webflux with different use cases

.gitignore

+HELP.md
+.gradle
+build/
+!gradle/wrapper/gradle-wrapper.jar
+!**/src/main/**
+!**/src/test/**
+
+### STS ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+
+### IntelliJ IDEA ###
+.idea
+*.iws
+*.iml
+*.ipr
+out/
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+
+### VS Code ###
+.vscode/

build.gradle

+plugins {
+	id 'org.springframework.boot' version '2.2.7.RELEASE'
+	id 'io.spring.dependency-management' version '1.0.9.RELEASE'
+	id 'java'
+}
+
+group = 'com.nisum'
+version = '0.0.1-SNAPSHOT'
+sourceCompatibility = '1.8'
+
+repositories {
+	mavenCentral()
+}
+
+dependencies {
+	implementation('org.springframework.boot:spring-boot-starter-data-mongodb-reactive')
+	implementation('org.springframework.boot:spring-boot-starter-webflux')
+	compileOnly('org.projectlombok:lombok')
+	testImplementation('org.springframework.boot:spring-boot-starter-test')
+	testImplementation('de.flapdoodle.embed:de.flapdoodle.embed.mongo')
+	testImplementation('io.projectreactor:reactor-test')
+}
+
+test {
+	useJUnitPlatform()
+}

gradle/wrapper/gradle-wrapper.properties

+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-bin.zip
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists

