Improve diagnostics in DevTools integration tests

See gh-10454

spring-boot-integration-tests/spring-boot-devtools-tests/src/test/java/org/springframework/boot/devtools/tests/DevToolsIntegrationTests.java

@@ -142,9 +142,11 @@ public class DevToolsIntegrationTests {
 			if (System.currentTimeMillis() > end) {
 				throw new IllegalStateException(String.format(
 						"server.port file was not written within 30 seconds. "
-								+ "Application output:%n%s",
+								+ "Application output:%n%s%s",
 						FileCopyUtils.copyToString(new FileReader(
-								this.launchedApplication.getStandardOut()))));
+								this.launchedApplication.getStandardOut())),
+						FileCopyUtils.copyToString(new FileReader(
+								this.launchedApplication.getStandardError()))));
 			}
 			Thread.sleep(100);
 		}

spring-boot-integration-tests/spring-boot-devtools-tests/src/test/java/org/springframework/boot/devtools/tests/JvmLauncher.java

@@ -49,10 +49,10 @@ class JvmLauncher implements TestRule {
 				.asList(System.getProperty("java.home") + "/bin/java", "-cp", classpath));
 		command.addAll(Arrays.asList(args));
 		File standardOut = new File(this.outputDirectory, name + ".out");
+		File standardError = new File(this.outputDirectory, name + ".err");
 		Process process = new ProcessBuilder(command.toArray(new String[command.size()]))
-				.redirectError(new File(this.outputDirectory, name + ".err"))
-				.redirectOutput(standardOut).start();
-		return new LaunchedJvm(process, standardOut);
+				.redirectError(standardError).redirectOutput(standardOut).start();
+		return new LaunchedJvm(process, standardOut, standardError);
 	}
 
 	static class LaunchedJvm {
@@ -61,9 +61,12 @@ class JvmLauncher implements TestRule {
 
 		private final File standardOut;
 
-		LaunchedJvm(Process process, File standardOut) {
+		private final File standardError;
+
+		LaunchedJvm(Process process, File standardOut, File standardError) {
 			this.process = process;
 			this.standardOut = standardOut;
+			this.standardError = standardError;
 		}
 
 		Process getProcess() {
@@ -74,6 +77,10 @@ class JvmLauncher implements TestRule {
 			return this.standardOut;
 		}
 
+		File getStandardError() {
+			return this.standardError;
+		}
+
 	}
 
 }

spring-boot-integration-tests/spring-boot-devtools-tests/src/test/java/org/springframework/boot/devtools/tests/LaunchedApplication.java

@@ -29,11 +29,15 @@ class LaunchedApplication {
 
 	private final File standardOut;
 
+	private final File standardError;
+
 	private final Process[] processes;
 
-	LaunchedApplication(File classesDirectory, File standardOut, Process... processes) {
+	LaunchedApplication(File classesDirectory, File standardOut, File standardError,
+			Process... processes) {
 		this.classesDirectory = classesDirectory;
 		this.standardOut = standardOut;
+		this.standardError = standardError;
 		this.processes = processes;
 	}
 
@@ -48,6 +52,10 @@ class LaunchedApplication {
 		return this.standardOut;
 	}
 
+	File getStandardError() {
+		return this.standardError;
+	}
+
 	File getClassesDirectory() {
 		return this.classesDirectory;
 	}

spring-boot-integration-tests/spring-boot-devtools-tests/src/test/java/org/springframework/boot/devtools/tests/LocalApplicationLauncher.java

@@ -37,7 +37,7 @@ public class LocalApplicationLauncher implements ApplicationLauncher {
 		LaunchedJvm jvm = jvmLauncher.launch("local", createApplicationClassPath(),
 				"com.example.DevToolsTestApplication", "--server.port=0");
 		return new LaunchedApplication(new File("target/app"), jvm.getStandardOut(),
-				jvm.getProcess());
+				jvm.getStandardError(), jvm.getProcess());
 	}
 
 	protected String createApplicationClassPath() throws Exception {

spring-boot-integration-tests/spring-boot-devtools-tests/src/test/java/org/springframework/boot/devtools/tests/RemoteApplicationLauncher.java

@@ -48,8 +48,8 @@ abstract class RemoteApplicationLauncher implements ApplicationLauncher {
 				"--spring.devtools.remote.secret=secret", "http://localhost:12345");
 		awaitRemoteSpringApplication(remoteSpringApplicationJvm.getStandardOut());
 		return new LaunchedApplication(new File("target/remote"),
-				applicationJvm.getStandardOut(), applicationJvm.getProcess(),
-				remoteSpringApplicationJvm.getProcess());
+				applicationJvm.getStandardOut(), applicationJvm.getStandardError(),
+				applicationJvm.getProcess(), remoteSpringApplicationJvm.getProcess());
 	}
 
 	protected abstract String createApplicationClassPath() throws Exception;