深入研究BufferedInputStream内幕
1 概述
最近研究JDK源码，发现IO体系中的BufferedInputStream，很有意思，平时对这个类有不少误解，于是写下这篇博客，以供学习
2 BufferedInputStream源码分析
/**
* 此类继承FilterInputStream，该类使用了装饰着设计模式，FilterInputStream的源码超级简单
*/
public class BufferedInputStream extends FilterInputStream {
// 默认的buf[]缓存数组大小
private static int DEFAULT_BUFFER_SIZE = 8192;
/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*
* buf[]缓存数组最大值 为什么会 减去8呢？因为一些JVM会数组头部存一些数据
*/
private static int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8;
/**
* The internal buffer array where the data is stored. When necessary,
* it may be replaced by another array of
* a different size.
*
* 缓存数组，核心成员变量，所有操作都是围绕buf[]
*/
protected volatile byte buf[];
/**
* Atomic updater to provide compareAndSet for buf. This is
* necessary because closes can be asynchronous. We use nullness
* of buf[] as primary indicator that this stream is closed. (The
* "in" field is also nulled out on close.)
*
* 多线程相关，确保操作线程安全
*/
private static final
AtomicReferenceFieldUpdater<BufferedInputStream, byte[]> bufUpdater =
AtomicReferenceFieldUpdater.newUpdater
(BufferedInputStream.class, byte[].class, "buf");
/**
* The index one greater than the index of the last valid byte in
* the buffer.
* This value is always
* in the range <code>0</code> through <code>buf.length</code>;
* elements <code>buf[0]</code> through <code>buf[count-1]
* </code>contain buffered input data obtained
* from the underlying input stream.
*
* buf[]数组中，有效数据的总数
*/
protected int count;
/**
* The current position in the buffer. This is the index of the next
* character to be read from the <code>buf</code> array.
* <p>
* This value is always in the range <code>0</code>
* through <code>count</code>. If it is less
* than <code>count</code>, then <code>buf[pos]</code>
* is the next byte to be supplied as input;
* if it is equal to <code>count</code>, then
* the next <code>read</code> or <code>skip</code>
* operation will require more bytes to be
* read from the contained input stream.
*
* @see java.io.BufferedInputStream#buf
*
* buf[]数组中，当前读取位置
*/
protected int pos;
/**
* The value of the <code>pos</code> field at the time the last
* <code>mark</code> method was called.
* <p>
* This value is always
* in the range <code>-1</code> through <code>pos</code>.
* If there is no marked position in the input
* stream, this field is <code>-1</code>. If
* there is a marked position in the input
* stream, then <code>buf[markpos]</code>
* is the first byte to be supplied as input
* after a <code>reset</code> operation. If
* <code>markpos</code> is not <code>-1</code>,
* then all bytes from positions <code>buf[markpos]</code>
* through <code>buf[pos-1]</code> must remain
* in the buffer array (though they may be
* moved to another place in the buffer array,
* with suitable adjustments to the values
* of <code>count</code>, <code>pos</code>,
* and <code>markpos</code>); they may not
* be discarded unless and until the difference
* between <code>pos</code> and <code>markpos</code>
* exceeds <code>marklimit</code>.
*
* @see java.io.BufferedInputStream#mark(int)
* @see java.io.BufferedInputStream#pos
*
* 最后一次，调用mark方法，标记的位置
*/
protected int markpos = -1;
/**
* The maximum read ahead allowed after a call to the
* <code>mark</code> method before subsequent calls to the
* <code>reset</code> method fail.
* Whenever the difference between <code>pos</code>
* and <code>markpos</code> exceeds <code>marklimit</code>,
* then the mark may be dropped by setting
* <code>markpos</code> to <code>-1</code>.
*
* @see java.io.BufferedInputStream#mark(int)
* @see java.io.BufferedInputStream#reset()
*
* 该变量唯一入口就是mark(int readLimit)，比如调用方法，mark(1024)，那么后面读取的数据如果
* 超过了1024字节，那么此次mark就为无效标记，子类可以选择抛弃该mark标记，从头开始。不过具体实现
* 跟具体的子类有关，在BufferedInputStream中，会抛弃mark标记，重新将markpos赋值为-1
*/
protected int marklimit;
/**
* Check to make sure that underlying input stream has not been
* nulled out due to close; if not return it;
*
* 获取真正的输入流
*/
private InputStream getInIfOpen() throws IOException {
InputStream input = in;
if (input == null)
throw new IOException("Stream closed");
return input;
}
/**
* Check to make sure that buffer has not been nulled out due to
* close; if not return it;
*
* 获取缓存数组
*/
private byte[] getBufIfOpen() throws IOException {
byte[] buffer = buf;
if (buffer == null)
throw new IOException("Stream closed");
return buffer;
}
/**
* Creates a <code>BufferedInputStream</code>
* and saves its argument, the input stream
* <code>in</code>, for later use. An internal
* buffer array is created and stored in <code>buf</code>.
