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/app/build.gradle

@@ -0,0 +1,18 @@
+plugins {
+	id "java"
+	id "org.springframework.boot"
+}
+
+apply plugin: "io.spring.dependency-management"
+
+repositories {
+	maven { url "file:${rootDir}/../int-test-maven-repository"}
+	mavenCentral()
+	maven { url "https://repo.spring.io/snapshot" }
+	maven { url "https://repo.spring.io/milestone" }
+}
+
+dependencies {
+	implementation("org.springframework.boot:spring-boot-starter-web")
+	implementation("org.webjars:jquery:3.5.0")
+}

spring-boot-tests/spring-boot-integration-tests/spring-boot-loader-tests/app/settings.gradle

@@ -0,0 +1,15 @@
+pluginManagement {
+	repositories {
+		maven { url "file:${rootDir}/../int-test-maven-repository"}
+		mavenCentral()
+		maven { url "https://repo.spring.io/snapshot" }
+		maven { url "https://repo.spring.io/milestone" }
+	}
+	resolutionStrategy {
+		eachPlugin {
+			if (requested.id.id == "org.springframework.boot") {
+				useModule "org.springframework.boot:spring-boot-gradle-plugin:${requested.version}"
+			}
+		}
+	}
+}

spring-boot-tests/spring-boot-integration-tests/spring-boot-loader-tests/app/src/main/java/org/springframework/boot/loaderapp/LoaderTestApplication.java

@@ -0,0 +1,50 @@
+/*
+ * 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.loaderapp;
+
+import java.net.URL;
+import java.util.Arrays;
+
+import javax.servlet.ServletContext;
+
+import org.springframework.boot.CommandLineRunner;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.context.annotation.Bean;
+import org.springframework.util.FileCopyUtils;
+
+@SpringBootApplication
+public class LoaderTestApplication {
+
+	@Bean
+	public CommandLineRunner commandLineRunner(ServletContext servletContext) {
+		return (args) -> {
+			URL resourceUrl = servletContext.getResource("webjars/jquery/3.5.0/jquery.js");
+			byte[] resourceContent = FileCopyUtils.copyToByteArray(resourceUrl.openStream());
+			URL directUrl = new URL(resourceUrl.toExternalForm());
+			byte[] directContent = FileCopyUtils.copyToByteArray(directUrl.openStream());
+			String message = (!Arrays.equals(resourceContent, directContent)) ? "NO MATCH"
+					: directContent.length + " BYTES";
+			System.out.println(">>>>> " + message + " from " + resourceUrl);
+		};
+	}
+
+	public static void main(String[] args) {
+		SpringApplication.run(LoaderTestApplication.class, args).stop();
+	}
+
+}

spring-boot-tests/spring-boot-integration-tests/spring-boot-loader-tests/build.gradle

@@ -0,0 +1,47 @@
+plugins {
+	id "java"
+	id "org.springframework.boot.conventions"
+	id "org.springframework.boot.integration-test"
+}
+
+description = "Spring Boot Loader Integration Tests"
+
+configurations {
+	app
+}
+
+dependencies {
+	app project(path: ":spring-boot-project:spring-boot-dependencies", configuration: "mavenRepository")
+	app project(path: ":spring-boot-project:spring-boot-tools:spring-boot-gradle-plugin", configuration: "mavenRepository")
+	app project(path: ":spring-boot-project:spring-boot-starters:spring-boot-starter-web", configuration: "mavenRepository")
+
+	intTestImplementation(enforcedPlatform(project(":spring-boot-project:spring-boot-parent")))
+	intTestImplementation(project(":spring-boot-project:spring-boot-tools:spring-boot-test-support"))
+	intTestImplementation(project(":spring-boot-project:spring-boot-starters:spring-boot-starter-test"))
+	intTestImplementation("org.testcontainers:junit-jupiter")
+	intTestImplementation("org.testcontainers:testcontainers")
+}
+
+task syncMavenRepository(type: Sync) {
+	from configurations.app
+	into "${buildDir}/int-test-maven-repository"
+}
+
+task syncAppSource(type: Sync) {
+	from "app"
+	into "${buildDir}/app"
+	filter { line ->
+		line.replace("id \"org.springframework.boot\"", "id \"org.springframework.boot\" version \"${project.version}\"")
+	}
+}
+
+task buildApp(type: GradleBuild) {
+	dependsOn syncAppSource, syncMavenRepository
+	dir = "${buildDir}/app"
+	startParameter.buildCacheEnabled = false
+	tasks  = ["build"]
+}
+
+intTest {
+	dependsOn buildApp
+}

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");
+	}
+
+}