DataOutputStream.java

/*
 * Copyright (c) 1994, 2004, 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;

/**
 * A data output stream lets an application write primitive Java data
 * types to an output stream in a portable way. An application can
 * then use a data input stream to read the data back in.
 *
 * @author unascribed
 * @see java.io.DataInputStream
 * @since 1.0
 */
// 基础数据输出流
public class DataOutputStream extends FilterOutputStream implements DataOutput {
    
    /**
     * The number of bytes written to the data output stream so far.
     * If this counter overflows, it will be wrapped to Integer.MAX_VALUE.
     */
    protected int written;  // 已写入的字节数量
    
    private byte[] writeBuffer = new byte[8];
    
    /**
     * bytearr is initialized on demand by writeUTF
     */
    // 缓存写入的按UTF8编码后的String
    private byte[] bytearr = null;
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a new data output stream to write data to the specified
     * underlying output stream. The counter <code>written</code> is
     * set to zero.
     *
     * @param out the underlying output stream, to be saved for later
     *            use.
     *
     * @see java.io.FilterOutputStream#out
     */
    public DataOutputStream(OutputStream out) {
        super(out);
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 写 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Writes a <code>boolean</code> to the underlying output stream as
     * a 1-byte value. The value <code>true</code> is written out as the
     * value <code>(byte)1</code>; the value <code>false</code> is
     * written out as the value <code>(byte)0</code>. If no exception is
     * thrown, the counter <code>written</code> is incremented by
     * <code>1</code>.
     *
     * @param v a <code>boolean</code> value to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入boolean值
    public final void writeBoolean(boolean v) throws IOException {
        out.write(v ? 1 : 0);
        incCount(1);
    }
    
    /**
     * Writes a <code>char</code> to the underlying output stream as a
     * 2-byte value, high byte first. If no exception is thrown, the
     * counter <code>written</code> is incremented by <code>2</code>.
     *
     * @param v a <code>char</code> value to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入char值（按大端法写入）
    public final void writeChar(int v) throws IOException {
        out.write((v >>> 8) & 0xFF);
        out.write((v >>> 0) & 0xFF);
        incCount(2);
    }
    
    /**
     * Writes out a <code>byte</code> to the underlying output stream as
     * a 1-byte value. If no exception is thrown, the counter
     * <code>written</code> is incremented by <code>1</code>.
     *
     * @param v a <code>byte</code> value to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入byte值
    public final void writeByte(int v) throws IOException {
        out.write(v);
        incCount(1);
    }
    
    /**
     * Writes a <code>short</code> to the underlying output stream as two
     * bytes, high byte first. If no exception is thrown, the counter
     * <code>written</code> is incremented by <code>2</code>.
     *
     * @param v a <code>short</code> to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入short值（按大端法写入）
    public final void writeShort(int v) throws IOException {
        out.write((v >>> 8) & 0xFF);
        out.write((v >>> 0) & 0xFF);
        incCount(2);
    }
    
    /**
     * Writes an <code>int</code> to the underlying output stream as four
     * bytes, high byte first. If no exception is thrown, the counter
     * <code>written</code> is incremented by <code>4</code>.
     *
     * @param v an <code>int</code> to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入int值（按大端法写入）
    public final void writeInt(int v) throws IOException {
        out.write((v >>> 24) & 0xFF);
        out.write((v >>> 16) & 0xFF);
        out.write((v >>> 8) & 0xFF);
        out.write((v >>> 0) & 0xFF);
        
        // 写入的字节计数：写入4个字节
        incCount(4);
    }
    
    /**
     * Writes a <code>long</code> to the underlying output stream as eight
     * bytes, high byte first. In no exception is thrown, the counter
     * <code>written</code> is incremented by <code>8</code>.
     *
     * @param v a <code>long</code> to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入long值（按大端法写入）
    public final void writeLong(long v) throws IOException {
        writeBuffer[0] = (byte) (v >>> 56);
        writeBuffer[1] = (byte) (v >>> 48);
        writeBuffer[2] = (byte) (v >>> 40);
        writeBuffer[3] = (byte) (v >>> 32);
        writeBuffer[4] = (byte) (v >>> 24);
        writeBuffer[5] = (byte) (v >>> 16);
        writeBuffer[6] = (byte) (v >>> 8);
        writeBuffer[7] = (byte) (v >>> 0);
        out.write(writeBuffer, 0, 8);
        incCount(8);
    }
    
