Fix jarmode support in unexploded jars

Update `LaunchedURLClassLoader` to ensure that the `JarModeLauncher`
is created in the correct classloader.

Prior to this commit the launcher was created by the application
classloader and did not have access to any of the required
`org.springframework` classes.

See gh-19848

Author: philwebb

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/LaunchedURLClassLoader.java

@@ -16,7 +16,9 @@
 
 package org.springframework.boot.loader;
 
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.io.InputStream;
 import java.net.JarURLConnection;
 import java.net.URL;
 import java.net.URLClassLoader;
@@ -38,6 +40,8 @@
  */
 public class LaunchedURLClassLoader extends URLClassLoader {
 
+	private static final int BUFFER_SIZE = 4096;
+
 	static {
 		ClassLoader.registerAsParallelCapable();
 	}
@@ -96,7 +100,7 @@ public Enumeration<URL> findResources(String name) throws IOException {
 	protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
 		if (name.startsWith("org.springframework.boot.loader.jarmode.")) {
 			try {
-				Class<?> result = findClass(name);
+				Class<?> result = loadClassInLaunchedClassLoader(name);
 				if (resolve) {
 					resolveClass(result);
 				}
@@ -129,6 +133,35 @@ protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundE
 		}
 	}
 
+	private Class<?> loadClassInLaunchedClassLoader(String name) throws ClassNotFoundException {
+		String internalName = name.replace('.', '/') + ".class";
+		InputStream inputStream = getParent().getResourceAsStream(internalName);
+		if (inputStream == null) {
+			throw new ClassNotFoundException(name);
+		}
+		try {
+			try {
+				ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+				byte[] buffer = new byte[BUFFER_SIZE];
+				int bytesRead = -1;
+				while ((bytesRead = inputStream.read(buffer)) != -1) {
+					outputStream.write(buffer, 0, bytesRead);
+				}
+				inputStream.close();
+				byte[] bytes = outputStream.toByteArray();
+				Class<?> definedClass = defineClass(name, bytes, 0, bytes.length);
+				definePackageIfNecessary(name);
+				return definedClass;
+			}
+			finally {
+				inputStream.close();
+			}
+		}
+		catch (IOException ex) {
+			throw new ClassNotFoundException("Cannot load resource for class [" + name + "]", ex);
+		}
+	}
+
 	/**
 	 * Define a package before a {@code findClass} call is made. This is necessary to
 	 * ensure that the appropriate manifest for nested JARs is associated with the