CharArrayReader.java

/*
 * Copyright (c) 1996, 2015, 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;

/**
 * This class implements a character buffer that can be used as a
 * character-input stream.
 *
 * @author Herb Jellinek
 * @since 1.1
 */
// 字符数组输入流：从给定的字符数组中读取字符
public class CharArrayReader extends Reader {
    
    /** The character buffer. */
    protected char[] buf;   // 输入源，存放待读字符
    
    /** The current buffer position. */
    protected int pos;      // 指向输入源下一个待读字符的游标
    
    /**
     * The index of the end of this buffer.  There is not valid
     * data at or beyond this index.
     */
    protected int count;    // 输入源中的游标上限
    
    /** The position of mark in buffer. */
    protected int markedPos = 0;
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a CharArrayReader from the specified array of chars.
     *
     * @param buf Input buffer (not copied)
     */
    public CharArrayReader(char[] buf) {
        this.buf = buf;
        this.pos = 0;
        this.count = buf.length;
    }
    
    /**
     * Creates a CharArrayReader from the specified array of chars.
     *
     * <p> The resulting reader will start reading at the given
     * {@code offset}.  The total number of {@code char} values that can be
     * read from this reader will be either {@code length} or
     * {@code buf.length-offset}, whichever is smaller.
     *
     * @param buf    Input buffer (not copied)
     * @param offset Offset of the first char to read
     * @param length Number of chars to read
     *
     * @throws IllegalArgumentException If {@code offset} is negative or greater than
     *                                  {@code buf.length}, or if {@code length} is negative, or if
     *                                  the sum of these two values is negative.
     */
    public CharArrayReader(char[] buf, int offset, int length) {
        if((offset<0) || (offset>buf.length) || (length<0) || ((offset + length)<0)) {
            throw new IllegalArgumentException();
        }
        this.buf = buf;
        this.pos = offset;
        this.count = Math.min(offset + length, buf.length);
        this.markedPos = offset;
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 读 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Reads a single character.
     *
     * @throws IOException If an I/O error occurs
     */
    // 返回从当前输入流中读取的一个char，返回-1表示读取失败
    public int read() throws IOException {
        synchronized(lock) {
            ensureOpen();
            
            if(pos >= count) {
                return -1;
            } else {
                return buf[pos++];
            }
        }
    }
    
    /**
     * Reads characters into a portion of an array.
     *
     * @param b   Destination buffer
     * @param off Offset at which to start storing characters
     * @param len Maximum number of characters to read
     *
     * @return The actual number of characters read, or -1 if
     * the end of the stream has been reached
     *
     * @throws IOException               If an I/O error occurs
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    // 尝试从当前输入流中读取len个char，并将其填充到b的off处。返回实际填充的字符数量，返回-1表示读取失败
    public int read(char[] b, int off, int len) throws IOException {
        synchronized(lock) {
            ensureOpen();
            
            if((off<0) || (off>b.length) || (len<0) || ((off + len)>b.length) || ((off + len)<0)) {
                throw new IndexOutOfBoundsException();
            } else if(len == 0) {
                return 0;
            }
            
            if(pos >= count) {
                return -1;
            }
            
            int avail = count - pos;
            if(len>avail) {
                len = avail;
            }
            
            if(len<=0) {
                return 0;
            }
            
            System.arraycopy(buf, pos, b, off, len);
            pos += len;
            return len;
        }
    }
    
    /*▲ 读 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 存档 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Tells whether this stream supports the mark() operation, which it does.
     */
    // 判断当前输入流是否支持存档标记，此处默认为true
    public boolean markSupported() {
        return true;
    }
    
    /**
     * Marks the present position in the stream.  Subsequent calls to reset()
     * will reposition the stream to this point.
     *
     * @param readAheadLimit Limit on the number of characters that may be
     *                       read while still preserving the mark.  Because
     *                       the stream's input comes from a character array,
     *                       there is no actual limit; hence this argument is
     *                       ignored.
     *
     * @throws IOException If an I/O error occurs
     */
    // 设置存档标记，readAheadLimit参数未使用
    public void mark(int readAheadLimit) throws IOException {
        synchronized(lock) {
            ensureOpen();
            markedPos = pos;
        }
    }
    
    /**
     * Resets the stream to the most recent mark, or to the beginning if it has
     * never been marked.
     *
     * @throws IOException If an I/O error occurs
     */
    // 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 void reset() throws IOException {
        synchronized(lock) {
            ensureOpen();
            pos = markedPos;
        }
    }
    
    /*▲ 存档 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Closes the stream and releases any system resources associated with
     * it.  Once the stream has been closed, further read(), ready(),
     * mark(), reset(), or skip() invocations will throw an IOException.
     * Closing a previously closed stream has no effect. This method will block
     * while there is another thread blocking on the reader.
     */
    // 关闭输入流
    public void close() {
        synchronized(lock) {
            buf = null;
        }
    }
    
    /**
     * Tells whether this stream is ready to be read.  Character-array readers
     * are always ready to be read.
     *
     * @throws IOException If an I/O error occurs
     */
    // 判断当前流是否已准备好被读取
    public boolean ready() throws IOException {
        synchronized(lock) {
            ensureOpen();
            return (count - pos)>0;
        }
    }
    
    /**
     * Skips characters.  Returns the number of characters that were skipped.
     *
     * <p>The <code>n</code> parameter may be negative, even though the
     * <code>skip</code> method of the {@link Reader} superclass throws
     * an exception in this case. If <code>n</code> is negative, then
     * this method does nothing and returns <code>0</code>.
     *
     * @param n The number of characters to skip
     *
     * @return The number of characters actually skipped
     *
     * @throws IOException If the stream is closed, or an I/O error occurs
     */
    // 读取中跳过n个字符，返回实际跳过的字符数
    public long skip(long n) throws IOException {
        synchronized(lock) {
            ensureOpen();
            
            long avail = count - pos;
            if(n>avail) {
                n = avail;
            }
            
            if(n<0) {
                return 0;
            }
            
            pos += n;
            return n;
        }
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /** Checks to make sure that the stream has not been closed */
    private void ensureOpen() throws IOException {
        if(buf == null) {
            throw new IOException("Stream closed");
        }
    }
    
}