Make TomcatEmbeddedWebappClassLoader parallel capable

Closes gh-10477

Author: wilkinsona

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatEmbeddedWebappClassLoader.java

@@ -21,24 +21,28 @@
 import java.util.Collections;
 import java.util.Enumeration;
 
-import org.apache.catalina.loader.WebappClassLoader;
+import org.apache.catalina.loader.ParallelWebappClassLoader;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
 /**
- * Extension of Tomcat's {@link WebappClassLoader} that does not consider the
+ * Extension of Tomcat's {@link ParallelWebappClassLoader} that does not consider the
  * {@link ClassLoader#getSystemClassLoader() system classloader}. This is required to
- * ensure that any custom context classloader is always used (as is the case with some
+ * ensure that any custom context class loader is always used (as is the case with some
  * executable archives).
  *
  * @author Phillip Webb
  * @since 2.0.0
  */
-public class TomcatEmbeddedWebappClassLoader extends WebappClassLoader {
+public class TomcatEmbeddedWebappClassLoader extends ParallelWebappClassLoader {
 
 	private static final Log logger = LogFactory
 			.getLog(TomcatEmbeddedWebappClassLoader.class);
 
+	static {
+		ClassLoader.registerAsParallelCapable();
+	}
+
 	public TomcatEmbeddedWebappClassLoader() {
 		super();
 	}
@@ -58,14 +62,16 @@ public Enumeration<URL> findResources(String name) throws IOException {
 	}
 
 	@Override
-	public synchronized Class<?> loadClass(String name, boolean resolve)
+	public Class<?> loadClass(String name, boolean resolve)
 			throws ClassNotFoundException {
-		Class<?> result = findExistingLoadedClass(name);
-		result = (result == null ? doLoadClass(name) : result);
-		if (result == null) {
-			throw new ClassNotFoundException(name);
+		synchronized (getClassLoadingLock(name)) {
+			Class<?> result = findExistingLoadedClass(name);
+			result = (result == null ? doLoadClass(name) : result);
+			if (result == null) {
+				throw new ClassNotFoundException(name);
+			}
+			return resolveIfNecessary(result, resolve);
 		}
-		return resolveIfNecessary(result, resolve);
 	}
 
 	private Class<?> findExistingLoadedClass(String name) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/embedded/tomcat/TomcatEmbeddedWebappClassLoaderTests.java

@@ -28,7 +28,7 @@
 import java.util.zip.ZipEntry;
 
 import org.apache.catalina.core.StandardContext;
-import org.apache.catalina.loader.WebappClassLoader;
+import org.apache.catalina.loader.ParallelWebappClassLoader;
 import org.apache.catalina.webresources.StandardRoot;
 import org.apache.catalina.webresources.WarResourceSet;
 import org.junit.Rule;
@@ -73,7 +73,7 @@ private void withWebappClassLoader(File war, ClassLoaderConsumer consumer)
 			throws Exception {
 		URLClassLoader parent = new URLClassLoader(
 				new URL[] { new URL(webInfClassesUrlString(war)) }, null);
-		try (WebappClassLoader classLoader = new TomcatEmbeddedWebappClassLoader(
+		try (ParallelWebappClassLoader classLoader = new TomcatEmbeddedWebappClassLoader(
 				parent)) {
 			StandardContext context = new StandardContext();
 			context.setName("test");