Writer.java

/*
 * Copyright (c) 1996, 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.io;

import java.util.Objects;

/**
 * Abstract class for writing to character streams.  The only methods that a
 * subclass must implement are write(char[], int, int), flush(), and close().
 * Most subclasses, however, will override some of the methods defined here in
 * order to provide higher efficiency, additional functionality, or both.
 *
 * @author Mark Reinhold
 * @see BufferedWriter
 * @see CharArrayWriter
 * @see FilterWriter
 * @see OutputStreamWriter
 * @see FileWriter
 * @see PipedWriter
 * @see PrintWriter
 * @see StringWriter
 * @see Reader
 * @since 1.1
 */
/*
 * 字符输出流，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，
 * [字符]的含义是本地写入的是字符(远端通常是数组、字节流、字符流、通道)
 *
 * 注：这里的字符指char
 */
public abstract class Writer implements Appendable, Closeable, Flushable {
    
    /**
     * Size of writeBuffer, must be >= 1
     */
    private static final int WRITE_BUFFER_SIZE = 1024;
    
    /**
     * The object used to synchronize operations on this stream.  For
     * efficiency, a character-stream object may use an object other than
     * itself to protect critical sections.  A subclass should therefore use
     * the object in this field rather than {@code this} or a synchronized
     * method.
     */
    protected Object lock;
    
    /**
     * Temporary buffer used to hold writes of strings and single characters
     */
    private char[] writeBuffer; // 临时存储待写字符序列
    
    
    
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    /**
     * Creates a new character-stream writer whose critical sections will
     * synchronize on the writer itself.
     */
    protected Writer() {
        this.lock = this;
    }
    
    /**
     * Creates a new character-stream writer whose critical sections will
     * synchronize on the given object.
     *
     * @param lock Object to synchronize on
     */
    protected Writer(Object lock) {
        if(lock == null) {
            throw new NullPointerException();
        }
        this.lock = lock;
    }
    
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    /**
     * Writes a single character.  The character to be written is contained in
     * the 16 low-order bits of the given integer value; the 16 high-order bits
     * are ignored.
     *
     * <p> Subclasses that intend to support efficient single-character output
     * should override this method.
     *
     * @param c int specifying a character to be written
     *
     * @throws IOException If an I/O error occurs
     */
    // 将指定的字符写入到输出流
    public void write(int c) throws IOException {
        synchronized(lock) {
            if(writeBuffer == null) {
                writeBuffer = new char[WRITE_BUFFER_SIZE];
            }
            writeBuffer[0] = (char) c;
            write(writeBuffer, 0, 1);
        }
    }
    
    /**
     * Writes an array of characters.
     *
     * @param cbuf Array of characters to be written
     *
     * @throws IOException If an I/O error occurs
     */
    // 将字符数组cbuf的内容写入到输出流
    public void write(char[] cbuf) throws IOException {
        write(cbuf, 0, cbuf.length);
    }
    
    /**
     * Writes a portion of an array of characters.
     *
     * @param cbuf Array of characters
     * @param off  Offset from which to start writing characters
     * @param len  Number of characters to write
     *
     * @throws IndexOutOfBoundsException Implementations should throw this exception
     *                                   if {@code off} is negative, or {@code len} is negative,
     *                                   or {@code off + len} is negative or greater than the length
     *                                   of the given array
     * @throws IOException               If an I/O error occurs
     */
    // 将字符数组cbuf中off处起的len个字符写入到输出流
    public abstract void write(char[] cbuf, int off, int len) throws IOException;
    
    
    /**
     * Writes a string.
     *
     * @param str String to be written
     *
     * @throws IOException If an I/O error occurs
     */
    // 将字符串中的字符写入到输出流
    public void write(String str) throws IOException {
        write(str, 0, str.length());
    }
    
    /**
     * Writes a portion of a string.
     *
     * @param str A String
     * @param off Offset from which to start writing characters
     * @param len Number of characters to write
     *
     * @throws IndexOutOfBoundsException Implementations should throw this exception
     *                                   if {@code off} is negative, or {@code len} is negative,
     *                                   or {@code off + len} is negative or greater than the length
     *                                   of the given string
     * @throws IOException               If an I/O error occurs
     * @implSpec The implementation in this class throws an
     * {@code IndexOutOfBoundsException} for the indicated conditions;
     * overriding methods may choose to do otherwise.
     */
    // 将字符串str中off处起的len个字符写入到输出流
    public void write(String str, int off, int len) throws IOException {
        synchronized(lock) {
            char[] cbuf;
            
            if(len<=WRITE_BUFFER_SIZE) {
                if(writeBuffer == null) {
                    writeBuffer = new char[WRITE_BUFFER_SIZE];
                }
                cbuf = writeBuffer;
            } else {
                // Don't permanently allocate very large buffers.
                cbuf = new char[len];
            }
            
            // 将String内部的字节批量转换为char后存入cbuf
            str.getChars(off, (off + len), cbuf, 0);
            
            write(cbuf, 0, len);
        }
    }
    
    
    /**
     * Appends the specified character to this writer.
     *
     * <p> An invocation of this method of the form {@code out.append(c)}
     * behaves in exactly the same way as the invocation
     *
     * <pre>
     *     out.write(c) </pre>
     *
     * @param c The 16-bit character to append
     *
     * @return This writer
     *
     * @throws IOException If an I/O error occurs
     * @since 1.5
     */
    // 将指定的字符写入到输出流
    public Writer append(char c) throws IOException {
        write(c);
        return this;
    }
    
