Page History

According to the Java API [API 06] Class for the class InputStream, with an array of bytes b as parameter, the read(b) method documentation:

...

\[read()\] Reads some number of bytes from the input stream and stores them into the buffer array b. The number of bytes actually read is returned as an integer. The number of bytes read is, at most, equal to the length of b.

Note that the read() methods will return as soon as they find some input data. By default, none of them guarantee that all the requested bytes will be read. It is left to the programmer to check the number of bytes read and call the read() method again as required. Ignoring the result returned by the read() methods is a direct violation of EXP02-J. Do not ignore values returned by methods.

Multibyte encodings like UTF-8 are used for character sets that require more than one byte to uniquely identify each constituting character. For example, the Japanese encoding Shift-JIS (shown below), supports multibyte encoding where the maximum character length is 2 two bytes (one leading and one trailing byte).

...

The trailing byte ranges overlap the range of both the single byte and lead byte characters. This can cause issues because if a multibyte character is separated between buffer boundaries, it will be interpreted differently, as defined by its composing bytes. [Phillips 05]

A third issue is caused due to because of the behavior of the String class constructor. According to the Java API [API 06] for the String class documentation:

The length of the new String is a function of the charset, and hence may not be equal to the length of the byte array. The behavior of this constructor when the given bytes are not valid in the given charset is unspecified.

...

This noncompliant snippet intends to read a specific number of bytes from a FileInputStream but suffers from a few pitfalls. The objective is to read 1024 bytes and return them as a String. Unfortunately, this won't may not happen because of the general contract of the read() methods.

The other A second issue involves multibyte character encoding. It is possible for the read() method to read data from the stream terminating the String buffer str with the leading byte of a multibyte character and in the next iteration reading the trailing bytes. Since the bytes are concatenated to str, the multibyte encoding information is lost as it does not extend across buffer boundaries.

Finally, the buffer str will contain data represented by the default encoding of the system as no specific encoding has been specified in the call to the String class constructor.

...

The space for the data byte buffer should be allocated depending upon the maximum number of bytes required to write an encoded character. For example, UTF-8 encoded data requires a maximum of 3 three bytes to denote one character. As Although it seems counter intuitive as it may sound, any character above U+FFFF requires a maximum of 4 four bytes. However, such a sequence is split into two separate char values of 2 two bytes each since Java internally uses UTF-16 for representing a char. Therefore the buffer size should be four times the size of a typical byte sequence.

This compliant solution also states the String str encoding explicitly to facilitate portability.

public static String readBytes(FileInputStream in) throws IOException {
  int offset = 0;
  int bytesRead = 0;
  byte[] data = new byte[1024];
  while (true) { 
    bytesRead += in.read(data, offset, data.length - offset);
    if (bytesRead == -1 || offset >= data.length)
      break;
    offset += bytesRead;
  }
  String str = new String(data, "UTF-8");
  return str;
}

...

The no argument and one argument readFully() methods of the DataInputStream class can be used to read all the requested data. An IOException gets EOFException is thrown if the byte array overflows or during the absence of incoming dataend of input is detected before the required number of bytes have been read, and an IOException is thrown if some other input/output error occurs. How to proceed is left to the exception handler to decide.

public static String readBytes(DataInputStream dis) throws IOException {    	      	  
{
  byte[] data = new byte[1024];
  dis.readFully(data);
  String str = new String(data,"UTF-8");
  return str;
}

...