Java-IO之BufferedInputStream（缓冲输入流）

BufferedInputStream是缓冲输入流，继承于FilterInputStream,作用是为另一个输入流添加一些功能，本质上是通过一个内部缓冲数组实现的。例如，在新建某输入流对应的BufferedInputStream后，当通过read()读取数据时，BufferedInputStream会将输入流的数据分批的填入到缓冲区中，每当缓冲区的数据读完之后，输入流会再次填充数据缓冲区，直到读完数据。

BufferedInputStream主要的函数列表：

BufferedInputStream(InputStream in)
BufferedInputStream(InputStream in, int size)
synchronized int available()
void close()
synchronized void mark(int readlimit)
boolean markSupported()
synchronized int read()
synchronized int read(byte[] buffer, int offset, int byteCount)
synchronized void reset()
synchronized long skip(long byteCount)

示例代码：

public class BufferedInputStreamTest {

private static final int LEN = 5;

public static void main(String[] args) {
testBufferedInputStream() ;
}
private static void testBufferedInputStream() {

// 创建BufferedInputStream字节流，内容是ArrayLetters数组
try {
File file = new File("file.txt");
InputStream in =new BufferedInputStream(new FileInputStream(file), 512);

// 从字节流中读取5个字节。“abcde”，a对应0x61，b对应0x62，依次类推...
for (int i=0; i<LEN; i++) {
// 若能继续读取下一个字节，则读取下一个字节
if (in.available() >= 0) {
// 读取“字节流的下一个字节”
int tmp = in.read();
System.out.printf("%d : 0x%s\n", i, Integer.toHexString(tmp));
}
}

// 若“该字节流”不支持标记功能，则直接退出
if (!in.markSupported()) {
System.out.println("make not supported!");
return ;
}

// 标记“当前索引位置”，即标记第6个位置的元素--“f”
// 1024对应marklimit
in.mark(1024);

// 跳过22个字节。
in.skip(22);

// 读取5个字节
byte[] buf = new byte[LEN];
in.read(buf, 0, LEN);
// 将buf转换为String字符串。
String str1 = new String(buf);
System.out.printf("str1=%s\n", str1);

// 重置“输入流的索引”为mark()所标记的位置，即重置到“f”处。
in.reset();
// 从“重置后的字节流”中读取5个字节到buf中。即读取“fghij”
in.read(buf, 0, LEN);
// 将buf转换为String字符串。
String str2 = new String(buf);
System.out.printf("str2=%s\n", str2);

in.close();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (SecurityException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}

运行结果：
0 : 0x61
1 : 0x62
2 : 0x63
3 : 0x64
4 : 0x65
str1=12345
str2=fghij

基于JDK8的BufferInputStream代码：

public class BufferedInputStream extends FilterInputStream {

private static int DEFAULT_BUFFER_SIZE = 8192;//默认缓冲区大小为8X1024
private static int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8;
protected volatile byte buf[]; //缓冲数组

/**
* 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.)
*/
private static final AtomicReferenceFieldUpdater<BufferedInputStream, byte[]> bufUpdater =
AtomicReferenceFieldUpdater.newUpdater(BufferedInputStream.class,  byte[].class, "buf");
//值在0到buf.len之间
protected int count;
//在buffer的当前位置，下一个字符被读取
protected int pos;
//值为mark函数最近被调用的时候，值为-1到pos
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()
*/

protected int marklimit;

/**
* Check to make sure that underlying input stream has not been
* nulled out due to close; if not return it;
*/
//
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;
*/
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.
*/
//带InputStream的构造函数
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}.
*/
//带InputStream和大小的构造函数
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.
*/
//
private void fill() throws IOException {
byte[] buffer = getBufIfOpen();
if (markpos < 0)
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;
} 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)
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;
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
*/
//读下一个字节，没有数据返回-1
public synchronized int read() throws IOException {
if (pos >= count) {
fill();
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. */
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.
*/
//
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n长的数据
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.
*/
//返回还有多少数据可以读
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()
*/
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()
*/
//是否支持标记
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.
*/
//关闭资源
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()
}
}
}