Java Converting String to and from ByteBuffer and associated problems

Question

I am using Java NIO for my socket connections  and my protocol is text based  so I need to be able to convert Strings to ByteBuffers before writing them to the SocketChannel  and convert the incoming ByteBuffers back to Strings   Currently  I am using this code   public static Charset charset   Charset forName  UTF-8    public static CharsetEncoder encoder   charset newEncoder    public static CharsetDecoder decoder   charset newDecoder     public static ByteBuffer str to bb String msg     try      return encoder encode CharBuffer wrap msg       catch Exception e  e printStackTrace       return null     public static String bb to str ByteBuffer buffer     String data         try      int old position   buffer position        data   decoder decode buffer  toString           reset buffer s position to its original so it is not altered      buffer position old position        catch  Exception e       e printStackTrace        return           return data      This works most of the time  but I question if this is the preferred  or simplest  way to do each direction of this conversion  or if there is another way to try   Occasionally  and seemingly at random  calls to encode   and decode   will throw a  java lang IllegalStateException  Current state   FLUSHED  new state   CODING END exception  or similar  even if I am using a new ByteBuffer object each time a conversion is done   Do I need to synchronize these methods   Any better way to convert between Strings and ByteBuffers   Thanks

User · Accepted Answer

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 reset method  unless it has not been used before  Invoke the encode method zero or more times  as long as additional input may be available  passing false for the endOfInput argument and filling the input buffer and flushing the output buffer between invocations  Invoke the encode method one final time  passing true for the endOfInput argument  and then Invoke the flush method 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

User · Answer

Answer by Adamski is a good one and describes the steps in an encoding operation when using the general encode method  that takes a byte buffer as one of the inputs   However  the method in question  in this discussion  is a variant of encode - encode CharBuffer in   This is a convenience method that implements the entire encoding operation   Please see java docs reference in P S    As per the docs  This method should therefore not be invoked if an encoding operation is already in progress  which is what is happening in ZenBlender s code -- using static encoder decoder in a multi threaded environment    Personally  I like to use convenience methods  over the more general encode decode methods  as they take away the burden by performing all the steps under the covers    ZenBlender and Adamski have already suggested multiple ways options to safely do this in their comments  Listing them all here    Create a new encoder decoder object when needed for each operation  not efficient as it could lead to a large number of objects   OR  Use a ThreadLocal to avoid creating new encoder decoder for each operation  OR  Synchronize the entire encoding decoding operation  this might not be preferred unless sacrificing some concurrency is ok for your program     P S   java docs references    Encode  convenience  method  http   docs oracle com javase 6 docs api java nio charset CharsetEncoder html encode 28java nio CharBuffer 29 General encode method  http   docs oracle com javase 6 docs api java nio charset CharsetEncoder html encode 28java nio CharBuffer  20java nio ByteBuffer  20boolean 29

User · Answer

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

[java] Java: Converting String to and from ByteBuffer and associated problems

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