Add LauncherSessionListener example to user guide

documentation/src/docs/asciidoc/user-guide/launcher-api.adoc

@@ -130,10 +130,48 @@ programmatically via the `{LauncherConfig}` that is passed to the `{LauncherFact
 they can be discovered at runtime via Java's `{ServiceLoader}` mechanism and automatically
 registered with `LauncherSession` (unless automatic registration is disabled.)
 
-For example, an `example.CustomLauncherSessionListener` class implementing
-`LauncherSessionListener` and declared within the
-`/META-INF/services/org.junit.platform.launcher.LauncherSessionListener` file is loaded
-and registered automatically.
+A `{LauncherSessionListener}` is well suited for implementing once-per-JVM setup/teardown
+behavior since it's called before the first and after the last test in a launcher session,
+respectively. The scope of a launcher session depends on the used IDE or build tool but
+usually corresponds to the lifecycle of the test JVM. A custom listener that starts an
+HTTP server before executing the first test and stops it after the last test has been
+executed, could look like this:
+
+[source,java]
+.src/test/java/example/session/GlobalSetupTeardownListener.java
+----
+package example.session;
+
+include::{testDir}/example/session/GlobalSetupTeardownListener.java[tags=user_guide]
+----
+<1> Start the HTTP server
+<2> Export its dynamic port as a system property for consumption by tests
+<3> Stop the HTTP server
+
+This sample uses the HTTP server implementation from the jdk.httpserver module that comes
+with the JDK but would work similarly with any other server or resource. In order for the
+listener to be picked up by JUnit Platform, you need to register it as a service by adding
+a resource file with the following name and contents to your test runtime classpath (e.g.
+by adding the file to `src/test/resources`):
+
+[source]
+.src/test/resources/META-INF/services/org.junit.platform.launcher.LauncherSessionListener
+----
+include::{testResourcesDir}/META-INF/services/org.junit.platform.launcher.LauncherSessionListener[]
+----
+
+You can now use the resource from your test:
+
+[source,java]
+.src/test/java/example/session/HttpTests.java
+----
+package example.session;
+
+include::{testDir}/example/session/HttpTests.java[tags=user_guide]
+----
+<1> Read the port of the server from the system property set by the listener
+<2> Send a request to the server
+<3> Check the status code of the response
 
 [[launcher-api-launcher-discovery-listeners-custom]]
 ==== Registering a LauncherDiscoveryListener

documentation/src/test/java/example/session/GlobalSetupTeardownListener.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2015-2021 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package example.session;
+
+//tag::user_guide[]
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.net.InetSocketAddress;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import com.sun.net.httpserver.HttpServer;
+
+import org.junit.platform.launcher.LauncherSession;
+import org.junit.platform.launcher.LauncherSessionListener;
+import org.junit.platform.launcher.TestExecutionListener;
+import org.junit.platform.launcher.TestPlan;
+
+public class GlobalSetupTeardownListener implements LauncherSessionListener {
+
+	private Fixture fixture;
+
+	@Override
+	public void launcherSessionOpened(LauncherSession session) {
+		// Avoid setup for test discovery by delaying it until tests are about to be executed
+		session.getLauncher().registerTestExecutionListeners(new TestExecutionListener() {
+			@Override
+			public void testPlanExecutionStarted(TestPlan testPlan) {
+				if (fixture == null) {
+					fixture = new Fixture();
+					fixture.setUp();
+				}
+			}
+		});
+	}
+
+	@Override
+	public void launcherSessionClosed(LauncherSession session) {
+		if (fixture != null) {
+			fixture.tearDown();
+			fixture = null;
+		}
+	}
+
+	static class Fixture {
+
+		private HttpServer server;
+		private ExecutorService executorService;
+
+		void setUp() {
+			try {
+				server = HttpServer.create(new InetSocketAddress(0), 0);
+			}
+			catch (IOException e) {
+				throw new UncheckedIOException("Failed to start HTTP server", e);
+			}
+			server.createContext("/test", exchange -> {
+				exchange.sendResponseHeaders(204, -1);
+				exchange.close();
+			});
+			executorService = Executors.newCachedThreadPool();
+			server.setExecutor(executorService);
+			server.start(); // <1>
+			int port = server.getAddress().getPort();
+			System.setProperty("http.server.port", String.valueOf(port)); // <2>
+		}
+
+		void tearDown() {
+			server.stop(0); // <3>
+			executorService.shutdownNow();
+		}
+	}
+
+}
+//end::user_guide[]

documentation/src/test/java/example/session/HttpTests.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2015-2021 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package example.session;
+
+//tag::user_guide[]
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.net.HttpURLConnection;
+import java.net.URL;
+
+import org.junit.jupiter.api.Test;
+
+class HttpTests {
+
+	@Test
+	void respondsWith204() throws Exception {
+		String port = System.getProperty("http.server.port"); // <1>
+		URL url = new URL("http://localhost:" + port + "/test");
+
+		HttpURLConnection connection = (HttpURLConnection) url.openConnection();
+		connection.setRequestMethod("GET");
+		int responseCode = connection.getResponseCode(); // <2>
+
+		assertEquals(204, responseCode); // <3>
+	}
+}
+//end::user_guide[]

documentation/src/test/resources/META-INF/services/org.junit.platform.launcher.LauncherSessionListener

@@ -0,0 +1 @@
+example.session.GlobalSetupTeardownListener