Tutorial Deleting a loaded plugin Jar file

I don't know if anyone has done this.

Basically, when a plugin is loaded, its corresponding jar file will be used by the server, hence the file lock is acquired and you cannot delete the file in anyway in Java. Here is how to release the corresponding resources.

ONLY DO THIS WHEN THE PLUGIN IS LOADED.

Obtain the responsible ClassLoader
First, obtain the class loader that is used to load the main class of the plugin, and cast it to URLClassLoader. If the action is done within the main class, do the following:

Code:java

URLClassLoader loader = (URLClassLoader) this.getClassLoader();

Otherwise, obtain the instance of the main class and do the following:

Code:java

URLClassLoader loader = plugin.getClass().getClassLoader();

The actual type of the class loader is PluginClassLoader. However, it is only accessible within org.bukkit.plugin.java, so we will cast the class loader to its superclass, URLClassLoader.

Then, you have two options:

Close with reflection
Using this method allows us to release only and the exact resources we want to. However, it may reduce performance insignificantly.

Code:java

((JarFile) loader.getClass().getField("jar").get(loader)).close();

Close with the overrided close() method
Using this method also closes the class loader, which is not what we want to do.

Code:java

loader.close()

How this works?
Take a look at the source code of the type of the class loader responsible for loading a plugin, PluginClassLoader:

Code:java

    private final JarFile jar;
    @Override
    public void close() throws IOException {
        try {
            super.close();
        } finally {
            jar.close();
        }
    }

What we actually want to do is to close the JarFile of the plugin jar file, instead of the class loader. Unfortunately, the jar field is private, so the only ways to close the JarFile is to invoke the overrided close() method, or to access the JarFile using reflection, then directly close it.

Consequences?
I currently cannot find any places after the plugin being loaded that will make use of the JarFile after we close it. If you find any, please tell me.

 

I just want to share what i know no similar things were shared before. Or in case someone needs this, idk.

 

I tried. If you dont close the JarFile deleteOnExit does not work.

 

Ofc ik. But did not work on exit.