SequenceInputStream.java

/*
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 * 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
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package java.io;

import java.util.Enumeration;
import java.util.Vector;

/**
 * A <code>SequenceInputStream</code> represents
 * the logical concatenation of other input
 * streams. It starts out with an ordered
 * collection of input streams and reads from
 * the first one until end of file is reached,
 * whereupon it reads from the second one,
 * and so on, until end of file is reached
 * on the last of the contained input streams.
 *
 * @author Author van Hoff
 * @since 1.0
 */
// 输入流序列(集合)，可以合并多个输入流并对其顺序读取
public class SequenceInputStream extends InputStream {
    
    Enumeration<? extends InputStream> list;    // 输入流集合
    InputStream in; // 下一个未处理的输入流
    
    /**
     * Initializes a newly
     * created <code>SequenceInputStream</code>
     * by remembering the two arguments, which
     * will be read in order, first <code>s1</code>
     * and then <code>s2</code>, to provide the
     * bytes to be read from this <code>SequenceInputStream</code>.
     *
     * @param s1 the first input stream to read.
     * @param s2 the second input stream to read.
     */
    public SequenceInputStream(InputStream s1, InputStream s2) {
        Vector<InputStream> v = new Vector<>(2);
        v.addElement(s1);
        v.addElement(s2);
        list = v.elements();
        peekNextStream();   // 查看是否存在下一个输入流
    }
    
    /**
     * Initializes a newly created <code>SequenceInputStream</code>
     * by remembering the argument, which must
     * be an <code>Enumeration</code>  that produces
     * objects whose run-time type is <code>InputStream</code>.
     * The input streams that are  produced by
     * the enumeration will be read, in order,
     * to provide the bytes to be read  from this
     * <code>SequenceInputStream</code>. After
     * each input stream from the enumeration
     * is exhausted, it is closed by calling its
     * <code>close</code> method.
     *
     * @param e an enumeration of input streams.
     *
     * @see java.util.Enumeration
     */
    public SequenceInputStream(Enumeration<? extends InputStream> list) {
        this.list = list;
        peekNextStream();   // 查看是否存在下一个输入流
    }
    
    /**
     * Reads the next byte of data from this input stream. The 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>
     * This method
     * tries to read one character from the current substream. If it
     * reaches the end of the stream, it calls the <code>close</code>
     * method of the current substream and begins reading from the next
     * substream.
     *
     * @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.
     */
    // 从输入流集合中读取一个字节并返回
    public int read() throws IOException {
        while(in != null) {
            int c = in.read();
            if(c != -1) {
                return c;
            }
            nextStream();   // 获取下一个输入流
        }
        
        return -1;
    }
    
    /**
     * Reads up to <code>len</code> bytes of data from this input stream
     * into an array of bytes.  If <code>len</code> is not zero, the method
     * blocks until at least 1 byte of input is available; otherwise, no
     * bytes are read and <code>0</code> is returned.
     * <p>
     * The <code>read</code> method of <code>SequenceInputStream</code>
     * tries to read the data from the current substream. If it fails to
     * read any characters because the substream has reached the end of
     * the stream, it calls the <code>close</code> method of the current
     * substream and begins reading from the next substream.
     *
     * @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 read.
     *
     * @return int   the number of bytes read.
     *
     * @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>
     * @throws IOException               if an I/O error occurs.
     */
    // 从输入流集合中读取len个字节，并将其存入字节数组b的off处，最后返回读到的字节数量
    public int read(byte[] b, int off, int len) throws IOException {
        if(in == null) {
            return -1;
        } else if(b == null) {
            throw new NullPointerException();
        } else if(off<0 || len<0 || len>b.length - off) {
            throw new IndexOutOfBoundsException();
        } else if(len == 0) {
            return 0;
        }
        
        do {
            int n = in.read(b, off, len);
            if(n>0) {
                return n;
            }
            nextStream();   // 获取下一个输入流
        } while(in != null);
        
        return -1;
    }
    
    /**
     * Returns an estimate of the number of bytes that can be read (or
     * skipped over) from the current underlying input stream without
     * blocking by the next invocation of a method for the current
     * underlying 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 simply calls {@code available} of the current underlying
     * input stream and returns the result.
     *
     * @return an estimate of the number of bytes that can be read (or
     * skipped over) from the current underlying input stream
     * without blocking or {@code 0} if this input stream
     * has been closed by invoking its {@link #close()} method
     *
     * @throws IOException if an I/O error occurs.
     * @since 1.1
     */
    // 返回当前剩余未读(可用)的字节数量
    public int available() throws IOException {
        if(in == null) {
            return 0; // no way to signal EOF from available()
        }
        
        return in.available();
    }
    
    /**
     * Closes this input stream and releases any system resources
     * associated with the stream.
     * A closed <code>SequenceInputStream</code>
     * cannot  perform input operations and cannot
     * be reopened.
     * <p>
     * If this stream was created
     * from an enumeration, all remaining elements
     * are requested from the enumeration and closed
     * before the <code>close</code> method returns.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 关闭所有输入流
    public void close() throws IOException {
        do {
            nextStream();   // 获取下一个输入流
        } while(in != null);
    }
    
    /**
     * Continues reading in the next stream if an EOF is reached.
     */
    // 获取下一个输入流
    final void nextStream() throws IOException {
        // 先把当前输入流关闭
        if(in != null) {
            in.close();
        }
        peekNextStream();   // 查看是否存在下一个输入流
    }
    
    // 查看是否存在下一个输入流
    private void peekNextStream() {
        // 如果存在未遍历元素
        if(list.hasMoreElements()) {
            // 获取下一个输入流
            in = list.nextElement();
            if(in == null) {
                throw new NullPointerException();
            }
        } else {
            in = null;
        }
    }
    
}