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JoyOfCodingPDX / java-koans

A framework and lessons to learn java syntax and idioms in a logical sequence.
Apache License 2.0
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NullPointerException when creating KoanClassLoader #7

Closed DavidWhitlock closed 10 years ago

DavidWhitlock commented 10 years ago

A couple of students have reported seeing this exception when they copy their koans project to a second machine using git.

https://plus.google.com/u/0/102032333290286425390/posts/gLxHo7vsnwj

java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:297)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
    at com.sandwich.koan.KoanClassLoader.buildClassPath(KoanClassLoader.java:57)
    at com.sandwich.koan.KoanClassLoader.<init>(KoanClassLoader.java:18)
    at com.sandwich.koan.KoanClassLoader.getInstance(KoanClassLoader.java:68)
    at com.sandwich.koan.runner.RunKoans.runKoans(RunKoans.java:54)
    at com.sandwich.koan.runner.RunKoans.run(RunKoans.java:46)
    at com.sandwich.koan.constant.ArgumentType.run(ArgumentType.java:76)
    at com.sandwich.koan.cmdline.CommandLineArgument.run(CommandLineArgument.java:38)
    at com.sandwich.koan.cmdline.CommandLineArgumentRunner.run(CommandLineArgumentRunner.java:26)
    at com.sandwich.koan.runner.AppLauncher.main(AppLauncher.java:47)
DavidWhitlock commented 10 years ago

Good news! I tried cloning my git repository (https://github.com/DavidWhitlock/PortlandStateJavaSummer2014) and I saw the same thing. Now I can debug it.

DavidWhitlock commented 10 years ago

Here's what I get from the debugging I added last night.

[WARNING] 
java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:483)
        at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:297)
        at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalStateException: Could not find any files in /u/whitlock/git/PortlandStateJavaSummer2014/koans/app/lib
        at com.sandwich.koan.KoanClassLoader.buildClassPath(KoanClassLoader.java:59)
        at com.sandwich.koan.KoanClassLoader.<init>(KoanClassLoader.java:20)
        at com.sandwich.koan.KoanClassLoader.getInstance(KoanClassLoader.java:72)
        at com.sandwich.koan.runner.RunKoans.runKoans(RunKoans.java:54)
        at com.sandwich.koan.runner.RunKoans.run(RunKoans.java:46)
        at com.sandwich.koan.constant.ArgumentType.run(ArgumentType.java:76)
        at com.sandwich.koan.cmdline.CommandLineArgument.run(CommandLineArgument.java:38)
        at com.sandwich.koan.cmdline.CommandLineArgumentRunner.run(CommandLineArgumentRunner.java:26)
        at com.sandwich.koan.runner.AppLauncher.main(AppLauncher.java:47)
        ... 6 more

And that makes sense because that directory contains jar files. And jar files are ignored by git. Derft.

DavidWhitlock commented 10 years ago

I was able to work around this problem by simply adding the jar files to git:

$ git add --force app/lib*.jar
$ git commit -m "Forcibly add jar files to work around https://github.com/DavidWhitlock/java-koans/issues/7"

But I want to look into this a little more. What, exactly, are in those jar files? Are they the stuff in the koans-lib?

DavidWhitlock commented 10 years ago

Yes, it looks like the classes in the four jar files in the app/lib directory are the one in the koans-lib artifact. Maybe I just need to change the KoanClassLoader to tolerate no classes being available in that directory (assuming that koans-lib is on the class path).

DavidWhitlock commented 10 years ago

I've been working with the koans and I don't think tolerating an empty "app/lib" directory will work. There are at least two places in the koan-lib code that expect that directory to be populated. I think it would be better to have the Maven build copy the koan-lib artifact into the app/lib directory.

Maybe something like this:

DavidWhitlock commented 10 years ago

Or I could just refactor the code for creating the classpath into one place. I should just look for usages of DirectoryManager.getProjectLibraryDir() and go from there.

DavidWhitlock commented 10 years ago

I think I finally have this problem sorted out.