Fix Tomcat 8 warnings and Jasper initialization

Fix MalformedURLException warnings caused by Tomcat 8 adding additional
URLs to the TomcatEmbeddedWebappClassLoader.

Add JasperInitializerLifecycleListener to call Tomcat 8's
JasperInitializer which is now required for JSP rendering.

Fixes gh-919

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/JasperInitializerLifecycleListener.java

@@ -0,0 +1,47 @@
+package org.springframework.boot.context.embedded.tomcat;
+
+import org.apache.catalina.LifecycleEvent;
+import org.apache.catalina.LifecycleListener;
+import org.springframework.boot.context.embedded.ServletContextInitializer;
+import org.springframework.util.ClassUtils;
+
+/**
+ * Tomcat {@link LifecycleListener} to initialize Jasper by calling the
+ * `JasperInitializer` used in Tomcat 8.
+ * 
+ * @author Phillip Webb
+ */
+class JasperInitializerLifecycleListener implements LifecycleListener {
+
+	private static final String JASPER_INITIALIZER_CLASS = "org.apache.jasper.servlet.JasperInitializer";
+
+	private final ServletContextInitializerLifecycleListener delegate;
+
+	public JasperInitializerLifecycleListener() {
+		ServletContextInitializer initializer = getJasperInitializer();
+		if (initializer != null) {
+			this.delegate = new ServletContextInitializerLifecycleListener(initializer);
+		}
+		else {
+			this.delegate = null;
+		}
+	}
+
+	@Override
+	public void lifecycleEvent(LifecycleEvent event) {
+		if (this.delegate != null) {
+			this.delegate.lifecycleEvent(event);
+		}
+	}
+
+	private ServletContextInitializer getJasperInitializer() {
+		try {
+			Class<?> jasperClass = ClassUtils.forName(JASPER_INITIALIZER_CLASS, null);
+			return (ServletContextInitializer) jasperClass.newInstance();
+		}
+		catch (Exception ex) {
+			return null;
+		}
+	}
+
+}

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatEmbeddedServletContainerFactory.java

@@ -230,6 +230,7 @@ public class TomcatEmbeddedServletContainerFactory extends
 			ServletContextInitializer[] initializers) {
 		context.addLifecycleListener(new ServletContextInitializerLifecycleListener(
 				initializers));
+		context.addLifecycleListener(new JasperInitializerLifecycleListener());
 		for (LifecycleListener lifecycleListener : this.contextLifecycleListeners) {
 			context.addLifecycleListener(lifecycleListener);
 		}

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

@@ -16,7 +16,11 @@
 
 package org.springframework.boot.context.embedded.tomcat;
 
+import java.net.URL;
+
 import org.apache.catalina.loader.WebappClassLoader;
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 
 /**
  * Extension of Tomcat's {@link WebappClassLoader} that does not consider the
@@ -28,6 +32,9 @@ import org.apache.catalina.loader.WebappClassLoader;
  */
 public class TomcatEmbeddedWebappClassLoader extends WebappClassLoader {
 
+	private static final Log logger = LogFactory
+			.getLog(TomcatEmbeddedWebappClassLoader.class);
+
 	public TomcatEmbeddedWebappClassLoader() {
 		super();
 	}
@@ -78,6 +85,14 @@ public class TomcatEmbeddedWebappClassLoader extends WebappClassLoader {
 		return (resultClass);
 	}
 
+	@Override
+	protected void addURL(URL url) {
+		// Ignore URLs added by the Tomcat 8 implementation (see gh-919)
+		if (logger.isTraceEnabled()) {
+			logger.trace("Ignoring request to add " + url + " to the tomcat classloader");
+		}
+	}
+
 	private Class<?> loadFromParent(String name) {
 		if (this.parent == null) {
 			return null;