Notice
Marked as a duplicate? I could definitely be wrong, but I’m not capable of seeing as to why the duplicate question would help. We’ve already established some sort of progress due to previous comments—comments that clearly have showed solutions that I’ve actually needed rather than marking it as a duplicate for some utterly useless information.
Creating object from class located inside JAR?
I’m currently attempting to create a system for managing .jar plugins from inside another Java project. The idea is that it should be able to create an object out of a class always named Plugin located inside another .jar file, and then load then initialize that class like you would any other Java class.
Progress
I’ve managed to locate the .jar I wish to load classes from using a File from the java.io package. I messed around with the JarFile for a short while, but realized I wasn’t going to be able to achieve what I’m after using it. Always fun playing around with something new!
Research
Over the period of time I’ve been sitting with this, I’ve found several posts here on Stackoverflow, that neither one can be used in order to achieve what I’m after:
- JAR plugins implementation—I can’t see how they’re loading a class from a
.jarfile in this one—it’s possible I’m wrong, should it prove to be the case, kindly inform me. - How should I load Jars dynamically at runtime?—could potentially be relevant—perhaps I’m too stupid to realize how this can be used for my task at hand.
It would seem to me as though it’s not an easy task. Any thoughts on these two resources would be highly appreciated should they prove to be of any use for my problem.
Moreover
It is a .jar file I’d like to load a class into an object from, whilst the application loading the class is up and running—in order for the user to be able to add more plugins, restart and enjoy!
Perhaps someone could point me in the right direction?
Example
Someone suggested I attach an example of what I’m currently attempting—that someone was also nice enough to link me to one of their answers to another question.
Here’s what I’ve tried using that approach:
// ...
// The ‘folderPlugins’ is a file set to the folder containing all plugins
File plugin = new File(folderPlugins.getPath() + "/" + jar);
URLClassLoader classLoader = null;
// First try / catch block used to instantiate the `URLClassLoader` works like a charm
try
{
classLoader = new URLClassLoader(new URL[] {plugin.toURI().toURL()});
}
catch (MalformedURLException ex)
{
System.out.println("0");
Logger.getLogger(Plugins.class.getName())
.log(Level.SEVERE, null, ex);
System.exit(0);
}
Class cls = null;
// Second try / catch block used to load up the class also works like a charm!
try
{
cls = classLoader.loadClass("x.X");
// This nicely outputs into the console as the loaded class ‘class x.X’
System.out.println("Cls: " + cls);
}
catch (ClassNotFoundException ex)
{
System.out.println("1");
Logger.getLogger(Plugins.class.getName())
.log(Level.SEVERE, null, ex);
System.exit(0);
}
// This is where ‘hell breaks lose’.
try
{
// Rather than ending up in here, the ‘catch’ block is used
Object object = cls.newInstance();
}
catch (InstantiationException | IllegalAccessException ex)
{
// Console prints ‘2’
System.out.println("2");
Logger.getLogger(Plugins.class.getName())
.log(Level.SEVERE, null, ex);
System.exit(0);
}
// ...
Part of the error generated by the last try / catch block is the following:
... java.lang.InstantiationException: textarea.TextArea ... Caused by: java.lang.NoSuchMethodException: textarea.TextArea.<init>() at java.lang.Class.getConstructor0(Class.java:3082) at java.lang.Class.newInstance(Class.java:412) ...
This error corresponds to the same line as the only one inside the last try block—that’s where the error occurs.
