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feat: add lazy initializer (#423)
* feat: add lazy initializer

* chore: run linter
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olavloite committed Sep 15, 2020
1 parent fd63673 commit e8522b9955c4a19fa7d6297fd463e9d2521dff92
@@ -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;
}
@@ -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();
}
}
@@ -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();
}
}

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