Question

I have a Serial-to-USB device with a similarly named device driver in the Windows device manager. The devices do not always grab the same COM port on system boot, so my program needs to identify it on start up.

I’ve tried using RXTX to enumerate the COM ports on the system, but this didn’t work because CommPortIdentifier.getName() simply returns the COM name (eg. COM1, COM2, etc.) I need to acquire either the driver manufacturer name, or the driver name as it appears in the device manager, and associate it with the COM name.

Can this easily be done in Java? (I’d be interested in any 3rd party Java libraries that support this.) Otherwise, how I could begin to accomplish this via the win32 API?

Answer 1

I achieved what I wanted by using the WinRegistry class provided by David in this SO question to obtain the FriendlyName from registry key associated with my USB device. I then parse out the COM number from the friendly name.

Some things to consider:

1. USB devices are located at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USB\ in the registry (tested on WinXP, Win7.)

2. I required the device VID + PID to identify the correct device key (eg. VID_xxxx&PID_xxxx.) Since VID and PID are device specific, this key should be reliable across multiple systems.

3. The VID_xxxx&PID_xxxx key contains another sub-key with device values. I had some trouble enumerating the sub-keys with WinRegistry, so I hard-coded the sub-key name as a quick hack during development. A much safer solution would search sub-keys to find the correct name.

4. The device keys exist in the registry regardless of whether the device is currently connected. This code makes the assumption that Windows will update FriendlyName if the device is reconnected to a different COM port. I haven’t verified this, but things looked good during use-testing.

Example

String keyPath = "SYSTEM\\CurrentControlSet\\Enum\\USB\\Vid_067b&Pid_2303\\";
String device1 = "5&75451e6&0&1";
System.out.println("First COM device: " + getComNumber(keyPath + device1));

Code

import java.util.regex.Pattern;
import java.util.regex.Matcher;

// Given a registry key, attempts to get the 'FriendlyName' value
// Returns null on failure.
//
public static String getFriendlyName(String registryKey) {
    if (registryKey == null || registryKey.isEmpty()) {
        throw new IllegalArgumentException("'registryKey' null or empty");
    }
    try {
        int hkey = WinRegistry.HKEY_LOCAL_MACHINE;
        return WinRegistry.readString(hkey, registryKey, "FriendlyName");
    } catch (Exception ex) { // catch-all: 
        // readString() throws IllegalArg, IllegalAccess, InvocationTarget
        System.err.println(ex.getMessage());
        return null;
    }
}

// Given a registry key, attempts to parse out the integer after
// substring "COM" in the 'FriendlyName' value; returns -1 on failure.
//
public static int getComNumber(String registryKey) {
    String friendlyName = getFriendlyName(registryKey);

    if (friendlyName != null && friendlyName.indexOf("COM") >= 0) {
        String substr = friendlyName.substring(friendlyName.indexOf("COM"));
        Matcher matchInt = Pattern.compile("\\d+").matcher(substr);
        if (matchInt.find()) {
            return Integer.parseInt(matchInt.group());
        }
    }
    return -1;
}