Add ResourceResolver implementation that resolves to a single (i.e. 'Singleton') Resource.

Resolve gh-92.

spring-geode/src/test/java/org/springframework/geode/core/io/support/SingleResourceResolverUnitTests.java

@@ -0,0 +1,62 @@
+/*
+ * 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.geode.core.io.support;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verifyNoInteractions;
+
+import org.junit.Test;
+
+import org.springframework.core.io.Resource;
+
+/**
+ * Unit Tests for {@link SingleResourceResolver}.
+ *
+ * @author John Blum
+ * @see org.junit.Test
+ * @see org.mockito.Mockito
+ * @see org.springframework.core.io.Resource
+ * @see org.springframework.geode.core.io.ResourceResolver
+ * @see org.springframework.geode.core.io.support.SingleResourceResolver
+ * @since 1.3.1
+ */
+public class SingleResourceResolverUnitTests {
+
+	@Test
+	public void constructsSingleResourceResolverWithNonNullResource() {
+
+		Resource mockResource = mock(Resource.class);
+
+		SingleResourceResolver resourceResolver = new SingleResourceResolver(mockResource);
+
+		assertThat(resourceResolver.resolve("/location/of/resource").orElse(null)).isEqualTo(mockResource);
+		assertThat(resourceResolver.resolve("/path/to/resource").orElse(null)).isEqualTo(mockResource);
+		assertThat(resourceResolver.resolve("/another/path/to/resource").orElse(null)).isEqualTo(mockResource);
+		assertThat(resourceResolver.resolve("/yet/another/path/to/resource").orElse(null)).isEqualTo(mockResource);
+
+		verifyNoInteractions(mockResource);
+	}
+
+	@Test
+	public void constructSingleResourceResolverWithNullResource() {
+
+		SingleResourceResolver resourceResolver = new SingleResourceResolver(null);
+
+		assertThat(resourceResolver.resolve("/location/of/resource").orElse(null)).isNull();
+		assertThat(resourceResolver.resolve("/path/to/resource").orElse(null)).isNull();
+	}
+}