[SOLVED] Why doesn't java honour the class path when executing a jar file with the -jar switch?

Why doesn't java honour the class path when executing a jar file with the -jar switch?

This works:

$ java -cp ".:/PATH/TO/RXTXcomm.jar:./jobexftp.jar" -Djava.library.path=/usr/lib/jni com.lhf.jobexftp.StandAloneApp
JObexFTP 2.0 beta (15/10/2010)
Java Obex File Transfer Protocol application and library
Developed under/using 100% free software.
For more information access: http://www.lhf.ind.br/jobexftp/

Usage: jobexftp <serialPort> [<commands>] [<options>]
...

This doesn't:

$ java -cp ".:/PATH/TO/RXTXcomm.jar" -Djava.library.path=/usr/lib/jni -jar jobexftp.jar
Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.NoClassDefFoundError: gnu/io/NoSuchPortException
    at java.lang.Class.getDeclaredMethods0(Native Method)
    at java.lang.Class.privateGetDeclaredMethods(Class.java:2701)
    at java.lang.Class.privateGetMethodRecursive(Class.java:3048)

I found the class name to execute (for the first example) in the META-INF/MANIFEST.MF file, inside the jar:

Main-Class: com.lhf.jobexftp.StandAloneApp

Why does executing the jar file, with the -jar switch seem to cause Java to ignore the classpath and fail to find the gnu/io/NoSuchPortException class in RXTXcomm.jar?

On an older machine, running Java 1.6, I can execute the jar file without needing a classpath. How does this work?

$ env | grep CLASS
$ env | grep JAVA
$ java -jar jobexftp.jar 
JObexFTP 2.0 beta (15/10/2010)
Java Obex File Transfer Protocol application and library
Developed under/using 100% free software.
For more information access: http://www.lhf.ind.br/jobexftp/

Usage: jobexftp <serialPort> [<commands>] [<options>]
...

The reason it worked on the old machine is that it had a copy of RXTXcomm.jar in /usr/lib/jvm/java-6-sun-1.6.0.21/jre/lib/ext/.

$ ls -lA /usr/lib/jvm/java-6-sun-1.6.0.21/jre/lib/ext/RXTXcomm.jar
-rwxr-xr-x 1 root root 137764 2011-02-17 16:58 /usr/lib/jvm/java-6-sun-1.6.0.21/jre/lib/ext/RXTXcomm.jar

Making a new ext directory in /usr/lib/jvm/java-8-oracle/jre and copying RXTXcomm.jar into it doesn't get rid of the error on Java 1.8.

Solution

The -jar switch is intended to launch the jar as a self contained program. As stated in the java man page:

When you use the -jar option, the specified JAR file is the source of all user classes, and other class path settings are ignored.

To provide a classpath, use the Class-Path entry in the manifest file.

Further reading: The Java™ Tutorials: Adding Classes to the JAR File's Classpath