RFR: Add test to verify Shenandoah is not enabled by default, and enabled with the flag

[Roman Kennke]

Ok


> Upstream had changed the GC selection logic a bit, and it is easy to miss the code that enables the
> GC: -XX:+UseShenandoahGC would be accepted, but Shenandoah would not enable! (Found this during
> Epsilon merges). Let's add a simple test that verifies GC is enabled:
> 
> diff -r 58b1ec268475 test/hotspot/jtreg/gc/shenandoah/options/TestEnabled.java
> --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
> +++ b/test/hotspot/jtreg/gc/shenandoah/options/TestEnabled.java	Fri Apr 13 10:50:12 2018 +0200
> @@ -0,0 +1,52 @@
> +/*
> + * Copyright (c) 2017, 2018, Red Hat, Inc. All rights reserved.
> + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
> + *
> + * This code is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License version 2 only, as
> + * published by the Free Software Foundation.
> + *
> + * This code is distributed in the hope that it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
> + * version 2 for more details (a copy is included in the LICENSE file that
> + * accompanied this code).
> + *
> + * You should have received a copy of the GNU General Public License version
> + * 2 along with this work; if not, write to the Free Software Foundation,
> + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
> + *
> + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
> + * or visit www.oracle.com if you need additional information or have any
> + * questions.
> + */
> +
> +import java.lang.management.GarbageCollectorMXBean;
> +import java.lang.management.ManagementFactory;
> +
> +/*
> + * @test TestEnabled
> + * @run main/othervm -Dexpected=false -Xmx64m                      TestEnabled
> + * @run main/othervm -Dexpected=true  -Xmx64m -XX:+UseShenandoahGC TestEnabled
> + */
> +
> +public class TestEnabled {
> +
> +  public static void main(String... args) {
> +    boolean expected = Boolean.getBoolean("expected");
> +    boolean actual = isEnabled();
> +    if (expected != actual) {
> +      throw new IllegalStateException("Error: expected = " + expected + ", actual = " + actual);
> +    }
> +  }
> +
> +  public static boolean isEnabled() {
> +    for (GarbageCollectorMXBean bean : ManagementFactory.getGarbageCollectorMXBeans()) {
> +      if (bean.getName().contains("Shenandoah")) {
> +        return true;
> +      }
> +    }
> +    return false;
> +  }
> +
> +}
> 
> 
> Thanks,
> -Aleksey
>