Java tutorial
/* * Copyright (c) 1994, 2019, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.io; import jdk.internal.misc.Unsafe; import jdk.internal.util.ArraysSupport; /** * 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 1.0 */ public class BufferedInputStream extends FilterInputStream { private static int DEFAULT_BUFFER_SIZE = 8192; /** * As this class is used early during bootstrap, it's motivated to use * Unsafe.compareAndSetObject instead of AtomicReferenceFieldUpdater * (or VarHandles) to reduce dependencies and improve startup time. */ private static final Unsafe U = Unsafe.getUnsafe(); private static final long BUF_OFFSET = U.objectFieldOffset(BufferedInputStream.class, "buf"); /** * The internal buffer array where the data is stored. When necessary, * it may be replaced by another array of * a different size. */ /* * We null this out with a CAS on close(), which is necessary since * 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.) */ protected volatile byte[] buf; /** * 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; /** * 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. */ 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]; } /** * 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 { /* grow buffer */ int nsz = ArraysSupport.newLength(pos, 1, /* minimum growth */ pos /* preferred growth */); if (nsz > marklimit) nsz = marklimit; byte[] nbuf = new byte[nsz]; System.arraycopy(buffer, 0, nbuf, 0, pos); if (!U.compareAndSetReference(this, BUF_OFFSET, 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 */ 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>. * * @throws IOException if this input stream has been closed by * invoking its {@link #close()} method, * {@code in.skip(n)} throws an IOException, * 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; } /** * 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; } /** * 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() */ 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. */ public void close() throws IOException { byte[] buffer; while ((buffer = buf) != null) { if (U.compareAndSetReference(this, BUF_OFFSET, buffer, null)) { InputStream input = in; in = null; if (input != null) input.close(); return; } // Else retry in case a new buf was CASed in fill() } } }