I'm trying to create a JNI project in Eclipse using CDT. I'm following this tutorial.

So first I created my project as a Java project and created a HelloJNI class in the default package, which contains some really basic code.

I then added a C nature to my project, selecting 'Makefile Project' in 'Project type' and 'Linux GCC' in 'Toolchains'.

Then I created a new directory called 'jni' to store all the C code. I then created a new file in eclipse called 'makefile' (note the lowercase name) and added this to it.

# Define a variable for classpath
CLASS_PATH = ../bin

# Define a virtual path for .class in the bin directory
vpath %.class $(CLASS_PATH)

# $* matches the target filename without the extension
HelloJNI.h : HelloJNI.class
    javac -h -classpath $(CLASS_PATH) $*

I then created a build target called 'HelloJNI.h' in the jni folder, which automatically used the makefile. I then attempted to build the target, but I got this error:

javac -h -classpath ../bin HelloJNI
javac: invalid flag: ../bin
Usage: javac <options> <source files>
use -help for a list of possible options
make: *** [makefile:9: HelloJNI.h] Error 2

I am puzzled by the error message. Does javac recognize ../bin as a flag? Is there some weird bug happening in Eclipse? If I try to do this in the terminal, I get the same error. I tried searching for answers, but found none.

This is my directory strcture:

-bin
    HelloJNI.class
-jni
    makefile
-src
    HelloJNI.java

Edit: Changing the javac call to

javac -h -classpath $CLASS_PATH $*

gave me the following error:

error: Class names, 'LASS_PATH,HelloJNI', are only accepted if annotation processing is explicitly requested

CodePudding user response：

Starting point

Looking at the tutorial you're following, your question lines up with "2. Getting Started", specifically "Step 2" – compiling Java and generating a C/C header file.

To revisit, here's what you tried:

javac -h -classpath ../bin HelloJNI

A few points..

• The -h option to javac specifies a directory. This directory is where the generated header file should go. It requires a directory name immediately after, like this: -h directory. In the attempt above, it's using the -h option, but doesn't specify a directory. That's an issue.
• This includes -classpath ../bin which isn't necessary for this example. It's not a problem, but it isn't necessary to include and can be removed.
• "HelloJNI" is not an appropriate input for the compiler - you need to specify a ".java" filename

The fix

In the tutorial, they show this example:

javac -h . HelloJNI.java

This works because:

• They're using -h with "." as the directory (where "." means "current directory").

• They're compiling the .java file – "HelloJNI.java"

• Also, they're not using -classpath. That option is used to designate where to find class files for compilation (as input to compilation itself), but that isn't relevant for this tutorial example – there aren't any necessary input classes for javac to work. So, just remove -classpath ../bin altogether.

In the end, things should work for you if you run the command as they posted in the tutorial:

javac -h . HelloJNI.java

Also: JDK version differences

You didn't specify which JDK version you're using, but if you're using an older (pre-8) version of the JDK, there are different steps to follow (namely, using javah in addition to javac).