*
* @param in the underlying input stream.
*
* 默认缓存数组大小为8kb
*/
public BufferedInputStream(InputStream in) {
this(in, DEFAULT_BUFFER_SIZE);
}
/**
* Creates a <code>BufferedInputStream</code>
* with the specified buffer size,
* and saves its argument, the input stream
* <code>in</code>, for later use. An internal
* buffer array of length <code>size</code>
* is created and stored in <code>buf</code>.
*
* @param in the underlying input stream.
* @param size the buffer size.
* @exception IllegalArgumentException if {@code size <= 0}.
*/
public BufferedInputStream(InputStream in, int size) {
super(in);
if (size <= 0) {
throw new IllegalArgumentException("Buffer size <= 0");
}
buf = new byte[size];
}
/**
* Fills the buffer with more data, taking into account
* shuffling and other tricks for dealing with marks.
* Assumes that it is being called by a synchronized method.
* This method also assumes that all data has already been read in,
* hence pos > count.
*
* 该方法作用，通过丢弃buf[]数据、增大buf[]数组，以腾出位置，将输入流中新的数据保存到buf[]缓存数组中
*/
private void fill() throws IOException {
byte[] buffer = getBufIfOpen();
if (markpos < 0)
// 因为没有mark标记，直接丢弃buf[]数据
pos = 0; /* no mark: throw away the buffer */
else if (pos >= buffer.length) /* no room left in buffer */
if (markpos > 0) { /* can throw away early part of the buffer */
int sz = pos - markpos;
System.arraycopy(buffer, markpos, buffer, 0, sz);
pos = sz;
markpos = 0;
// ！！！往下执行，markpos全部等于0
} else if (buffer.length >= marklimit) {
markpos = -1; /* buffer got too big, invalidate mark */
pos = 0; /* drop buffer contents */
} else if (buffer.length >= MAX_BUFFER_SIZE) {
throw new OutOfMemoryError("Required array size too large");
} else { /* grow buffer */
int nsz = (pos <= MAX_BUFFER_SIZE - pos) ?
pos * 2 : MAX_BUFFER_SIZE;
if (nsz > marklimit)
// buf[]长度不超过marklimit，这样mark标记始终有效
nsz = marklimit;
byte nbuf[] = new byte[nsz];
System.arraycopy(buffer, 0, nbuf, 0, pos);
if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {
// Can't replace buf if there was an async close.
// Note: This would need to be changed if fill()
// is ever made accessible to multiple threads.
// But for now, the only way CAS can fail is via close.
// assert buf == null;
throw new IOException("Stream closed");
}
buffer = nbuf;
}
count = pos;
// 将输入流中的数据独到buf[]数组中
int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
if (n > 0)
count = n + pos;
}
/**
* See
* the general contract of the <code>read</code>
* method of <code>InputStream</code>.
*
* @return the next byte of data, or <code>-1</code> if the end of the
* stream is reached.
* @exception IOException if this input stream has been closed by
* invoking its {@link #close()} method,
* or an I/O error occurs.
* @see java.io.FilterInputStream#in
*/
public synchronized int read() throws IOException {
// 说明当前buf[]数组大小不够了，需要fill()
if (pos >= count) {
fill();
// 说明没有读取到任何数据
if (pos >= count)
return -1;
}
return getBufIfOpen()[pos++] & 0xff;
}
/**
* Read characters into a portion of an array, reading from the underlying
* stream at most once if necessary.
*/
private int read1(byte[] b, int off, int len) throws IOException {
int avail = count - pos;
if (avail <= 0) {
/* If the requested length is at least as large as the buffer, and
if there is no mark/reset activity, do not bother to copy the
bytes into the local buffer. In this way buffered streams will
cascade harmlessly. */
// ！！！这个位置代码很重要
// ！！！这个位置代码很重要
// ！！！这个位置代码很重要
/**
* 当写入指定数组b的长度大小超过BufferedInputStream中核心缓存数组buf[]的大小并且
* markpos < 0，那么就直接从数据流中读取数据给b数组，而不通过buf[]缓存数组，避免buf[]数组急剧增大
*
*/
if (len >= getBufIfOpen().length && markpos < 0) {
return getInIfOpen().read(b, off, len);
}
fill();
avail = count - pos;
if (avail <= 0) return -1;
}
int cnt = (avail < len) ? avail : len;
System.arraycopy(getBufIfOpen(), pos, b, off, cnt);
pos += cnt;
return cnt;
}
/**
* Reads bytes from this byte-input stream into the specified byte array,
* starting at the given offset.
