
    private String prefix;

    /**
     * The temporary file suffix.
     */
    private String suffix;

    /**
     * The directory to use for temporary files.
     */
    private File directory;

    
    /**
     * True when close() has been called successfully.
     */
    private boolean closed = false;

    // ----------------------------------------------------------- Constructors


    /**
     * Constructs an instance of this class which will trigger an event at the
     * specified threshold, and save data to a file beyond that point.
     *
     * @param threshold  The number of bytes at which to trigger an event.
     * @param outputFile The file to which data is saved beyond the threshold.
     */
    public DeferredFileOutputStream(int threshold, File outputFile)
    {
        super(threshold);
        this.outputFile = outputFile;

        memoryOutputStream = new ByteArrayOutputStream();
        currentOutputStream = memoryOutputStream;
    }


    /**
     * Constructs an instance of this class which will trigger an event at the
     * specified threshold, and save data to a temporary file beyond that point.
     *
     * @param threshold  The number of bytes at which to trigger an event.
     * @param prefix Prefix to use for the temporary file.
     * @param suffix Suffix to use for the temporary file.
     * @param directory Temporary file directory.
     *
     * @since Commons IO 1.4
     */
    public DeferredFileOutputStream(int threshold, String prefix, String suffix, File directory)
    {
        this(threshold, (File)null);
        if (prefix == null) {
            throw new IllegalArgumentException("Temporary file prefix is missing");
        }
        this.prefix = prefix;
        this.suffix = suffix;
        this.directory = directory;
    }


    // --------------------------------------- ThresholdingOutputStream methods


    /**
     * Returns the current output stream. This may be memory based or disk
     * based, depending on the current state with respect to the threshold.
     *
     * @return The underlying output stream.
     *
     * @exception IOException if an error occurs.
     */
    protected OutputStream getStream() throws IOException
    {
        return currentOutputStream;
    }


    /**
     * Switches the underlying output stream from a memory based stream to one
     * that is backed by disk. This is the point at which we realise that too
     * much data is being written to keep in memory, so we elect to switch to
     * disk-based storage.
     *
     * @exception IOException if an error occurs.
     */
    protected void thresholdReached() throws IOException
    {
        if (prefix != null) {
            outputFile = File.createTempFile(prefix, suffix, directory);
        }
        FileOutputStream fos = new FileOutputStream(outputFile);
        memoryOutputStream.writeTo(fos);
        currentOutputStream = fos;
        memoryOutputStream = null;
    }


    // --------------------------------------------------------- Public methods


    /**
     * Determines whether or not the data for this output stream has been
     * retained in memory.
     *
     * @return <code>true</code> if the data is available in memory;
     *         <code>false</code> otherwise.
     */
    public boolean isInMemory()
    {
        return (!isThresholdExceeded());
    }


    /**
     * Returns the data for this output stream as an array of bytes, assuming
     * that the data has been retained in memory. If the data was written to
     * disk, this method returns <code>null</code>.
     *
     * @return The data for this output stream, or <code>null</code> if no such
     *         data is available.
     */
    public byte[] getData()
    {
        if (memoryOutputStream != null)
        {
            return memoryOutputStream.toByteArray();
        }
        return null;
    }


    /**
     * Returns either the output file specified in the constructor or
     * the temporary file created or null.
     * <p>
     * If the constructor specifying the file is used then it returns that
     * same output file, even when threashold has not been reached.
     * <p>
     * If constructor specifying a temporary file prefix/suffix is used
     * then the temporary file created once the threashold is reached is returned
     * If the threshold was not reached then <code>null</code> is returned.
     *
     * @return The file for this output stream, or <code>null</code> if no such
     *         file exists.
     */
    public File getFile()
    {
        return outputFile;
    }
    
        
    /**
     * Closes underlying output stream, and mark this as closed
     *
     * @exception IOException if an error occurs.
     */
    public void close() throws IOException
    {
        super.close();
        closed = true;
    }
    
    
    /**
     * Writes the data from this output stream to the specified output stream,
     * after it has been closed.
     *
     * @param out output stream to write to.
     * @exception IOException if this stream is not yet closed or an error occurs.
     */
    public void writeTo(OutputStream out) throws IOException 
    {
        // we may only need to check if this is closed if we are working with a file
        // but we should force the habit of closing wether we are working with
        // a file or memory.
        if (!closed)
        {
            throw new IOException("Stream not closed");
        }
        
        if(isInMemory())
        {
            memoryOutputStream.writeTo(out);
        }
        else
        {
            FileInputStream fis = new FileInputStream(outputFile);
            try {
                IOUtils.copy(fis, out);
            } finally {
                IOUtils.closeQuietly(fis);
            }
        }
    }
}