InputStream.java

/*
 * Copyright (c) 1994, 2018, 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 java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Objects;

/**
 * This abstract class is the superclass of all classes representing
 * an input stream of bytes.
 *
 * <p> Applications that need to define a subclass of <code>InputStream</code>
 * must always provide a method that returns the next byte of input.
 *
 * @author Arthur van Hoff
 * @see java.io.BufferedInputStream
 * @see java.io.ByteArrayInputStream
 * @see java.io.DataInputStream
 * @see java.io.FilterInputStream
 * @see java.io.InputStream#read()
 * @see java.io.OutputStream
 * @see java.io.PushbackInputStream
 * @since 1.0
 */
/*
 * 字节输入流，可以从中读取数据
 *
 * 本地 <--------- 远端
 *
 * 字节输入流的解释：
 * [输入流]的含义是从远端读取数据到本地，
 * [字节]的含义是本地获取到的是字节(远端通常是数组、字节流、通道、)
 */
public abstract class InputStream implements Closeable {
    
    // MAX_SKIP_BUFFER_SIZE is used to determine the maximum buffer size to use when skipping.
    private static final int MAX_SKIP_BUFFER_SIZE = 2048;
    
    private static final 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 final int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8;
    
    
    
    /*▼ 读 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Reads the next byte of data from the input stream. The value byte is
     * returned as an <code>int</code> in the range <code>0</code> to
     * <code>255</code>. If no byte is available because the end of the stream
     * has been reached, the value <code>-1</code> is returned. This method
     * blocks until input data is available, the end of the stream is detected,
     * or an exception is thrown.
     *
     * <p> A subclass must provide an implementation of this method.
     *
     * @return the next byte of data, or <code>-1</code> if the end of the
     * stream is reached.
     *
     * @throws IOException if an I/O error occurs.
     */
    /*
     * 尝试从当前输入流读取一个字节，读取成功直接返回，读取失败返回-1
     */
    public abstract int read() throws IOException;
    
    /**
     * Reads some number of bytes from the input stream and stores them into
     * the buffer array <code>b</code>. The number of bytes actually read is
     * returned as an integer.  This method blocks until input data is
     * available, end of file is detected, or an exception is thrown.
     *
     * <p> If the length of <code>b</code> is zero, then no bytes are read and
     * <code>0</code> is returned; otherwise, there is an attempt to read at
     * least one byte. If no byte is available because the stream is at the
     * end of the file, the value <code>-1</code> is returned; otherwise, at
     * least one byte is read and stored into <code>b</code>.
     *
     * <p> The first byte read is stored into element <code>b[0]</code>, the
     * next one into <code>b[1]</code>, and so on. The number of bytes read is,
     * at most, equal to the length of <code>b</code>. Let <i>k</i> be the
     * number of bytes actually read; these bytes will be stored in elements
     * <code>b[0]</code> through <code>b[</code><i>k</i><code>-1]</code>,
     * leaving elements <code>b[</code><i>k</i><code>]</code> through
     * <code>b[b.length-1]</code> unaffected.
     *
     * <p> The <code>read(b)</code> method for class <code>InputStream</code>
     * has the same effect as: <pre><code> read(b, 0, b.length) </code></pre>
     *
     * @param b the buffer into which the data is read.
     *
     * @return the total number of bytes read into the buffer, or
     * <code>-1</code> if there is no more data because the end of
     * the stream has been reached.
     *
     * @throws IOException          If the first byte cannot be read for any reason
     *                              other than the end of the file, if the input stream has been closed, or
     *                              if some other I/O error occurs.
     * @throws NullPointerException if <code>b</code> is <code>null</code>.
     * @see java.io.InputStream#read(byte[], int, int)
     */
    /*
     * 尝试从当前输入流读取dst.length个字节，并将读到的内容插入到dst的起点处
     * 返回值表示成功读取的字节数量(可能小于预期值)，返回-1表示已经没有可读内容了
     */
    public int read(byte[] dst) throws IOException {
        return read(dst, 0, dst.length);
    }
    
