gzipoutputstream.java

java源代码 请看看啊 提点宝贵的意见

/*
 * @(#)GZIPOutputStream.java	1.21 03/01/23
 *
 * Copyright 2003 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util.zip;

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

/**
 * This class implements a stream filter for writing compressed data in
 * the GZIP file format.
 * @version 	1.21, 01/23/03
 * @author 	David Connelly
 *
 */
public
class GZIPOutputStream extends DeflaterOutputStream {
    /**
     * CRC-32 of uncompressed data.
     */
    protected CRC32 crc = new CRC32();

    /*
     * GZIP header magic number.
     */
    private final static int GZIP_MAGIC = 0x8b1f;

    /*
     * Trailer size in bytes.
     *
     */
    private final static int TRAILER_SIZE = 8;

    /**
     * Creates a new output stream with the specified buffer size.
     * @param out the output stream
     * @param size the output buffer size
     * @exception IOException If an I/O error has occurred.
     * @exception IllegalArgumentException if size is <= 0
     */
    public GZIPOutputStream(OutputStream out, int size) throws IOException {
	super(out, new Deflater(Deflater.DEFAULT_COMPRESSION, true), size);
        usesDefaultDeflater = true;
	writeHeader();
	crc.reset();
    }

    /**
     * Creates a new output stream with a default buffer size.
     * @param out the output stream
     * @exception IOException If an I/O error has occurred.
     */
    public GZIPOutputStream(OutputStream out) throws IOException {
	this(out, 512);
    }

    /**
     * Writes array of bytes to the compressed output stream. This method
     * will block until all the bytes are written.
     * @param buf the data to be written
     * @param off the start offset of the data
     * @param len the length of the data
     * @exception IOException If an I/O error has occurred.
     */
    public synchronized void write(byte[] buf, int off, int len)
	throws IOException
    {
	super.write(buf, off, len);
	crc.update(buf, off, len);
    }

    /**
     * Finishes writing compressed data to the output stream without closing
     * the underlying stream. Use this method when applying multiple filters
     * in succession to the same output stream.
     * @exception IOException if an I/O error has occurred
     */
    public void finish() throws IOException {
	if (!def.finished()) {
	    def.finish();
	    while (!def.finished()) {
                int len = def.deflate(buf, 0, buf.length);
                if (def.finished() && len <= buf.length - TRAILER_SIZE) {
                    // last deflater buffer. Fit trailer at the end 
                    writeTrailer(buf, len);
                    len = len + TRAILER_SIZE;
                    out.write(buf, 0, len);
                    return;
                }
                if (len > 0)
                    out.write(buf, 0, len);
	    }
            // if we can't fit the trailer at the end of the last
            // deflater buffer, we write it separately
            byte[] trailer = new byte[TRAILER_SIZE];
	    writeTrailer(trailer, 0);
            out.write(trailer);
	}
    }
  
    /*
     * Writes GZIP member header.
     */

    private final static byte[] header = {
        (byte) GZIP_MAGIC,                // Magic number (short)
        (byte)(GZIP_MAGIC >> 8),          // Magic number (short)
        Deflater.DEFLATED,                // Compression method (CM)
        0,                                // Flags (FLG)
        0,                                // Modification time MTIME (int)
        0,                                // Modification time MTIME (int)
        0,                                // Modification time MTIME (int)
        0,                                // Modification time MTIME (int)
        0,                                // Extra flags (XFLG)
        0                                 // Operating system (OS)
    };

    private void writeHeader() throws IOException {
        out.write(header);
    }

    /*
     * Writes GZIP member trailer to a byte array, starting at a given
     * offset.
     */
    private void writeTrailer(byte[] buf, int offset) throws IOException {
        writeInt((int)crc.getValue(), buf, offset); // CRC-32 of uncompr. data
        writeInt(def.getTotalIn(), buf, offset + 4); // Number of uncompr. bytes
    }

    /*
     * Writes integer in Intel byte order to a byte array, starting at a
     * given offset.
     */
    private void writeInt(int i, byte[] buf, int offset) throws IOException {
        writeShort(i & 0xffff, buf, offset);
        writeShort((i >> 16) & 0xffff, buf, offset + 2);
    }

    /*
     * Writes short integer in Intel byte order to a byte array, starting
     * at a given offset
     */
    private void writeShort(int s, byte[] buf, int offset) throws IOException {
        buf[offset] = (byte)(s & 0xff);
        buf[offset + 1] = (byte)((s >> 8) & 0xff);
    }
}