There is simpler approach to decode a ByteBuffer into a String without any problems, mentioned by Andy Thomas.
String s = StandardCharsets.UTF_8.decode(byteBuffer).toString();
There is simpler approach to decode a ByteBuffer into a String without any problems, mentioned by Andy Thomas.
String s = StandardCharsets.UTF_8.decode(byteBuffer).toString();
EDIT (2018): The edited sibling answer by @xinyongCheng is a simpler approach, and should be the accepted answer.
Your approach would be reasonable if you knew the bytes are in the platform's default charset. In your example, this is true because k.getBytes() returns the bytes in the platform's default charset.
More frequently, you'll want to specify the encoding. However, there's a simpler way to do that than the question you linked. The String API provides methods that converts between a String and a byte[] array in a particular encoding. These methods suggest using CharsetEncoder/CharsetDecoder "when more control over the decoding [encoding] process is required."
To get the bytes from a String in a particular encoding, you can use a sibling getBytes() method:
byte[] bytes = k.getBytes( StandardCharsets.UTF_8 );
To put bytes with a particular encoding into a String, you can use a different String constructor:
String v = new String( bytes, StandardCharsets.UTF_8 );
Note that ByteBuffer.array() is an optional operation. If you've constructed your ByteBuffer with an array, you can use that array directly. Otherwise, if you want to be safe, use ByteBuffer.get(byte[] dst, int offset, int length) to get bytes from the buffer into a byte array.
ByteBuffer to String
ByteBuffer to String in Java - Stack Overflow
arrays - Converting a String to ByteBuffer in Java - Stack Overflow
java - Put a StringBuffer in a ByteBuffer - Stack Overflow
Check out the CharsetEncoder and CharsetDecoder API descriptions - You should follow a specific sequence of method calls to avoid this problem. For example, for CharsetEncoder:
- Reset the encoder via the
resetmethod, unless it has not been used before; - Invoke the
encodemethod zero or more times, as long as additional input may be available, passingfalsefor the endOfInput argument and filling the input buffer and flushing the output buffer between invocations; - Invoke the
encodemethod one final time, passingtruefor the endOfInput argument; and then - Invoke the
flushmethod so that the encoder can flush any internal state to the output buffer.
By the way, this is the same approach I am using for NIO although some of my colleagues are converting each char directly to a byte in the knowledge they are only using ASCII, which I can imagine is probably faster.
Unless things have changed, you're better off with
public static ByteBuffer str_to_bb(String msg, Charset charset){
return ByteBuffer.wrap(msg.getBytes(charset));
}
public static String bb_to_str(ByteBuffer buffer, Charset charset){
byte[] bytes;
if(buffer.hasArray()) {
bytes = buffer.array();
} else {
bytes = new byte[buffer.remaining()];
buffer.get(bytes);
}
return new String(bytes, charset);
}
Usually buffer.hasArray() will be either always true or always false depending on your use case. In practice, unless you really want it to work under any circumstances, it's safe to optimize away the branch you don't need.
Hi Guys,
Could anyone please tell me how I can convert a ByteBuffer to a String? I thought it would be a simple function but I can't seem to work it out.
This is just a part of what I have written. I am essentially trying to read a message sent via TCP. There were intermediate steps which I didn't think were necessary to include here.
ServerSocketChannel tcpserver = ServerSocketChannel.open(); SocketChannel connectionSocket = tcpserver.accept(); // create buffer to hold message ByteBuffer buf = ByteBuffer.allocate(48); int bytesRead = connectionSocket.read(buf);
While googling around, there was talk of paying attention to character encoding. Could someone please explain how this factors into the problem at hand?
Any help would be greatly appreciated!!
Kind Regards,
Giri
A way to convert a ByteBuffer to a String would be to use a Charset to decode the bytes:
Charset charset = Charset.forName("ISO-8859-1");
ByteBuffer m_buffer = ...;
String text = charset.decode(m_buffer).toString();
The decoding creates a CharBuffer which you can conveniently convert to a String. You can reuse the CharSet and it is threadsafe. I wouldn't worry too much about performance (re "fastest way") unless you have really a problem in that area. A general advice, when you want to use ByteBuffer, do the to-String conversion as late as possible, and only when the String is needed.
As 14jbella mentioned, Strings are immutable. There is no way of creating a String from an array (char or byte) that does not include copying the data because arrays are mutable. So no, there is no way to do it without copying.
Further you should take into consideration, that m_buffer.array() returns the internal array of the ByteBuffer, which may be much more than the actual data stored in the buffer. Creating a String from that array might lead to a potentially huge memory allocation, because the data gets copied into a new array. For example if you're using a 256 MB ByteBuffer somewhere in your code, and you get a slice() of 32 bytes named m_buffer from that buffer to convert to a String, your invocation of new String(m_buffer.array()) would allocate a new byte array of the size of the original, backing byte array, which is 256 MB, which is probably not that fast, if it requires a GC.
Btw. new String(byte[]) internally uses a CharSet decoder on a ByteBuffer wrapped around your input byte array.
Java's String class is immutable. In order to maintain this guarantee, String has to have its own reference to the char[] backing it, and nobody else may have that reference. If it were to share an array with the ByteBuffer, the String class could not guarantee that it would not be modified. In addition, char, and byte are not the same in Java.