Java IO.pdf - Nguyen Dang Binh

Java I/O
off in the modem so that you don't see duplicates of everything you type. (Even my password
originally appeared on the screen in clear text. I replaced it with asterisks manually.) And no
effort at all is made to perform terminal emulation of any sort. Furthermore, there's no way to
exit the program and close the port. Terminating it with a Ctrl-C forces abnormal execution
that fails to release control of the port. Nonetheless, it's amazing just how quick and easy it is
to write a program that communicates with a simple serial port device. Communicating with a
basic daisy-wheel printer would be no harder.
17.3.2 Port Properties
The javax.comm.CommPort class has a number of driver-dependent methods for adjusting the
properties of the port. These properties are mostly generic characteristics, like buffer size, that
may be implemented in software. More specific properties of a particular type of port, like the
baud rate of a serial port or the mode of the parallel port, must be set using a more specific
subclass, like javax.comm.SerialPort or javax.comm.ParallelPort.
The five generic properties are receive threshold, time-out value, receive framing byte, input
buffer size, and output buffer size. Four of these properties—receive threshold, receive timeout,
receive framing and input buffer size—determine exactly how and when the input stream
blocks. The receive threshold specifies the number of bytes that must be available before a
call to read() returns. The receive time-out specifies the number of milliseconds that must
pass before a call to read() returns. The input buffer size specifies how large a buffer will be
provided for the serial port. If the buffer fills up, the read() method returns.
For instance, if the receive threshold is set to 5, read() won't return until at least 5 bytes are
available. If the receive timeout is set to 10 milliseconds, read() will wait 10 milliseconds
before returning. However, if data becomes available before 10 milliseconds are up, read()
returns immediately. For example, if the receive threshold is set to 5 bytes and the receive
time-out is set to 10 milliseconds, then read() will wait until either 10 milliseconds pass or 5
bytes are available before returning. If the input buffer size is set and the receive threshold is
set, the lower of the two values must be reached before read() will return. Finally, if receive
framing is enabled, all reads return immediately, regardless of the other values. Table 17.1
summarizes.
Receive
Threshold
Receive
Time-out
Table 17.1. When Does read() Return?
Receive
Framing
Input
Buffer Size
read( ) Returns When
disabled disabled disabled b bytes Returns when any data is available.
n bytes disabled disabled b bytes
Returns when either n or b bytes are available,
whichever is less.
disabled t ms disabled b bytes
Returns after t milliseconds or when any data is
available.
n bytes t ms disabled b bytes
Returns after t milliseconds or when either n or b
bytes are available, whichever is less.
disabled disabled enabled b bytes Returns immediately.
n bytes disabled enabled b bytes Returns immediately.
disabled t ms enabled b bytes Returns immediately.
n bytes t ms enabled b bytes Returns immediately.
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