    /**
     * Reads up to <code>len</code> bytes of data from the input stream into
     * an array of bytes.  An attempt is made to read as many as
     * <code>len</code> bytes, but a smaller number may be read.
     * The number of bytes actually read is returned as an integer.
     *
     * <p> This method blocks until input data is available, end of file is
     * detected, or an exception is thrown.
     *
     * <p> If <code>len</code> is zero, then no bytes are read and
     * <code>0</code> is returned; otherwise, there is an attempt to read at
     * least one byte. If no byte is available because the stream is at end of
     * file, the value <code>-1</code> is returned; otherwise, at least one
     * byte is read and stored into <code>b</code>.
     *
     * <p> The first byte read is stored into element <code>b[off]</code>, the
     * next one into <code>b[off+1]</code>, and so on. The number of bytes read
     * is, at most, equal to <code>len</code>. Let <i>k</i> be the number of
     * bytes actually read; these bytes will be stored in elements
     * <code>b[off]</code> through <code>b[off+</code><i>k</i><code>-1]</code>,
     * leaving elements <code>b[off+</code><i>k</i><code>]</code> through
     * <code>b[off+len-1]</code> unaffected.
     *
     * <p> In every case, elements <code>b[0]</code> through
     * <code>b[off]</code> and elements <code>b[off+len]</code> through
     * <code>b[b.length-1]</code> are unaffected.
     *
     * <p> The <code>read(b,</code> <code>off,</code> <code>len)</code> method
     * for class <code>InputStream</code> simply calls the method
     * <code>read()</code> repeatedly. If the first such call results in an
     * <code>IOException</code>, that exception is returned from the call to
     * the <code>read(b,</code> <code>off,</code> <code>len)</code> method.  If
     * any subsequent call to <code>read()</code> results in a
     * <code>IOException</code>, the exception is caught and treated as if it
     * were end of file; the bytes read up to that point are stored into
     * <code>b</code> and the number of bytes read before the exception
     * occurred is returned. The default implementation of this method blocks
     * until the requested amount of input data <code>len</code> has been read,
     * end of file is detected, or an exception is thrown. Subclasses are
     * encouraged to provide a more efficient implementation of this method.
     *
     * @param b   the buffer into which the data is read.
     * @param off the start offset in array <code>b</code>
     *            at which the data is written.
     * @param len the maximum number of bytes to read.
     *
     * @return the total number of bytes read into the buffer, or
     * <code>-1</code> if there is no more data because the end of
     * the stream has been reached.
     *
     * @throws IOException               If the first byte cannot be read for any reason
     *                                   other than end of file, or if the input stream has been closed, or if
     *                                   some other I/O error occurs.
     * @throws NullPointerException      If <code>b</code> is <code>null</code>.
     * @throws IndexOutOfBoundsException If <code>off</code> is negative,
     *                                   <code>len</code> is negative, or <code>len</code> is greater than
     *                                   <code>b.length - off</code>
     * @see java.io.InputStream#read()
     */
    /*
     * 尝试从当前输入流读取len个字节，并将读到的内容插入到字节数组b的off索引处
     * 返回值表示成功读取的字节数量(可能小于预期值)，返回-1表示已经没有可读内容了
     */
    public int read(byte[] b, int off, int len) throws IOException {
        Objects.checkFromIndexSize(off, len, b.length);
        
        if(len == 0) {
            return 0;
        }
        
        // 先尝试读取一个字节
        int c = read();
        if(c == -1) {
            return -1;
        }
        
        b[off] = (byte) c;
        
        int i = 1;
        
        try {
            // 尽力凑够len个字节
            for(; i<len; i++) {
                c = read();
                if(c == -1) {
                    break;
                }
                b[off + i] = (byte) c;
            }
        } catch(IOException ee) {
        }
        
