8154096: Extend WhiteBox API with methods which retrieve from VM information about available GC

Reviewed-by: kbarrett, iignatyev

test/lib/sun/hotspot/WhiteBox.java

@@ -377,6 +377,12 @@ public class WhiteBox {
   public native long incMetaspaceCapacityUntilGC(long increment);
   public native long metaspaceCapacityUntilGC();
 
+  // Don't use these methods directly
+  // Use sun.hotspot.gc.GC class instead.
+  public native int currentGC();
+  public native int allSupportedGC();
+  public native boolean gcSelectedByErgo();
+
   // Force Young GC
   public native void youngGC();
 

test/lib/sun/hotspot/gc/GC.java

@@ -0,0 +1,104 @@
+/*
+ * Copyright (c) 2016, Oracle and/or its affiliates. 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.
+ *
+ */
+
+package sun.hotspot.gc;
+
+import java.util.ArrayList;
+import java.util.List;
+import sun.hotspot.WhiteBox;
+
+/**
+ * API to obtain information about selected and supported Garbage Collectors
+ * retrieved from the VM with the WhiteBox API.
+ */
+public enum GC {
+    Serial(1),
+    Parallel(2),
+    ConcMarkSweep(4),
+    G1(8);
+
+    private static final GC CURRENT_GC;
+    private static final int ALL_GC_CODES;
+    private static final boolean IS_BY_ERGO;
+    static {
+        WhiteBox WB = WhiteBox.getWhiteBox();
+        ALL_GC_CODES = WB.allSupportedGC();
+        IS_BY_ERGO = WB.gcSelectedByErgo();
+
+        int currentCode = WB.currentGC();
+        GC tmp = null;
+        for (GC gc: GC.values()) {
+            if (gc.code == currentCode) {
+                tmp = gc;
+                break;
+            }
+        }
+        if (tmp == null) {
+            throw new Error("Unknown current GC code " + currentCode);
+        }
+        CURRENT_GC = tmp;
+    }
+
+    private final int code;
+    private GC(int code) {
+        this.code = code;
+    }
+
+    /**
+     * @return true if the collector is supported by the VM, false otherwise.
+     */
+    public boolean isSupported() {
+        return (ALL_GC_CODES & code) != 0;
+    }
+
+
+    /**
+     * @return the current collector used by VM.
+     */
+    public static GC current() {
+        return CURRENT_GC;
+    }
+
+    /**
+     * @return true if GC was selected by ergonomic, false if specified
+     * explicitly by the command line flag.
+     */
+    public static boolean currentSetByErgo() {
+        return IS_BY_ERGO;
+    }
+
+    /**
+     * @return List of collectors supported by the VM.
+     */
+    public static List<GC> allSupported() {
+        List<GC> list = new ArrayList<>();
+        for (GC gc: GC.values()) {
+            if (gc.isSupported()) {
+                list.add(gc);
+            }
+        }
+        return list;
+    }
+}
+