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.

16 files changed, 482 insertions, 74 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();
+    }
+  }
 }
diff --git a/api/src/test/java/io/opencensus/metrics/export/MetricProducerManagerTest.java b/api/src/test/java/io/opencensus/metrics/export/MetricProducerManagerTest.java
index b79de213..07854927 100644
--- a/api/src/test/java/io/opencensus/metrics/export/MetricProducerManagerTest.java
+++ b/api/src/test/java/io/opencensus/metrics/export/MetricProducerManagerTest.java
@@ -19,7 +19,6 @@ package io.opencensus.metrics.export;
 import static com.google.common.truth.Truth.assertThat;
 
 import io.opencensus.metrics.MetricProducer;
-import java.util.Set;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -32,9 +31,9 @@ import org.mockito.MockitoAnnotations;
 /** Unit tests for {@link MetricProducerManager}. */
 @RunWith(JUnit4.class)
 public class MetricProducerManagerTest {
-  private final MetricProducerManager metricProducerManager = new MetricProducerManager();
+  private final MetricProducerManager metricProducerManager =
+      MetricProducerManager.newNoopMetricProducerManager();
   @Mock private MetricProducer metricProducer;
-  @Mock private MetricProducer metricProducerOther;
 
   @Rule public final ExpectedException thrown = ExpectedException.none();
 
@@ -52,34 +51,13 @@ public class MetricProducerManagerTest {
   @Test
   public void add() {
     metricProducerManager.add(metricProducer);
-    assertThat(metricProducerManager.getAllMetricProducer()).containsExactly(metricProducer);
-  }
-
-  @Test
-  public void add_DuplicateElement() {
-    metricProducerManager.add(metricProducer);
-    Set<MetricProducer> metricProducerSet = metricProducerManager.getAllMetricProducer();
-    assertThat(metricProducerSet).containsExactly(metricProducer);
-    metricProducerManager.add(metricProducer);
-    // Returns the same object.
-    assertThat(metricProducerManager.getAllMetricProducer()).isSameAs(metricProducerSet);
-  }
-
-  @Test
-  public void add_MultipleElements() {
-    metricProducerManager.add(metricProducer);
-    Set<MetricProducer> metricProducerSet = metricProducerManager.getAllMetricProducer();
-    assertThat(metricProducerSet).containsExactly(metricProducer);
-    metricProducerManager.add(metricProducerOther);
-    // Returns the same object.
-    assertThat(metricProducerManager.getAllMetricProducer())
-        .containsExactly(metricProducer, metricProducerOther);
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
   }
 
   @Test
   public void addAndRemove() {
     metricProducerManager.add(metricProducer);
-    assertThat(metricProducerManager.getAllMetricProducer()).containsExactly(metricProducer);
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
     metricProducerManager.remove(metricProducer);
     assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
   }
@@ -97,16 +75,6 @@ public class MetricProducerManagerTest {
   }
 
   @Test
-  public void remove_NotPresent() {
-    metricProducerManager.add(metricProducer);
-    Set<MetricProducer> metricProducerSet = metricProducerManager.getAllMetricProducer();
-    assertThat(metricProducerSet).containsExactly(metricProducer);
-    metricProducerManager.remove(metricProducerOther);
-    // Returns the same object.
