Java的DataInputStream的readUTF方法是怎么读取字符串的？？？

Question

大神们好，我是新手小白，今天在学习Java的IO操作中遇到一个百思不得其解的问题，下面的代码是今天做的关于DataInputStream类的练习，我很不解为什么DataInputStream的readUTF方法不需要任何参数，但是却在读取的时候可以知道自己读取多长，不明白他是怎么知道我当初在写入的时候写入的长度的，他是靠什么控制读取的范围的呀？？？比如下面的程序，readUTF方法就可以知道读取"昨天"这两个字，是怎么知道我就刚好需要读这两个字，而不会把下面的内容给读出。很是感谢大家的回答

package com.zhang.hello;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class IODataStreamTest {
    public static void main(String[] args) throws IOException {

        File file=new File("f:/zhang.dat");
        if(!file.exists()){
            file.createNewFile();
        }
        DataInputStream dis=new DataInputStream(new FileInputStream(file));
        DataOutputStream dos=new DataOutputStream(new FileOutputStream(file));

        int testInt=2016;
        double testDouble=10.12;
        long testLong=20161012;

        dos.writeInt(testInt);
        dos.writeDouble(testDouble);
        dos.writeLong(testLong);
        //utf-8 一个中文占3个字节
        dos.writeUTF("昨天");
        dos.writeUTF("今天是个好日子");
        dos.close();
        
        int testRInt=dis.readInt();
        System.out.println(testRInt);
        double testRDouble=dis.readDouble();
        System.out.println(testRDouble);
        long testRLong=dis.readLong();
        System.out.println(testRLong);
        String testRStr=dis.readUTF();//为什么readUTF知道读取多长
        System.out.println(testRStr);
        dis.close();
    }
}

执行结果：
2016
10.12
20161012
昨天

Answer 1

DataOutputStream

static int writeUTF(String str, DataOutput out) throws IOException {
    int strlen = str.length();
    int utflen = 0;
    int c, count = 0;

    /* use charAt instead of copying String to char array */
    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");

    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];
    }

    // 将字符串的字节长度写入流中
    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);
    // 写入的长度在字符串中增加了2，即字节长度标识所占用的资源
    return utflen + 2;
}

DataInputStream

public final static String readUTF(DataInput in) throws IOException {
    // 读取字符串字节长度
    int utflen = in.readUnsignedShort();
    //...
}

实际上在调用writeUTF写入时jdk内部有将字符串的字节数写入流中，读取时先读取到字节长度，按照指定的字节长度读取出相应的字符串。

Answer 2

看源码，调用的第一句就获取了长度
int utflen = in.readUnsignedShort();
这个方法的Doc ：

Reads two input bytes and returns an int value in the range 0 through

1. Let a be the first byte read and b be the second byte. The value returned is:

(((a & 0xff) << 8) | (b & 0xff)) This method is suitable for reading
the bytes written by the writeShort method of interface DataOutput if
the argument to writeShort was intended to be a value in the range 0
through 65535. Returns: the unsigned 16-bit value read. Throws:
EOFException - if this stream reaches the end before reading all the

1. IOException - if an I/O error occurs.

readUTF的Doc：

Reads from the stream in a representation of a Unicode character
string encoded in modified UTF-8 format; this string of characters is
then returned as a String. The details of the modified UTF-8
representation are exactly the same as for the readUTF method of
DataInput.