Java OutputStream 缓冲区大小

Question

Java 中的OutputStream 有一个名为flush() 的方法。根据其文档:

Flushes this output stream and forces any buffered output bytes to be written out.

我如何了解这个缓冲区有多少多字节容量？

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额外说明:我从 HttpURLConnection getOutputStream() 方法中获得了自己的 OutputStream。

Answer 1

这取决于您使用的 OutputStream 类型。

让我们从基础开始，分析 OutputStream 的刷新建议作为契约(Contract):

public void flush() throws IOException

Flushes this output stream and forces any buffered output bytes to be written out. The general contract of flush is that calling it is an indication that, if any bytes previously written have been buffered by the implementation of the output stream, such bytes should immediately be written to their intended destination.

If the intended destination of this stream is an abstraction provided by the underlying operating system, for example a file, then flushing the stream guarantees only that bytes previously written to the stream are passed to the operating system for writing; it does not guarantee that they are actually written to a physical device such as a disk drive.

The flush method of OutputStream does nothing.

如果你看到 OutputStream 的 flush 方法，它实际上什么也没做:

public void flush() throws IOException {
}

想法是，装饰 OutputStream 的实现必须处理其刷新，然后将其级联到其他 OutputStreams，直到它到达操作系统(如果是这种情况)。

所以它做了一些事情!通过实现它的人。具体类将覆盖 flush 以执行诸如将数据移动到磁盘或通过网络发送数据(您的情况)之类的操作。

如果检查 BufferedOutputStream 的刷新:

/**
 * Flushes this buffered output stream. This forces any buffered
 * output bytes to be written out to the underlying output stream.
 *
 * @exception  IOException  if an I/O error occurs.
 * @see        java.io.FilterOutputStream#out
 */
public synchronized void flush() throws IOException {
    flushBuffer();
    out.flush();
}

/** Flush the internal buffer */
private void flushBuffer() throws IOException {
    if (count > 0) {
        out.write(buf, 0, count);
        count = 0;
    }
}

您可能会看到它正在将自己缓冲区的内容写入包装的 OutputStream。你可以看到它的缓冲区的默认大小(或者你可以改变它)，看到它的构造函数:

/**
 * The internal buffer where data is stored.
 */
protected byte buf[];

/**
 * Creates a new buffered output stream to write data to the
 * specified underlying output stream.
 *
 * @param   out   the underlying output stream.
 */
public BufferedOutputStream(OutputStream out) {
    this(out, 8192);
}

/**
 * Creates a new buffered output stream to write data to the
 * specified underlying output stream with the specified buffer
 * size.
 *
 * @param   out    the underlying output stream.
 * @param   size   the buffer size.
 * @exception IllegalArgumentException if size <= 0.
 */
public BufferedOutputStream(OutputStream out, int size) {
    super(out);
    if (size <= 0) {
        throw new IllegalArgumentException("Buffer size <= 0");
    }
    buf = new byte[size];
}

因此，BufferedInputStream 缓冲区的默认大小为 8192 字节。

既然您已经掌握了要点，请查看 HttpURLConnection 中为您使用的 OutputStream 代码，以熟悉其缓冲区(如果它有一个).

在您的 Java 之旅中，您可能会得到一些将刷新操作委托(delegate)给操作系统的 native 代码。在那种情况下，您可能必须检查您的操作系统是否正在使用某个缓冲区以及在处理 IO 时它的大小是多少。我知道这部分答案听起来可能很宽泛，但事实就是如此。您需要知道您正在使用什么才能了解​​其背后的原因。

看看这个问题: What is the purpose of flush() in Java streams?

还有这篇文章: http://www.oracle.com/technetwork/articles/javase/perftuning-137844.html

干杯!