    /**
     * Converts the float argument to an <code>int</code> using the
     * <code>floatToIntBits</code> method in class <code>Float</code>,
     * and then writes that <code>int</code> value to the underlying
     * output stream as a 4-byte quantity, high byte first. If no
     * exception is thrown, the counter <code>written</code> is
     * incremented by <code>4</code>.
     *
     * @param v a <code>float</code> value to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     * @see java.lang.Float#floatToIntBits(float)
     */
    // 向包装的输出流写入float值（先转为二进制，再按大端法写入）
    public final void writeFloat(float v) throws IOException {
        writeInt(Float.floatToIntBits(v));
    }
    
    /**
     * Converts the double argument to a <code>long</code> using the
     * <code>doubleToLongBits</code> method in class <code>Double</code>,
     * and then writes that <code>long</code> value to the underlying
     * output stream as an 8-byte quantity, high byte first. If no
     * exception is thrown, the counter <code>written</code> is
     * incremented by <code>8</code>.
     *
     * @param v a <code>double</code> value to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     * @see java.lang.Double#doubleToLongBits(double)
     */
    // 向包装的输出流写入double值（先转为二进制，按大端法写入）
    public final void writeDouble(double v) throws IOException {
        writeLong(Double.doubleToLongBits(v));
    }
    
    /**
     * Writes out the string to the underlying output stream as a
     * sequence of bytes. Each character in the string is written out, in
     * sequence, by discarding its high eight bits. If no exception is
     * thrown, the counter <code>written</code> is incremented by the
     * length of <code>s</code>.
     *
     * @param s a string of bytes to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入String值(按字节写入)
    public final void writeBytes(String s) throws IOException {
        int len = s.length();
        for(int i = 0; i<len; i++) {
            out.write((byte) s.charAt(i));
        }
        incCount(len);
    }
    
    /**
     * Writes a string to the underlying output stream as a sequence of
     * characters. Each character is written to the data output stream as
     * if by the <code>writeChar</code> method. If no exception is
     * thrown, the counter <code>written</code> is incremented by twice
     * the length of <code>s</code>.
     *
     * @param s a <code>String</code> value to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.DataOutputStream#writeChar(int)
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入String值(按字符写入)
    public final void writeChars(String s) throws IOException {
        int len = s.length();
        for(int i = 0; i<len; i++) {
            int v = s.charAt(i);
            out.write((v >>> 8) & 0xFF);
            out.write((v >>> 0) & 0xFF);
        }
        incCount(len * 2);
    }
    
    
    /**
     * Writes the specified byte (the low eight bits of the argument
     * <code>b</code>) to the underlying output stream. If no exception
     * is thrown, the counter <code>written</code> is incremented by
     * <code>1</code>.
     * <p>
     * Implements the <code>write</code> method of <code>OutputStream</code>.
     *
     * @param b the <code>byte</code> to be written.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入一个byte
    public synchronized void write(int b) throws IOException {
        out.write(b);
        incCount(1);
    }
    
    /**
     * Writes <code>len</code> bytes from the specified byte array
     * starting at offset <code>off</code> to the underlying output stream.
     * If no exception is thrown, the counter <code>written</code> is
     * incremented by <code>len</code>.
     *
     * @param b   the data.
     * @param off the start offset in the data.
     * @param len the number of bytes to write.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     */
    // 向包装的输出流写入b中off处起的len个byte
    public synchronized void write(byte[] b, int off, int len) throws IOException {
        out.write(b, off, len);
        incCount(len);
    }
    
    
    /**
     * Writes a string to the underlying output stream using
     * <a href="DataInput.html#modified-utf-8">modified UTF-8</a>
     * encoding in a machine-independent manner.
     * <p>
     * First, two bytes are written to the output stream as if by the
     * <code>writeShort</code> method giving the number of bytes to
     * follow. This value is the number of bytes actually written out,
     * not the length of the string. Following the length, each character
     * of the string is output, in sequence, using the modified UTF-8 encoding
     * for the character. If no exception is thrown, the counter
     * <code>written</code> is incremented by the total number of
     * bytes written to the output stream. This will be at least two
     * plus the length of <code>str</code>, and at most two plus
     * thrice the length of <code>str</code>.
     *
     * @param str a string to be written.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 将String按UTF8编码后写入当前输出流
    public final void writeUTF(String str) throws IOException {
        writeUTF(str, this);
    }
    
