Merge pull request #10865 from singhkaushal01/master

BAEL-4126: what does Mono.defer() do?

reactor-core/pom.xml

@@ -32,6 +32,12 @@
             <version>${assertj.version}</version>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <version>RELEASE</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <properties>

reactor-core/src/test/java/com/baeldung/mono/MonoUnitTest.java

@@ -1,5 +1,6 @@
 package com.baeldung.mono;
 
+import lombok.extern.slf4j.Slf4j;
 import org.junit.Test;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -13,6 +14,7 @@ import java.util.Optional;
 
 import static org.junit.Assert.assertEquals;
 
+@Slf4j
 public class MonoUnitTest {
     @Test
     public void whenMonoProducesString_thenBlockAndConsume() {
@@ -80,4 +82,84 @@ public class MonoUnitTest {
 
         return Mono.just(list);
     }
+
+    @Test
+    public void whenEmptyList_thenMonoDeferExecuted() {
+
+        Mono<List<String>> emptyList = Mono.defer(() -> monoOfEmptyList());
+
+        //Empty list, hence Mono publisher in switchIfEmpty executed after condition evaluation
+        Flux<String> emptyListElements = emptyList.flatMapIterable(l -> l)
+          .switchIfEmpty(Mono.defer(() -> sampleMsg("EmptyList")))
+          .log();
+
+        StepVerifier.create(emptyListElements)
+          .expectNext("EmptyList")
+          .verifyComplete();
+    }
+
+    @Test
+    public void whenNonEmptyList_thenMonoDeferNotExecuted() {
+
+        Mono<List<String>> nonEmptyist = Mono.defer(() -> monoOfList());
+
+        //Non empty list, hence Mono publisher in switchIfEmpty won't evaluated.
+        Flux<String> listElements = nonEmptyist.flatMapIterable(l -> l)
+          .switchIfEmpty(Mono.defer(() -> sampleMsg("NonEmptyList")))
+          .log();
+
+        StepVerifier.create(listElements)
+          .expectNext("one", "two", "three", "four")
+          .verifyComplete();
+    }
+
+    private Mono<List<String>> monoOfEmptyList() {
+        List<String> list = new ArrayList<>();
+        return Mono.just(list);
+    }
+
+    @Test
+    public void whenUsingMonoJust_thenEagerEvaluation() throws InterruptedException {
+
+        Mono<String> msg = sampleMsg("Eager Publisher");
+
+        log.debug("Intermediate Test Message....");
+
+        StepVerifier.create(msg)
+          .expectNext("Eager Publisher")
+          .verifyComplete();
+
+        Thread.sleep(5000);
+
+        StepVerifier.create(msg)
+          .expectNext("Eager Publisher")
+          .verifyComplete();
+    }
+
+    @Test
+    public void whenUsingMonoDefer_thenLazyEvaluation() throws InterruptedException {
+
+        Mono<String> deferMsg = Mono.defer(() -> sampleMsg("Lazy Publisher"));
+
+        log.debug("Intermediate Test Message....");
+
+        StepVerifier.create(deferMsg)
+          .expectNext("Lazy Publisher")
+          .verifyComplete();
+
+        Thread.sleep(5000);
+
+        StepVerifier.create(deferMsg)
+          .expectNext("Lazy Publisher")
+          .verifyComplete();
+
+    }
+
+    private Mono<String> sampleMsg(String str) {
+        log.debug("Call to Retrieve Sample Message!! --> {} at: {}", str, System.currentTimeMillis());
+        return Mono.just(str);
+    }
 }
+
+
+