*
* <p> This method implements the general contract of the corresponding
* <code>{@link InputStream#read(byte[], int, int) read}</code> method of
* the <code>{@link InputStream}</code> class. As an additional
* convenience, it attempts to read as many bytes as possible by repeatedly
* invoking the <code>read</code> method of the underlying stream. This
* iterated <code>read</code> continues until one of the following
* conditions becomes true: <ul>
*
* <li> The specified number of bytes have been read,
*
* <li> The <code>read</code> method of the underlying stream returns
* <code>-1</code>, indicating end-of-file, or
*
* <li> The <code>available</code> method of the underlying stream
* returns zero, indicating that further input requests would block.
*
* </ul> If the first <code>read</code> on the underlying stream returns
* <code>-1</code> to indicate end-of-file then this method returns
* <code>-1</code>. Otherwise this method returns the number of bytes
* actually read.
*
* <p> Subclasses of this class are encouraged, but not required, to
* attempt to read as many bytes as possible in the same fashion.
*
* @param b destination buffer.
* @param off offset at which to start storing bytes.
* @param len maximum number of bytes to read.
* @return the number of bytes read, or <code>-1</code> if the end of
* the stream has been reached.
* @exception IOException if this input stream has been closed by
* invoking its {@link #close()} method,
* or an I/O error occurs.
*
* 该方法主要调用read1(byte[] b, int off, int len)
*/
public synchronized int read(byte b[], int off, int len)
throws IOException
{
getBufIfOpen(); // Check for closed stream
if ((off | len | (off + len) | (b.length - (off + len))) < 0) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return 0;
}
int n = 0;
for (;;) {
int nread = read1(b, off + n, len - n);
if (nread <= 0)
return (n == 0) ? nread : n;
n += nread;
if (n >= len)
return n;
// if not closed but no bytes available, return
InputStream input = in;
if (input != null && input.available() <= 0)
return n;
}
}
/**
* See the general contract of the <code>skip</code>
* method of <code>InputStream</code>.
*
* @exception IOException if the stream does not support seek,
* or if this input stream has been closed by
* invoking its {@link #close()} method, or an
* I/O error occurs.
*
* 跳过流中指定字节数，感觉该方法用处不大，至少到目前为止，我本人还从来没有用过skip方法
*/
public synchronized long skip(long n) throws IOException {
getBufIfOpen(); // Check for closed stream
if (n <= 0) {
return 0;
}
long avail = count - pos;
if (avail <= 0) {
// If no mark position set then don't keep in buffer
if (markpos <0)
return getInIfOpen().skip(n);
// Fill in buffer to save bytes for reset
fill();
avail = count - pos;
if (avail <= 0)
return 0;
}
long skipped = (avail < n) ? avail : n;
pos += skipped;
return skipped;
}
/**
* Returns an estimate of the number of bytes that can be read (or
* skipped over) from this input stream without blocking by the next
* invocation of a method for this input stream. The next invocation might be
* the same thread or another thread. A single read or skip of this
* many bytes will not block, but may read or skip fewer bytes.
* <p>
* This method returns the sum of the number of bytes remaining to be read in
* the buffer (<code>count - pos</code>) and the result of calling the
* {@link java.io.FilterInputStream#in in}.available().
*
* @return an estimate of the number of bytes that can be read (or skipped
* over) from this input stream without blocking.
* @exception IOException if this input stream has been closed by
* invoking its {@link #close()} method,
* or an I/O error occurs.
*
* buf[]数组剩余字节数+输入流中剩余字节数
*/
public synchronized int available() throws IOException {
int n = count - pos;
int avail = getInIfOpen().available();
return n > (Integer.MAX_VALUE - avail)
? Integer.MAX_VALUE
: n + avail;
}
/**
* See the general contract of the <code>mark</code>
* method of <code>InputStream</code>.
*
* @param readlimit the maximum limit of bytes that can be read before
* the mark position becomes invalid.
