问题
I've been using RXTX for about a year now, without too many problems. I just started a new program to interact with a new piece of hardware, so I reused the connect() method I've used on my other projects, but I have a weird problem I've never seen before.
The Problem
The device works fine, because when I connect with HyperTerminal, I send things and receive what I expect, and Serial Port Monitor(SPM) reflects this.
However, when I run the simple HyperTerminal-clone I wrote to diagnose the problem I'm having with my main app, bytes are sent, according to SPM, but nothing is received, and my SerialPortEventListener never fires. Even when I check for available data in the main loop, reader.ready() returns false. If I ignore this check, then I get an exception, details below.
Relevant section of connect() method
// Configure and open port
port = (SerialPort) CommPortIdentifier.getPortIdentifier(name)
.open(owner,1000)
port.setSerialPortParams(baud, databits, stopbits, parity);
port.setFlowControlMode(fc_mode);
final BufferedReader br = new BufferedReader(
new InputStreamReader(
port.getInputStream(),
"US-ASCII"));
// Add listener to print received characters to screen
port.addEventListener(new SerialPortEventListener(){
public void serialEvent(SerialPortEvent ev) {
try {
System.out.println("Received: "+br.readLine());
} catch (IOException e) { e.printStackTrace(); }
}
});
port.notifyOnDataAvailable();
Exception
java.io.IOException: Underlying input stream returned zero bytes
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:268)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:306)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158)
at java.io.InputStreamReader.read(InputStreamReader.java:167)
at java.io.BufferedReader.fill(BufferedReader.java:136)
at java.io.BufferedReader.read(BufferedReader.java:157)
at <my code>
The big question (again)
I think I've eliminated all possible hardware problems, so what could be wrong with my code, or the RXTX library?
Edit: something interesting
When I open HyperTerminal after sending a bunch of commands from java that should have gotten responses, all of the responses appear immediately, as if they had been put in the buffer somewhere, but unavailable.
Edit 2: Tried something new, same results
I ran the code example found here, with the same results. No data came in, but when I switched to a new program, it came all at once.
Edit 3
The hardware is fine, and even a different computer has the same problem. I am not using any sort of USB adapter.
I've started using PortMon, too, and it's giving me some interesting results. HyperTerminal and RXTX are not using the same settings, and RXTX always polls the port, unlike HyperTerminal, but I still can't see what settings would affect this. As soon as I can isolate the configuration from the constant polling, I'll post my PortMon logs.
Edit 4
Is it possible that some sort of Windows update in the last 3 months could have caused this? It has screwed up one of my MATLAB mex-based programs once.
Edit 5
I've also noticed some things that are different between HyperTerminal, RXTX, and a separate program I found that communicates with the device (but doesn't do what I want, which is why I'm rolling my own program)
- HyperTerminal - set to no flow control, but Serial Port Monitor's RTS and DTR indicators are green
- Other program - not sure what settings it thinks it's using, but only SPM's RTS indicator is green
- RXTX - no matter what flow control I set, only SPM's CTS and DTR indicators are on.
From Serial Port Monitor's help files (paraphrased):
the indicators display the state of the serial control lines
RTS - Request To Send
CTS - Clear To Send
DTR - Data Terminal Ready
回答1:
OK, sorry it's taken me so long to come back to this question. Here's how I got things working.
Note: This method will NOT work for everyone, please read below before copy/pasting into your own code
public void connect(CommPortIdentifier portId) throws Failure {
if (portId == null)
throw new Failure("No port set");
try { port = (SerialPort) portId.open(getClass().getName(), 10000); }
catch (PortInUseException e) {
throw new Failure("Port in use by " + e.currentOwner,e); }
try {
port.setSerialPortParams(9600, SerialPort.DATABITS_8,
SerialPort.STOPBITS_1, SerialPort.PARITY_NONE);
port.setFlowControlMode(SerialPort.FLOWCONTROL_RTSCTS_IN
| SerialPort.FLOWCONTROL_RTSCTS_OUT);
} catch (UnsupportedCommOperationException e) { throw new Failure(e); }
port.setRTS(true);
// More setup
}
So, in my case, the problem was that my particular device requires RTS flow control. Other devices may require different things (CTS, XON/XOFF), so check that device's manual. By default, RXTX disables all flow control mechanisms (unlike Hypertrm or other programs). Enabling each one is a two-step process.
- Once you have a SerialPort object, call the
setFlowControlMode()method, and bitwise-OR ('|') the necessarySerialPort.FLOWCONTROL_constants - Set the appropriate flow control to true or false (like I did with
port.setRTS(true))
For the others with similar problems, if this doesn't work, I suggest
- Using a serial port monitoring program like Serial Port Monitor and/or PortMon (both Windows) to see what is actually going on.
- Emailing the RXTX developers at rxtx@qbang.org (they are very helpful)
回答2:
There is a simpler solution to this problem. This is what I did:
BufferedReader br = new BufferedReader(new InputStreamReader(in));
String line;
while (keepRunning) {
try {
while ((br.ready()) && (line = br.readLine()) != null) {
....
}
If you check that the buffer "is ready" before you read it there should be no problem.
回答3:
Ok, I do realize this thread is extremely old, but none of these solutions worked for me. I had the same problem and I tried everything to fix it, to no avail. Then I did some research on what causes the problem, and, when not dealing with Serial Communication, it happens at the end of a file. So, I figured I needed to add an ending to whatever is being received by the Java Application, specifically, a line return (\n). And sure enough, it fixed the problem for me! Hopefully this helps someone new, as I'm not expecting this to help anyone already on this thread...
回答4:
(might be too simple, but might as well start somewhere...)
Is the port in use? Rather than:
port = (SerialPort) CommPortIdentifier.getPortIdentifier(name)
.open(owner,1000)
what about:
CommPortIdentifier portIdentifier;
try {
portIdentifier = CommPortIdentifier.getPortIdentifier(name);
} catch (NoSuchPortException nspe) {
// handle?
}
if (portIdentifier.isCurrentlyOwned()) {
// handle?
}
port = portIdentifier.open(owner, 1000);
if (!(port instanceof SerialPort)) {
// handle?
}
Are you swallowing any exceptions?
回答5:
I tried RXTX a few months ago and ran into similar problems. I suggest two things:
Create a virtual comport using com0com. Enable trace logging. Compare the logs for when you use Hyperterminal versus when you run your own program. The difference will highlight what you are doing wrong.
In my humble opinion, RXTX's design is flawed and its implementation is quite buggy (take a look at its source-code, what a mess!). I've published an alternative library at http://kenai.com/projects/jperipheral with the following caveats: It's Windows-only and there are no pre-built binaries. Both of these will change in the near future. If you are interested in trying it out send me an email using http://desktopbeautifier.com/Main/contactus and I'll send you a pre-built version.
回答6:
If anyone is still getting java.io.IOException: Underlying input stream returned zero bytes after you've read your characters using br.readline() for RXTX (even when you are checking first to see if br.readline() == null), just do this simple fix with a try/catch:
String line;
while (true){
try{
line = br.readLine();
}catch(IOException e){
System.out.println("No more characters received");
break;
}
//Print the line read
if (line.length() != 0)
System.out.println(line);
}
I've done some searching and it appears that this is the best/easiest way to get around this problem.
EDIT : I take that back. I tried this and still ended up having some problems. I'd recommend working with the raw InputStream directly, and implementing your own read/readLine method using InputStream.read(). That worked for me.
来源：https://stackoverflow.com/questions/1391402/issues-receiving-in-rxtx