Cas collect compatibility with earlier jvms (#399)

! core:  introduce CAS for safer counter collection and inline implementation of Unsafe.getAndSetLong for <jdk8 compatibility.

3 files changed, 35 insertions, 1 deletions

diff --git a/kamon-core/src/main/java/kamon/jsr166/LongAdder.java b/kamon-core/src/main/java/kamon/jsr166/LongAdder.java
index 18f91759..7e47ae63 100644
--- a/kamon-core/src/main/java/kamon/jsr166/LongAdder.java
+++ b/kamon-core/src/main/java/kamon/jsr166/LongAdder.java
@@ -151,6 +151,21 @@ public class LongAdder extends Striped64 implements Serializable {
         return sum;
     }
 
+    public long sumAndReset() {
+        long sum = getAndSetBase(0L);
+        Cell[] as = cells;
+        if (as != null) {
+            int n = as.length;
+            for (int i = 0; i < n; ++i) {
+                Cell a = as[i];
+                if (a != null) {
+                    sum += a.getAndSet(0L);
+                }
+            }
+        }
+        return sum;
+    }
+
     /**
      * Returns the String representation of the {@link #sum}.
      * @return the String representation of the {@link #sum}
diff --git a/kamon-core/src/main/java/kamon/jsr166/Striped64.java b/kamon-core/src/main/java/kamon/jsr166/Striped64.java
index 4840c8d7..8fbfa4ba 100644
--- a/kamon-core/src/main/java/kamon/jsr166/Striped64.java
+++ b/kamon-core/src/main/java/kamon/jsr166/Striped64.java
@@ -105,6 +105,14 @@ abstract class Striped64 extends Number {
             return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
         }
 
+        final long getAndSet(long val) {
+            long v;
+            do {
+                v = UNSAFE.getLongVolatile(this, valueOffset);
+            } while (!UNSAFE.compareAndSwapLong(this, valueOffset, v, val));
+            return v;
+        }
+
         // Unsafe mechanics
         private static final sun.misc.Unsafe UNSAFE;
         private static final long valueOffset;
@@ -182,6 +190,17 @@ abstract class Striped64 extends Number {
     }
 
     /**
+     * CASes the base field.
+     */
+    final long getAndSetBase(long val) {
+        long v;
+        do {
+            v = UNSAFE.getLongVolatile(this, baseOffset);
+        } while (!UNSAFE.compareAndSwapLong(this, baseOffset, v, val));
+        return v;
+    }
+
+    /**
      * CASes the busy field from 0 to 1 to acquire lock.
      */
     final boolean casBusy() {
diff --git a/kamon-core/src/main/scala/kamon/metric/instrument/Counter.scala b/kamon-core/src/main/scala/kamon/metric/instrument/Counter.scala
index 349a12bd..b7ab60de 100644
--- a/kamon-core/src/main/scala/kamon/metric/instrument/Counter.scala
+++ b/kamon-core/src/main/scala/kamon/metric/instrument/Counter.scala
@@ -48,7 +48,7 @@ class LongAdderCounter extends Counter {
     counter.add(times)
   }
 
-  def collect(context: CollectionContext): Counter.Snapshot = CounterSnapshot(counter.sumThenReset())
+  def collect(context: CollectionContext): Counter.Snapshot = CounterSnapshot(counter.sumAndReset())
 
   def cleanup: Unit = {}
 }