fix: Implement ChannelCache, which aggregates stubs to a single chann…

…el, and properly cleans them up on teardown. (#72)

google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/Stubs.java

@@ -17,19 +17,21 @@
 package com.google.cloud.pubsublite;
 
 import com.google.auth.oauth2.GoogleCredentials;
+import com.google.cloud.pubsublite.internal.ChannelCache;
 import com.google.common.collect.ImmutableList;
 import io.grpc.Channel;
-import io.grpc.ManagedChannelBuilder;
 import io.grpc.auth.MoreCallCredentials;
 import io.grpc.stub.AbstractStub;
 import java.io.IOException;
 import java.util.function.Function;
 
 public class Stubs {
+  private static final ChannelCache channels = new ChannelCache();
+
   public static <StubT extends AbstractStub<StubT>> StubT defaultStub(
       String target, Function<Channel, StubT> stubFactory) throws IOException {
     return stubFactory
-        .apply(ManagedChannelBuilder.forTarget(target).build())
+        .apply(channels.get(target))
         .withCallCredentials(
             MoreCallCredentials.from(
                 GoogleCredentials.getApplicationDefault()

google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/internal/ChannelCache.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * 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 com.google.cloud.pubsublite.internal;
+
+import io.grpc.Channel;
+import io.grpc.ManagedChannel;
+import io.grpc.ManagedChannelBuilder;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.TimeUnit;
+
+/** A ChannelCache creates and stores default channels for use with api methods. */
+public class ChannelCache {
+  private final ConcurrentHashMap<String, ManagedChannel> channels = new ConcurrentHashMap<>();
+
+  public ChannelCache() {
+    Runtime.getRuntime().addShutdownHook(new Thread(this::onShutdown));
+  }
+
+  private void onShutdown() {
+    channels.forEachValue(
+        channels.size(),
+        channel -> {
+          try {
+            channel.shutdownNow().awaitTermination(60, TimeUnit.SECONDS);
+          } catch (InterruptedException e) {
+            e.printStackTrace();
+          }
+        });
+  }
+
+  public Channel get(String target) {
+    return channels.computeIfAbsent(target, this::newChannel);
+  }
+
+  private ManagedChannel newChannel(String target) {
+    return ManagedChannelBuilder.forTarget(target).build();
+  }
+}