java io系列15之 DataOutputStream详解

本章介绍DataOutputStream。我们先对DataOutputStream有个大致认识,然后再深入学习它的源码,最后通过示例加深对它的了解。

目录
1. DataOutputStream 介绍
2. DataOutputStream 源码分析(基于jdk1.7.40)
3. 示例代码

1. DataOutputStream 介绍

DataOutputStream 是数据输出流。它继承于FilterOutputStream。

DataOutputStream 是用来装饰其它输出流,将DataOutputStream和DataInputStream输入流配合使用,“允许应用程序以与机器无关方式从底层输入流中读写基本 Java 数据类型”。

2. DataOutputStream 源码分析(基于jdk1.7.40)

package java.io;

public class DataOutputStream extends FilterOutputStream implements DataOutput {
    // “数据输出流”的字节数
    protected int written;

    // “数据输出流”对应的字节数组
    private byte[] bytearr = null;

    // 构造函数
    public DataOutputStream(OutputStream out) {
        super(out);
    }

    // 增加“输出值”
    private void incCount(int value) {
        int temp = written + value;
        if (temp < 0) {
            temp = Integer.MAX_VALUE;
        }
        written = temp;
    }

    // 将int类型的值写入到“数据输出流”中
    public synchronized void write(int b) throws IOException {
        out.write(b);
        incCount(1);
    }

    // 将字节数组b从off开始的len个字节,都写入到“数据输出流”中
    public synchronized void write(byte b[], int off, int len)
        throws IOException
    {
        out.write(b, off, len);
        incCount(len);
    }

    // 清空缓冲,即将缓冲中的数据都写入到输出流中
    public void flush() throws IOException {
        out.flush();
    }

    // 将boolean类型的值写入到“数据输出流”中
    public final void writeBoolean(boolean v) throws IOException {
        out.write(v ? 1 : 0);
        incCount(1);
    }

    // 将byte类型的值写入到“数据输出流”中
    public final void writeByte(int v) throws IOException {
        out.write(v);
        incCount(1);
    }

    // 将short类型的值写入到“数据输出流”中
    // 注意:short占2个字节
    public final void writeShort(int v) throws IOException {
        // 写入 short高8位 对应的字节
        out.write((v >>> 8) & 0xFF);
        // 写入 short低8位 对应的字节
        out.write((v >>> 0) & 0xFF);
        incCount(2);
    }

    // 将char类型的值写入到“数据输出流”中
    // 注意:char占2个字节
    public final void writeChar(int v) throws IOException {
        // 写入 char高8位 对应的字节
        out.write((v >>> 8) & 0xFF);
        // 写入 char低8位 对应的字节
        out.write((v >>> 0) & 0xFF);
        incCount(2);
    }

    // 将int类型的值写入到“数据输出流”中
    // 注意:int占4个字节
    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);
        incCount(4);
    }

    private byte writeBuffer[] = new byte[8];

    // 将long类型的值写入到“数据输出流”中
    // 注意:long占8个字节
    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);
    }

    // 将float类型的值写入到“数据输出流”中
    public final void writeFloat(float v) throws IOException {
        writeInt(Float.floatToIntBits(v));
    }

    // 将double类型的值写入到“数据输出流”中
    public final void writeDouble(double v) throws IOException {
        writeLong(Double.doubleToLongBits(v));
    }

    // 将String类型的值写入到“数据输出流”中
    // 实际写入时,是将String对应的每个字符转换成byte数据后写入输出流中。
    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);
    }

    // 将String类型的值写入到“数据输出流”中
    // 实际写入时,是将String对应的每个字符转换成char数据后写入输出流中。
    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);
    }

    // 将UTF-8类型的值写入到“数据输出流”中
    public final void writeUTF(String str) throws IOException {
        writeUTF(str, this);
    }

    // 将String数据以UTF-8类型的形式写入到“输出流out”中
    static int writeUTF(String str, DataOutput out) throws IOException {
        //获取String的长度
        int strlen = str.length();
        int utflen = 0;
        int c, count = 0;

        // 由于UTF-8是1~4个字节不等;
        // 这里,根据UTF-8首字节的范围,判断UTF-8是几个字节的。
        for (int i = 0; i < strlen; i++) {
            c = str.charAt(i);
            if ((c >= 0x0001) && (c <= 0x007F)) {
                utflen++;
            } else if (c > 0x07FF) {
                utflen += 3;
            } else {
                utflen += 2;
            }
        }

        if (utflen > 65535)
            throw new UTFDataFormatException(
                "encoded string too long: " + utflen + " bytes");

        // 新建“字节数组bytearr”
        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];
        }

        // “字节数组”的前2个字节保存的是“UTF-8数据的长度”
        bytearr[count++] = (byte) ((utflen >>> 8) & 0xFF);
        bytearr[count++] = (byte) ((utflen >>> 0) & 0xFF);

