How to remove module layer in Java 9?

Question

I have three modules: module-a, module-b, module-c. Module-a and module-b are in boot layer. Layer for module-c I create myself. Module-c has JPMS implementation of the service which interface is in module-a.

This is the way I create layer with module-c in module-b.

ModuleFinder finder = ModuleFinder.of(moduleCPath);
ModuleLayer parent = ModuleLayer.boot();
Configuration cf = parent.configuration().resolve(finder, ModuleFinder.of(), Set.of("module-c"));
ClassLoader scl = ClassLoader.getSystemClassLoader();
ModuleLayer layer = parent.defineModulesWithOneLoader(cf, scl);

Then in module-b I call service from module-c. After service execution completed I don't need module-c and new created layer any more. How to remove it from JVM and release all resources? Is it enough to do layer = null;?

Does creating a new `ModuleLayer` without `module-c` not work? — Naman, Sep 20 '17 at 10:09
Could you update the question with what exactly you tried and what did it result in please. — Naman, Sep 20 '17 at 10:12
@nullpointer My question has all the information, what I do and what is the question about. If you don't understand something, please say what you don't understand. — Pavel_K, Sep 20 '17 at 10:13
Could you include in the question what are you trying to achieve by removing the module, like what is it that the `layer` after that be used for. Or would it not be used even? — Naman, Sep 20 '17 at 10:20
@nullpointer Yes, you are right. `layer` will not be used anymore. What I want to do is to remove this `layer` from memory and release all resources. — Pavel_K, Sep 20 '17 at 10:23

score 5 · Accepted Answer · answered Sep 20 '17 at 11:13

5

The module layer, the modules in the layer, and class loaders supporting the layer, are eligible to be GC'ed/unloaded when they are no longer reachable.

If you want to prove this to yourself then create a weak reference to the layer object and you should see that the reference is cleared (and queued if you are using a reference queue) when the layer is GC'ed.

answered Sep 20 '17 at 11:13

Alan Bateman

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What happened if singleton object (from module) loaded to ram , then remove module ? singleton removed immediately ? – mah454 May 26 '22 at 15:16

Naman · Answer 2 · 2017-09-20T12:26:58.103

An EMPTY_LAYER shall solve your use-case(from one of the comments on the question, trying to assign new HashSet<> as roots) here, wherein the references to other layers no more handled within the layer that you created :

layer = ModuleLayer.empty();

Returns the empty layer. There are no modules in the empty layer. It has no parents.

On the thought of being able to remove a layer explicitly form the JVM, I would not probably expect such an API exposed publicly since a JVM is supposed to have at least one non-empty layer, the boot layer, that is created when the Java virtual machine is started.

And if such a method is exposed, I wonder if users can try and remove this layer as well. Though I am trying to be technically hypothetical on this part.

How to remove module layer in Java 9?

2 Answers2