* @see java.io.BufferedInputStream#reset()
*
* 标记位置，marklimit只有在这里才能够被赋值，readlimit表示mark()方法执行后，最多能够从流中
* 读取的数据，如果超过该字节大小，那么在fill()的时候，就会认为此mark()标记无效，重新将
* markpos = -1，pos = 0
*/
public synchronized void mark(int readlimit) {
marklimit = readlimit;
markpos = pos;
}
/**
* See the general contract of the <code>reset</code>
* method of <code>InputStream</code>.
* <p>
* If <code>markpos</code> is <code>-1</code>
* (no mark has been set or the mark has been
* invalidated), an <code>IOException</code>
* is thrown. Otherwise, <code>pos</code> is
* set equal to <code>markpos</code>.
*
* @exception IOException if this stream has not been marked or,
* if the mark has been invalidated, or the stream
* has been closed by invoking its {@link #close()}
* method, or an I/O error occurs.
* @see java.io.BufferedInputStream#mark(int)
*/
public synchronized void reset() throws IOException {
getBufIfOpen(); // Cause exception if closed
if (markpos < 0)
throw new IOException("Resetting to invalid mark");
pos = markpos;
}
/**
* Tests if this input stream supports the <code>mark</code>
* and <code>reset</code> methods. The <code>markSupported</code>
* method of <code>BufferedInputStream</code> returns
* <code>true</code>.
*
* @return a <code>boolean</code> indicating if this stream type supports
* the <code>mark</code> and <code>reset</code> methods.
* @see java.io.InputStream#mark(int)
* @see java.io.InputStream#reset()
*/
public boolean markSupported() {
return true;
}
/**
* Closes this input stream and releases any system resources
* associated with the stream.
* Once the stream has been closed, further read(), available(), reset(),
* or skip() invocations will throw an IOException.
* Closing a previously closed stream has no effect.
*
* @exception IOException if an I/O error occurs.
*/
public void close() throws IOException {
byte[] buffer;
while ( (buffer = buf) != null) {
if (bufUpdater.compareAndSet(this, buffer, null)) {
InputStream input = in;
in = null;
if (input != null)
input.close();
return;
}
// Else retry in case a new buf was CASed in fill()
}
}
}
3 BufferedInputStream在实际场景中，没有太多用处
网上很多博客，说BufferedInputStream很有用，可以一次性从IO中读入很多数据，然后缓存在buf[]中，这样就减少了IO消耗，很多博主，甚至给出了一些代码实操，证明BufferedInputStream确实可以提高效率，这本身没有任何问题，但是经我深入源码研究过后，却发现实际场景中，该类使用频率很少，根本不需要BufferedInputStream
我将结合代码，进行更有力的说明：
// file文件大小1个G
private static String file = "D:\\StudySoftware\\VMware_virtualbox\\Data_vmware\\VMwareMachine\\kafka_single\\kafka-single-103-da5cf665.vmem";
private static void file() throws IOException{
long beginTime = System.currentTimeMillis();
FileInputStream input = new FileInputStream(file);
byte[] bytes = new byte[1024 * 1];
int read = 0;
while ((read = input.read(bytes, 0, bytes.length)) != -1) {
// 不执行任何操作，仅仅读取文件
}
long endTime = System.currentTimeMillis();
System.out.println("file: 耗费时间：" + (endTime - beginTime));
}
private static void bufferd() throws IOException{
long beginTime = System.currentTimeMillis();
FileInputStream input = new FileInputStream(file);
BufferedInputStream bufferedInput = new BufferedInputStream(input);
byte[] bytes = new byte[1024 * 1];
int read = 0;
while ((read = bufferedInput.read(bytes, 0, bytes.length)) != -1) {
//不执行任何操作，仅仅读取文件
}
long endTime = System.currentTimeMillis();
System.out.println("buffered: 耗费时间：" + (endTime - beginTime));
}
注意：
代码操作的时候，两个方法不能够对同一个文件进行操作，防止JVM会自动优化，因为第一个方法读完整个文件，第二个方法再读的时候，JVM可能保存了部分信息，从而造成测试数据的不准确。并且为了最大程度保证测试数据的准确性，一次JVM启动，只测试一个方法
结果：
①当 byte[] bytes = new byte[1024 * 1]; 数组大小为1024
buffered: 耗费时间：855
file: 耗费时间：3073
②当 byte[] bytes = new byte[1024 * 2]; 数组大小为2018
buffered: 耗费时间：813
file: 耗费时间：1909
③当 byte[] bytes = new byte[1024 * 3]; 数组大小为3072
buffered: 耗费时间：1304
file: 耗费时间：1476
④当 byte[] bytes = new byte[1024 * 4]; 数组大小为4096
buffered: 耗费时间：844
file: 耗费时间：1287
⑤当 byte[] bytes = new byte[1024 * 5]; 数组大小为5120
buffered: 耗费时间：1343
file: 耗费时间：1061
⑥当 byte[] bytes = new byte[1024 * 6]; 数组大小为6144
buffered: 耗费时间：1280
file: 耗费时间：985
⑦当 byte[] bytes = new byte[1024 * 7]; 数组大小为7168
buffered: 耗费时间：1443
file: 耗费时间：851
⑧当 byte[] bytes = new byte[1024 * 8]; 数组大小为8192
buffered: 耗费时间：774
file: 耗费时间：739
⑨当 byte[] bytes = new byte[1024 * 9]; 数组大小为9216
buffered: 耗费时间：734
file: 耗费时间：749
⑩当 byte[] bytes = new byte[1024 * 10]; 数组大小为10240
buffered: 耗费时间：739
file: 耗费时间：697
... ... ...
