Deregister JDBC drivers when deployed war's ServletContext is destroyed

Closes gh-21221

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/support/SpringBootServletInitializer.java

@@ -16,6 +16,9 @@
 
 package org.springframework.boot.web.servlet.support;
 
+import java.sql.Driver;
+import java.sql.DriverManager;
+import java.sql.SQLException;
 import java.util.Collections;
 
 import javax.servlet.Filter;
@@ -98,14 +101,47 @@ public void contextInitialized(ServletContextEvent event) {
 					// no-op because the application context is already initialized
 				}
 
+				@Override
+				public void contextDestroyed(ServletContextEvent event) {
+					try {
+						super.contextDestroyed(event);
+					}
+					finally {
+						deregisterJdbcDrivers(event.getServletContext());
+					}
+				}
+
 			});
+
 		}
 		else {
 			this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not "
 					+ "return an application context");
 		}
 	}
 
+	/**
+	 * Deregisters the JDBC drivers that were registered by the application represented by
+	 * the given {@code servletContext}. The default implementation
+	 * {@link DriverManager#deregisterDriver(Driver) deregisters} every {@link Driver}
+	 * that was loaded by the {@link ServletContext#getClassLoader web application's class
+	 * loader}.
+	 * @param servletContext the web application's servlet context
+	 * @since 2.3.0
+	 */
+	protected void deregisterJdbcDrivers(ServletContext servletContext) {
+		for (Driver driver : Collections.list(DriverManager.getDrivers())) {
+			if (driver.getClass().getClassLoader() == servletContext.getClassLoader()) {
+				try {
+					DriverManager.deregisterDriver(driver);
+				}
+				catch (SQLException ex) {
+					// Continue
+				}
+			}
+		}
+	}
+
 	protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
 		SpringApplicationBuilder builder = createSpringApplicationBuilder();
 		builder.main(getClass());

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/servlet/support/SpringBootServletInitializerTests.java

@@ -17,12 +17,18 @@
 package org.springframework.boot.web.servlet.support;
 
 import java.util.Collections;
+import java.util.Vector;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import javax.servlet.ServletContext;
+import javax.servlet.ServletContextEvent;
+import javax.servlet.ServletContextListener;
+import javax.servlet.ServletException;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.builder.SpringApplicationBuilder;
@@ -46,6 +52,7 @@
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 import static org.mockito.BDDMockito.given;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
 
 /**
  * Tests for {@link SpringBootServletInitializer}.
@@ -142,6 +149,31 @@ void servletContextPropertySourceIsAvailablePriorToRefresh() {
 		}
 	}
 
+	@Test
+	void whenServletContextIsDestroyedThenJdbcDriversAreDeregistered() throws ServletException {
+		ServletContext servletContext = mock(ServletContext.class);
+		given(servletContext.getInitParameterNames()).willReturn(new Vector<String>().elements());
+		given(servletContext.getAttributeNames()).willReturn(new Vector<String>().elements());
+		AtomicBoolean driversDeregistered = new AtomicBoolean();
+		new SpringBootServletInitializer() {
+
+			@Override
+			protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
+				return builder.sources(Config.class);
+			}
+
+			@Override
+			protected void deregisterJdbcDrivers(ServletContext servletContext) {
+				driversDeregistered.set(true);
+			}
+
+		}.onStartup(servletContext);
+		ArgumentCaptor<ServletContextListener> captor = ArgumentCaptor.forClass(ServletContextListener.class);
+		verify(servletContext).addListener(captor.capture());
+		captor.getValue().contextDestroyed(new ServletContextEvent(servletContext));
+		assertThat(driversDeregistered).isTrue();
+	}
+
 	static class PropertySourceVerifyingSpringBootServletInitializer extends SpringBootServletInitializer {
 
 		@Override