ByteArrayInputStream.java

/*
 * Copyright (c) 1994, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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.
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 * 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.Arrays;
import java.util.Objects;

/**
 * A {@code ByteArrayInputStream} contains an internal buffer that contains bytes that may be read from the stream.
 * An internal counter keeps track of the next byte to be supplied by the {@code read} method.
 * <p>
 * Closing a {@code ByteArrayInputStream} has no effect.
 * The methods in this class can be called after the stream has been closed without generating an {@code IOException}.
 *
 * @author Arthur van Hoff
 * @see java.io.StringBufferInputStream
 * @since 1.0
 */
// 字节数组输入流：将字节数组作为输入源
public class ByteArrayInputStream extends InputStream {
    
    /**
     * An array of bytes that was provided
     * by the creator of the stream. Elements {@code buf[0]}
     * through {@code buf[count-1]} are the
     * only bytes that can ever be read from the
     * stream;  element {@code buf[pos]} is
     * the next byte to be read.
     */
    protected byte[] buf;   // 提供输入源的字节数组
    
    /**
     * The index one greater than the last valid character in the input
     * stream buffer.
     * This value should always be nonnegative
     * and not larger than the length of {@code buf}.
     * It  is one greater than the position of
     * the last byte within {@code buf} that
     * can ever be read  from the input stream buffer.
     */
    protected int count;    // 可读数据的上界
    
    /**
     * The index of the next character to read from the input stream buffer.
     * This value should always be nonnegative
     * and not larger than the value of {@code count}.
     * The next byte to be read from the input stream buffer
     * will be {@code buf[pos]}.
     */
    protected int pos;      // "读游标"
    
    /**
     * The currently marked position in the stream.
     * ByteArrayInputStream objects are marked at position zero by
     * default when constructed.  They may be marked at another
     * position within the buffer by the {@code mark()} method.
     * The current buffer position is set to this point by the
     * {@code reset()} method.
     * <p>
     * If no mark has been set, then the value of mark is the offset
     * passed to the constructor (or 0 if the offset was not supplied).
     *
     * @since 1.1
     */
    protected int mark = 0; // 存档标记
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a {@code ByteArrayInputStream}
     * so that it  uses {@code buf} as its
     * buffer array.
     * The buffer array is not copied.
     * The initial value of {@code pos}
     * is {@code 0} and the initial value
     * of  {@code count} is the length of
     * {@code buf}.
     *
     * @param buf the input buffer.
     */
    public ByteArrayInputStream(byte[] buf) {
        this.buf = buf;
        this.pos = 0;
        this.count = buf.length;
    }
    
    /**
     * Creates {@code ByteArrayInputStream}
     * that uses {@code buf} as its
     * buffer array. The initial value of {@code pos}
     * is {@code offset} and the initial value
     * of {@code count} is the minimum of {@code offset+length}
     * and {@code buf.length}.
     * The buffer array is not copied. The buffer's mark is
     * set to the specified offset.
     *
     * @param buf    the input buffer.
     * @param offset the offset in the buffer of the first byte to read.
     * @param length the maximum number of bytes to read from the buffer.
     */
    public ByteArrayInputStream(byte[] buf, int offset, int length) {
        this.buf = buf;
        this.pos = offset;
        this.count = Math.min(offset + length, buf.length);
        this.mark = offset;
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 读 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Reads the next byte of data from this input stream. The value
     * byte is returned as an {@code int} in the range
     * {@code 0} to {@code 255}. If no byte is available
     * because the end of the stream has been reached, the value
     * {@code -1} is returned.
     * <p>
     * This {@code read} method
     * cannot block.
     *
     * @return the next byte of data, or {@code -1} if the end of the
     * stream has been reached.
     */
    /*
     * 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 synchronized int read() {
        return (pos<count) ? (buf[pos++] & 0xff) : -1;
    }
    
