Add ResolverProviderTest unit test (#337)

Author: Bouncheck

core/src/main/java/com/datastax/oss/driver/internal/core/resolver/ResolverProvider.java

@@ -1,6 +1,7 @@
 package com.datastax.oss.driver.internal.core.resolver;
 
 import com.datastax.oss.driver.internal.core.resolver.defaultResolver.DefaultResolverFactory;
+import com.datastax.oss.driver.shaded.guava.common.annotations.VisibleForTesting;
 
 /**
  * Entry point for driver components to getting the {@link Resolver} instances. By default returns
@@ -45,4 +46,10 @@ public static synchronized void setDefaultResolverFactory(
     alreadySet = true;
     defaultResolverFactoryImpl = resolverFactoryImpl;
   }
+
+  @VisibleForTesting
+  static synchronized void resetBooleansForTesting() {
+    alreadyInUse = false;
+    alreadySet = false;
+  }
 }

core/src/test/java/com/datastax/oss/driver/internal/core/resolver/ResolverProviderTest.java

@@ -0,0 +1,52 @@
+package com.datastax.oss.driver.internal.core.resolver;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+import com.datastax.oss.driver.internal.core.resolver.mockResolver.MockResolverFactory;
+import org.junit.Before;
+import org.junit.Test;
+
+public class ResolverProviderTest {
+
+  @Before
+  public void resetState() {
+    ResolverProvider.resetBooleansForTesting();
+  }
+
+  @Test(expected = IllegalStateException.class)
+  public void should_not_allow_setting_twice() {
+    try {
+      MockResolverFactory first = new MockResolverFactory();
+      MockResolverFactory second = new MockResolverFactory();
+      ResolverProvider.setDefaultResolverFactory(first);
+      ResolverProvider.setDefaultResolverFactory(second);
+    } catch (IllegalStateException ex) {
+      assertThat(ex.getMessage())
+          .isEqualTo(
+              "Cannot change default resolver factory: this method has already been called. You can set "
+                  + "default resolver factory only once.");
+      throw ex;
+    }
+  }
+
+  @Test(expected = IllegalStateException.class)
+  public void should_not_allow_setting_once_in_use() {
+    try {
+      ResolverProvider.getResolver(ResolverProviderTest.class);
+      MockResolverFactory first = new MockResolverFactory();
+      ResolverProvider.setDefaultResolverFactory(first);
+    } catch (IllegalStateException ex) {
+      assertThat(ex.getMessage())
+          .isEqualTo(
+              "Cannot change default resolver factory: ResolverProvider has already returned an instance of a "
+                  + "Resolver to use. Default resolver factory needs to be set up before first use by any class.");
+      throw ex;
+    }
+  }
+
+  @Test
+  public void should_allow_setting_once() {
+    MockResolverFactory first = new MockResolverFactory();
+    ResolverProvider.setDefaultResolverFactory(first);
+  }
+}