Refine ApplicationHome detection logic

Update the detection logic used in ApplicationHome to:
- Deal with `!/` elements in URLs so that `BOOT-INF/classes` packaging
  works as expected.
- Use the `start-class` when no explicit source class is provided to
  prevent accidentally picking a home next to a `spring-boot.jar` that
  happens to be on the classpath.
- Ignore search logic when running from a unit test.

Fixes gh-6129

.gitignore

@@ -35,3 +35,4 @@ overridedb.*
 .DS_Store
 .factorypath
 dump.rdb
+transaction-logs

spring-boot/src/main/java/org/springframework/boot/ApplicationHome.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -18,12 +18,17 @@ package org.springframework.boot;
 
 import java.io.File;
 import java.io.IOException;
+import java.io.InputStream;
 import java.net.JarURLConnection;
 import java.net.URL;
 import java.net.URLConnection;
 import java.security.CodeSource;
 import java.security.ProtectionDomain;
+import java.util.Enumeration;
+import java.util.jar.JarFile;
+import java.util.jar.Manifest;
 
+import org.springframework.util.ClassUtils;
 import org.springframework.util.StringUtils;
 
 /**
@@ -51,17 +56,51 @@ public class ApplicationHome {
 	 * @param sourceClass the source class or {@code null}
 	 */
 	public ApplicationHome(Class<?> sourceClass) {
-		this.source = findSource(sourceClass == null ? getClass() : sourceClass);
+		this.source = findSource(sourceClass == null ? getStartClass() : sourceClass);
 		this.dir = findHomeDir(this.source);
 	}
 
+	private Class<?> getStartClass() {
+		try {
+			ClassLoader classLoader = getClass().getClassLoader();
+			return getStartClass(classLoader.getResources("META-INF/MANIFEST.MF"));
+		}
+		catch (Exception ex) {
+			return null;
+		}
+	}
+
+	private Class<?> getStartClass(Enumeration<URL> manifestResources) {
+		while (manifestResources.hasMoreElements()) {
+			try {
+				InputStream inputStream = manifestResources.nextElement().openStream();
+				try {
+					Manifest manifest = new Manifest(inputStream);
+					String startClass = manifest.getMainAttributes()
+							.getValue("Start-Class");
+					if (startClass != null) {
+						return ClassUtils.forName(startClass,
+								getClass().getClassLoader());
+					}
+				}
+				finally {
+					inputStream.close();
+				}
+			}
+			catch (Exception ex) {
+			}
+		}
+		return null;
+	}
+
 	private File findSource(Class<?> sourceClass) {
 		try {
-			ProtectionDomain protectionDomain = sourceClass.getProtectionDomain();
-			CodeSource codeSource = protectionDomain.getCodeSource();
+			ProtectionDomain domain = (sourceClass == null ? null
+					: sourceClass.getProtectionDomain());
+			CodeSource codeSource = (domain == null ? null : domain.getCodeSource());
 			URL location = (codeSource == null ? null : codeSource.getLocation());
 			File source = (location == null ? null : findSource(location));
-			if (source != null && source.exists()) {
+			if (source != null && source.exists() && !isUnitTest()) {
 				return source.getAbsoluteFile();
 			}
 			return null;
@@ -71,14 +110,36 @@ public class ApplicationHome {
 		}
 	}
 
+	private boolean isUnitTest() {
+		try {
+			for (StackTraceElement element : Thread.currentThread().getStackTrace()) {
+				if (element.getClassName().startsWith("org.junit.")) {
+					return true;
+				}
+			}
+		}
+		catch (Exception ex) {
+		}
+		return false;
+	}
+
 	private File findSource(URL location) throws IOException {
 		URLConnection connection = location.openConnection();
 		if (connection instanceof JarURLConnection) {
-			return new File(((JarURLConnection) connection).getJarFile().getName());
+			return getRootJarFile(((JarURLConnection) connection).getJarFile());
 		}
 		return new File(location.getPath());
 	}
 
+	private File getRootJarFile(JarFile jarFile) {
+		String name = jarFile.getName();
+		int separator = name.indexOf("!/");
+		if (separator > 0) {
+			name = name.substring(0, separator);
+		}
+		return new File(name);
+	}
+
 	private File findHomeDir(File source) {
 		File homeDir = source;
 		homeDir = (homeDir == null ? findDefaultHomeDir() : homeDir);