Why is using BufferedInputStream to read a file byte by byte faster than using FileInputStream

本文解释了使用BufferedInputStream按字节读取文件为何比FileInputStream更快。通过内部缓存大量数据减少对操作系统的调用次数，从而提高效率。

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refer to http://stackoverflow.com/questions/18600331/why-is-using-bufferedinputstream-to-read-a-file-byte-by-byte-faster-than-using-f

In FileInputStream, the method read() reads a single byte. From the source code:

/**
 * Reads a byte of data from this input stream. This method blocks
 * if no input is yet available.
 *
 * @return     the next byte of data, or <code>-1</code> if the end of the
 *             file is reached.
 * @exception  IOException  if an I/O error occurs.
 */
public native int read() throws IOException;

This is a native call to the OS which uses the disk to read the single byte. This is a heavy operation.

With a BufferedInputStream, the method delegates to an overloaded read() method that reads 8192 amount of bytes and buffers them until they are needed. It still returns only the single byte (but keeps the others in reserve). This way the BufferedInputStream makes less native calls to the OS to read from the file.

For example, your file is 32768 bytes long. To get all the bytes in memory with a FileInputStream, you will require 32768 native calls to the OS. With a BufferedInputStream, you will only require 4, regardless of the number of read() calls you will do (still 32768).

As to how to make it faster, you might want to consider Java 7's NIO FileChannel class, but I have no evidence to support this.

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Aah I see, I should have checked the API first before asking. So it's simply an 8K internal buffer. That makes sense. Thanks. As for the "more efficient" part, it's not necessary, but I thought my code might have been overly redundant in some way. I guess it's not. – ZimZim Sep 3 '13 at 21:59

7

@user1007059 You're welcome. Note that if you used FileInputStream's read(byte[], int, int) method directly instead, with a byte[>8192] you wouldn't need a BufferedInputStream wrapping it. – Sotirios Delimanolis Sep 4 '13 at 4:01

@SotiriosDelimanolis When to use read() byte by byte and when to use read(byte[]) array of byte. As I think reading array is always better. then can you give me example where to use read() byte by byte OR read(byte[]) array of byte. OR BufferedInputStream.? – UnKnown Apr 1 at 13:47

@UnKnown Don't have a great example. Maybe the first byte contains some flag about the content of the file or some other metadata. I don't think anyone would ever read an entire file using read(). – Sotirios Delimanolis Apr 27 at 19:04