-    assertThat(metricProducerManager.getAllMetricProducer()).isSameAs(metricProducerSet);
-  }
-
-  @Test
   public void getAllMetricProducer_empty() {
     assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
   }
diff --git a/buildscripts/import-control.xml b/buildscripts/import-control.xml
index 66c561a4..7c8251bb 100644
--- a/buildscripts/import-control.xml
+++ b/buildscripts/import-control.xml
@@ -46,6 +46,7 @@ General guidelines on imports:
   <subpackage name="stats">
     <allow pkg="io.opencensus.common"/>
     <allow pkg="io.opencensus.internal"/>
+    <allow pkg="io.opencensus.metrics"/>
     <allow pkg="io.opencensus.stats"/>
     <allow pkg="io.opencensus.tags"/>
   </subpackage>
@@ -178,6 +179,7 @@ General guidelines on imports:
     <allow pkg="io.opencensus.common"/>
     <allow pkg="io.opencensus.impl"/>
     <allow pkg="io.opencensus.implcore"/>
+    <allow pkg="io.opencensus.metrics"/>
     <allow pkg="io.opencensus.stats"/>
     <allow pkg="io.opencensus.tags"/>
     <allow pkg="io.opencensus.trace"/>
@@ -186,6 +188,7 @@ General guidelines on imports:
     <allow pkg="io.opencensus.common"/>
     <allow pkg="io.opencensus.implcore"/>
     <allow pkg="io.opencensus.impllite"/>
+    <allow pkg="io.opencensus.metrics"/>
     <allow pkg="io.opencensus.stats"/>
     <allow pkg="io.opencensus.tags"/>
     <allow pkg="io.opencensus.trace"/>
diff --git a/impl/src/main/java/io/opencensus/impl/metrics/MetricsComponentImpl.java b/impl/src/main/java/io/opencensus/impl/metrics/MetricsComponentImpl.java
new file mode 100644
index 00000000..bd8b6154
--- /dev/null
+++ b/impl/src/main/java/io/opencensus/impl/metrics/MetricsComponentImpl.java
@@ -0,0 +1,23 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.impl.metrics;
+
+import io.opencensus.implcore.metrics.MetricsComponentImplBase;
+import io.opencensus.metrics.MetricsComponent;
+
+/** Implementation of {@link MetricsComponent}. */
+public final class MetricsComponentImpl extends MetricsComponentImplBase {}
diff --git a/impl/src/test/java/io/opencensus/impl/metrics/MetricsTest.java b/impl/src/test/java/io/opencensus/impl/metrics/MetricsTest.java
new file mode 100644
index 00000000..d27fa83d
--- /dev/null
+++ b/impl/src/test/java/io/opencensus/impl/metrics/MetricsTest.java
@@ -0,0 +1,36 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.impl.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import io.opencensus.implcore.metrics.export.ExportComponentImpl;
+import io.opencensus.metrics.Metrics;
+import io.opencensus.metrics.MetricsComponent;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Test for accessing the {@link MetricsComponent} through the {@link Metrics} class. */
+@RunWith(JUnit4.class)
+public class MetricsTest {
+
+  @Test
+  public void getExportComponent() {
+    assertThat(Metrics.getExportComponent()).isInstanceOf(ExportComponentImpl.class);
+  }
+}
diff --git a/impl_core/src/main/java/io/opencensus/implcore/metrics/MetricsComponentImplBase.java b/impl_core/src/main/java/io/opencensus/implcore/metrics/MetricsComponentImplBase.java
new file mode 100644
index 00000000..312b702e
--- /dev/null
+++ b/impl_core/src/main/java/io/opencensus/implcore/metrics/MetricsComponentImplBase.java
@@ -0,0 +1,42 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.metrics;
+
+import io.opencensus.implcore.metrics.export.ExportComponentImpl;
+import io.opencensus.metrics.MetricRegistry;
+import io.opencensus.metrics.MetricsComponent;
+
+/** Implementation of {@link MetricsComponent}. */
+public class MetricsComponentImplBase extends MetricsComponent {
+
+  private final ExportComponentImpl exportComponent;
+
+  @Override
+  public ExportComponentImpl getExportComponent() {
+    return exportComponent;
+  }
+
+  @Override
+  public MetricRegistry getMetricRegistry() {
+    // TODO: implement this method
+    throw new UnsupportedOperationException("not implemented");
+  }
+
+  public MetricsComponentImplBase() {
+    exportComponent = new ExportComponentImpl();
+  }
+}
diff --git a/impl_core/src/main/java/io/opencensus/implcore/metrics/export/ExportComponentImpl.java b/impl_core/src/main/java/io/opencensus/implcore/metrics/export/ExportComponentImpl.java
new file mode 100644
index 00000000..173c3aec
--- /dev/null
+++ b/impl_core/src/main/java/io/opencensus/implcore/metrics/export/ExportComponentImpl.java
@@ -0,0 +1,31 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.metrics.export;
+
+import io.opencensus.metrics.export.ExportComponent;
+import io.opencensus.metrics.export.MetricProducerManager;
+
+/** Implementation of {@link ExportComponent}. */
+public final class ExportComponentImpl extends ExportComponent {
+
+  private final MetricProducerManager metricProducerManager = new MetricProducerManagerImpl();
+
+  @Override
+  public MetricProducerManager getMetricProducerManager() {
+    return metricProducerManager;
+  }
+}
