GZIPOutputStream.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
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 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
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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.
 *
 * @author David Connelly
 * @since 1.1
 */
// gzip输出流：读取指定内存处的原始数据，将其压缩为gzip文件后写入最终输出流(适用于对单个文件的压缩)
public class GZIPOutputStream extends DeflaterOutputStream {
    
    /**
     * CRC-32 of uncompressed data.
     */
    protected CRC32 crc = new CRC32();  // 数据校验
    
    /*
     * GZIP header magic number.
     */
    private static final int GZIP_MAGIC = 0x8b1f;   //gzip文件的魔数
    
    /*
     * Trailer size in bytes.
     *
     */
    private static final int TRAILER_SIZE = 8;  // gzip文件尾信息的字节长度
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a new output stream with a default buffer size.
     *
     * <p>The new output stream instance is created as if by invoking
     * the 2-argument constructor GZIPOutputStream(out, false).
     *
     * @param out the output stream
     *
     * @throws IOException If an I/O error has occurred.
     */
    // 用指定的最终输出流构造gzip输出流
    public GZIPOutputStream(OutputStream out) throws IOException {
        this(out, 512, false);
    }
    
    /**
     * Creates a new output stream with the specified buffer size.
     *
     * <p>The new output stream instance is created as if by invoking
     * the 3-argument constructor GZIPOutputStream(out, size, false).
     *
     * @param out  the output stream
     * @param size the output buffer size
     *
     * @throws IOException              If an I/O error has occurred.
     * @throws IllegalArgumentException if {@code size <= 0}
     */
    // 用指定的最终输出流和缓冲区容量构造gzip输出流
    public GZIPOutputStream(OutputStream out, int size) throws IOException {
        this(out, size, false);
    }
    
    /**
     * Creates a new output stream with a default buffer size and
     * the specified flush mode.
     *
     * @param out       the output stream
     * @param syncFlush if {@code true} invocation of the inherited
     *                  {@link DeflaterOutputStream#flush() flush()} method of
     *                  this instance flushes the compressor with flush mode
     *                  {@link Deflater#SYNC_FLUSH} before flushing the output
     *                  stream, otherwise only flushes the output stream
     *
     * @throws IOException If an I/O error has occurred.
     * @since 1.7
     */
    // 用指定的最终输出流构造gzip输出流，syncFlush指示是否开启同步刷新
    public GZIPOutputStream(OutputStream out, boolean syncFlush) throws IOException {
        this(out, 512, syncFlush);
    }
    
    /**
     * Creates a new output stream with the specified buffer size and
     * flush mode.
     *
     * @param out       the output stream
     * @param size      the output buffer size
     * @param syncFlush if {@code true} invocation of the inherited
     *                  {@link DeflaterOutputStream#flush() flush()} method of
     *                  this instance flushes the compressor with flush mode
     *                  {@link Deflater#SYNC_FLUSH} before flushing the output
     *                  stream, otherwise only flushes the output stream
     *
     * @throws IOException              If an I/O error has occurred.
     * @throws IllegalArgumentException if {@code size <= 0}
     * @since 1.7
     */
    /*
     * 构造gzip输出流
     *
     * out       - 最终输出流，压缩后的数据会写入此处
     * size      - 压缩缓冲区容量
     * syncFlush - 是否开启同步刷新
     */
    public GZIPOutputStream(OutputStream out, int size, boolean syncFlush) throws IOException {
        super(out, new Deflater(Deflater.DEFAULT_COMPRESSION, true), size, syncFlush);
        
        usesDefaultDeflater = true; // 使用了具有默认压缩级别的压缩器
        
        // 压缩前：写入gzip文件头信息，主要是写入一个魔数和使用的压缩算法
        writeHeader();
        
        // 重置校验和
        crc.reset();
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 写/压缩 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     *
     * @throws IOException If an I/O error has occurred.
     */
    // 将字节数组buf中off处起的len个字压缩后写入最终输出流。该方法将阻塞，直到可以写入字节为止
    public synchronized void write(byte[] buf, int off, int len) throws IOException {
        super.write(buf, off, len);
        
        // 用字节数组b中off处起的len个字节更新当前校验和
        crc.update(buf, off, len);
    }
    
    /*▲ 写/压缩 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 压缩后 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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.
     *
     * @throws IOException if an I/O error has occurred
     */
    // 结束压缩，并将剩余待压缩数据压缩后写入最终输出流，最后写入gzip文件尾信息
    public void finish() throws IOException {
        // 如果压缩器已经完成压缩，直接返回
        if(def.finished()) {
            return;
        }
        
        // 设置刷新模式为FINISH
        def.finish();
        
        // 如果压缩器还未完成压缩
        while(!def.finished()) {
            // 向压缩缓冲区buf中填充压缩后的数据(默认刷新模式为NO_FLUSH)，返回实际填充的字节数
            int len = def.deflate(buf, 0, buf.length);
            
            // 如果压缩器已经完成压缩，且压缩缓冲区中可以塞得下gzip文件尾信息
            if(def.finished() && len + TRAILER_SIZE<=buf.length) {
                // 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 */
        // 如果前面未能成功写入gzip文件尾信息，则在这里写入
        byte[] trailer = new byte[TRAILER_SIZE];
        writeTrailer(trailer, 0);
        out.write(trailer);
    }
    
    /*▲ 压缩后 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*
     * Writes GZIP member header.
     */
    // 压缩前：写入gzip文件头信息，主要是写入一个魔数和使用的压缩算法
    private void writeHeader() throws IOException {
        byte[] header = new byte[]{
            (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)
        };
        out.write(header);
    }
    
    /*
     * Writes GZIP member trailer to a byte array, starting at a given offset.
     */
    // 压缩后：写入gzip文件尾信息，主要是写入数据校验和，以及累计写入了多少压缩前的字节
    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);
    }
    
}