Java IO学习之缓冲输入流（BufferedInputStream）

Java IO学习之缓冲输入流（BufferedInputStream）

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

// 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 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

* mark method before subsequent calls to the

* reset method fail.

* Whenever the difference between pos

* and markpos exceeds marklimit,

* then the mark may be dropped by setting

* markpos to -1.

*

* @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 BufferedInputStream

* and saves its argument, the input stream

* in, for later use. An internal

* buffer array is created and stored in buf.

*

* @param in the underlying input stream.

*/

//带InputStream的构造函数

public BufferedInputStream(InputStream in) {

this(in, DEFAULT_BUFFER_SIZE);

}

/**

* Creates a BufferedInputStream

* with the specified buffer size,

* and saves its argument, the input stream

* in, for later use. An internal

* buffer array of length size

* is created and stored in buf.

*

* @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 read

* method of InputStream.

*

* @return the next byte of data, or -1 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.

*

*

This method implements the general contract of the corresponding

* {@link InputStream#read(byte[], int, int) read} method of

* the {@link InputStream} class. As an additional

* convenience, it attempts to read as many bytes as possible by repeatedly

* invoking the read method of the underlying stream. This

* iterated read continues until one of the following

* conditions becomes true:

// 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 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

* mark method before subsequent calls to the

* reset method fail.

* Whenever the difference between pos

* and markpos exceeds marklimit,

* then the mark may be dropped by setting

* markpos to -1.

*

* @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 BufferedInputStream

* and saves its argument, the input stream

* in, for later use. An internal

* buffer array is created and stored in buf.

*

* @param in the underlying input stream.

*/

//带InputStream的构造函数

public BufferedInputStream(InputStream in) {

this(in, DEFAULT_BUFFER_SIZE);

}

/**

* Creates a BufferedInputStream

* with the specified buffer size,

* and saves its argument, the input stream

* in, for later use. An internal

* buffer array of length size

* is created and stored in buf.

*

* @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 read

* method of InputStream.

*

* @return the next byte of data, or -1 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.

*

*

This method implements the general contract of the corresponding

* {@link InputStream#read(byte[], int, int) read} method of

* the {@link InputStream} class. As an additional

* convenience, it attempts to read as many bytes as possible by repeatedly

* invoking the read method of the underlying stream. This

* iterated read continues until one of the following

* conditions becomes true:

*

*

*

*

* -1, indicating end-of-file, or

*

*

* returns zero, indicating that further input requests would block.

*

* If the first read on the underlying stream returns

* -1 to indicate end-of-file then this method returns

* -1. Otherwise this method returns the number of bytes

* actually read.

*

*

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 -1 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 skip

* method of InputStream.

*

* @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.

*

* This method returns the sum of the number of bytes remaining to be read in

* the buffer (count - pos) 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 mark

* method of InputStream.

*

* @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;

}

/**

http:// * See the general contract of the reset

* method of InputStream.

*

* If markpos is -1

* (no mark has been set or the mark has been

* invalidated), an IOException

* is thrown. Otherwise, pos is

* set equal to markpos.

*

* @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 mark

* and reset methods. The markSupported

* method of BufferedInputStream returns

* true.

*

* @return a boolean indicating if this stream type supports

* the mark and reset 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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