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 : 0x611 : 0x622 : 0x633 : 0x644 : 0x65str1=12345str2=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; else if (pos >= buffer.length) if (markpos > 0) { int sz = pos - markpos; System.arraycopy(buffer, markpos, buffer, 0, sz); pos = sz; markpos = 0; } else if (buffer.length >= marklimit) { markpos = -1; pos = 0; } else if (buffer.length >= MAX_BUFFER_SIZE) { throw new OutOfMemoryError("Required array size too large"); } else { 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 (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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