Improve error message used in TestDatabaseAutoConfiguration

By default, `@DataJpaTest` (and `@AutoConfigureTestDatabase`) attempt to
replace any existing `DataSource` by an embedded one. Previously, if
there is was no embedded database on the classpath, the exception message
did not provide that context in the error message.

This commit clarifies the error message to conduct
`TestDatabaseAutoConfiguration` (that is replacing the existing
`DataSource`).

Closes gh-7797

spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/orm/jpa/TestDatabaseAutoConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 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.
@@ -193,9 +193,10 @@ public class TestDatabaseAutoConfiguration {
 				connection = EmbeddedDatabaseConnection.get(getClass().getClassLoader());
 			}
 			Assert.state(connection != EmbeddedDatabaseConnection.NONE,
-					"Cannot determine embedded database for tests. If you want "
-							+ "an embedded database please put a supported one "
-							+ "on the classpath.");
+					"Failed to replace DataSource with an embedded database for tests. If "
+							+ "you want an embedded database please put a supported one "
+							+ "on the classpath or tune the replace attribute of "
+							+ "@AutoconfigureTestDatabase.");
 			return new EmbeddedDatabaseBuilder().generateUniqueName(true)
 					.setType(connection.getType()).build();
 		}

spring-boot-test-autoconfigure/src/test/java/org/springframework/boot/test/autoconfigure/orm/jpa/TestDatabaseAutoConfigurationNoEmbeddedTests.java

@@ -0,0 +1,106 @@
+/*
+ * Copyright 2012-2017 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
+ *
+ *      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 org.springframework.boot.test.autoconfigure.orm.jpa;
+
+import javax.sql.DataSource;
+
+import org.junit.After;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.beans.factory.BeanCreationException;
+import org.springframework.boot.test.util.EnvironmentTestUtils;
+import org.springframework.boot.testutil.ClassPathExclusions;
+import org.springframework.boot.testutil.FilteredClassPathRunner;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Specific tests for {@link TestDatabaseAutoConfiguration} when no embedded database is
+ * available.
+ *
+ * @author Stephane Nicoll
+ */
+@RunWith(FilteredClassPathRunner.class)
+@ClassPathExclusions({ "h2-*.jar", "hsqldb-*.jar", "derby-*.jar" })
+public class TestDatabaseAutoConfigurationNoEmbeddedTests {
+
+	private ConfigurableApplicationContext context;
+
+	@After
+	public void closeContext() {
+		if (this.context != null) {
+			this.context.close();
+		}
+	}
+
+	@Test
+	public void applyAnyReplace() {
+		try {
+			load(ExistingDataSourceConfiguration.class);
+		}
+		catch (BeanCreationException ex) {
+			String message = ex.getMessage();
+			assertThat(message).contains(
+					"Failed to replace DataSource with an embedded database for tests.");
+			assertThat(message).contains(
+					"If you want an embedded database please put a supported one on the "
+							+ "classpath");
+			assertThat(message).contains(
+					"or tune the replace attribute of @AutoconfigureTestDatabase.");
+		}
+	}
+
+	@Test
+	public void applyNoReplace() {
+		load(ExistingDataSourceConfiguration.class, "spring.test.database.replace=NONE");
+		assertThat(this.context.getBeansOfType(DataSource.class)).hasSize(1);
+		assertThat(this.context.getBean(DataSource.class)).isSameAs(
+				this.context.getBean("myCustomDataSource"));
+	}
+
+	public void load(Class<?> config, String... environment) {
+		this.context = doLoad(config, environment);
+	}
+
+	public ConfigurableApplicationContext doLoad(Class<?> config, String... environment) {
+		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
+		if (config != null) {
+			ctx.register(config);
+		}
+		ctx.register(TestDatabaseAutoConfiguration.class);
+		EnvironmentTestUtils.addEnvironment(ctx, environment);
+		ctx.refresh();
+		return ctx;
+	}
+
+	@Configuration
+	static class ExistingDataSourceConfiguration {
+
+		@Bean
+		public DataSource myCustomDataSource() {
+			return mock(DataSource.class);
+		}
+
+	}
+
+}