DATAGEODE-318 - Add abstract caching implementation of the CacheResolver interface.

This will enable concrete implementations to cache the result of their (potentially expensive) GemFireCache resolution (lookup) process.

spring-data-geode/src/main/java/org/springframework/data/gemfire/support/AbstractCachingCacheResolver.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2017-2020 the original author or 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
+ *
+ *      https://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 org.springframework.data.gemfire.support;
+
+import org.apache.geode.cache.GemFireCache;
+
+import org.springframework.data.gemfire.CacheResolver;
+
+/**
+ * Thread-safe, abstract {@link CacheResolver} implementation to "cache" the instance reference to the (single)
+ * {@link GemFireCache} so that the {@link GemFireCache} object is only ever resolved once.
+ *
+ * @author John Blum
+ * @see org.apache.geode.cache.GemFireCache
+ * @see org.springframework.data.gemfire.CacheResolver
+ * @since 2.3.0
+ */
+public abstract class AbstractCachingCacheResolver<T extends GemFireCache> implements CacheResolver<T> {
+
+	private T cacheReference;
+
+	/**
+	 * @inheritDoc
+	 */
+	@Override
+	public synchronized T resolve() {
+
+		if (this.cacheReference == null) {
+			this.cacheReference = doResolve();
+		}
+
+		return this.cacheReference;
+	}
+
+	/**
+	 * Performs the actual resolution process of the {@link GemFireCache} object iff the cache reference
+	 * is not already cached.
+	 *
+	 * @see #resolve()
+	 */
+	protected abstract T doResolve();
+
+}

spring-data-geode/src/test/java/org/springframework/data/gemfire/support/AbstractCachingCacheResolverUnitTests.java

@@ -0,0 +1,132 @@
+/*
+ * Copyright 2020 the original author or 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
+ *
+ *      https://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 org.springframework.data.gemfire.support;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import edu.umd.cs.mtc.MultithreadedTestCase;
+import edu.umd.cs.mtc.TestFramework;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+
+import org.apache.geode.cache.GemFireCache;
+
+/**
+ * Unit Tests for {@link AbstractCachingCacheResolver}.
+ *
+ * @author John Blum
+ * @see org.junit.Test
+ * @see org.mockito.Mockito
+ * @see org.mockito.junit.MockitoJUnitRunner
+ * @see edu.umd.cs.mtc.MultithreadedTestCase
+ * @see edu.umd.cs.mtc.TestFramework
+ * @see org.springframework.data.gemfire.CacheResolver
+ * @see org.springframework.data.gemfire.support.AbstractCachingCacheResolver
+ * @since 2.3.0
+ */
+@RunWith(MockitoJUnitRunner.class)
+public class AbstractCachingCacheResolverUnitTests {
+
+	@Mock
+	private GemFireCache mockCache;
+
+	@Test
+	@SuppressWarnings("rawtypes")
+	public void resolveCachesTheGemFireCacheObject() {
+
+		AbstractCachingCacheResolver cacheResolver = mock(AbstractCachingCacheResolver.class);
+
+		when(cacheResolver.doResolve()).thenReturn(this.mockCache);
+		when(cacheResolver.resolve()).thenCallRealMethod();
+
+		assertThat(cacheResolver.resolve()).isEqualTo(this.mockCache);
+		assertThat(cacheResolver.resolve()).isSameAs(this.mockCache);
+
+		verify(cacheResolver, times(2)).resolve();
+		verify(cacheResolver, times(1)).doResolve();
+	}
+
+	@Test
+	public void abstractCachinCacheResolverIsThreadSafe() throws Throwable {
+		TestFramework.runOnce(new AbstractCachingCacheResolverMultithreadedTestCase());
+	}
+
+	@SuppressWarnings("unused")
+	public final class AbstractCachingCacheResolverMultithreadedTestCase extends MultithreadedTestCase {
+
+		private AbstractCachingCacheResolver<?> mockCacheResolver;
+
+		private AtomicReference<GemFireCache> gemfireCacheFromThreadOne = new AtomicReference<>(null);
+		private AtomicReference<GemFireCache> gemfireCacheFromThreadTwo = new AtomicReference<>(null);
+
+		@Override
+		public void initialize() {
+
+			super.initialize();
+
+			this.mockCacheResolver = mock(AbstractCachingCacheResolver.class);
+
+			when(this.mockCacheResolver.resolve()).thenCallRealMethod();
+
+			when(this.mockCacheResolver.doResolve()).thenAnswer(invocation -> {
+
+				waitForTick(2);
+
+				return AbstractCachingCacheResolverUnitTests.this.mockCache;
+			});
+		}
+
+		public void thread1() {
+
+			Thread.currentThread().setName("GemFireCache Access Thread 1");
+
+			assertTick(0);
+
+			this.gemfireCacheFromThreadOne.set(this.mockCacheResolver.resolve());
+		}
+
+		public void thread2() {
+
+			Thread.currentThread().setName("GemFireCache Access Thread 2");
+
+			assertTick(0);
+			waitForTick(1);
+
+			this.gemfireCacheFromThreadTwo.set(this.mockCacheResolver.resolve());
+		}
+
+		@Override
+		public void finish() {
+
+			GemFireCache gemfireCacheFromThreadOne = this.gemfireCacheFromThreadOne.get();
+
+			assertThat(gemfireCacheFromThreadOne).isNotNull();
+			assertThat(gemfireCacheFromThreadOne).isEqualTo(this.gemfireCacheFromThreadTwo.get());
+
+			verify(this.mockCacheResolver, times(2)).resolve();
+			verify(this.mockCacheResolver, times(1)).doResolve();
+		}
+	}
+}