diff --git a/impl_core/src/main/java/io/opencensus/implcore/metrics/export/MetricProducerManagerImpl.java b/impl_core/src/main/java/io/opencensus/implcore/metrics/export/MetricProducerManagerImpl.java
new file mode 100644
index 00000000..fc9db5c1
--- /dev/null
+++ b/impl_core/src/main/java/io/opencensus/implcore/metrics/export/MetricProducerManagerImpl.java
@@ -0,0 +1,64 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.metrics.export;
+
+import com.google.common.base.Preconditions;
+import io.opencensus.metrics.MetricProducer;
+import io.opencensus.metrics.export.MetricProducerManager;
+import java.util.Collections;
+import java.util.LinkedHashSet;
+import java.util.Set;
+import javax.annotation.concurrent.ThreadSafe;
+
+/** Implementation of {@link MetricProducerManager}. */
+@ThreadSafe
+public final class MetricProducerManagerImpl extends MetricProducerManager {
+
+  private volatile Set<MetricProducer> metricProducers =
+      Collections.unmodifiableSet(new LinkedHashSet<MetricProducer>());
+
+  @Override
+  public synchronized void add(MetricProducer metricProducer) {
+    Preconditions.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);
+  }
+
+  @Override
+  public synchronized void remove(MetricProducer metricProducer) {
+    Preconditions.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);
+  }
+
+  @Override
+  public Set<MetricProducer> getAllMetricProducer() {
+    return metricProducers;
+  }
+}
diff --git a/impl_core/src/test/java/io/opencensus/implcore/metrics/MetricsComponentImplBaseTest.java b/impl_core/src/test/java/io/opencensus/implcore/metrics/MetricsComponentImplBaseTest.java
new file mode 100644
index 00000000..a86a53d9
--- /dev/null
+++ b/impl_core/src/test/java/io/opencensus/implcore/metrics/MetricsComponentImplBaseTest.java
@@ -0,0 +1,36 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import io.opencensus.implcore.metrics.export.ExportComponentImpl;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link MetricsComponentImplBase}. */
+@RunWith(JUnit4.class)
+public class MetricsComponentImplBaseTest {
+
+  @Test
+  public void getExportComponent() {
+    MetricsComponentImplBase metricsComponentImplBase = new MetricsComponentImplBase();
+    assertThat(metricsComponentImplBase.getExportComponent())
+        .isInstanceOf(ExportComponentImpl.class);
+  }
+}
diff --git a/impl_core/src/test/java/io/opencensus/implcore/metrics/export/ExportComponentImplTest.java b/impl_core/src/test/java/io/opencensus/implcore/metrics/export/ExportComponentImplTest.java
new file mode 100644
index 00000000..fb91641c
--- /dev/null
+++ b/impl_core/src/test/java/io/opencensus/implcore/metrics/export/ExportComponentImplTest.java
@@ -0,0 +1,35 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.metrics.export;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link ExportComponentImpl}. */
+@RunWith(JUnit4.class)
+public class ExportComponentImplTest {
+
+  @Test
+  public void getMetricProducerManager() {
+    ExportComponentImpl exportComponent = new ExportComponentImpl();
+    assertThat(exportComponent.getMetricProducerManager())
+        .isInstanceOf(MetricProducerManagerImpl.class);
+  }
+}
diff --git a/impl_core/src/test/java/io/opencensus/implcore/metrics/export/MetricProducerManagerImplTest.java b/impl_core/src/test/java/io/opencensus/implcore/metrics/export/MetricProducerManagerImplTest.java
new file mode 100644
index 00000000..7f2ec153
--- /dev/null
+++ b/impl_core/src/test/java/io/opencensus/implcore/metrics/export/MetricProducerManagerImplTest.java
@@ -0,0 +1,115 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.metrics.export;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import io.opencensus.metrics.MetricProducer;
+import io.opencensus.metrics.export.MetricProducerManager;
+import java.util.Set;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+
+/** Unit tests for {@link MetricProducerManagerImpl}. */
+@RunWith(JUnit4.class)
+public class MetricProducerManagerImplTest {
+
+  private final MetricProducerManager metricProducerManager = new MetricProducerManagerImpl();
+  @Mock private MetricProducer metricProducer;
+  @Mock private MetricProducer metricProducerOther;
+
+  @Rule public final ExpectedException thrown = ExpectedException.none();
+
+  @Before
+  public void setUp() {
+    MockitoAnnotations.initMocks(this);
+  }
+
+  @Test
+  public void add_DisallowsNull() {
+    thrown.expect(NullPointerException.class);
+    metricProducerManager.add(null);
+  }
+
+  @Test
+  public void add() {
+    metricProducerManager.add(metricProducer);
+    assertThat(metricProducerManager.getAllMetricProducer()).containsExactly(metricProducer);
+  }
+
+  @Test
+  public void add_DuplicateElement() {
+    metricProducerManager.add(metricProducer);
+    Set<MetricProducer> metricProducerSet = metricProducerManager.getAllMetricProducer();
+    assertThat(metricProducerSet).containsExactly(metricProducer);
+    metricProducerManager.add(metricProducer);
+    // Returns the same object.
