Allow empty URIs to create embedded datasource

Refine `DataSourceAutoConfiguration` conditions so that an empty
`spring.datasource.url` property can be used to enable the embedded
source.

Fixed gh-20996

Author: philwebb

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/DataSourceAutoConfiguration.java

@@ -36,8 +36,10 @@
 import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
+import org.springframework.core.env.Environment;
 import org.springframework.core.type.AnnotatedTypeMetadata;
 import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType;
+import org.springframework.util.StringUtils;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for {@link DataSource}.
@@ -135,8 +137,7 @@ static class EmbeddedDatabaseCondition extends SpringBootCondition {
 		@Override
 		public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
 			ConditionMessage.Builder message = ConditionMessage.forCondition("EmbeddedDataSource");
-			boolean hasDatasourceUrl = context.getEnvironment().containsProperty(DATASOURCE_URL_PROPERTY);
-			if (hasDatasourceUrl) {
+			if (hasDataSourceUrlProperty(context)) {
 				return ConditionOutcome.noMatch(message.because(DATASOURCE_URL_PROPERTY + " is set"));
 			}
 			if (anyMatches(context, metadata, this.pooledCondition)) {
@@ -149,6 +150,19 @@ public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeM
 			return ConditionOutcome.match(message.found("embedded database").items(type));
 		}
 
+		private boolean hasDataSourceUrlProperty(ConditionContext context) {
+			Environment environment = context.getEnvironment();
+			if (environment.containsProperty(DATASOURCE_URL_PROPERTY)) {
+				try {
+					return StringUtils.hasText(environment.getProperty(DATASOURCE_URL_PROPERTY));
+				}
+				catch (IllegalArgumentException ex) {
+					// Ignore unresolvable placeholder errors
+				}
+			}
+			return false;
+		}
+
 	}
 
 }

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/jdbc/DataSourceAutoConfigurationTests.java

@@ -48,6 +48,7 @@
 import org.springframework.context.annotation.Configuration;
 import org.springframework.jdbc.core.JdbcTemplate;
 import org.springframework.jdbc.datasource.SimpleDriverDataSource;
+import org.springframework.jdbc.datasource.embedded.EmbeddedDatabase;
 import org.springframework.util.StringUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -201,6 +202,12 @@ public void whenThereIsAUserProvidedDataSourceAnUnresolvablePlaceholderDoesNotCa
 				.run((context) -> assertThat(context).getBean(DataSource.class).isInstanceOf(BasicDataSource.class));
 	}
 
+	@Test
+	public void whenThereIsAnEmptyUserProvidedDataSource() {
+		this.contextRunner.with(hideConnectionPools()).withPropertyValues("spring.datasource.url:")
+				.run((context) -> assertThat(context).getBean(DataSource.class).isInstanceOf(EmbeddedDatabase.class));
+	}
+
 	@Test
 	public void testDataSourceIsInitializedEarly() {
 		this.contextRunner.withUserConfiguration(TestInitializedDataSourceConfiguration.class)