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使用方法
BufferedInputStream继承于FilterInputStream，提供缓冲输入流功能。缓冲输入流相对于普通输入流的优势是，它提供了一个缓冲数组，每次调用read方法的时候，它首先尝试从缓冲区里读取数据，若读取失败（缓冲区无可读数据），则选择从物理数据源（譬如文件）读取新数据（这里会尝试尽可能读取多的字节）放入到缓冲区中，最后再将缓冲区中的内容部分或全部返回给用户.由于从缓冲区里读取数据远比直接从物理数据源（譬如文件）读取速度快。
方法介绍
BufferedInputStream提供的API如下：
//构造方法
BufferedInputStream(InputStream in)
BufferedInputStream(InputStream in, int size)
//下一字节是否可读
synchronized int available()
//关闭
void close()
//标记, readlimit为mark后最多可读取的字节数
synchronized void mark(int readlimit)
//是否支持mark, true
boolean markSupported()
//读取一个字节
synchronized int read()
//读取多个字节到b
synchronized int read(byte[] b, int off, int len)
//重置会mark位置
synchronized void reset()
//跳过n个字节
synchronized long skip(long n)使用示例
public void testBufferedInput() {
try {
/**
* 建立输入流 BufferedInputStream, 缓冲区大小为8
* buffer.txt内容为
* abcdefghij
*/
InputStream in = new BufferedInputStream(new FileInputStream(new File("buff.txt")), 8);
/*从字节流中读取5个字节*/
byte [] tmp = new byte[5];
in.read(tmp, 0, 5);
System.out.println("字节流的前5个字节为: " + new String(tmp));
/*标记测试*/
in.mark(6);
/*读取5个字节*/
in.read(tmp, 0, 5);
System.out.println("字节流中第6到10个字节为: " + new String(tmp));
/*reset*/
in.reset();
System.out.printf("reset后读取的第一个字节为: %c" , in.read());
} catch (Exception e) {
e.printStackTrace();
}
}运行结果如下：
字节流的前5个字节为: abcde
字节流中第6到10个字节为: fghij
reset后读取的第一个字节为: f源码分析
构造方法
BufferedInputStream的构造方法有两个，区别是缓冲区大小设置。
/**
* 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.
*/
public BufferedInputStream(InputStream in) {
this(in, DEFAULT_BUFFER_SIZE); //默认8192, 8M
}
/**
* 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];
}read方法
read方法有每次读取一个字节和一次读取多个字节两种重载。下面主要分析读取多个字节的read方法。重点在于fill()方法。
/**
* 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>
*
* @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); //读取len长度的字节到b中
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;
}
}
/**
* 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;
}
/**
* 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; //缓冲区中可读字节数
if (avail <= 0) { //没可读字节
/* 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) { //没mark并且请求长度大于buff长度
return getInIfOpen().read(b, off, len); //直接从文件中读取,不走缓冲区
}
fill(); //修改或者扩展缓冲区
avail = count - pos; //可读字节数
if (avail <= 0) return -1;
}
int cnt = (avail < len) ? avail : len; //取最小值, 缓冲区中可能没有足够可读的字节
System.arraycopy(getBufIfOpen(), pos, b, off, cnt); //复制
pos += cnt;
return cnt;
}
/**
* 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 {
/**
* 填充字符时如果没有mark标记, 则直接清空缓冲区,然后将输入流的数据写入缓冲区
* 如果有mark标记,则分如下几种情况
* 1 普通mark,直接将标记以前的字符用标记以后的字符覆盖,剩余的空间读取输入流的内容填充
* 2 当前位置pos >= buffer的长度 >= marklimit,说明mark已经失效,直接清空缓冲区,然后读取输入流内容
* 3 buffer长度超出限制,抛出异常
* 4 marklimit比buffer的长度还大,此时mark还没失效,则扩大buffer空间
*/
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; //扩大后的大小
if (nsz > marklimit)
nsz = marklimit;
byte nbuf[] = new byte[nsz];
System.arraycopy(buffer, 0, nbuf, 0, pos); //将buffer的数据复制到nbuf中
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; //修改缓冲区
}
count = pos;
int n = getInIfOpen().read(buffer, pos, buffer.length - pos); //读取输入流中内容填充缓冲区
if (n > 0)
count = n + pos;
}mark\reset方法
/**
* 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;
}参考：
[1] http://zhhphappy.iteye.com/blog/1562427
[2] http://blog.sina.com.cn/s/blog_67f995260101huxz.html
[3] http://www.cnblogs.com/skywang12345/p/io_12.html