OutputStream.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
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package java.io;

import java.util.Objects;

/**
 * This abstract class is the superclass of all classes representing
 * an output stream of bytes. An output stream accepts output bytes
 * and sends them to some sink.
 * <p>
 * Applications that need to define a subclass of
 * <code>OutputStream</code> must always provide at least a method
 * that writes one byte of output.
 *
 * @author  Arthur van Hoff
 * @see     java.io.BufferedOutputStream
 * @see     java.io.ByteArrayOutputStream
 * @see     java.io.DataOutputStream
 * @see     java.io.FilterOutputStream
 * @see     java.io.InputStream
 * @see     java.io.OutputStream#write(int)
 * @since   1.0
 */
/*
 * 字节输出流，可以向它写入数据
 *
 * 本地 ---------> 远端
 *
 * 字节输出流的解释：
 * [输出流]的含义是将本地数据写入到远端，
 * [字节]的含义是本地写入的是字节(远端通常是数组、字节流、通道)
 */
public abstract class OutputStream implements Closeable, Flushable {
    
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    /**
     * Writes the specified byte to this output stream. The general
     * contract for <code>write</code> is that one byte is written
     * to the output stream. The byte to be written is the eight
     * low-order bits of the argument <code>b</code>. The 24
     * high-order bits of <code>b</code> are ignored.
     * <p>
     * Subclasses of <code>OutputStream</code> must provide an
     * implementation for this method.
     *
     * @param      b   the <code>byte</code>.
     * @exception  IOException  if an I/O error occurs. In particular,
     *             an <code>IOException</code> may be thrown if the
     *             output stream has been closed.
     */
    // 将指定的字节写入到输出流
    public abstract void write(int b) throws IOException;
    
    /**
     * Writes <code>b.length</code> bytes from the specified byte array
     * to this output stream. The general contract for <code>write(b)</code>
     * is that it should have exactly the same effect as the call
     * <code>write(b, 0, b.length)</code>.
     *
     * @param      b   the data.
     * @exception  IOException  if an I/O error occurs.
     * @see        java.io.OutputStream#write(byte[], int, int)
     */
    // 将字节数组b的内容写入到输出流
    public void write(byte[] b) throws IOException {
        write(b, 0, b.length);
    }
    
    /**
     * Writes <code>len</code> bytes from the specified byte array
     * starting at offset <code>off</code> to this output stream.
     * The general contract for <code>write(b, off, len)</code> is that
     * some of the bytes in the array <code>b</code> are written to the
     * output stream in order; element <code>b[off]</code> is the first
     * byte written and <code>b[off+len-1]</code> is the last byte written
     * by this operation.
     * <p>
     * The <code>write</code> method of <code>OutputStream</code> calls
     * the write method of one argument on each of the bytes to be
     * written out. Subclasses are encouraged to override this method and
     * provide a more efficient implementation.
     * <p>
     * If <code>b</code> is <code>null</code>, a
     * <code>NullPointerException</code> is thrown.
     * <p>
     * If <code>off</code> is negative, or <code>len</code> is negative, or
     * <code>off+len</code> is greater than the length of the array
     * {@code b}, then an {@code IndexOutOfBoundsException} is thrown.
     *
     * @param      b     the data.
     * @param      off   the start offset in the data.
     * @param      len   the number of bytes to write.
     * @exception  IOException  if an I/O error occurs. In particular,
     *             an <code>IOException</code> is thrown if the output
     *             stream is closed.
     */
    // 将字节数组b中off处起的len个字节写入到输出流
    public void write(byte[] b, int off, int len) throws IOException {
        Objects.checkFromIndexSize(off, len, b.length);
        // len == 0 condition implicitly handled by loop bounds
        for (int i = 0 ; i < len ; i++) {
            write(b[off + i]);
        }
    }
    
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    /**
     * Flushes this output stream and forces any buffered output bytes
     * to be written out. The general contract of <code>flush</code> is
     * that calling it is an indication that, if any bytes previously
     * written have been buffered by the implementation of the output
     * stream, such bytes should immediately be written to their
     * intended destination.
     * <p>
     * If the intended destination of this stream is an abstraction provided by
     * the underlying operating system, for example a file, then flushing the
     * stream guarantees only that bytes previously written to the stream are
     * passed to the operating system for writing; it does not guarantee that
     * they are actually written to a physical device such as a disk drive.
     * <p>
     * The <code>flush</code> method of <code>OutputStream</code> does nothing.
     *
     * @exception  IOException  if an I/O error occurs.
     */
    // 将内部缓冲区中的字节写入到输出流
    public void flush() throws IOException {
    }
    
    /**
     * Closes this output stream and releases any system resources
     * associated with this stream. The general contract of <code>close</code>
     * is that it closes the output stream. A closed stream cannot perform
     * output operations and cannot be reopened.
     * <p>
     * The <code>close</code> method of <code>OutputStream</code> does nothing.
     *
     * @exception  IOException  if an I/O error occurs.
     */
    // 关闭输出流
    public void close() throws IOException {
    }
    
    /**
     * Returns a new {@code OutputStream} which discards all 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 write(int)}, {@code
     * write(byte[])}, and {@code write(byte[], int, int)} methods do nothing.
     * After the stream has been closed, these methods all throw {@code
     * IOException}.
     *
     * <p> The {@code flush()} method does nothing.
     *
     * @return an {@code OutputStream} which discards all bytes
     *
     * @since 11
     */
    // 返回一个不包含有效字节的输出流
    public static OutputStream nullOutputStream() {
        return new OutputStream() {
            private volatile boolean closed;
            
            private void ensureOpen() throws IOException {
                if (closed) {
                    throw new IOException("Stream closed");
                }
            }
            
            @Override
            public void write(int b) throws IOException {
                ensureOpen();
            }
            
            @Override
            public void write(byte b[], int off, int len) throws IOException {
                Objects.checkFromIndexSize(off, len, b.length);
                ensureOpen();
            }
            
            @Override
            public void close() {
                closed = true;
            }
        };
    }
    
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}