    /**
     * Reads up to {@code len} bytes of data into an array of bytes from this
     * input stream.  If {@code pos} equals {@code count}, then {@code -1} is
     * returned to indicate end of file.  Otherwise, the  number {@code k} of
     * bytes read is equal to the smaller of {@code len} and {@code count-pos}.
     * If {@code k} is positive, then bytes {@code buf[pos]} through
     * {@code buf[pos+k-1]} are copied into {@code b[off]} through
     * {@code b[off+k-1]} in the manner performed by {@code System.arraycopy}.
     * The value {@code k} is added into {@code pos} and {@code k} is returned.
     * <p>
     * This {@code read} method cannot block.
     *
     * @param b   the buffer into which the data is read.
     * @param off the start offset in the destination array {@code b}
     * @param len the maximum number of bytes read.
     *
     * @return the total number of bytes read into the buffer, or
     * {@code -1} if there is no more data because the end of
     * the stream has been reached.
     *
     * @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}
     */
    /*
     * 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 synchronized int read(byte[] b, int off, int len) {
        Objects.checkFromIndexSize(off, len, b.length);
        
        if(pos >= count) {
            return -1;
        }
        
        int avail = count - pos;
        if(len>avail) {
            len = avail;
        }
        
        if(len<=0) {
            return 0;
        }
        
        // 复制数据
        System.arraycopy(buf, pos, b, off, len);
        
        pos += len;
        
        return len;
    }
    
    // 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 synchronized byte[] readAllBytes() {
        byte[] result = Arrays.copyOfRange(buf, pos, count);
        pos = count;
        return result;
    }
    
    /*
     * 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) {
        int n = read(b, off, len);
        return n == -1 ? 0 : n;
    }
    
    /*▲ 读 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 存档 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Tests if this {@code InputStream} supports mark/reset. The
     * {@code markSupported} method of {@code ByteArrayInputStream}
     * always returns {@code true}.
     *
     * @since 1.1
     */
    // 判断当前输入流是否支持存档标记
    public boolean markSupported() {
        return true;
    }
    
    /**
     * Set the current marked position in the stream.
     * ByteArrayInputStream objects are marked at position zero by
     * default when constructed.  They may be marked at another
     * position within the buffer by this method.
     * <p>
     * If no mark has been set, then the value of the mark is the
     * offset passed to the constructor (or 0 if the offset was not
     * supplied).
     *
     * <p> Note: The {@code readAheadLimit} for this class
     * has no meaning.
     *
     * @since 1.1
     */
    // 设置存档标记，此处未设存档上限
    public void mark(int readAheadLimit) {
        mark = pos;
    }
    
    /**
     * Resets the buffer to the marked position.  The marked position
     * is 0 unless another position was marked or an offset was specified
     * in the constructor.
     */
    // 重置"读游标"到存档区的起始位置
    public synchronized void reset() {
        pos = mark;
    }
    
    /*▲ 存档 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 转移 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    // 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 synchronized long transferTo(OutputStream out) throws IOException {
        int len = count - pos;
        out.write(buf, pos, len);
        pos = count;
        return len;
    }
    
    /*▲ 转移 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Closing a {@code ByteArrayInputStream} has no effect. The methods in
     * this class can be called after the stream has been closed without
     * generating an {@code IOException}.
     */
    // 关闭输入流
    public void close() throws IOException {
    }
    
    /**
     * Returns the number of remaining bytes that can be read (or skipped over)
     * from this input stream.
     * <p>
     * The value returned is {@code count - pos},
     * which is the number of bytes remaining to be read from the input buffer.
     *
     * @return the number of remaining bytes that can be read (or skipped
     * over) from this input stream without blocking.
     */
    // 返回可读的字节数量
    public synchronized int available() {
        return count - pos;
    }
    
    /**
     * Skips {@code n} bytes of input from this input stream. Fewer
     * bytes might be skipped if the end of the input stream is reached.
     * The actual number {@code k}
     * of bytes to be skipped is equal to the smaller
     * of {@code n} and  {@code count-pos}.
     * The value {@code k} is added into {@code pos}
     * and {@code k} is returned.
     *
     * @param n the number of bytes to be skipped.
     *
     * @return the actual number of bytes skipped.
     */
    // 读取中跳过n个字节，返回实际跳过的字节数
    public synchronized long skip(long n) {
        long k = count - pos;
        if(n<k) {
            k = n<0 ? 0 : n;
        }
        
        pos += k;
        return k;
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
}