Explain missing JDBC driver for R2DBC users of MySQL, MariaDB and MSSQL

docs/modules/databases/mariadb.md

@@ -44,4 +44,5 @@ Add the following dependency to your `pom.xml`/`build.gradle` file:
 
 !!! hint
     Adding this Testcontainers library JAR will not automatically add a database driver JAR to your project. You should ensure that your project also has a suitable database driver as a dependency.
+    The JDBC driver is required even if your application only uses R2DBC, because the container startup check connects over JDBC.
 

docs/modules/databases/mssqlserver.md

@@ -44,6 +44,7 @@ Add the following dependency to your `pom.xml`/`build.gradle` file:
 
 !!! hint
     Adding this Testcontainers library JAR will not automatically add a database driver JAR to your project. You should ensure that your project also has a suitable database driver as a dependency.
+    The JDBC driver is required even if your application only uses R2DBC, because the container startup check connects over JDBC.
 
 ## License
 

docs/modules/databases/mysql.md

@@ -54,3 +54,4 @@ Add the following dependency to your `pom.xml`/`build.gradle` file:
 
 !!! hint
     Adding this Testcontainers library JAR will not automatically add a database driver JAR to your project. You should ensure that your project also has a suitable database driver as a dependency.
+    The JDBC driver is required even if your application only uses R2DBC, because the container startup check connects over JDBC.

docs/modules/databases/r2dbc.md

@@ -14,6 +14,9 @@ The started container will be terminated when the `ConnectionFactory` is closed.
 !!! warning
     Both the database module (e.g. `org.testcontainers:testcontainers-mysql`) **and** `org.testcontainers:testcontainers-r2dbc` need to be on your application's classpath at runtime.
 
+!!! warning
+    For MySQL, MariaDB and MS SQL Server, the JDBC driver also needs to be on the test classpath, even if your application only uses R2DBC. Testcontainers checks that these databases are ready by connecting over JDBC; without the driver, the container fails to start with `Could not get Driver`.
+
 **Original URL**: `r2dbc:mysql://localhost:3306/databasename`
 
 1. Insert `tc:` after `r2dbc:` as follows. Note that the hostname, port and database name will be ignored; you can leave these as-is or set them to any value.

modules/jdbc/src/main/java/org/testcontainers/containers/JdbcDatabaseContainer.java

@@ -232,7 +232,14 @@ public Driver getJdbcDriverInstance() throws NoDriverFoundException {
                 try {
                     driver = (Driver) Class.forName(this.getDriverClassName()).newInstance();
                 } catch (InstantiationException | IllegalAccessException | ClassNotFoundException e) {
-                    throw new NoDriverFoundException("Could not get Driver", e);
+                    throw new NoDriverFoundException(
+                        String.format(
+                            "Could not get Driver %s. The JDBC driver must be on the classpath because the " +
+                            "container startup check connects over JDBC, even if the application only uses R2DBC",
+                            this.getDriverClassName()
+                        ),
+                        e
+                    );
                 }
             }
         }

modules/jdbc/src/test/java/org/testcontainers/jdbc/MissingJdbcDriverTest.java

@@ -10,6 +10,7 @@
 import java.util.concurrent.atomic.AtomicInteger;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.assertj.core.api.Assertions.fail;
 
 class MissingJdbcDriverTest {
@@ -35,6 +36,17 @@ void shouldFailFastIfNoDriverFound() {
             .isEqualTo(1);
     }
 
+    @Test
+    void shouldNameTheMissingDriverClass() {
+        final MissingDriverContainer container = new MissingDriverContainer();
+
+        assertThatThrownBy(container::getJdbcDriverInstance)
+            .isInstanceOf(JdbcDatabaseContainer.NoDriverFoundException.class)
+            .hasMessageContaining("nonexistent.ClassName")
+            .hasMessageContaining("R2DBC")
+            .hasCauseInstanceOf(ClassNotFoundException.class);
+    }
+
     /**
      * Container class for the purposes of testing, with a known non-existent driver
      */