writeSQLCAGRP() writes Strings into the message being built. Profiling shows that it is more expensive to write a String than a byte because the String must be converted to UTF8. writeSQLCAGRP() writes 5 bytes for SQLState, and this is done by either writing a String constant, or the return value from SQLException.getSQLState(). For the common case where there is no exception (SQLState = 5xspace), or the exception is a "dummy" exception (SQLState=00000 or 02000, End of Data), this is wasteful because the String has to be converted to byte each time, and in the case of the dummy exception, a new String object will be created each time getSQLState() is called, even if the exception object is the same (there is no caching, which is reasonable since exceptions are meant to be thrown, not kept around for a long time).
A solution is to keep the commonly used SQLStates as byte constants that can be inserted into the message with writeBytes().
If writeSQLCAGRP() is called with no SQLException (null) there is no attempt to put an internationalized error message into the outgoing message (The third argument to writeSQLCAXGRP() is null). This is reasonable, but the same optimization is not done when the exception is one of the dummy exceptions mentioned previously. In this case an internationalized version of the message "End of Data" is constructed and inserted into the message. It would be better to call writeSQLCAXGRP(..,null,..) in this case as well, since it isn't needed by the client in this case.
Finally, writeSQLCAERRWARN() uses writeScalarPaddedBytes() to write values that also can be stored as byte constants, and written faster with writeBytes()