Set Thread context class loader while Tomcat starts up

Fixes gh-1085

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

@@ -18,6 +18,7 @@ package org.springframework.boot.context.embedded.tomcat;
 
 import org.apache.catalina.Container;
 import org.apache.catalina.core.StandardContext;
+import org.springframework.util.ClassUtils;
 
 /**
  * Tomcat {@link StandardContext} used by {@link TomcatEmbeddedServletContainer} to
@@ -32,7 +33,20 @@ class TomcatEmbeddedContext extends StandardContext {
 	}
 
 	public void deferredLoadOnStartup() {
+		// Some older Servlet frameworks (e.g. Struts, BIRT) use the Thread context class
+		// loader to create servlet instances in this phase. If they do that and then try
+		// to initialize them later the class loader may have changed, so wrap the call to
+		// loadOnStartup in what we think its going to be the main webapp classloader at
+		// runtime.
+		ClassLoader classLoader = getLoader().getClassLoader();
+		ClassLoader existingLoader = null;
+		if (classLoader != null) {
+			existingLoader = ClassUtils.overrideThreadContextClassLoader(classLoader);
+		}
 		super.loadOnStartup(findChildren());
+		if (existingLoader != null) {
+			ClassUtils.overrideThreadContextClassLoader(existingLoader);
+		}
 	}
 
 }