    /**
     * Appends the specified character sequence to this writer.
     *
     * <p> An invocation of this method of the form {@code out.append(csq)}
     * behaves in exactly the same way as the invocation
     *
     * <pre>
     *     out.write(csq.toString()) </pre>
     *
     * <p> Depending on the specification of {@code toString} for the
     * character sequence {@code csq}, the entire sequence may not be
     * appended. For instance, invoking the {@code toString} method of a
     * character buffer will return a subsequence whose content depends upon
     * the buffer's position and limit.
     *
     * @param csq The character sequence to append.  If {@code csq} is
     *            {@code null}, then the four characters {@code "null"} are
     *            appended to this writer.
     *
     * @return This writer
     *
     * @throws IOException If an I/O error occurs
     * @since 1.5
     */
    // 将字符序列csq的字符写入到输出流
    public Writer append(CharSequence csq) throws IOException {
        write(String.valueOf(csq));
        return this;
    }
    
    /**
     * Appends a subsequence of the specified character sequence to this writer.
     * {@code Appendable}.
     *
     * <p> An invocation of this method of the form
     * {@code out.append(csq, start, end)} when {@code csq}
     * is not {@code null} behaves in exactly the
     * same way as the invocation
     *
     * <pre>{@code
     *     out.write(csq.subSequence(start, end).toString())
     * }</pre>
     *
     * @param csq   The character sequence from which a subsequence will be
     *              appended.  If {@code csq} is {@code null}, then characters
     *              will be appended as if {@code csq} contained the four
     *              characters {@code "null"}.
     * @param start The index of the first character in the subsequence
     * @param end   The index of the character following the last character in the
     *              subsequence
     *
     * @return This writer
     *
     * @throws IndexOutOfBoundsException If {@code start} or {@code end} are negative, {@code start}
     *                                   is greater than {@code end}, or {@code end} is greater than
     *                                   {@code csq.length()}
     * @throws IOException               If an I/O error occurs
     * @since 1.5
     */
    // 将字符序列csq[start, end)范围的字符写入到输出流
    public Writer append(CharSequence csq, int start, int end) throws IOException {
        if(csq == null) {
            csq = "null";
        }
        return append(csq.subSequence(start, end));
    }
    
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    /**
     * Flushes the stream.  If the stream has saved any characters from the
     * various write() methods in a buffer, write them immediately to their
     * intended destination.  Then, if that destination is another character or
     * byte stream, flush it.  Thus one flush() invocation will flush all the
     * buffers in a chain of Writers and OutputStreams.
     *
     * <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.
     *
     * @throws IOException If an I/O error occurs
     */
    // 将内部缓冲区中的字符写入到输出流
    public abstract void flush() throws IOException;
    
    /**
     * Closes the stream, flushing it first. Once the stream has been closed,
     * further write() or flush() invocations will cause an IOException to be
     * thrown. Closing a previously closed stream has no effect.
     *
     * @throws IOException If an I/O error occurs
     */
    // 关闭输出流
    public abstract void close() throws IOException;
    
    /**
     * Returns a new {@code Writer} which discards all characters.  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 append(char)}, {@code
     * append(CharSequence)}, {@code append(CharSequence, int, int)},
     * {@code flush()}, {@code write(int)}, {@code write(char[])}, and
     * {@code write(char[], int, int)} methods do nothing. After the stream
     * has been closed, these methods all throw {@code IOException}.
     *
     * <p> The {@link #lock object} used to synchronize operations on the
     * returned {@code Writer} is not specified.
     *
     * @return a {@code Writer} which discards all characters
     *
     * @since 11
     */
    // 返回一个不包含有效字符的输出流
    public static Writer nullWriter() {
        return new Writer() {
            private volatile boolean closed;
            
            @Override
            public Writer append(char c) throws IOException {
                ensureOpen();
                return this;
            }
            
            @Override
            public Writer append(CharSequence csq) throws IOException {
                ensureOpen();
                return this;
            }
            
            @Override
            public Writer append(CharSequence csq, int start, int end) throws IOException {
                ensureOpen();
                if(csq != null) {
                    Objects.checkFromToIndex(start, end, csq.length());
                }
                return this;
            }
            
            @Override
            public void write(int c) throws IOException {
                ensureOpen();
            }
            
            @Override
            public void write(char[] cbuf, int off, int len) throws IOException {
                Objects.checkFromIndexSize(off, len, cbuf.length);
                ensureOpen();
            }
            
            @Override
            public void write(String str) throws IOException {
                Objects.requireNonNull(str);
                ensureOpen();
            }
            
            @Override
            public void write(String str, int off, int len) throws IOException {
                Objects.checkFromIndexSize(off, len, str.length());
                ensureOpen();
            }
            
            @Override
            public void flush() throws IOException {
                ensureOpen();
            }
            
            @Override
            public void close() throws IOException {
                closed = true;
            }
            
            private void ensureOpen() throws IOException {
                if(closed) {
                    throw new IOException("Stream closed");
                }
            }
        };
    }
    
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}