        return i;
    }
    
    /**
     * Reads all remaining bytes from the input stream. This method blocks until
     * all remaining bytes have been read and end of stream is detected, or an
     * exception is thrown. This method does not close the input stream.
     *
     * <p> When this stream reaches end of stream, further invocations of this
     * method will return an empty byte array.
     *
     * <p> Note that this method is intended for simple cases where it is
     * convenient to read all bytes into a byte array. It is not intended for
     * reading input streams with large amounts of data.
     *
     * <p> The behavior for the case where the input stream is <i>asynchronously
     * closed</i>, or the thread interrupted during the read, is highly input
     * stream specific, and therefore not specified.
     *
     * <p> If an I/O error occurs reading from the input stream, then it may do
     * so after some, but not all, bytes have been read. Consequently the input
     * stream may not be at end of stream and may be in an inconsistent state.
     * It is strongly recommended that the stream be promptly closed if an I/O
     * error occurs.
     *
     * @return a byte array containing the bytes read from this input stream
     *
     * @throws IOException      if an I/O error occurs
     * @throws OutOfMemoryError if an array of the required size cannot be
     *                          allocated.
     * @implSpec This method invokes {@link #readNBytes(int)} with a length of
     * {@link Integer#MAX_VALUE}.
     * @since 9
     */
    // 尝试从输入流读取全部字节，成功读取到的内容(可能少于预期)会被存入字节数组后返回
    public byte[] readAllBytes() throws IOException {
        return readNBytes(Integer.MAX_VALUE);
    }
    
    /**
     * Reads up to a specified number of bytes from the input stream. This
     * method blocks until the requested number of bytes have been read, end
     * of stream is detected, or an exception is thrown. This method does not
     * close the input stream.
     *
     * <p> The length of the returned array equals the number of bytes read
     * from the stream. If {@code len} is zero, then no bytes are read and
     * an empty byte array is returned. Otherwise, up to {@code len} bytes
     * are read from the stream. Fewer than {@code len} bytes may be read if
     * end of stream is encountered.
     *
     * <p> When this stream reaches end of stream, further invocations of this
     * method will return an empty byte array.
     *
     * <p> Note that this method is intended for simple cases where it is
     * convenient to read the specified number of bytes into a byte array. The
     * total amount of memory allocated by this method is proportional to the
     * number of bytes read from the stream which is bounded by {@code len}.
     * Therefore, the method may be safely called with very large values of
     * {@code len} provided sufficient memory is available.
     *
     * <p> The behavior for the case where the input stream is <i>asynchronously
     * closed</i>, or the thread interrupted during the read, is highly input
     * stream specific, and therefore not specified.
     *
     * <p> If an I/O error occurs reading from the input stream, then it may do
     * so after some, but not all, bytes have been read. Consequently the input
     * stream may not be at end of stream and may be in an inconsistent state.
     * It is strongly recommended that the stream be promptly closed if an I/O
     * error occurs.
     *
     * @param len the maximum number of bytes to read
     *
     * @return a byte array containing the bytes read from this input stream
     *
     * @throws IllegalArgumentException if {@code length} is negative
     * @throws IOException              if an I/O error occurs
     * @throws OutOfMemoryError         if an array of the required size cannot be
     *                                  allocated.
     * @implNote The number of bytes allocated to read data from this stream and return
     * the result is bounded by {@code 2*(long)len}, inclusive.
     * @since 11
     */
    // 尝试从输入流读取len个字节，成功读取到的内容(可能少于预期)会被存入字节数组后返回
    public byte[] readNBytes(int len) throws IOException {
        if(len<0) {
            throw new IllegalArgumentException("len < 0");
        }
        
        // 存储每轮读取中使用的缓冲区
        List<byte[]> bufs = null;
        
        // 第一轮读取中使用的缓冲区
        byte[] result = null;
        
        // 总共读到的字节数
        int total = 0;
        
        // 剩余应读取的字节数量
        int remaining = len;
        
        int n;
        
        do {
            // 本轮读取使用的缓冲区
            byte[] buf = new byte[Math.min(remaining, DEFAULT_BUFFER_SIZE)];
            
