Fixing the ConcurrentModificationException on init

src/main/java/spark/route/Routes.java

@@ -21,6 +21,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 import spark.FilterImpl;
 import spark.RouteImpl;
@@ -49,7 +50,7 @@ public static Routes create() {
      * Constructor
      */
     protected Routes() {
-        routes = new ArrayList<>();
+        routes = new CopyOnWriteArrayList<>();
     }
 
     /**

src/test/java/spark/ConcurrentInitializationTest.java

@@ -0,0 +1,75 @@
+package spark;
+
+import org.junit.AfterClass;
+import org.junit.Assert;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import spark.util.SparkTestUtil;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+
+import static org.junit.Assert.assertEquals;
+import static spark.Spark.get;
+
+public class ConcurrentInitializationTest {
+
+    private static final int NUM_HANDLERS = 10000;
+    private static final Logger LOGGER = LoggerFactory.getLogger(ConcurrentInitializationTest.class);
+
+    private volatile boolean isClientRunning = true;
+
+
+    /**
+     * Simulates the client which check the server health check awaiting for it to be up and running,
+     * for example k8s or other orchestrator or load balancer. It call health check in loop to
+     * figure out that the service is started
+     */
+    private void client(CountDownLatch latch, List<Object> exceptionList) {
+        SparkTestUtil testUtil = new SparkTestUtil(4567);
+        while (isClientRunning) {
+            try {
+                SparkTestUtil.UrlResponse response = testUtil.doMethod("GET", "/healthcheck", null);
+                Assert.assertEquals(200, response.status);
+                Assert.assertEquals("Hello World!", response.body);
+                latch.countDown();
+            } catch (Exception | Error e) {
+                exceptionList.add(e);
+                LOGGER.error("Client error", e);
+            }
+        }
+    }
+
+    /**
+     * Simulates the server with a long list of APIs to be initialized
+     */
+    private void initHandlers() {
+        for (int i = 0; i < NUM_HANDLERS; i++) {
+            get("/api" + i, (q, a) -> "Hello World!");
+        }
+    }
+
+    @Test
+    public void testInitialization() throws InterruptedException {
+        CountDownLatch latch = new CountDownLatch(5);
+        List<Object> exceptionList = new ArrayList<>();
+        get("/healthcheck", (q, a) -> "Hello World!");
+        // Run the client in its own Thread
+        Thread clientThread = new Thread(() -> client(latch, exceptionList));
+        clientThread.start();
+        latch.await();
+        initHandlers();
+        isClientRunning = false;
+        clientThread.join();
+        assertEquals(0, exceptionList.size());
+    }
+
+    @AfterClass
+    public static void tearDown() {
+        Spark.stop();
+    }
+
+}