testing with StepVerifier (#5673)

* testing with StepVerifier

* Test publisher

spring-5-reactive/src/test/java/com/baeldung/stepverifier/PostExecutionUnitTest.java

@@ -0,0 +1,34 @@
+package com.baeldung.stepverifier;
+
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+import java.time.Duration;
+
+public class PostExecutionUnitTest {
+
+    Flux<Integer> source = Flux.<Integer>create(emitter -> {
+        emitter.next(1);
+        emitter.next(2);
+        emitter.next(3);
+        emitter.complete();
+        try {
+            Thread.sleep(1000);
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+        emitter.next(4);
+    }).filter(number -> number % 2 == 0);
+
+    @Test
+    public void droppedElements() {
+        StepVerifier.create(source)
+          .expectNext(2)
+          .expectComplete()
+          .verifyThenAssertThat()
+          .hasDropped(4)
+          .tookLessThan(Duration.ofMillis(1050));
+    }
+
+}