            // 本轮读取累计读到的字节数
            int nread = 0;
            
            /* read to EOF which may read more or less than buffer size */
            /*
             * 从当前输入流读取数据，尽量填满缓冲区buf，返回值含义：
             * >0：成功读取到n个字节
             * =0：缓冲区已经填满，但还没有读完
             * =-1：数据已经读完
             */
            while((n = read(buf, nread, Math.min(buf.length - nread, remaining)))>0) {
                nread += n;
                remaining -= n;
            }
            
            // 如果读到了有效数据
            if(nread>0) {
                if(MAX_BUFFER_SIZE - total<nread) {
                    throw new OutOfMemoryError("Required array size too large");
                }
                
                total += nread;
                if(result == null) {
                    result = buf;
                } else {
                    if(bufs == null) {
                        bufs = new ArrayList<>();
                        bufs.add(result);
                    }
                    bufs.add(buf);
                }
            }
            
            /* if the last call to read returned -1 or the number of bytes requested have been read then break */
            // 如果读取的字节数还未达标，则需要开启下一轮读取
        } while(n >= 0 && remaining>0);
        
        // 如果只用了一轮就读取完了
        if(bufs == null) {
            // 如果没有读到任何有效字节
            if(result == null) {
                return new byte[0];
            }
            
            // 返回一个字节数组，包含了成功读到的字节
            return result.length == total ? result : Arrays.copyOf(result, total);
        }
        
        result = new byte[total];
        
        /* 如果读取了多轮，则需要拼接各轮读取中读到的字节，然后返回 */
        
        int offset = 0;
        remaining = total;
        for(byte[] b : bufs) {
            int count = Math.min(b.length, remaining);
            System.arraycopy(b, 0, result, offset, count);
            offset += count;
            remaining -= count;
        }
        
        return result;
    }
    
    /**
     * Reads the requested number of bytes from the input stream into the given
     * byte array. This method blocks until {@code len} bytes of input data have
     * been read, end of stream is detected, or an exception is thrown. The
     * number of bytes actually read, possibly zero, is returned. This method
     * does not close the input stream.
     *
     * <p> In the case where end of stream is reached before {@code len} bytes
     * have been read, then the actual number of bytes read will be returned.
     * When this stream reaches end of stream, further invocations of this
     * method will return zero.
     *
     * <p> If {@code len} is zero, then no bytes are read and {@code 0} is
     * returned; otherwise, there is an attempt to read up to {@code len} bytes.
     *
     * <p> The first byte read is stored into element {@code b[off]}, the next
     * one in to {@code b[off+1]}, and so on. The number of bytes read is, at
     * most, equal to {@code len}. Let <i>k</i> be the number of bytes actually
     * read; these bytes will be stored in elements {@code b[off]} through
     * {@code b[off+}<i>k</i>{@code -1]}, leaving elements {@code b[off+}<i>k</i>
     * {@code ]} through {@code b[off+len-1]} unaffected.
     *
     * <p> The behavior for the case where the input stream is <i>asynchronously
     * closed</i>, or the thread interrupted during the read, is highly input
     * stream specific, and therefore not specified.
     *
     * <p> If an I/O error occurs reading from the input stream, then it may do
     * so after some, but not all, bytes of {@code b} have been updated with
     * data from the input stream. Consequently the input stream and {@code b}
     * may be in an inconsistent state. It is strongly recommended that the
     * stream be promptly closed if an I/O error occurs.
     *
     * @param b   the byte array into which the data is read
     * @param off the start offset in {@code b} at which the data is written
     * @param len the maximum number of bytes to read
     *
     * @return the actual number of bytes read into the buffer
     *
     * @throws IOException               if an I/O error occurs
     * @throws NullPointerException      if {@code b} is {@code null}
     * @throws IndexOutOfBoundsException If {@code off} is negative, {@code len}
     *                                   is negative, or {@code len} is greater than {@code b.length - off}
     * @since 9
     */
    /*
     * 从当前输入流读取len个字节，并将读到的内容插入到字节数组b的off索引处
     * 返回值表示成功读取的字节数量(可能小于预期值)，返回0表示已经没有可读内容了
     *
     * 与read(byte[], int, int)不同的是，在遇到IO异常时，read方法会直接退出，
     * 而readNBytes方法会忽略该异常，继续读取，直到读够len个字节，除非，已经没有可读数据后才会结束。
     */
    public int readNBytes(byte[] b, int off, int len) throws IOException {
        Objects.checkFromIndexSize(off, len, b.length);
        
