Class - java.io.BufferedInputStream
Created by : Mr Dk.
2020 / 07 / 27 14:36
Nanjing, Jiangsu, China
Definition
这个类为输入流维护了一个内部 buffer，随着字节的读取或跳过，buffer 会不断被重新填充。Buffer 使得 mark() 和 reset() 操作能够被实现。
/**
* A <code>BufferedInputStream</code> adds
* functionality to another input stream-namely,
* the ability to buffer the input and to
* support the <code>mark</code> and <code>reset</code>
* methods. When the <code>BufferedInputStream</code>
* is created, an internal buffer array is
* created. As bytes from the stream are read
* or skipped, the internal buffer is refilled
* as necessary from the contained input stream,
* many bytes at a time. The <code>mark</code>
* operation remembers a point in the input
* stream and the <code>reset</code> operation
* causes all the bytes read since the most
* recent <code>mark</code> operation to be
* reread before new bytes are taken from
* the contained input stream.
*
* @author Arthur van Hoff
* @since JDK1.0
*/
public
class BufferedInputStream extends FilterInputStream {
}
Fields
内部维护的字节 buffer。这个 buffer 数组有默认的大小，也可以通过构造函数指定大小。
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
*/
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.
*/
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.)
*/
private static final
AtomicReferenceFieldUpdater<BufferedInputStream, byte[]> bufUpdater =
AtomicReferenceFieldUpdater.newUpdater
(BufferedInputStream.class, byte[].class, "buf");
与 stream 相关的状态变量：
count- 表示 buffer 中的有效长度，范围在[0]到[count - 1]之间pos- 流的目前位置，是下一个从流中被读取的字节位置markpos- 最后一次调用mark()时的pos位置，-1代表不存在标记位置marklimit- 如果pos和markpos之间的举例超过了marklimit，那么markpos将被丢弃
/**
* 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.
*/
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
*/
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
*/
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;
获取底层数据结构
获取父类 FilterInputStream 中维护的 InputStream。获取本类中的 buffer。
/**
* 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;
}
构造函数
/**
* 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);
}
/**
* 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];
}
填充 buffer
用新的数据填充 buffer。这里显然需要考虑带有 mark 标记的情况，至少需要保留从 mark 标记开始的字节，直到超过 marklimit。
/**
* 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)
// 没有 mark，那么直接丢弃数组中所有的内容
pos = 0; /* no mark: throw away the buffer */
else if (pos >= buffer.length) /* no room left in buffer */
// 有 mark，且当前位置已经超出 buffer 长度，开始按情况处理
if (markpos > 0) { /* can throw away early part of the buffer */
// 如果 markpos 的位置不在开头，那么至少可以丢弃 buffer 开头部分到 markpos 的字节
// 把从 markpos 开始的字节移动到 buffer 开头处
int sz = pos - markpos;
System.arraycopy(buffer, markpos, buffer, 0, sz);
pos = sz;
markpos = 0;
} else if (buffer.length >= marklimit) {
// 如果 marklimit 小于 buffer 的空间，说明 buffer 开辟大了
// 那么直接丢弃缓存的字节，将下一个读取的位置 pos 设置到数组开头
markpos = -1; /* buffer got too big, invalidate mark */
pos = 0; /* drop buffer contents */
} else if (buffer.length >= MAX_BUFFER_SIZE) {
// buffer 大小超出极限
throw new OutOfMemoryError("Required array size too large");
} else { /* grow buffer */
// buffer 扩大两倍，同时保证不超过 marklimit
// 重新开辟 buffer，并从 pos 位置开始将字节复制到新数组
// 用 CAS 操作替换新数组
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;
}
// 将有效长度 count 设置到 pos 的位置，读取字节存放到 buffer 中
// 如果读取到了新字节，那么更新 buffer 的有效长度 count
count = pos;
int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
if (n > 0)
count = n + pos;
}
Read
读取 stream 中的下一个字节。如果读取的位置已经超出了 buffer 有效长度，那么调用一次 fill() 重新填充 buffer。
/**
* 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 {
if (pos >= count) {
fill();
if (pos >= count)
return -1;
}
return getBufIfOpen()[pos++] & 0xff;
}
将 buffer 中指定数量的字节读取到一个字节数组的指定偏移位置。如果 buffer 中已经没有字节可以读取，那么就调用一次 fill() 再读取。fill() 最多只能调用一次。最后更新 pos 的位置。这是一个内部函数。
/**
* 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; // 计算 buffer 中还可以读取的字节数
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;
}
以下函数是真正实现 read() 接口的函数。该函数会循环调用底层的 read()，直到以下条件之一满足：
- 指定长度的字节已经被读取 (
len) - 底层
read()返回-1，说明读到了结尾 - 底层 stream 的
available()函数返回0，说明读取之后的内容将会阻塞
/**
* 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;
}
}
Skip
跳过指定数量的字节。同样，如果接触到了 buffer 的尾部，那么调用一次 fill() 读取新内容。
/**
* 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.
*/
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;
}
Available
返回剩余还可以不阻塞地被读取或跳过的字节数。不考虑边界条件，应当会返回输入流中的可读字节数 + buffer 中剩余可读字节数。
/**
* 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.
*/
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;
}
Mark && Reset
mark() 将当前的 pos 记录到 markpos 中。在 reset() 中，将 pos 重新设置为 markpos。
/**
* 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;
}
Mark Support
显然，该类支持 mark/reset 操作。
/**
* 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;
}
Close
通过 CAS 操作将 buffer 数组设置为 null。另外将底层的 InputStream 也设置为 null。如果流还没有关闭，那么调用流的 close() 函数。
/**
* 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()
}
}