Update App Engine samples to listen to "PORT" env var (#1511)

Author: i0n-

appengine-java11/appengine-simple-jetty-main/src/main/java/com/example/appengine/demo/jettymain/Main.java

@@ -32,11 +32,13 @@ public static void main(String[] args) throws Exception {
     System.setProperty("org.eclipse.jetty.util.log.class", "org.eclipse.jetty.util.log.StrErrLog");
     System.setProperty("org.eclipse.jetty.LEVEL", "INFO");
 
-    // Create a basic Jetty server object that will listen on port 8080.
+    // Create a basic Jetty server object that will listen on port defined by
+    // the PORT environment variable when present, otherwise on 8080.
     // Note: If you set this to port 0, a randomly available port will be
     // assigned. You can find the assigned port in the logs or programmatically
     // obtain it.
-    Server server = new Server(8080);
+    int port = Integer.parseInt(System.getenv().getOrDefault("PORT", "8080"));
+    Server server = new Server(port);
 
     // The WebAppContext is the interface to provide configuration for a web
     // application. In this example, the context path is being set to "/" so

appengine-java11/custom-entrypoint/Main.java

@@ -23,8 +23,10 @@
 public class Main {
 
   public static void main(String[] args) throws IOException {
-    // Create an instance of HttpServer bound to port 8080.
-    HttpServer server = HttpServer.create(new InetSocketAddress(8080), 0);
+    // Create an instance of HttpServer bound to port defined by the 
+    // PORT environment variable when present, otherwise on 8080.
+    int port = Integer.parseInt(System.getenv().getOrDefault("PORT", "8080"));
+    HttpServer server = HttpServer.create(new InetSocketAddress(port), 0);
 
     // Set root URI path.
     server.createContext("/", (var t) -> {

appengine-java11/http-server/src/main/java/com/example/appengine/Main.java

@@ -24,8 +24,10 @@
 public class Main {
 
   public static void main(String[] args) throws IOException {
-    // Create an instance of HttpServer with port 8080.
-    HttpServer server = HttpServer.create(new InetSocketAddress(8080), 0);
+    // Create an instance of HttpServer bound to port defined by the 
+    // PORT environment variable when present, otherwise on 8080.
+    int port = Integer.parseInt(System.getenv().getOrDefault("PORT", "8080"));
+    HttpServer server = HttpServer.create(new InetSocketAddress(port), 0);
 
     // Set root URI path.
     server.createContext("/", (var t) -> {

appengine-java11/sparkjava-helloworld/src/main/java/com/example/appengine/sparkdemo/Main.java

@@ -21,8 +21,10 @@
 public class Main {
 
   public static void main(String[] args) {
-    // Starts the webapp on localhost:8080.
-    Spark.port(8080);
+    // Starts the webapp on localhost and the port defined by the PORT
+    // environment variable when present, otherwise on 8080.
+    int port = Integer.parseInt(System.getenv().getOrDefault("PORT", "8080"));
+    Spark.port(port);
     Spark.get("/", (req, res) -> "Hello World!");
   }
 }