问题
I put rxtxcomm.jar into jre/lib/ext folder, but I still get NoClassDefFoundError Isn't this folder automatically taken into the global classpath?
Thanks
回答1:
yes it is taken automatically to classpath, but RXTXcomm uses JNI /native external libraries (.so and .dll files), you must provide the path to them when running your program in command line:
java -jar yourprogram.jar -Djava.library.path="PATH_TO_EXTERNAL_LIBRARIES"
for linux:
suppose you unpacked the rxtx.zip to /home/user/
if you have 32bit x86 platofrm:
PATH_TO_EXTERNAL_LIBRARIES = /home/user/Linux/i686-unknown-linux-gnu/
if you have 64bit x86 platform the it would be:
PATH_TO_EXTERNAL_LIBRARIES = /home/user/Linux/x86_64-unknown-linux-gnu/
for windows:
suppose you downloaded and unpacked it to C:\rxtxt
PATH_TO_EXTERNAL_LIBRARIES = C:\rxtxt\Windows\i368-mingw32\
If you find it cumbersome to do it from command line you can do it from yout code (before opening port via RXTXcomm):
System.setProperty("java.library.path","PATH_TO_EXTERNAL_LIBRARIES");
EDIT:
of course, you must put RXTXcomm.jar in your classpath in addition to all of the above. If running from command line as jar packaged program - yourprogram.jar - inside the jar you must have a META-INF folder that contains MANIFEST.MF with the following entries:
Class-Path: lib/RXTXcomm.jar
Main-Class: pkg.Main
and yourprogram.jar must be in folder which has folder lib in which is RXTXcomm.jar, also the class with
public static void main(String[] args)
method must be called Main and reside in package named pkg (just replace pkg.Main with whatever you have).
Then you can run your program succesfully and open a serial port if you have one. This approach also eliminates the need to copy anything in jre/lib/ext folder
EDIT^2:
or, if you don't want to pack your program in jar, position yourself in folder which contains the folder pkg and write:
java -cp PATH_TO_RXTX/RXTXcomm.jar -Djava.library.path="PATH_TO_EXTERNAL_LIBRARIES" pkg.Main
(all paths can be relative or absolute)
EDIT^3:
but I would recommend Java Simple Serial Connector instead of RXTXcomm:
- it handles heavy load from multiple threads as opposed to RXTXcomm (tested in production)
- external libraries are packed in jar so there is no need for setting java.library.path
来源：https://stackoverflow.com/questions/12543202/rxtx-installation-on-windows-java-lang-noclassdeffounderror-gnu-io-commport