ScanResult leaks shutdown hooks

This code shows that a shutdown hook (ie unstarted thread) is created but not destroyed for each scan, even when using try-with-resources for ScanResult.

Seems like Runtime.getRuntime().removeShutdownHook() might be needed. [EDIT: was addShutdownHook, nonsensically.]

import java.lang.reflect.Field;
import java.util.Map;

import io.github.classgraph.ClassGraph;
import io.github.classgraph.ScanResult;

public class ClassGraphShutdownHooks {
    public static void main(String[] args) throws Exception {
        for (int i = 0; i < 100_000; i++) {
            try (ScanResult scan = new ClassGraph().scan()) {
            }
            System.out.printf("Number of shutdown hooks: %d%n", countShutdownHooks());
        }
    }

    // thanks to https://stackoverflow.com/a/6865509/14379
    // works on Java 8
    private static int countShutdownHooks() throws ReflectiveOperationException {
        Class clazz = Class.forName("java.lang.ApplicationShutdownHooks");
        Field field = clazz.getDeclaredField("hooks");
        field.setAccessible(true);
        Map<?, ?> hooks = (Map<?, ?>) field.get(null);
        return hooks.size();
    }
}

Output:

Number of shutdown hooks: 1
Number of shutdown hooks: 2
Number of shutdown hooks: 3
Number of shutdown hooks: 4
Number of shutdown hooks: 5
Number of shutdown hooks: 6
Number of shutdown hooks: 7
Number of shutdown hooks: 8
Number of shutdown hooks: 9
Number of shutdown hooks: 10
...

(Related to https://github.com/classgraph/classgraph/issues/233)

[seanf]

I don’t really trust WeakReferences either 😃, so I modified my code:

import java.lang.reflect.Field;
import java.util.Map;
import java.util.Set;

import io.github.classgraph.ClassGraph;
import io.github.classgraph.ScanResult;

public class ClassGraphShutdownHooks {
    public static void main(String[] args) throws Exception {
        for (int i = 0; i < 100_000; i++) {
//            try (ScanResult scan = new ClassGraph().scan()) {
//            }
            new ClassGraph().scan();
            System.out.printf("Number of references: %d%n", countRefs());
            System.out.printf("Number of shutdown hooks: %d%n", countShutdownHooks());
        }
    }

    // thanks to https://stackoverflow.com/a/6865509/14379
    // works on Java 8
    private static int countShutdownHooks() throws ReflectiveOperationException {
        Class clazz = Class.forName("java.lang.ApplicationShutdownHooks");
        Field field = clazz.getDeclaredField("hooks");
        field.setAccessible(true);
        Map<?, ?> hooks = (Map<?, ?>) field.get(null);
        return hooks.size();
    }

    private static int countRefs() throws ReflectiveOperationException {
        Field field = ScanResult.class.getDeclaredField("nonClosedWeakReferences");
        field.setAccessible(true);
        Set hooks = (Set) field.get(null);
        return hooks.size();
    }

}

Now if I use try-with-resources as recommended, references don’t accumulate at all (nor hooks!):

Number of references: 0
Number of shutdown hooks: 1
Number of references: 0
Number of shutdown hooks: 1
Number of references: 0
Number of shutdown hooks: 1
Number of references: 0
Number of shutdown hooks: 1
Number of references: 0
Number of shutdown hooks: 1
...

On the other hand, if I fail to close the ScanResults, the finalizer keeps the weak-refs down:

...
Number of references: 10
Number of shutdown hooks: 1
Number of references: 11
Number of shutdown hooks: 1
Number of references: 12
Number of shutdown hooks: 1
Number of references: 1
Number of shutdown hooks: 1
Number of references: 2
Number of shutdown hooks: 1
...

Ah, I get it. Didn’t notice the bit in close() which was calling nonClosedWeakReferences.remove(weakReference).

Excellent. Thanks @lukehutch!

[lukehutch]

Good to know. I have never needed them before, and there are tons of gotchas to using them right, so this has been a step learning curve. It was a good catch, thanks.