Java的DataInputStream的readUTF方法是如何读取字符串的

在使用DataInputStream时候，经常会使用到readUTF来读取一段字符串，但是readUTF是如何知道该字符串有多长的呢？

下面的例子是对DataInputStream的简单使用：

package s1;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

public class X {
	public static void main(String[] args) {
		DataOutputStream out = null;
		DataInputStream in = null;
		try {
			out = new DataOutputStream(new BufferedOutputStream(new FileOutputStream("test.txt")));
			out.writeDouble(3.14159);
			out.writeUTF("This is PI");
			out.writeDouble(1.41413);
			out.writeUTF("Square root of 2");
			
			in = new DataInputStream(new BufferedInputStream(new  FileInputStream("test.txt")));
			System.out.println(in.readDouble());
                        // 如何知道字符串有多长的？？？
			System.out.println(in.readUTF());
			System.out.println(in.readDouble());
			System.out.println(in.readUTF());
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			try {
				if(out != null) {
					out.close();
				}
				if(in != null) {
					in.close();
				}
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}
}

其实原因并不复杂，因为在writeUTF的时候并不是简简单单地写入字符串内容，其实还有字符串的长度。可以参考源码：

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;
}

在readUTF时候，先读取字符串的长度，然后再读取字符串。源码：

    public final static String readUTF(DataInput in) throws IOException {
        // 读取字符串的长度
        int utflen = in.readUnsignedShort();
        // 读取字符串内容
        .
        .
        .
    }