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I've looked here and the wider web to find a solution to this. There's related material, but I've been unable to find anything useful about my specific question.

I'm working on some Java software that needs to accept plugins. I don't want to use a fancy framework like OSGi, and ServiceLoader seems to offer the right level of support. I've basically got it working but am having a problem with classpaths. My directory structure is as follows:

progfolder |___________ plugintest.jar |___________/plugins |________ plugin1.jar |________ plugin2.jar

If I run plugintest.jar with java -jar plugintest.jar then it doesn't find the plugins even if I add ./plugins/ (or variations of this) to the Class-Path: in the manifest. Reading suggests that this only works for classes, not jars, so I've tried putting the classes from the two plugins inside plugins both directly and within their full path of directories, but with no success.

I'm not allowed to add -cp plugins/* to add the plugins folder to the classpath if I'm using the -jar option. To get round this, I can run using java -cp plugintest.jar;plugins/* com.plugin.test.Main and this works as expected - the two plugins are detected and accessible via code, but the command line is a bit clunky, although I could live with it, if it's the best option.

I found another solution where I create a classloader for jars found in plugins, which works in this simple case, but reading suggests I might run into security issues in a more complex application.

Is there a way to fix things so I can simply run with java -jar plugintest.jar without having to do my own class loading or is this just the way it is?

SteveR
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2 Answers2

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Ok, so at least a partial answer, following more experimentation. Putting the class files in the plugins directory does work, after all, but you have to remember to include the META-INF directory and META-INF/services. The file in the services directory has to include references to all the plugins.

It would be nice if there was a solution that allowed the plugin jar files to be used directly, but creating a class loader seems to be the only way to do this (that I've found, at least), and this may cause security issues, as previously noted.

SteveR
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last time I faced with similar problem [1]. I found answer in java documentation [2]:

Note: The Class-Path header points to classes or JAR files on the local network, not JAR files within the JAR file or classes accessible over Internet protocols. To load classes in JAR files within a JAR file into the class path, you must write custom code to load those classes. For example, if MyJar.jar contains another JAR file called MyUtils.jar, you cannot use the Class-Path header in MyJar.jar's manifest to load classes in MyUtils.jar into the class path.

[1] https://github.com/narvi-blog/01-exec-jar#dependency-jar-files-within-an-executable-jar-are-not-so-easy
[2] https://docs.oracle.com/javase/tutorial/deployment/jar/downman.html

Narvi Blog
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