Add OnceTest

proxy/src/test/java/com/velocitypowered/proxy/util/concurrent/OnceTest.java

@@ -0,0 +1,43 @@
+package com.velocitypowered.proxy.util.concurrent;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.fail;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+import org.junit.jupiter.api.Test;
+
+public class OnceTest {
+  @Test
+  void uncontendedOnce() {
+    AtomicInteger i = new AtomicInteger();
+    Once once = new Once();
+    once.run(i::incrementAndGet);
+    assertEquals(1, i.get(), "Integer didn't increment");
+  }
+
+  @Test
+  void contendedOnce() throws Exception {
+    ExecutorService service = Executors.newFixedThreadPool(10);
+    AtomicInteger i = new AtomicInteger();
+    Once once = new Once();
+    for (int i1 = 0; i1 < 10; i1++) {
+      service.execute(() -> once.run(i::incrementAndGet));
+    }
+    service.shutdown();
+    service.awaitTermination(500, TimeUnit.MILLISECONDS);
+    assertEquals(1, i.get(), "Integer is not equal to one");
+  }
+
+  @Test
+  void handlesException() {
+    Once once = new Once();
+    assertThrows(RuntimeException.class, () -> once.run(() -> {
+      throw new RuntimeException("exception");
+    }));
+    once.run(() -> fail("Once.run() ran twice"));
+  }
+}