        int n = 0;
        
        while(n<len) {
            // 从当前输入流开始读len个字节，读到的内容存入字节数组的[off, off+len-1]范围
            int count = read(b, off + n, len - n);
            
            // 如果没有可读的数据，则结束读取
            if(count<0) {
                break;
            }
            
            n += count;
        }
        
        return n;
    }
    
    /*▲ 读 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 存档 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Tests if this input stream supports the <code>mark</code> and
     * <code>reset</code> methods. Whether or not <code>mark</code> and
     * <code>reset</code> are supported is an invariant property of a
     * particular input stream instance. The <code>markSupported</code> method
     * of <code>InputStream</code> returns <code>false</code>.
     *
     * @return <code>true</code> if this stream instance supports the mark
     * and reset methods; <code>false</code> otherwise.
     *
     * @see java.io.InputStream#mark(int)
     * @see java.io.InputStream#reset()
     */
    // 判断当前输入流是否支持存档标记
    public boolean markSupported() {
        return false;
    }
    
    /**
     * Marks the current position in this input stream.
     * A subsequent call to the <code>reset</code> method repositions this stream at the last marked position
     * so that subsequent reads re-read the same bytes.
     *
     * <p> The <code>readlimit</code> arguments tells this input stream to
     * allow that many bytes to be read before the mark position gets
     * invalidated.
     *
     * <p> The general contract of <code>mark</code> is that, if the method
     * <code>markSupported</code> returns <code>true</code>, the stream somehow
     * remembers all the bytes read after the call to <code>mark</code> and
     * stands ready to supply those same bytes again if and whenever the method
     * <code>reset</code> is called.  However, the stream is not required to
     * remember any data at all if more than <code>readlimit</code> bytes are
     * read from the stream before <code>reset</code> is called.
     *
     * <p> Marking a closed stream should not have any effect on the stream.
     *
     * <p> The <code>mark</code> method of <code>InputStream</code> does
     * nothing.
     *
     * @param readlimit the maximum limit of bytes that can be read before
     *                  the mark position becomes invalid.
     *
     * @see java.io.InputStream#reset()
     */
    // 设置存档标记，readlimit是存档上限
    public synchronized void mark(int readlimit) {
    }
    
    /**
     * Repositions this stream to the position at the time the
     * <code>mark</code> method was last called on this input stream.
     *
     * <p> The general contract of <code>reset</code> is:
     *
     * <ul>
     * <li> If the method <code>markSupported</code> returns
     * <code>true</code>, then:
     *
     * <ul><li> If the method <code>mark</code> has not been called since
     * the stream was created, or the number of bytes read from the stream
     * since <code>mark</code> was last called is larger than the argument
     * to <code>mark</code> at that last call, then an
     * <code>IOException</code> might be thrown.
     *
     * <li> If such an <code>IOException</code> is not thrown, then the
     * stream is reset to a state such that all the bytes read since the
     * most recent call to <code>mark</code> (or since the start of the
     * file, if <code>mark</code> has not been called) will be resupplied
     * to subsequent callers of the <code>read</code> method, followed by
     * any bytes that otherwise would have been the next input data as of
     * the time of the call to <code>reset</code>. </ul>
     *
     * <li> If the method <code>markSupported</code> returns
     * <code>false</code>, then:
     *
     * <ul><li> The call to <code>reset</code> may throw an
     * <code>IOException</code>.
     *
     * <li> If an <code>IOException</code> is not thrown, then the stream
     * is reset to a fixed state that depends on the particular type of the
     * input stream and how it was created. The bytes that will be supplied
     * to subsequent callers of the <code>read</code> method depend on the
     * particular type of the input stream. </ul></ul>
     *
     * <p>The method <code>reset</code> for class <code>InputStream</code>
     * does nothing except throw an <code>IOException</code>.
     *
     * @throws IOException if this stream has not been marked or if the
     *                     mark has been invalidated.
     * @see java.io.InputStream#mark(int)
     * @see java.io.IOException
     */
    // 对于支持设置存档的输入流，可以重置其"读游标"到存档区的起始位置
    public synchronized void reset() throws IOException {
        throw new IOException("mark/reset not supported");
    }
    
