Metrics: Implement ExportComponent, make ProducerManager abstract. (#1347)

* Metrics: Implement ExportComponent, make ProducerManager abstract.

* Metrics: Add impl and impllite.

* Resolve two TODOs.

* Fix import path, leave a TODO for getMetricRegistry.

* Make NoopMetricProducerManager private. Add missing since tag.

4 files changed, 34 insertions, 38 deletions

diff --git a/api/src/main/java/io/opencensus/common/ToDoubleFunction.java b/api/src/main/java/io/opencensus/common/ToDoubleFunction.java
index 479270f1..eac85793 100644
--- a/api/src/main/java/io/opencensus/common/ToDoubleFunction.java
+++ b/api/src/main/java/io/opencensus/common/ToDoubleFunction.java
@@ -16,10 +16,8 @@
 
 package io.opencensus.common;
 
-// TODO(bdrutu): Change @code to @link when metrics is moved in the API.
-
 /**
- * Represents a function that produces a double-valued result. See {@code
+ * Represents a function that produces a double-valued result. See {@link
  * io.opencensus.metrics.MetricRegistry} for an example of its use.
  *
  * <p>Note: This class is based on the java.util.ToDoubleFunction class added in Java 1.8. We cannot
diff --git a/api/src/main/java/io/opencensus/common/ToLongFunction.java b/api/src/main/java/io/opencensus/common/ToLongFunction.java
index 9a6e9b19..26a27b18 100644
--- a/api/src/main/java/io/opencensus/common/ToLongFunction.java
+++ b/api/src/main/java/io/opencensus/common/ToLongFunction.java
@@ -16,10 +16,8 @@
 
 package io.opencensus.common;
 
-// TODO(bdrutu): Change @code to @link when metrics is moved in the API.
-
 /**
- * Represents a function that produces a long-valued result. See {@code
+ * Represents a function that produces a long-valued result. See {@link
  * io.opencensus.metrics.MetricRegistry} for an example of its use.
  *
  * <p>Note: This class is based on the java.util.ToLongFunction class added in Java 1.8. We cannot
diff --git a/api/src/main/java/io/opencensus/metrics/export/ExportComponent.java b/api/src/main/java/io/opencensus/metrics/export/ExportComponent.java
index f1511543..bab65d13 100644
--- a/api/src/main/java/io/opencensus/metrics/export/ExportComponent.java
+++ b/api/src/main/java/io/opencensus/metrics/export/ExportComponent.java
@@ -50,7 +50,7 @@ public abstract class ExportComponent {
   private static final class NoopExportComponent extends ExportComponent {
 
     private static final MetricProducerManager METRIC_PRODUCER_MANAGER =
-        new MetricProducerManager();
+        MetricProducerManager.newNoopMetricProducerManager();
 
     @Override
     public MetricProducerManager getMetricProducerManager() {
diff --git a/api/src/main/java/io/opencensus/metrics/export/MetricProducerManager.java b/api/src/main/java/io/opencensus/metrics/export/MetricProducerManager.java
index 9600a023..c5acf174 100644
--- a/api/src/main/java/io/opencensus/metrics/export/MetricProducerManager.java
+++ b/api/src/main/java/io/opencensus/metrics/export/MetricProducerManager.java
@@ -20,7 +20,6 @@ import io.opencensus.common.ExperimentalApi;
 import io.opencensus.internal.Utils;
 import io.opencensus.metrics.MetricProducer;
 import java.util.Collections;
-import java.util.LinkedHashSet;
 import java.util.Set;
 import javax.annotation.concurrent.ThreadSafe;
 
@@ -32,9 +31,7 @@ import javax.annotation.concurrent.ThreadSafe;
  */
 @ExperimentalApi
 @ThreadSafe
-public final class MetricProducerManager {
-  private volatile Set<MetricProducer> metricProducers =
-      Collections.unmodifiableSet(new LinkedHashSet<MetricProducer>());
+public abstract class MetricProducerManager {
 
   /**
    * Adds the {@link MetricProducer} to the manager if it is not already present.
@@ -42,34 +39,15 @@ public final class MetricProducerManager {
    * @param metricProducer the {@code MetricProducer} to be added to the manager.
    * @since 0.16
    */
-  public synchronized void add(MetricProducer metricProducer) {
-    Utils.checkNotNull(metricProducer, "metricProducer");
-    // Updating the set of MetricProducers happens under a lock to avoid multiple add or remove
-    // operations to happen in the same time.
-    Set<MetricProducer> newMetricProducers = new LinkedHashSet<MetricProducer>(metricProducers);
-    if (!newMetricProducers.add(metricProducer)) {
-      // The element already present, no need to update the current set of MetricProducers.
-      return;
-    }
-    metricProducers = Collections.unmodifiableSet(newMetricProducers);
-  }
+  public abstract void add(MetricProducer metricProducer);
 
   /**
    * Removes the {@link MetricProducer} to the manager if it is present.
    *
    * @param metricProducer the {@code MetricProducer} to be removed from the manager.
+   * @since 0.16
    */
-  public synchronized void remove(MetricProducer metricProducer) {
-    Utils.checkNotNull(metricProducer, "metricProducer");
-    // Updating the set of MetricProducers happens under a lock to avoid multiple add or remove
-    // operations to happen in the same time.
-    Set<MetricProducer> newMetricProducers = new LinkedHashSet<MetricProducer>(metricProducers);
-    if (!newMetricProducers.remove(metricProducer)) {
-      // The element not present, no need to update the current set of MetricProducers.
-      return;
-    }
-    metricProducers = Collections.unmodifiableSet(newMetricProducers);
-  }
+  public abstract void remove(MetricProducer metricProducer);
 
   /**
    * Returns all registered {@link MetricProducer}s that should be exported.
@@ -78,12 +56,34 @@ public final class MetricProducerManager {
    * {@code MetricProducer} registered with the {@code MetricProducerManager}.
    *
    * @return all registered {@code MetricProducer}s that should be exported.
+   * @since 0.16
    */
-  public Set<MetricProducer> getAllMetricProducer() {
-    return metricProducers;
+  public abstract Set<MetricProducer> getAllMetricProducer();
+
+  /**
+   * Returns a no-op implementation for {@link MetricProducerManager}.
+   *
+   * @return a no-op implementation for {@code MetricProducerManager}.
+   */
+  static MetricProducerManager newNoopMetricProducerManager() {
+    return new NoopMetricProducerManager();
   }
 
-  // Package protected to allow us to possibly change this to an abstract class in the future. This
-  // ensures that nobody can create an instance of this class except ExportComponent.
-  MetricProducerManager() {}
+  private static final class NoopMetricProducerManager extends MetricProducerManager {
+
+    @Override
+    public void add(MetricProducer metricProducer) {
+      Utils.checkNotNull(metricProducer, "metricProducer");
+    }
+
+    @Override
+    public void remove(MetricProducer metricProducer) {
+      Utils.checkNotNull(metricProducer, "metricProducer");
+    }
+
+    @Override
+    public Set<MetricProducer> getAllMetricProducer() {
+      return Collections.emptySet();
+    }
+  }
 }