Remove dependency on sun.misc.Unsafe (#547)

This replaces `sun.misc.Unsafe` with
`java.util.concurrent.atomic.AtomicLongFieldUpdater` and
`AtomicIntegerFieldUpdater`. This is to enable compatibility with Jigsaw
modules where `sun.misc.Unsafe` is no longer available.

Closes #468

Signed-off-by: Andre Masella <andre@masella.name>

simpleclient/src/main/java/io/prometheus/client/Striped64.java

@@ -9,6 +9,8 @@
 package io.prometheus.client;
 
 import java.util.Random;
+import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
+import java.util.concurrent.atomic.AtomicLongFieldUpdater;
 
 /**
  * A package-local class holding common representation and mechanics
@@ -94,22 +96,10 @@ static final class Cell {
         Cell(long x) { value = x; }
 
         final boolean cas(long cmp, long val) {
-            return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
+            return CAS_VALUE.compareAndSet(this, cmp, val);
         }
 
-        // Unsafe mechanics
-        private static final sun.misc.Unsafe UNSAFE;
-        private static final long valueOffset;
-        static {
-            try {
-                UNSAFE = getUnsafe();
-                Class<?> ak = Cell.class;
-                valueOffset = UNSAFE.objectFieldOffset
-                        (ak.getDeclaredField("value"));
-            } catch (Exception e) {
-                throw new Error(e);
-            }
-        }
+        private static final AtomicLongFieldUpdater<Cell> CAS_VALUE = AtomicLongFieldUpdater.newUpdater(Cell.class, "value");
 
     }
 
@@ -155,14 +145,14 @@ final boolean cas(long cmp, long val) {
      * CASes the base field.
      */
     final boolean casBase(long cmp, long val) {
-        return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val);
+        return CAS_BASE.compareAndSet(this, cmp, val);
     }
 
     /**
      * CASes the busy field from 0 to 1 to acquire lock.
      */
     final boolean casBusy() {
-        return UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1);
+        return CAS_BUSY.compareAndSet(this, 0, 1);
     }
 
     /**
@@ -287,50 +277,7 @@ final void internalReset(long initialValue) {
         }
     }
 
-    // Unsafe mechanics
-    private static final sun.misc.Unsafe UNSAFE;
-    private static final long baseOffset;
-    private static final long busyOffset;
-    static {
-        try {
-            UNSAFE = getUnsafe();
-            Class<?> sk = Striped64.class;
-            baseOffset = UNSAFE.objectFieldOffset
-                    (sk.getDeclaredField("base"));
-            busyOffset = UNSAFE.objectFieldOffset
-                    (sk.getDeclaredField("busy"));
-        } catch (Exception e) {
-            throw new Error(e);
-        }
-    }
+    private static final AtomicLongFieldUpdater<Striped64> CAS_BASE = AtomicLongFieldUpdater.newUpdater(Striped64.class, "base");
+    private static final AtomicIntegerFieldUpdater<Striped64> CAS_BUSY = AtomicIntegerFieldUpdater.newUpdater(Striped64.class, "busy");
 
-    /**
-     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
-     * Replace with a simple call to Unsafe.getUnsafe when integrating
-     * into a jdk.
-     *
-     * @return a sun.misc.Unsafe
-     */
-    private static sun.misc.Unsafe getUnsafe() {
-        try {
-            return sun.misc.Unsafe.getUnsafe();
-        } catch (SecurityException tryReflectionInstead) {}
-        try {
-            return java.security.AccessController.doPrivileged
-                    (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
-                        public sun.misc.Unsafe run() throws Exception {
-                            Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
-                            for (java.lang.reflect.Field f : k.getDeclaredFields()) {
-                                f.setAccessible(true);
-                                Object x = f.get(null);
-                                if (k.isInstance(x))
-                                    return k.cast(x);
-                            }
-                            throw new NoSuchFieldError("the Unsafe");
-                        }});
-        } catch (java.security.PrivilegedActionException e) {
-            throw new RuntimeException("Could not initialize intrinsics",
-                    e.getCause());
-        }
-    }
 }