Tighten up PropertiesLauncher's contract

The main changes are:

- Switch to `loader.properties` instead of `application.properties`
- Search for `loader.properties` in `loader.home` as well as in
  the classpath
- Placeholder replacements in MANIFEST.MF (using `loader.properties`
  or system/env vars)

See gh-7221
Closes gh-8346

spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/PropertiesLauncherTests.java

@@ -31,6 +31,7 @@ import org.junit.After;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.ExpectedException;
 import org.junit.rules.TemporaryFolder;
 import org.mockito.MockitoAnnotations;
 
@@ -50,6 +51,9 @@ public class PropertiesLauncherTests {
 	@Rule
 	public InternalOutputCapture output = new InternalOutputCapture();
 
+	@Rule
+	public ExpectedException expected = ExpectedException.none();
+
 	@Rule
 	public TemporaryFolder temporaryFolder = new TemporaryFolder();
 
@@ -72,9 +76,28 @@ public class PropertiesLauncherTests {
 
 	@Test
 	public void testDefaultHome() {
+		System.clearProperty("loader.home");
+		PropertiesLauncher launcher = new PropertiesLauncher();
+		assertThat(launcher.getHomeDirectory())
+				.isEqualTo(new File(System.getProperty("user.dir")));
+	}
+
+	@Test
+	public void testAlternateHome() throws Exception {
+		System.setProperty("loader.home", "src/test/resources/home");
 		PropertiesLauncher launcher = new PropertiesLauncher();
 		assertThat(launcher.getHomeDirectory())
 				.isEqualTo(new File(System.getProperty("loader.home")));
+		assertThat(launcher.getMainClass()).isEqualTo("demo.HomeApplication");
+	}
+
+	@Test
+	public void testNonExistentHome() throws Exception {
+		System.setProperty("loader.home", "src/test/resources/nonexistent");
+		this.expected.expectMessage("Invalid source folder");
+		PropertiesLauncher launcher = new PropertiesLauncher();
+		assertThat(launcher.getHomeDirectory())
+				.isNotEqualTo(new File(System.getProperty("loader.home")));
 	}
 
 	@Test
@@ -93,6 +116,13 @@ public class PropertiesLauncherTests {
 				.isEqualTo("[etc/]");
 	}
 
+	@Test
+	public void testRootOfClasspathFirst() throws Exception {
+		System.setProperty("loader.config.name", "bar");
+		PropertiesLauncher launcher = new PropertiesLauncher();
+		assertThat(launcher.getMainClass()).isEqualTo("my.BarApplication");
+	}
+
 	@Test
 	public void testUserSpecifiedDotPath() throws Exception {
 		System.setProperty("loader.path", ".");
@@ -232,6 +262,13 @@ public class PropertiesLauncherTests {
 				.containsExactly("/foo.jar", "/bar/");
 	}
 
+	@Test
+	public void testManifestWithPlaceholders() throws Exception {
+		System.setProperty("loader.home", "src/test/resources/placeholders");
+		PropertiesLauncher launcher = new PropertiesLauncher();
+		assertThat(launcher.getMainClass()).isEqualTo("demo.FooApplication");
+	}
+
 	private void waitFor(String value) throws Exception {
 		int count = 0;
 		boolean timeout = false;

spring-boot-tools/spring-boot-loader/src/test/resources/BOOT-INF/classes/application.properties

@@ -1 +1 @@
-loader.main: demo.Application
+loader.main: demo.NoSuchApplication

spring-boot-tools/spring-boot-loader/src/test/resources/BOOT-INF/classes/bar.properties

@@ -0,0 +1 @@
+loader.main: my.BootInfBarApplication

spring-boot-tools/spring-boot-loader/src/test/resources/BOOT-INF/classes/loader.properties

@@ -0,0 +1 @@
+loader.main: demo.Application

spring-boot-tools/spring-boot-loader/src/test/resources/home/loader.properties

@@ -0,0 +1 @@
+loader.main: demo.HomeApplication

spring-boot-tools/spring-boot-loader/src/test/resources/placeholders/loader.properties

@@ -0,0 +1 @@
+foo.main: demo.FooApplication