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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 . . . }