FilterInputStream.java

/*
 * Copyright (c) 1994, 2017, 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
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package java.io;

/**
 * A <code>FilterInputStream</code> contains
 * some other input stream, which it uses as
 * its  basic source of data, possibly transforming
 * the data along the way or providing  additional
 * functionality. The class <code>FilterInputStream</code>
 * itself simply overrides all  methods of
 * <code>InputStream</code> with versions that
 * pass all requests to the contained  input
 * stream. Subclasses of <code>FilterInputStream</code>
 * may further override some of  these methods
 * and may also provide additional methods
 * and fields.
 *
 * @author Jonathan Payne
 * @since 1.0
 */
// 对输入流的简单包装，由子类实现具体的包装行为
public class FilterInputStream extends InputStream {
    
    /**
     * The input stream to be filtered.
     */
    protected volatile InputStream in;      // 包装的输入流
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a <code>FilterInputStream</code>
     * by assigning the  argument <code>in</code>
     * to the field <code>this.in</code> so as
     * to remember it for later use.
     *
     * @param in the underlying input stream, or <code>null</code> if
     *           this instance is to be created without an underlying stream.
     */
    protected FilterInputStream(InputStream in) {
        this.in = in;
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 读 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Reads the next byte of data from this 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>
     * This method
     * simply performs <code>in.read()</code> and returns the result.
     *
     * @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.
     * @see java.io.FilterInputStream#in
     */
    /*
     * 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 int read() throws IOException {
        return in.read();
    }
    
    /**
     * Reads up to <code>b.length</code> bytes of data from this
     * input stream into an array of bytes. This method blocks until some
     * input is available.
     * <p>
     * This method simply performs the call
     * <code>read(b, 0, b.length)</code> and returns
     * the  result. It is important that it does
     * <i>not</i> do <code>in.read(b)</code> instead;
     * certain subclasses of  <code>FilterInputStream</code>
     * depend on the implementation strategy actually
     * used.
     *
     * @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 an I/O error occurs.
     * @see java.io.FilterInputStream#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[] b) throws IOException {
        return read(b, 0, b.length);
    }
    
    /**
     * 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 some input is available; otherwise, no
     * bytes are read and <code>0</code> is returned.
     * <p>
     * This method simply performs <code>in.read(b, off, len)</code>
     * and returns the result.
     *
     * @param b   the buffer into which the data is read.
     * @param off the start offset in the destination array <code>b</code>
     * @param len the maximum number of bytes 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 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.
     * @see java.io.FilterInputStream#in
     */
    /*
     * 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 {
        return in.read(b, off, len);
    }
    
    /*▲ 读 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 存档 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Tests if this input stream supports the <code>mark</code>
     * and <code>reset</code> methods.
     * This method
     * simply performs <code>in.markSupported()</code>.
     *
     * @return <code>true</code> if this stream type supports the
     * <code>mark</code> and <code>reset</code> method;
     * <code>false</code> otherwise.
     *
     * @see java.io.FilterInputStream#in
     * @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 in.markSupported();
    }
    
    /**
     * 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> argument tells this input stream to
     * allow that many bytes to be read before the mark position gets
     * invalidated.
     * <p>
     * This method simply performs <code>in.mark(readlimit)</code>.
     *
     * @param readlimit the maximum limit of bytes that can be read before
     *                  the mark position becomes invalid.
     *
     * @see java.io.FilterInputStream#in
     * @see java.io.FilterInputStream#reset()
     */
    // 设置存档标记，readlimit是存档上限
    public synchronized void mark(int readlimit) {
        in.mark(readlimit);
    }
    
    /**
     * Repositions this stream to the position at the time the
     * <code>mark</code> method was last called on this input stream.
     * <p>
     * This method
     * simply performs <code>in.reset()</code>.
     * <p>
     * Stream marks are intended to be used in
     * situations where you need to read ahead a little to see what's in
     * the stream. Often this is most easily done by invoking some
     * general parser. If the stream is of the type handled by the
     * parse, it just chugs along happily. If the stream is not of
     * that type, the parser should toss an exception when it fails.
     * If this happens within readlimit bytes, it allows the outer
     * code to reset the stream and try another parser.
     *
     * @throws IOException if the stream has not been marked or if the
     *                     mark has been invalidated.
     * @see java.io.FilterInputStream#in
     * @see java.io.FilterInputStream#mark(int)
     */
    // 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 {
        in.reset();
    }
    
    /*▲ 存档 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Closes this input stream and releases any system resources
     * associated with the stream.
     * This
     * method simply performs <code>in.close()</code>.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterInputStream#in
     */
    // 关闭输入流
    public void close() throws IOException {
        in.close();
    }
    
    /**
     * Returns an estimate of the number of bytes that can be read (or
     * skipped over) from this input stream without blocking by the next
     * caller of a method for this input stream. The next caller 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 result of {@link #in in}.available().
     *
     * @return an estimate of the number of bytes that can be read (or skipped
     * over) from this input stream without blocking.
     *
     * @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 in.available();
    }
    
    /**
     * Skips over and discards <code>n</code> bytes of data from the
     * 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>. The actual number of bytes skipped is
     * returned.
     * <p>
     * This method simply performs <code>in.skip(n)</code>.
     *
     * @param n the number of bytes to be skipped.
     *
     * @return the actual number of bytes skipped.
     *
     * @throws IOException if {@code in.skip(n)} throws an IOException.
     */
    // 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 {
        return in.skip(n);
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
}