+    assertThat(metricProducerManager.getAllMetricProducer()).isSameAs(metricProducerSet);
+  }
+
+  @Test
+  public void add_MultipleElements() {
+    metricProducerManager.add(metricProducer);
+    Set<MetricProducer> metricProducerSet = metricProducerManager.getAllMetricProducer();
+    assertThat(metricProducerSet).containsExactly(metricProducer);
+    metricProducerManager.add(metricProducerOther);
+    // Returns the same object.
+    assertThat(metricProducerManager.getAllMetricProducer())
+        .containsExactly(metricProducer, metricProducerOther);
+  }
+
+  @Test
+  public void addAndRemove() {
+    metricProducerManager.add(metricProducer);
+    assertThat(metricProducerManager.getAllMetricProducer()).containsExactly(metricProducer);
+    metricProducerManager.remove(metricProducer);
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
+  }
+
+  @Test
+  public void remove_DisallowsNull() {
+    thrown.expect(NullPointerException.class);
+    metricProducerManager.remove(null);
+  }
+
+  @Test
+  public void remove_FromEmpty() {
+    metricProducerManager.remove(metricProducer);
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
+  }
+
+  @Test
+  public void remove_NotPresent() {
+    metricProducerManager.add(metricProducer);
+    Set<MetricProducer> metricProducerSet = metricProducerManager.getAllMetricProducer();
+    assertThat(metricProducerSet).containsExactly(metricProducer);
+    metricProducerManager.remove(metricProducerOther);
+    // Returns the same object.
+    assertThat(metricProducerManager.getAllMetricProducer()).isSameAs(metricProducerSet);
+  }
+
+  @Test
+  public void getAllMetricProducer_empty() {
+    assertThat(metricProducerManager.getAllMetricProducer()).isEmpty();
+  }
+}
diff --git a/impl_lite/src/main/java/io/opencensus/impllite/metrics/MetricsComponentImplLite.java b/impl_lite/src/main/java/io/opencensus/impllite/metrics/MetricsComponentImplLite.java
new file mode 100644
index 00000000..e755088d
--- /dev/null
+++ b/impl_lite/src/main/java/io/opencensus/impllite/metrics/MetricsComponentImplLite.java
@@ -0,0 +1,23 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.impllite.metrics;
+
+import io.opencensus.implcore.metrics.MetricsComponentImplBase;
+import io.opencensus.metrics.MetricsComponent;
+
+/** Android-compatible implementation of {@link MetricsComponent}. */
+public final class MetricsComponentImplLite extends MetricsComponentImplBase {}
diff --git a/impl_lite/src/test/java/io/opencensus/impllite/metrics/MetricsTest.java b/impl_lite/src/test/java/io/opencensus/impllite/metrics/MetricsTest.java
new file mode 100644
index 00000000..ab8889a9
--- /dev/null
+++ b/impl_lite/src/test/java/io/opencensus/impllite/metrics/MetricsTest.java
@@ -0,0 +1,36 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.impllite.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import io.opencensus.implcore.metrics.export.ExportComponentImpl;
+import io.opencensus.metrics.Metrics;
+import io.opencensus.metrics.MetricsComponent;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Test for accessing the {@link MetricsComponent} through the {@link Metrics} class. */
+@RunWith(JUnit4.class)
+public class MetricsTest {
+
+  @Test
+  public void getExportComponent() {
+    assertThat(Metrics.getExportComponent()).isInstanceOf(ExportComponentImpl.class);
+  }
+}