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feat: add lazy initializer #423
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55 changes: 55 additions & 0 deletions
55
google-cloud-spanner/src/main/java/com/google/cloud/spanner/AbstractLazyInitializer.java
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/* | ||
* 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. | ||
*/ | ||
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package com.google.cloud.spanner; | ||
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/** | ||
* 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; | ||
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/** 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; | ||
} | ||
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/** | ||
* 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; | ||
} |
29 changes: 29 additions & 0 deletions
29
google-cloud-spanner/src/main/java/com/google/cloud/spanner/LazySpannerInitializer.java
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/* | ||
* 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. | ||
*/ | ||
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package com.google.cloud.spanner; | ||
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/** 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(); | ||
} | ||
} |
119 changes: 119 additions & 0 deletions
119
google-cloud-spanner/src/test/java/com/google/cloud/spanner/LazySpannerInitializerTest.java
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/* | ||
* 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. | ||
*/ | ||
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package com.google.cloud.spanner; | ||
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import static com.google.common.truth.Truth.assertThat; | ||
import static org.junit.Assert.fail; | ||
import static org.mockito.Mockito.mock; | ||
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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; | ||
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@RunWith(JUnit4.class) | ||
public class LazySpannerInitializerTest { | ||
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@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); | ||
} | ||
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@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); | ||
} | ||
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@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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Could we use composition instead of inheritance (pass in the initialiser function instead of overriding it)?
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That would be possible, but as we have to support Java 7, that would mean that we would have to use
com.google.cloud.base.Function
in the public interface. I think that it is more idiomatic to use inheritance than composition for Java 7 (once we drop Java 7 support, I would agree that using composition would be a lot nicer in this case).