ExternalCleaner.java

import java.lang.ref.Cleaner;

public class ExternalCleaner {

  public static void main(String... args) throws Exception {
    while (true) {
      SensitiveData data = new SensitiveData();
      StateCleaner cleaner = new StateCleaner(data);

      Thread.sleep(100);
      System.gc();
    }
  }

  static class SensitiveData {

    public void clear() {
      System.out.println("I'm getting cleared - yay!");
    }
  }

  static class StateCleaner implements AutoCloseable {
    private static final Cleaner cleaner = Cleaner.create();

    static class State implements Runnable {

      private final SensitiveData data;

      State(SensitiveData data) {
        this.data = data;
      }

      public void run() {
        data.clear();
      }
    }

    private final Cleaner.Cleanable cleanable;

    public StateCleaner(final SensitiveData data) {
      State state = new State(data);
      this.cleanable = cleaner.register(this, state);
    }

    public void close() {
      cleanable.clean();
    }

  }

}