使用-jar开关执行jar文件时,为什么java不尊重类路径？

$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>]
...

这不是：

$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)

我在jar中找到了要在META-INF / MANIFEST.MF文件中执行的类名(对于第一个示例)：

Main-Class: com.lhf.jobexftp.StandAloneApp

为什么使用-jar开关执行jar文件似乎会导致Java忽略类路径并且无法在RXTXcomm.jar中找到gnu / io / NoSuchPortException类？

在运行Java 1.6的旧机器上,我可以执行jar文件而无需类路径.这是如何运作的？

$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>]
...

它在旧机器上工作的原因是它在/usr/lib/jvm/java-6-sun-1.6.0.21/jre/lib/ext/中有一份RXTXcomm.jar.

$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

在/usr/lib / jvm / java-8-oracle / jre中创建一个新的ext目录并将RXTXcomm.jar复制到它中并没有消除Java 1.8上的错误.