Prevent Tomcat URL "reflective access" warnings

Update the jar `Handler` class to support a non-reflective fallback
mechanism when possible. The updated code attempts to capture a regular
jar URL before our handler is installed. It can then use that URL as
context when creating the a fallback URL. The JDK jar `Handler` will
be copied from the context URL to the fallback URL.

Without this commit, resolving new Tomcat URLs of the form
`jar:war:file:...` would result in an ugly "Illegal reflective access"
warning.

Fixes gh-18631

spring-boot-tests/spring-boot-integration-tests/spring-boot-loader-tests/src/intTest/java/org/springframework/boot/loader/LoaderIntegrationTests.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2012-2020 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
+ *
+ *      https://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.loader;
+
+import java.io.File;
+import java.time.Duration;
+
+import org.junit.jupiter.api.Test;
+import org.testcontainers.containers.GenericContainer;
+import org.testcontainers.containers.output.ToStringConsumer;
+import org.testcontainers.containers.startupcheck.OneShotStartupCheckStrategy;
+import org.testcontainers.junit.jupiter.Container;
+import org.testcontainers.junit.jupiter.Testcontainers;
+import org.testcontainers.utility.DockerImageName;
+import org.testcontainers.utility.MountableFile;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Integration tests loader that supports fat jars.
+ *
+ * @author Phillip Webb
+ */
+@Testcontainers(disabledWithoutDocker = true)
+class LoaderIntegrationTests {
+
+	private static final DockerImageName JRE = DockerImageName.parse("adoptopenjdk:15-jre-hotspot");
+
+	private static ToStringConsumer output = new ToStringConsumer();
+
+	@Container
+	public static GenericContainer<?> container = new GenericContainer<>(JRE).withLogConsumer(output)
+			.withCopyFileToContainer(MountableFile.forHostPath(findApplication().toPath()), "/app.jar")
+			.withStartupCheckStrategy(new OneShotStartupCheckStrategy().withTimeout(Duration.ofMinutes(5)))
+			.withCommand("java", "-jar", "app.jar");
+
+	private static File findApplication() {
+		File appJar = new File("build/app/build/libs/app.jar");
+		if (appJar.isFile()) {
+			return appJar;
+		}
+		throw new IllegalStateException(
+				"Could not find test application in build/app/build/libs directory. Have you built it?");
+	}
+
+	@Test
+	void readUrlsWithoutWarning() {
+		assertThat(output.toUtf8String()).contains(">>>>> 287649 BYTES from").doesNotContain("WARNING:")
+				.doesNotContain("illegal");
+	}
+
+}