Improve startup failure diagnostics

A typical cause of startup failure is an incorrectly configured
classpath. Previously the classpath used to launch the application
wasn't easy for a user to discover. This made problem diagnosis
difficult. This commit adds an application listener that, in the event
of a start error, logs the thread context class loader's classpath at
INFO level. The classpath is also logged at DEBUG level when an
application starts successfully.

spring-boot/src/main/java/org/springframework/boot/context/listener/ClasspathLoggingApplicationListener.java

+/*
+ * Copyright 2012-2013 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.context.listener;
+
+import java.net.URLClassLoader;
+import java.util.Arrays;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import org.springframework.boot.SpringApplicationErrorEvent;
+import org.springframework.boot.SpringApplicationStartEvent;
+import org.springframework.context.ApplicationEvent;
+import org.springframework.context.event.SmartApplicationListener;
+
+/**
+ * A {@link SmartApplicationListener} that reacts to {@link SpringApplicationStartEvent
+ * start events} by logging the classpath of the thread context class loader (TCCL) at
+ * {@code DEBUG} level and to {@link SpringApplicationErrorEvent error events} by logging
+ * the TCCL's classpath at {@code INFO} level.
+ * 
+ * @author Andy Wilkinson
+ */
+public final class ClasspathLoggingApplicationListener implements
+		SmartApplicationListener {
+
+	private static final int ORDER = Integer.MIN_VALUE + 12;
+
+	private final Log logger = LogFactory.getLog(getClass());
+
+	@Override
+	public void onApplicationEvent(ApplicationEvent event) {
+		if (event instanceof SpringApplicationStartEvent) {
+			if (this.logger.isDebugEnabled()) {
+				this.logger
+						.debug("Application started with classpath: " + getClasspath());
+			}
+		}
+		else if (event instanceof SpringApplicationErrorEvent) {
+			if (this.logger.isInfoEnabled()) {
+				this.logger.info("Application failed to start with classpath: "
+						+ getClasspath());
+			}
+		}
+	}
+
+	@Override
+	public int getOrder() {
+		return ORDER;
+	}
+
+	@Override
+	public boolean supportsEventType(Class<? extends ApplicationEvent> type) {
+		return SpringApplicationStartEvent.class.isAssignableFrom(type)
+				|| SpringApplicationErrorEvent.class.isAssignableFrom(type);
+	}
+
+	@Override
+	public boolean supportsSourceType(Class<?> sourceType) {
+		return true;
+	}
+
+	private String getClasspath() {
+		ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
+		if (classLoader instanceof URLClassLoader) {
+			return Arrays.toString(((URLClassLoader) classLoader).getURLs());
+		}
+		else {
+			return "unknown";
+		}
+	}
+}

spring-boot/src/main/resources/META-INF/spring.factories

@@ -5,9 +5,11 @@ org.springframework.boot.context.initializer.EnvironmentDelegateApplicationConte
 
 # Application Listeners
 org.springframework.context.ApplicationListener=\
-org.springframework.boot.context.listener.FileEncodingApplicationListener,\
-org.springframework.boot.context.listener.VcapApplicationListener,\
+org.springframework.boot.context.listener.ClasspathLoggingApplicationListener,\
 org.springframework.boot.context.listener.ConfigFileApplicationListener,\
+org.springframework.boot.context.listener.EnvironmentDelegateApplicationListener,\
+org.springframework.boot.context.listener.FileEncodingApplicationListener,\
 org.springframework.boot.context.listener.LoggingApplicationListener,\
-org.springframework.boot.liquibase.LiquibaseServiceLocatorInitializer,\
-org.springframework.boot.context.listener.EnvironmentDelegateApplicationListener
+org.springframework.boot.context.listener.VcapApplicationListener,\
+org.springframework.boot.liquibase.LiquibaseServiceLocatorInitializer
+