# Ignition 8 - unloading module using dll throws an exception

**URL:** <https://forum.inductiveautomation.com/t/ignition-8-unloading-module-using-dll-throws-an-exception/24891>\
**Category:** Module Development\
**Created:** [May 10, 2019, 4:56pm UTC](https://forum.inductiveautomation.com/t/ignition-8-unloading-module-using-dll-throws-an-exception/24891 "2019-05-10T16:56:55Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![florint](https://avatars.discourse-cdn.com/v4/letter/f/3ab097/32.png) [@florint](https://forum.inductiveautomation.com/u/florint)\
**Post date:** [May 10, 2019, 4:56pm UTC](https://forum.inductiveautomation.com/t/ignition-8-unloading-module-using-dll-throws-an-exception/24891/1 "2019-05-10T16:56:55Z")

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Hi,

There are instances when a module uses a custom dll/library loaded using System.loadLibrary(“abcd”) .

The dll must be unloaded when the shutting down/removing a module, or else subsequent loadLibrary() calls won’t work.

A nice hack was posted a while back, it worked fine in Ignition 7: [Module using dll - unloading?](http://forum.inductiveautomation.com/t/module-using-dll-unloading/6183).

```
try {
				ClassLoader classLoader = this.getClass().getClassLoader();
				Field field = ClassLoader.class.getDeclaredField("nativeLibraries");
				field.setAccessible(true);
				@SuppressWarnings("unchecked")
				Vector<Object> libs = (Vector<Object>) field.get(classLoader);
				for (Object o : libs) {
					Method finalize = o.getClass().getDeclaredMethod("finalize", new Class[0]);
					finalize.setAccessible(true);
					finalize.invoke(o, new Object[0]);
				}
			} catch (Throwable t) {
				lg.tologFatal(true, "Error unloading native resources. " + t.toString());
			}

```

This hack does not work in Ignition 8, is there an alternative solution?

The line that fails is : Vector libs = (Vector) field.get(classLoader);

And the error is:  
java.lang.ClassCastException: class java.util.concurrent.ConcurrentHashMap cannot be cast to class java.util.Vector (java.util.concurrent.ConcurrentHashMap and java.util.Vector are in module java.base of loader ‘bootstrap’)

Thanks

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<div class="post-metadata">

**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\
**Post date:** [May 10, 2019, 5:13pm UTC](https://forum.inductiveautomation.com/t/ignition-8-unloading-module-using-dll-throws-an-exception/24891/2 "2019-05-10T17:13:25Z")

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There doesn’t appear to be a neat hack to workaround this any more.

As of Java 9 native libraries are supposed to get unloaded when the ClassLoader that loaded them gets garbage collected. This means that unless your module has a leak that causes the ClassLoader to hang around, it should _eventually_ get garbage collected and unloaded.

You can test this out by uninstalling your module instead, waiting for a garbage collection event to happen, and then installing it and seeing if you get an error. You can use a tool like VisualVM or JProfiler to trigger a GC or just watch the memory graph in the gateway to see when a GC has happened.