    /*▲ 存档 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 转移 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Reads all bytes from this input stream and writes the bytes to the
     * given output stream in the order that they are read. On return, this
     * input stream will be at end of stream. This method does not close either
     * stream.
     * <p>
     * This method may block indefinitely reading from the input stream, or
     * writing to the output stream. The behavior for the case where the input
     * and/or output stream is <i>asynchronously closed</i>, or the thread
     * interrupted during the transfer, is highly input and output stream
     * specific, and therefore not specified.
     * <p>
     * If an I/O error occurs reading from the input stream or writing to the
     * output stream, then it may do so after some bytes have been read or
     * written. Consequently the input stream may not be at end of stream and
     * one, or both, streams may be in an inconsistent state. It is strongly
     * recommended that both streams be promptly closed if an I/O error occurs.
     *
     * @param out the output stream, non-null
     *
     * @return the number of bytes transferred
     *
     * @throws IOException          if an I/O error occurs when reading or writing
     * @throws NullPointerException if {@code out} is {@code null}
     * @since 9
     */
    // 将当前输入流中的字节转移到输出流中，返回值表示成功转移的字节数
    public long transferTo(OutputStream out) throws IOException {
        Objects.requireNonNull(out, "out");
        long transferred = 0;
        byte[] buffer = new byte[DEFAULT_BUFFER_SIZE];
        int read;
        while((read = this.read(buffer, 0, DEFAULT_BUFFER_SIZE)) >= 0) {
            out.write(buffer, 0, read);
            transferred += read;
        }
        return transferred;
    }
    
    /*▲ 转移 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Closes this input stream and releases any system resources associated
     * with the stream.
     *
     * <p> The <code>close</code> method of <code>InputStream</code> does
     * nothing.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 关闭输入流
    public void close() throws IOException {
    }
    
    /**
     * Returns an estimate of the number of bytes that can be read (or skipped
     * over) from this input stream without blocking, which may be 0, or 0 when
     * end of stream is detected.  The read might be on 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> Note that while some implementations of {@code InputStream} will
     * return the total number of bytes in the stream, many will not.  It is
     * never correct to use the return value of this method to allocate
     * a buffer intended to hold all data in this stream.
     *
     * <p> A subclass's implementation of this method may choose to throw an
     * {@link IOException} if this input stream has been closed by invoking the
     * {@link #close()} method.
     *
     * <p> The {@code available} method of {@code InputStream} always returns
     * {@code 0}.
     *
     * <p> This method should be overridden by subclasses.
     *
     * @return an estimate of the number of bytes that can be read (or
     * skipped over) from this input stream without blocking or
     * {@code 0} when it reaches the end of the input stream.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 返回剩余可不被阻塞地读取（或跳过）的字节数（估计值）
    public int available() throws IOException {
        return 0;
    }
    