        // 对UTF-8中的单字节数据进行预处理
        int i=0;
        for (i=0; i<strlen; i++) {
           c = str.charAt(i);
           if (!((c >= 0x0001) && (c <= 0x007F))) break;
           bytearr[count++] = (byte) c;
        }

        // 对预处理后的数据,接着进行处理
        for (;i < strlen; i++){
            c = str.charAt(i);
            // UTF-8数据是1个字节的情况
            if ((c >= 0x0001) && (c <= 0x007F)) {
                bytearr[count++] = (byte) c;

            } else if (c > 0x07FF) {
                // UTF-8数据是3个字节的情况
                bytearr[count++] = (byte) (0xE0 | ((c >> 12) & 0x0F));
                bytearr[count++] = (byte) (0x80 | ((c >>  6) & 0x3F));
                bytearr[count++] = (byte) (0x80 | ((c >>  0) & 0x3F));
            } else {
                // UTF-8数据是2个字节的情况
                bytearr[count++] = (byte) (0xC0 | ((c >>  6) & 0x1F));
                bytearr[count++] = (byte) (0x80 | ((c >>  0) & 0x3F));
            }
        }
        // 将字节数组写入到“数据输出流”中
        out.write(bytearr, 0, utflen+2);
        return utflen + 2;
    }

    public final int size() {
        return written;
    }
}

3. 示例代码

关于DataOutStream中API的详细用法,参考示例代码(DataInputStreamTest.java):

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.InputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.FileNotFoundException;
import java.lang.SecurityException;

/**
 * DataInputStream 和 DataOutputStream测试程序
 *
 * @author skywang
 */
public class DataInputStreamTest {

    private static final int LEN = 5;

    public static void main(String[] args) {
        // 测试DataOutputStream,将数据写入到输出流中。
        testDataOutputStream() ;
        // 测试DataInputStream,从上面的输出流结果中读取数据。
        testDataInputStream() ;
    }

    /**
     * DataOutputStream的API测试函数
     */
    private static void testDataOutputStream() {

        try {
            File file = new File("file.txt");
            DataOutputStream out =
                  new DataOutputStream(
                      new FileOutputStream(file));

            out.writeBoolean(true);
            out.writeByte((byte)0x41);
            out.writeChar((char)0x4243);
            out.writeShort((short)0x4445);
            out.writeInt(0x12345678);
            out.writeLong(0x0FEDCBA987654321L);

            out.writeUTF("abcdefghijklmnopqrstuvwxyz严12");

            out.close();
       } catch (FileNotFoundException e) {
           e.printStackTrace();
       } catch (SecurityException e) {
           e.printStackTrace();
       } catch (IOException e) {
           e.printStackTrace();
       }
    }
    /**
     * DataInputStream的API测试函数
     */
    private static void testDataInputStream() {

        try {
            File file = new File("file.txt");
            DataInputStream in =
                  new DataInputStream(
                      new FileInputStream(file));

            System.out.printf("byteToHexString(0x8F):0x%s\n", byteToHexString((byte)0x8F));
            System.out.printf("charToHexString(0x8FCF):0x%s\n", charToHexString((char)0x8FCF));

            System.out.printf("readBoolean():%s\n", in.readBoolean());
            System.out.printf("readByte():0x%s\n", byteToHexString(in.readByte()));
            System.out.printf("readChar():0x%s\n", charToHexString(in.readChar()));
            System.out.printf("readShort():0x%s\n", shortToHexString(in.readShort()));
            System.out.printf("readInt():0x%s\n", Integer.toHexString(in.readInt()));
            System.out.printf("readLong():0x%s\n", Long.toHexString(in.readLong()));
            System.out.printf("readUTF():%s\n", in.readUTF());

            in.close();
       } catch (FileNotFoundException e) {
           e.printStackTrace();
       } catch (SecurityException e) {
           e.printStackTrace();
       } catch (IOException e) {
           e.printStackTrace();
       }
    }

    // 打印byte对应的16进制的字符串
    private static String byteToHexString(byte val) {
        return Integer.toHexString(val & 0xff);
    }

    // 打印char对应的16进制的字符串
    private static String charToHexString(char val) {
        return Integer.toHexString(val);
    }

    // 打印short对应的16进制的字符串
    private static String shortToHexString(short val) {
        return Integer.toHexString(val & 0xffff);
    }
}

运行结果:

byteToHexString(0x8F):0x8f
charToHexString(0x8FCF):0x8fcf
readBoolean():true
readByte():0x41
readChar():0x4243
readShort():0x4445
readInt():0x12345678
readLong():0xfedcba987654321
readUTF():abcdefghijklmnopqrstuvwxyz严12
by skywang
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