在 Java 中强制刷新 GZIPOutputStream

Question

我们正在开发一个需要刷新（强制压缩和发送数据）GZIPOutputStream 的程序。 问题是， GZIPOutputStream 的刷新方法没有按预期工作（强制压缩并发送数据），而是 Stream 等待更多数据以进行有效的数据压缩。

当您调用完成时，数据被压缩并通过输出流发送，但 GZIPOutputStream（不是底层流）将关闭，因此我们无法写入更多数据，直到我们创建新的 GZIPOutputStream，这会消耗时间和性能。

希望任何人都可以帮助解决这个问题。

最好的问候。

Answer 1

我还没有尝试过这个，这个建议在我们手头有 Java 7 之前不会有用，但是从DeflaterOutputStream继承的GZIPOutputStream的flush()方法的文档依赖于在构建时使用syncFlush指定的刷新模式参数（与Deflater#SYNC_FLUSH相关）来决定是否刷新待压缩的待处理数据。 这个syncFlush参数在构建时也被GZIPOutputStream接受。

听起来您想使用Deflator#SYNC_FLUSH甚至Deflater#FULL_FLUSH ，但是，在深入研究之前，首先尝试使用两个参数或四个参数的GZIPOutputStream构造函数，并为syncFlush参数传递true 。 这将激活您想要的冲洗行为。

Answer 2

我没有找到其他工作答案。 它仍然拒绝刷新，因为 GZIPOutputStream 使用的本机代码保留了数据。

幸运的是，我发现有人实现了 FlushableGZIPOutputStream 作为 Apache Tomcat 项目的一部分。 这是神奇的部分：

@Override
public synchronized void flush() throws IOException {
    if (hasLastByte) {
        // - do not allow the gzip header to be flushed on its own
        // - do not do anything if there is no data to send

        // trick the deflater to flush
        /**
         * Now this is tricky: We force the Deflater to flush its data by
         * switching compression level. As yet, a perplexingly simple workaround
         * for
         * http://developer.java.sun.com/developer/bugParade/bugs/4255743.html
         */
        if (!def.finished()) {
            def.setLevel(Deflater.NO_COMPRESSION);
            flushLastByte();
            flagReenableCompression = true;
        }
    }
    out.flush();
}

你可以在这个 jar 中找到整个类（如果你使用 Maven）：

<dependency>
    <groupId>org.apache.tomcat</groupId>
    <artifactId>tomcat-coyote</artifactId>
    <version>7.0.8</version>
</dependency>

或者只是去获取源代码FlushableGZIPOutputStream.java

它是在 Apache-2.0 许可下发布的。

Answer 3

这段代码在我的应用程序中对我很有用。

public class StreamingGZIPOutputStream extends GZIPOutputStream {

    public StreamingGZIPOutputStream(OutputStream out) throws IOException {
        super(out);
    }

    @Override
    protected void deflate() throws IOException {
        // SYNC_FLUSH is the key here, because it causes writing to the output
        // stream in a streaming manner instead of waiting until the entire
        // contents of the response are known.  for a large 1 MB json example
        // this took the size from around 48k to around 50k, so the benefits
        // of sending data to the client sooner seem to far outweigh the
        // added data sent due to less efficient compression
        int len = def.deflate(buf, 0, buf.length, Deflater.SYNC_FLUSH);
        if (len > 0) {
            out.write(buf, 0, len);
        }
    }

}

Answer 4

Android也有同样的问题。 接受者的回答不起作用，因为def.setLevel(Deflater.NO_COMPRESSION); 抛出异常。 根据flush方法，它改变了Deflater压缩级别。 所以我认为应该在写入数据之前调用更改压缩，但我不确定。

还有 2 个其他选项：

• 如果您的应用程序的 API 级别高于 19，那么您可以尝试使用带有 syncFlush 参数的构造函数
• 另一个解决方案是使用jzlib 。

Answer 5

错误 ID 4813885处理此问题。 2006 年 9 月 9 日提交的“DamonHD”评论（大约是错误报告的一半）包含一个FlushableGZIPOutputStream示例，该示例构建在Jazzlib 的net.sf.jazzlib.DeflaterOutputStream之上。

作为参考，这里有一个（重新格式化的）摘录：

/**
 * Substitute for GZIPOutputStream that maximises compression and has a usable
 * flush(). This is also more careful about its output writes for efficiency,
 * and indeed buffers them to minimise the number of write()s downstream which
 * is especially useful where each write() has a cost such as an OS call, a disc
 * write, or a network packet.
 */
public class FlushableGZIPOutputStream extends net.sf.jazzlib.DeflaterOutputStream {
    private final CRC32 crc = new CRC32();
    private final static int GZIP_MAGIC = 0x8b1f;
    private final OutputStream os;

    /** Set when input has arrived and not yet been compressed and flushed downstream. */
    private boolean somethingWritten;

    public FlushableGZIPOutputStream(final OutputStream os) throws IOException {
        this(os, 8192);
    }

    public FlushableGZIPOutputStream(final OutputStream os, final int bufsize) throws IOException {
        super(new FilterOutputStream(new BufferedOutputStream(os, bufsize)) {
            /** Suppress inappropriate/inefficient flush()es by DeflaterOutputStream. */
            @Override
            public void flush() {
            }
        }, new net.sf.jazzlib.Deflater(net.sf.jazzlib.Deflater.BEST_COMPRESSION, true));
        this.os = os;
        writeHeader();
        crc.reset();
    }

    public synchronized void write(byte[] buf, int off, int len) throws IOException {
        somethingWritten = true;
        super.write(buf, off, len);
        crc.update(buf, off, len);
    }

    /**
     * Flush any accumulated input downstream in compressed form. We overcome
     * some bugs/misfeatures here so that:
     * <ul>
     * <li>We won't allow the GZIP header to be flushed on its own without real compressed
     * data in the same write downstream. 
     * <li>We ensure that any accumulated uncompressed data really is forced through the 
     * compressor.
     * <li>We prevent spurious empty compressed blocks being produced from successive 
     * flush()es with no intervening new data.
     * </ul>
     */
    @Override
    public synchronized void flush() throws IOException {
        if (!somethingWritten) { return; }

        // We call this to get def.flush() called,
        // but suppress the (usually premature) out.flush() called internally.
        super.flush();

        // Since super.flush() seems to fail to reliably force output, 
        // possibly due to over-cautious def.needsInput() guard following def.flush(),
        // we try to force the issue here by bypassing the guard.
        int len;
        while((len = def.deflate(buf, 0, buf.length)) > 0) {
            out.write(buf, 0, len);
        }

        // Really flush the stream below us...
        os.flush();

        // Further flush()es ignored until more input data data written.
        somethingWritten = false;
    }

    public synchronized void close() throws IOException {
        if (!def.finished()) {
            def.finish();
            do {
                int len = def.deflate(buf, 0, buf.length);
                if (len <= 0) { 
                    break;
                }
                out.write(buf, 0, len);
            } while (!def.finished());
        }

        // Write trailer
        out.write(generateTrailer());

        out.close();
    }

    // ...
}

你可能会发现它很有用。

Answer 6

正如@seh 所说，这很好用：

ByteArrayOutputStream stream = new ByteArrayOutputStream();

// the second param need to be true
GZIPOutputStream gzip = new GZIPOutputStream(stream,  true);
gzip.write( .. );
gzip.flush();

...
gzip.close()