Java 8 上 Windows 10 - CLASSPATH 不起作用

Question

Windows 10 1803 64-bit, Java JRE 8. No JDK installed.

I have a.jar in a folder:

c:\users\myuser\KickAssembler\KickAss.jar

Under System environment variables I have the CLASSPATH as:

c:\users\myuser\KickAssembler\

At a command prompt the CLASSPATH is set (checked with ECHO).

At a prompt when I try:

java -jar KickAss.jar

It returns:

Error: Unable to access jarfile KickAss.jar

If I CD into the directory first, then run it, it works fine. So it looks like CLASSPATH is not being used somehow. I have added the location into the standard Windows PATH also, still doesn't work.

Different combinations of casing for the filename doesn't work. Windows is set to show hidden files. It can't be a permissions issue because it works when I change into the directory first.

Anything else I can try?

Answer 1

CLASSPATH is used to define the path from which Java will load classes. CLASSPATH can contain folders and JAR files, such as: C:\mybin;c:\myjars\some.jar This classpath would load any fully qualified class file residing in or beneath c:\mybin and any fully qualified class within c:\myjars\some.jar . CLASSPATH will not allow JAVA to find your JAR file as you are expecting when using the -jar switch, it does not search for any JAR file along the classpath, it will only look in the ones explicitly stated in the CLASSPATH and then only for class files within them. Note, fully qualified means package + class , such as: com.myorg.somepackage.someclass , not just someclass .

As you observed, if you are in the folder where kickass.jar resides your command line works as the JAR file is present. If you fully reference the JAR file while executing the command from another folder the command line should work as well.

See https://docs.oracle.com/javase/8/docs/technotes/tools/windows/classpath.html for more detail on how CLASSPATH works. It is important to have a solid understanding of CLASSPATH when using JAVA.