Java IO学习之缓冲输入流（BufferedInputStream）
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2024-03-06 14:21:08
java io bufferedinputstream
概要：
bufferedinputstream是缓冲输入流，继承于filterinputstream...
java io bufferedinputstream
概要：
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()
}
}
}
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