Address comments, perform deep copies and support synchronization for…

… mutable BoundedTrieData

runners/core-java/src/main/java/org/apache/beam/runners/core/metrics/BoundedTrieCell.java

@@ -19,10 +19,8 @@
 
 import java.util.Arrays;
 import java.util.Objects;
-import java.util.concurrent.atomic.AtomicReference;
 import org.apache.beam.sdk.metrics.BoundedTrie;
 import org.apache.beam.sdk.metrics.MetricName;
-import org.apache.beam.sdk.metrics.MetricsContainer;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -33,44 +31,37 @@
  * In that case retrieving the underlying cell and reporting directly to it avoids a step of
  * indirection.
  */
+// TODO: Write multi-threaded test in MetricContainerImp for this Cell class too.
 public class BoundedTrieCell implements BoundedTrie, MetricCell<BoundedTrieData> {
 
   private final DirtyState dirty = new DirtyState();
-  private final AtomicReference<BoundedTrieData> setValue =
-      new AtomicReference<>(BoundedTrieData.empty());
+  private final BoundedTrieData value;
   private final MetricName name;
 
-  /**
-   * Generally, runners should construct instances using the methods in {@link
-   * MetricsContainerImpl}, unless they need to define their own version of {@link
-   * MetricsContainer}. These constructors are *only* public so runners can instantiate.
-   */
   public BoundedTrieCell(MetricName name) {
     this.name = name;
+    this.value = new BoundedTrieData();
+  }
+
+  public void update(BoundedTrieCell other) {
+    this.value.combine(other.value);
+    dirty.afterModification();
   }
 
   @Override
   public void reset() {
-    setValue.set(BoundedTrieData.empty());
+    value.clear();
     dirty.reset();
   }
 
-  void update(BoundedTrieData data) {
-    BoundedTrieData original;
-    do {
-      original = setValue.get();
-    } while (!setValue.compareAndSet(original, original.combine(data)));
-    dirty.afterModification();
-  }
-
   @Override
   public DirtyState getDirty() {
     return dirty;
   }
 
   @Override
   public BoundedTrieData getCumulative() {
-    return setValue.get();
+    return value.getCumulative();
   }
 
   @Override
@@ -83,23 +74,20 @@ public boolean equals(@Nullable Object object) {
     if (object instanceof BoundedTrieCell) {
       BoundedTrieCell boundedTrieCell = (BoundedTrieCell) object;
       return Objects.equals(dirty, boundedTrieCell.dirty)
-          && Objects.equals(setValue.get(), boundedTrieCell.setValue.get())
+          && Objects.equals(value, boundedTrieCell.value)
           && Objects.equals(name, boundedTrieCell.name);
     }
     return false;
   }
 
   @Override
   public int hashCode() {
-    return Objects.hash(dirty, setValue.get(), name);
+    return Objects.hash(dirty, value, name);
   }
 
   @Override
   public void add(Iterable<String> values) {
-    BoundedTrieData original;
-    do {
-      original = setValue.get();
-    } while (!setValue.compareAndSet(original, original.add(values)));
+    this.value.add(values);
     dirty.afterModification();
   }