就性能而言，用 BufferedOutputStream 包装 FileOutputStream 在什么时候有意义？

BufferedOutputStream 在写入小于缓冲区大小（例如 8 KB）时提供帮助。对于较大的写入，它无济于事，也不会使情况变得更糟。如果所有写入都大于缓冲区大小，或者每次写入后您总是 flush（），我不会使用缓冲区。但是，如果很大一部分写入小于缓冲区大小，并且您不是每次都使用 flush（），则值得拥有。

您可能会发现将缓冲区大小增加到 32 KB 或更大会带来边际改进，或者使其变得更糟。断续器

您可能会发现 BufferedOutputStream.write 的代码很有用

/**
 * Writes <code>len</code> bytes from the specified byte array
 * starting at offset <code>off</code> to this buffered output stream.
 *
 * <p> Ordinarily this method stores bytes from the given array into this
 * stream's buffer, flushing the buffer to the underlying output stream as
 * needed.  If the requested length is at least as large as this stream's
 * buffer, however, then this method will flush the buffer and write the
 * bytes directly to the underlying output stream.  Thus redundant
 * <code>BufferedOutputStream</code>s will not copy data unnecessarily.
 *
 * @param      b     the data.
 * @param      off   the start offset in the data.
 * @param      len   the number of bytes to write.
 * @exception  IOException  if an I/O error occurs.
 */
public synchronized void write(byte b[], int off, int len) throws IOException {
    if (len >= buf.length) {
        /* If the request length exceeds the size of the output buffer,
           flush the output buffer and then write the data directly.
           In this way buffered streams will cascade harmlessly. */
        flushBuffer();
        out.write(b, off, len);
        return;
    }
    if (len > buf.length - count) {
        flushBuffer();
    }
    System.arraycopy(b, off, buf, count, len);
    count += len;
}

我最近一直在尝试探索 IO 性能。从我所观察到的，直接写到a已经导致了更好的结果;我将其归因于 的本地调用。此外，我还观察到，当 的延迟开始收敛于直接延迟时，它会波动得多，即它可以突然加倍（我还没有能够找出原因）。FileOutputStreamFileOutputStreamwrite(byte[], int, int)BufferedOutputStreamFileOutputStream

附言：我正在使用Java 8，现在无法评论我的观察结果是否适用于以前的Java版本。

这是我测试的代码，其中我的输入是一个大约10KB的文件

public class WriteCombinationsOutputStreamComparison {
    private static final Logger LOG = LogManager.getLogger(WriteCombinationsOutputStreamComparison.class);

public static void main(String[] args) throws IOException {

    final BufferedInputStream input = new BufferedInputStream(new FileInputStream("src/main/resources/inputStream1.txt"), 4*1024);
    final ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
    int data = input.read();
    while (data != -1) {
        byteArrayOutputStream.write(data); // everything comes in memory
        data = input.read();
    }
    final byte[] bytesRead = byteArrayOutputStream.toByteArray();
    input.close();

    /*
     * 1. WRITE USING A STREAM DIRECTLY with entire byte array --> FileOutputStream directly uses a native call and writes
     */
    try (OutputStream outputStream = new FileOutputStream("src/main/resources/outputStream1.txt")) {
        final long begin = System.nanoTime();
        outputStream.write(bytesRead);
        outputStream.flush();
        final long end = System.nanoTime();
        LOG.info("Total time taken for file write, writing entire array [nanos=" + (end - begin) + "], [bytesWritten=" + bytesRead.length + "]");
        if (LOG.isDebugEnabled()) {
            LOG.debug("File reading result was: \n" + new String(bytesRead, Charset.forName("UTF-8")));
        }
    }

    /*
     * 2. WRITE USING A BUFFERED STREAM, write entire array
     */

    // changed the buffer size to different combinations --> write latency fluctuates a lot for same buffer size over multiple runs
    try (BufferedOutputStream outputStream = new BufferedOutputStream(new FileOutputStream("src/main/resources/outputStream1.txt"), 16*1024)) {
        final long begin = System.nanoTime();
        outputStream.write(bytesRead);
        outputStream.flush();
        final long end = System.nanoTime();
        LOG.info("Total time taken for buffered file write, writing entire array [nanos=" + (end - begin) + "], [bytesWritten=" + bytesRead.length + "]");
        if (LOG.isDebugEnabled()) {
            LOG.debug("File reading result was: \n" + new String(bytesRead, Charset.forName("UTF-8")));
        }
    }
}
}

输出：

2017-01-30 23:38:59.064 [INFO] [main] [WriteCombinationsOutputStream] - Total time taken for file write, writing entire array [nanos=100990], [bytesWritten=11059]

2017-01-30 23:38:59.086 [INFO] [main] [WriteCombinationsOutputStream] - Total time taken for buffered file write, writing entire array [nanos=142454], [bytesWritten=11059]