A DESCRIPTION OF THE PROBLEM :
java.beans.Introspector is using a new cache mechanism since Java9 in com.sun.beans.introspect.ClassInfo::CACHE which uses a SoftReference cache over the class, which makes the class to leak, unless clearing the cache manually or the reference.

Projects such as tomcat or classloader-leak-prevention will be required to clear that cache manually for them to work under Java11

I think the fix is:
1. Make java.beans.Introspector::flushCaches() also clear the cache in com.sun.beans.introspect.ClassInfo::CACHE. The flushCaches() is a function which called already by current Java WebApplication projects, and is expected to clear any caches of the Introspector

2. And, consider making a WeakReference to the class in ClassInfo code, but I'm not 100% sure what will be the implications of that. But as far as I understand as long as its classloader is loaded, the class will remain loaded, thus its weak reference will remain valid.

REGRESSION : Last worked in version 8u172

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Any classloader which loads classes, and then call java.beans.Introspector.getBeanInfo() will leak its classes forever, even if called java.beans.Introspector.flushCaches(); which should flush all caches completely.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
After java.beans.Introspector.flushCaches() all Introspector caches will be purged, but its not, which will cause the class, and its classloader to be freed and unloaded successfully
ACTUAL -
The new cache in com.sun.beans.introspect.ClassInfo::CACHE isn't cleared since it is using a SoftReference to the class.

---------- BEGIN SOURCE ----------
import java.lang.ref.WeakReference;
import java.net.URL;
import java.net.URLClassLoader;

public class TestLeak {
  public static void main(String[] args) throws Exception {
    WeakReference<ClassLoader> loader = getLoader();

    // Doesn't clear the cache in com.sun.beans.introspect.ClassInfo::CACHE
    java.beans.Introspector.flushCaches();

    while (loader.get() != null) {
      System.gc();
      Thread.sleep(1000);
      System.out.println("Not freed :(");
    }

    System.out.println("No leak!");
  }


  public static void test() {
    try {
      // Leaks the class loaded under the classloader
      java.beans.Introspector.getBeanInfo(TestLeak.class);

      // The class is still leaked
      java.beans.Introspector.flushFromCaches(TestLeak.class);
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

  private static WeakReference<ClassLoader> getLoader() throws Exception {
    URL url = TestLeak.class.getProtectionDomain().getCodeSource().getLocation();
    URLClassLoader loader = new URLClassLoader(new URL[] {url}, null);
    Class<?> workerClass = Class.forName("TestLeak", true, loader);
    workerClass.getDeclaredMethod("test").invoke(null);
    loader.close();
    return new WeakReference<>(loader);
  }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Clear the cache in "com.sun.beans.introspect.ClassInfo" manually using reflection, which is prune to break.

FREQUENCY : always