feat: add lazy initializer (#423)

* feat: add lazy initializer

* chore: run linter

google-cloud-spanner/src/main/java/com/google/cloud/spanner/AbstractLazyInitializer.java

@@ -0,0 +1,55 @@
+/*
+ * 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.spanner;
+
+/**
+ * Generic {@link AbstractLazyInitializer} for any heavy-weight object that might throw an exception
+ * during initialization. The underlying object is initialized at most once.
+ */
+public abstract class AbstractLazyInitializer<T> {
+  private final Object lock = new Object();
+  private volatile boolean initialized;
+  private volatile T object;
+  private volatile Exception error;
+
+  /** Returns an initialized instance of T. */
+  T get() throws Exception {
+    // First check without a lock to improve performance.
+    if (!initialized) {
+      synchronized (lock) {
+        if (!initialized) {
+          try {
+            object = initialize();
+          } catch (Exception e) {
+            error = e;
+          }
+          initialized = true;
+        }
+      }
+    }
+    if (error != null) {
+      throw error;
+    }
+    return object;
+  }
+
+  /**
+   * Initializes the actual object that should be returned. Is called once the first time an
+   * instance of T is required.
+   */
+  public abstract T initialize() throws Exception;
+}

google-cloud-spanner/src/main/java/com/google/cloud/spanner/LazySpannerInitializer.java

@@ -0,0 +1,29 @@
+/*
+ * 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.spanner;
+
+/** Default implementation of {@link AbstractLazyInitializer} for a {@link Spanner} instance. */
+public class LazySpannerInitializer extends AbstractLazyInitializer<Spanner> {
+  /**
+   * Initializes a default {@link Spanner} instance. Override this method to create an instance with
+   * custom configuration.
+   */
+  @Override
+  public Spanner initialize() throws Exception {
+    return SpannerOptions.newBuilder().build().getService();
+  }
+}

google-cloud-spanner/src/test/java/com/google/cloud/spanner/LazySpannerInitializerTest.java

@@ -0,0 +1,119 @@
+/*
+ * 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.spanner;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.fail;
+import static org.mockito.Mockito.mock;
+
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.google.common.util.concurrent.ListeningExecutorService;
+import com.google.common.util.concurrent.MoreExecutors;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Callable;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class LazySpannerInitializerTest {
+
+  @Test
+  public void testGet_shouldReturnSameInstance() throws Throwable {
+    final LazySpannerInitializer initializer =
+        new LazySpannerInitializer() {
+          @Override
+          public Spanner initialize() {
+            return mock(Spanner.class);
+          }
+        };
+    Spanner s1 = initializer.get();
+    Spanner s2 = initializer.get();
+    assertThat(s1).isSameInstanceAs(s2);
+  }
+
+  @Test
+  public void testGet_shouldThrowErrorFromInitializeMethod() {
+    final LazySpannerInitializer initializer =
+        new LazySpannerInitializer() {
+          @Override
+          public Spanner initialize() throws IOException {
+            throw new IOException("Could not find credentials file");
+          }
+        };
+    Throwable t1 = null;
+    try {
+      initializer.get();
+      fail("Missing expected exception");
+    } catch (Throwable t) {
+      t1 = t;
+    }
+    Throwable t2 = null;
+    try {
+      initializer.get();
+      fail("Missing expected exception");
+    } catch (Throwable t) {
+      t2 = t;
+    }
+    assertThat(t1).isSameInstanceAs(t2);
+  }
+
+  @Test
+  public void testGet_shouldInvokeInitializeOnlyOnce()
+      throws InterruptedException, ExecutionException {
+    final AtomicInteger count = new AtomicInteger();
+    final LazySpannerInitializer initializer =
+        new LazySpannerInitializer() {
+          @Override
+          public Spanner initialize() {
+            count.incrementAndGet();
+            return mock(Spanner.class);
+          }
+        };
+    final int threads = 16;
+    final CountDownLatch latch = new CountDownLatch(threads);
+    ListeningExecutorService executor =
+        MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(threads));
+    List<ListenableFuture<Spanner>> futures = new ArrayList<>(threads);
+    for (int i = 0; i < threads; i++) {
+      futures.add(
+          executor.submit(
+              new Callable<Spanner>() {
+                @Override
+                public Spanner call() throws Exception {
+                  latch.countDown();
+                  latch.await(10L, TimeUnit.SECONDS);
+                  return initializer.get();
+                }
+              }));
+    }
+    assertThat(Futures.allAsList(futures).get()).hasSize(threads);
+    for (int i = 0; i < threads - 1; i++) {
+      assertThat(futures.get(i).get()).isSameInstanceAs(futures.get(i + 1).get());
+    }
+    assertThat(count.get()).isEqualTo(1);
+    executor.shutdown();
+  }
+}