    /**
     * Skips over and discards <code>n</code> bytes of data from this input
     * stream. The <code>skip</code> method may, for a variety of reasons, end
     * up skipping over some smaller number of bytes, possibly <code>0</code>.
     * This may result from any of a number of conditions; reaching end of file
     * before <code>n</code> bytes have been skipped is only one possibility.
     * The actual number of bytes skipped is returned. If {@code n} is
     * negative, the {@code skip} method for class {@code InputStream} always
     * returns 0, and no bytes are skipped. Subclasses may handle the negative
     * value differently.
     *
     * <p> The <code>skip</code> method implementation of this class creates a
     * byte array and then repeatedly reads into it until <code>n</code> bytes
     * have been read or the end of the stream has been reached. Subclasses are
     * encouraged to provide a more efficient implementation of this method.
     * For instance, the implementation may depend on the ability to seek.
     *
     * @param n the number of bytes to be skipped.
     *
     * @return the actual number of bytes skipped.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 读取中跳过n个字节，返回实际跳过的字节数
    public long skip(long n) throws IOException {
        long remaining = n;
        int nr;
        
        if(n<=0) {
            return 0;
        }
        
        int size = (int) Math.min(MAX_SKIP_BUFFER_SIZE, remaining);
        
        byte[] skipBuffer = new byte[size];
        
        while(remaining>0) {
            /*
             * 尝试从当前输入流读取len个字节，成功读到的内容存入字节数组的[0, len-1]范围
             * 返回值表示成功读取的字节数量，返回-1表示已经没有可读内容了
             */
            nr = read(skipBuffer, 0, (int) Math.min(size, remaining));
            if(nr<0) {
                break;
            }
            
            remaining -= nr;
        }
        
        return n - remaining;
    }
    
    /**
     * Returns a new {@code InputStream} that reads no bytes. The returned
     * stream is initially open.  The stream is closed by calling the
     * {@code close()} method.  Subsequent calls to {@code close()} have no
     * effect.
     *
     * <p> While the stream is open, the {@code available()}, {@code read()},
     * {@code read(byte[])}, {@code read(byte[], int, int)},
     * {@code readAllBytes()}, {@code readNBytes(byte[], int, int)},
     * {@code readNBytes(int)}, {@code skip(long)}, and
     * {@code transferTo()} methods all behave as if end of stream has been
     * reached.  After the stream has been closed, these methods all throw
     * {@code IOException}.
     *
     * <p> The {@code markSupported()} method returns {@code false}.  The
     * {@code mark()} method does nothing, and the {@code reset()} method
     * throws {@code IOException}.
     *
     * @return an {@code InputStream} which contains no bytes
     *
     * @since 11
     */
    // 返回一个不包含有效字节的输入流
    public static InputStream nullInputStream() {
        return new InputStream() {
            private volatile boolean closed;
            
            @Override
            public int available() throws IOException {
                ensureOpen();
                return 0;
            }
            
            @Override
            public int read() throws IOException {
                ensureOpen();
                return -1;
            }
            
            @Override
            public int read(byte[] b, int off, int len) throws IOException {
                Objects.checkFromIndexSize(off, len, b.length);
                if(len == 0) {
                    return 0;
                }
                ensureOpen();
                return -1;
            }
            
            @Override
            public byte[] readAllBytes() throws IOException {
                ensureOpen();
                return new byte[0];
            }
            
            @Override
            public int readNBytes(byte[] b, int off, int len) throws IOException {
                Objects.checkFromIndexSize(off, len, b.length);
                ensureOpen();
                return 0;
            }
            
            @Override
            public byte[] readNBytes(int len) throws IOException {
                if(len<0) {
                    throw new IllegalArgumentException("len < 0");
                }
                ensureOpen();
                return new byte[0];
            }
            
            @Override
            public long skip(long n) throws IOException {
                ensureOpen();
                return 0L;
            }
            
            @Override
            public long transferTo(OutputStream out) throws IOException {
                Objects.requireNonNull(out);
                ensureOpen();
                return 0L;
            }
            
            @Override
            public void close() throws IOException {
                closed = true;
            }
            
            private void ensureOpen() throws IOException {
                if(closed) {
                    throw new IOException("Stream closed");
                }
            }
        };
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
}