    /**
     * Writes a string to the specified DataOutput using
     * <a href="DataInput.html#modified-utf-8">modified UTF-8</a>
     * encoding in a machine-independent manner.
     * <p>
     * First, two bytes are written to out as if by the <code>writeShort</code>
     * method giving the number of bytes to follow. This value is the number of
     * bytes actually written out, not the length of the string. Following the
     * length, each character of the string is output, in sequence, using the
     * modified UTF-8 encoding for the character. If no exception is thrown, the
     * counter <code>written</code> is incremented by the total number of
     * bytes written to the output stream. This will be at least two
     * plus the length of <code>str</code>, and at most two plus
     * thrice the length of <code>str</code>.
     *
     * @param str a string to be written.
     * @param out destination to write to
     *
     * @return The number of bytes written out.
     *
     * @throws IOException if an I/O error occurs.
     */
    // 将String按UTF8编码后写入out输出流
    static int writeUTF(String str, DataOutput out) throws IOException {
        int strlen = str.length();
        int utflen = 0; // 记录str按UTF8编码的字节数
        int c, count = 0;
        
        /* use charAt instead of copying String to char array */
        for(int i = 0; i<strlen; i++) {
            c = str.charAt(i);
            
            // 单字节字符
            if((c >= 0x0001) && (c<=0x007F)) {
                utflen++;
                
                // 三字节字符
            } else if(c>0x07FF) {
                utflen += 3;
                
                // 双字节字符
            } else {
                utflen += 2;
            }
        }
        
        if(utflen>65535) {
            throw new UTFDataFormatException("encoded string too long: " + utflen + " bytes");
        }
        
        byte[] bytearr = null;
        if(out instanceof DataOutputStream) {
            DataOutputStream dos = (DataOutputStream) out;
            if(dos.bytearr == null || (dos.bytearr.length<(utflen + 2))) {
                dos.bytearr = new byte[(utflen * 2) + 2];
            }
            bytearr = dos.bytearr;
        } else {
            bytearr = new byte[utflen + 2];
        }
        
        // 按大端法写入字节数量信息(占用2字节)
        bytearr[count++] = (byte) ((utflen >>> 8) & 0xFF);
        bytearr[count++] = (byte) ((utflen >>> 0) & 0xFF);
        
        int i = 0;
        for(i = 0; i<strlen; i++) {
            c = str.charAt(i);
            
            if(!((c >= 0x0001) && (c<=0x007F))) {
                break;
            }
            
            bytearr[count++] = (byte) c;
        }
        
        for(; i<strlen; i++) {
            c = str.charAt(i);
            if((c >= 0x0001) && (c<=0x007F)) {
                bytearr[count++] = (byte) c;
            } else if(c>0x07FF) {
                bytearr[count++] = (byte) (0xE0 | ((c >> 12) & 0x0F));
                bytearr[count++] = (byte) (0x80 | ((c >> 6) & 0x3F));
                bytearr[count++] = (byte) (0x80 | ((c >> 0) & 0x3F));
            } else {
                bytearr[count++] = (byte) (0xC0 | ((c >> 6) & 0x1F));
                bytearr[count++] = (byte) (0x80 | ((c >> 0) & 0x3F));
            }
        }
        
        out.write(bytearr, 0, utflen + 2);
        
        return utflen + 2;
    }
    
    /*▲ 写 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Flushes this data output stream. This forces any buffered output
     * bytes to be written out to the stream.
     * <p>
     * The <code>flush</code> method of <code>DataOutputStream</code>
     * calls the <code>flush</code> method of its underlying output stream.
     *
     * @throws IOException if an I/O error occurs.
     * @see java.io.FilterOutputStream#out
     * @see java.io.OutputStream#flush()
     */
    // 刷新缓冲区
    public void flush() throws IOException {
        out.flush();
    }
    
    /**
     * Returns the current value of the counter <code>written</code>,
     * the number of bytes written to this data output stream so far.
     * If the counter overflows, it will be wrapped to Integer.MAX_VALUE.
     *
     * @return the value of the <code>written</code> field.
     *
     * @see java.io.DataOutputStream#written
     */
    // 返回已写入的字节数量
    public final int size() {
        return written;
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Increases the written counter by the specified value
     * until it reaches Integer.MAX_VALUE.
     */
    private void incCount(int value) {
        int temp = written + value;
        if(temp<0) {
            temp = Integer.MAX_VALUE;
        }
        
        written = temp;
    }
    
}