Add jarmode support to the loader code

Update the `Launcher` class to allow a packaged jar to be  launched in
a different mode. The launcher now checks for a `jarmode` property and
attempts to find a `JarMode` implementation using the standard
`spring.factories` mechanism.

Closes gh-19848

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jarmode/LauncherJarModeTests.java

@@ -0,0 +1,86 @@
+/*
+ * 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.jarmode;
+
+import java.util.Collections;
+import java.util.Iterator;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+
+import org.springframework.boot.loader.Launcher;
+import org.springframework.boot.loader.archive.Archive;
+import org.springframework.boot.testsupport.system.CapturedOutput;
+import org.springframework.boot.testsupport.system.OutputCaptureExtension;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link Launcher} with jar mode support.
+ *
+ * @author Phillip Webb
+ */
+@ExtendWith(OutputCaptureExtension.class)
+class LauncherJarModeTests {
+
+	@BeforeEach
+	void setup() {
+		System.setProperty(JarModeLauncher.DISABLE_SYSTEM_EXIT, "true");
+	}
+
+	@AfterEach
+	void cleanup() {
+		System.clearProperty("jarmode");
+		System.clearProperty(JarModeLauncher.DISABLE_SYSTEM_EXIT);
+	}
+
+	@Test
+	void launchWhenJarModePropertyIsSetLaunchesJarMode(CapturedOutput out) throws Exception {
+		System.setProperty("jarmode", "test");
+		new TestLauncher().launch(new String[] { "boot" });
+		assertThat(out).contains("running in test jar mode [boot]");
+	}
+
+	@Test
+	void launchWhenJarModePropertyIsNotAcceptedThrowsException(CapturedOutput out) throws Exception {
+		System.setProperty("jarmode", "idontexist");
+		new TestLauncher().launch(new String[] { "boot" });
+		assertThat(out).contains("Unsupported jarmode 'idontexist'");
+	}
+
+	private static class TestLauncher extends Launcher {
+
+		@Override
+		protected String getMainClass() throws Exception {
+			throw new IllegalStateException("Should not be called");
+		}
+
+		@Override
+		protected Iterator<Archive> getClassPathArchivesIterator() throws Exception {
+			return Collections.emptyIterator();
+		}
+
+		@Override
+		protected void launch(String[] args) throws Exception {
+			super.launch(args);
+		}
+
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/resources/META-INF/spring.factories

@@ -0,0 +1,2 @@
+org.springframework.boot.loader.jarmode.JarMode=\
+org.springframework.boot.loader.jarmode.TestJarMode