我们可以得出以下重要结论：
当bytes比较小时，使用BufferedInputStream确实读取文件时要快不少，可是当bytes逐步增大，尤其是达到8kb的时候，我们会发现 BufferedInputStream和FileInputStream读取文件速度差不多了，没有明显差异
我们深入源码，即可发现：
因此当我们把 while ((read = input.read(bytes, 0, bytes.length)) != -1)中的bytes增大时，BufferedInputStream没有任何作用(除非有mark、reset需求)
有的小伙伴，肯定会说，那我将BufferedInputStream中的buf[]大小提高不就行了吗？
可以是可以，但是我将 while ((read = input.read(bytes, 0, bytes.length)) != -1)中的bytes大小增大不就行了？ 说到底都是字节数组，一个是在BufferedInputStream外面，一个是在BufferedInputStream内部，而现在我们进行流读取的时候，很多时候是不需要mark、reset操作的，并且我们设置外部bytes大小通常会比较大，这个时候，完全可以不使用BufferedInputStream
4 BufferedInputStream使用场景
①第一种使用场景，就是当我们需要mark、reset特性时。不过要特别注意，mark、reset的使用，里面涉及到很多东西，特别是当BufferedInputStream执行fill()操作时
public static void main(String[] args) {
try {
final byte[] src = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20};
final ByteArrayInputStream bis = new ByteArrayInputStream(src);
final BufferedInputStream bufis = new BufferedInputStream(bis, 5);
int data = -1;
int i = 0;
while((data = bufis.read()) != -1) {
if(data == 4) {
bufis.mark(2);
}
if(i++ == 9) {
bufis.reset();
}
System.out.printf("%d", data);
}
} catch(IOException ioex) {
ioex.printStackTrace();
}
}
// 原文链接：https://blog.csdn.net/qq_26971305/article/details/79472696
有兴趣的朋友，可以debug上面的代码，debug下面的情况，相应你对BufferedInputStream有更深的理解
if(i++ == 5)
if(i++ == 6)
if(i++ == 7)
if(i++ == 8)
if(i++ == 9)
if(i++ == 10)
... ... ... 时间多的朋友，可以设置BufferedInputStream中buf[]的大小长度和if(i++ == xx)判断语句中的值来看看BufferedInputStream类的执行流程
mark、reset特性不可乱用，不然会抛出异常的
public synchronized void reset() throws IOException {
getBufIfOpen(); // Cause exception if closed
if (markpos < 0)
throw new IOException("Resetting to invalid mark");
pos = markpos;
}
②第二种使用场景，当BufferedInputStream配合DataInputStream和ObjectOutputStream使用时
ObjectInputStream input = new ObjectInputStream(new BufferedInputStream(new FileInputStream("E:\\obejct.txt")));
DataInputStream input = new DataInputStream(new BufferedInputStream(new FileInputStream("E:\\obejct.txt")));
当DataXxxStream从管道读取字节流的时候，是一个一个字节读取的
而ObjectInputStream底层依赖了DataXxxStream对象
参考链接：
作者：一杯热咖啡AAA
出处：https://www.cnblogs.com/AdaiCoffee/
本文以学习、研究和分享为主，欢迎转载。如果文中有不妥或者错误的地方还望指出，以免误人子弟。如果你有更好的想法和意见，可以留言讨论，谢谢！
出处：https://www.cnblogs.com/AdaiCoffee/
本文以学习、研究和分享为主，欢迎转载。如果文中有不妥或者错误的地方还望指出，以免误人子弟。如果你有更好的想法和意见，可以留言讨论，谢谢！