gradlew

+#!/usr/bin/env sh
+
+#
+# Copyright 2015 the original author or authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+##############################################################################
+##
+##  Gradle start up script for UN*X
+##
+##############################################################################
+
+# Attempt to set APP_HOME
+# Resolve links: $0 may be a link
+PRG="$0"
+# Need this for relative symlinks.
+while [ -h "$PRG" ] ; do
+    ls=`ls -ld "$PRG"`
+    link=`expr "$ls" : '.*-> \(.*\)$'`
+    if expr "$link" : '/.*' > /dev/null; then
+        PRG="$link"
+    else
+        PRG=`dirname "$PRG"`"/$link"
+    fi
+done
+SAVED="`pwd`"
+cd "`dirname \"$PRG\"`/" >/dev/null
+APP_HOME="`pwd -P`"
+cd "$SAVED" >/dev/null
+
+APP_NAME="Gradle"
+APP_BASE_NAME=`basename "$0"`
+
+# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+DEFAULT_JVM_OPTS='"-Xmx64m" "-Xms64m"'
+
+# Use the maximum available, or set MAX_FD != -1 to use that value.
+MAX_FD="maximum"
+
+warn () {
+    echo "$*"
+}
+
+die () {
+    echo
+    echo "$*"
+    echo
+    exit 1
+}
+
+# OS specific support (must be 'true' or 'false').
+cygwin=false
+msys=false
+darwin=false
+nonstop=false
+case "`uname`" in
+  CYGWIN* )
+    cygwin=true
+    ;;
+  Darwin* )
+    darwin=true
+    ;;
+  MINGW* )
+    msys=true
+    ;;
+  NONSTOP* )
+    nonstop=true
+    ;;
+esac
+
+CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
+
+# Determine the Java command to use to start the JVM.
+if [ -n "$JAVA_HOME" ] ; then
+    if [ -x "$JAVA_HOME/jre/sh/java" ] ; then
+        # IBM's JDK on AIX uses strange locations for the executables
+        JAVACMD="$JAVA_HOME/jre/sh/java"
+    else
+        JAVACMD="$JAVA_HOME/bin/java"
+    fi
+    if [ ! -x "$JAVACMD" ] ; then
+        die "ERROR: JAVA_HOME is set to an invalid directory: $JAVA_HOME
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+    fi
+else
+    JAVACMD="java"
+    which java >/dev/null 2>&1 || die "ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+fi
+
+# Increase the maximum file descriptors if we can.
+if [ "$cygwin" = "false" -a "$darwin" = "false" -a "$nonstop" = "false" ] ; then
+    MAX_FD_LIMIT=`ulimit -H -n`
+    if [ $? -eq 0 ] ; then
+        if [ "$MAX_FD" = "maximum" -o "$MAX_FD" = "max" ] ; then
+            MAX_FD="$MAX_FD_LIMIT"
+        fi
+        ulimit -n $MAX_FD
+        if [ $? -ne 0 ] ; then
+            warn "Could not set maximum file descriptor limit: $MAX_FD"
+        fi
+    else
+        warn "Could not query maximum file descriptor limit: $MAX_FD_LIMIT"
+    fi
+fi
+
+# For Darwin, add options to specify how the application appears in the dock
+if $darwin; then
+    GRADLE_OPTS="$GRADLE_OPTS \"-Xdock:name=$APP_NAME\" \"-Xdock:icon=$APP_HOME/media/gradle.icns\""
+fi
+
+# For Cygwin or MSYS, switch paths to Windows format before running java
+if [ "$cygwin" = "true" -o "$msys" = "true" ] ; then
+    APP_HOME=`cygpath --path --mixed "$APP_HOME"`
+    CLASSPATH=`cygpath --path --mixed "$CLASSPATH"`
+    JAVACMD=`cygpath --unix "$JAVACMD"`
+
+    # We build the pattern for arguments to be converted via cygpath
+    ROOTDIRSRAW=`find -L / -maxdepth 1 -mindepth 1 -type d 2>/dev/null`
+    SEP=""
+    for dir in $ROOTDIRSRAW ; do
+        ROOTDIRS="$ROOTDIRS$SEP$dir"
+        SEP="|"
+    done
+    OURCYGPATTERN="(^($ROOTDIRS))"
+    # Add a user-defined pattern to the cygpath arguments
+    if [ "$GRADLE_CYGPATTERN" != "" ] ; then
+        OURCYGPATTERN="$OURCYGPATTERN|($GRADLE_CYGPATTERN)"
+    fi
+    # Now convert the arguments - kludge to limit ourselves to /bin/sh
+    i=0
+    for arg in "$@" ; do
+        CHECK=`echo "$arg"|egrep -c "$OURCYGPATTERN" -`
+        CHECK2=`echo "$arg"|egrep -c "^-"`                                 ### Determine if an option
+
+        if [ $CHECK -ne 0 ] && [ $CHECK2 -eq 0 ] ; then                    ### Added a condition
+            eval `echo args$i`=`cygpath --path --ignore --mixed "$arg"`
+        else
+            eval `echo args$i`="\"$arg\""
+        fi
+        i=`expr $i + 1`
+    done
+    case $i in
+        0) set -- ;;
+        1) set -- "$args0" ;;
+        2) set -- "$args0" "$args1" ;;
+        3) set -- "$args0" "$args1" "$args2" ;;
+        4) set -- "$args0" "$args1" "$args2" "$args3" ;;
+        5) set -- "$args0" "$args1" "$args2" "$args3" "$args4" ;;
+        6) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" ;;
+        7) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" ;;
+        8) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" "$args7" ;;
+        9) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" "$args7" "$args8" ;;
+    esac
+fi
+
+# Escape application args
+save () {
+    for i do printf %s\\n "$i" | sed "s/'/'\\\\''/g;1s/^/'/;\$s/\$/' \\\\/" ; done
+    echo " "
+}
+APP_ARGS=`save "$@"`
+
+# Collect all arguments for the java command, following the shell quoting and substitution rules
+eval set -- $DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS "\"-Dorg.gradle.appname=$APP_BASE_NAME\"" -classpath "\"$CLASSPATH\"" org.gradle.wrapper.GradleWrapperMain "$APP_ARGS"
+
+exec "$JAVACMD" "$@"

gradlew.bat

+@rem
+@rem Copyright 2015 the original author or authors.
+@rem
+@rem Licensed under the Apache License, Version 2.0 (the "License");
+@rem you may not use this file except in compliance with the License.
+@rem You may obtain a copy of the License at
+@rem
+@rem      https://www.apache.org/licenses/LICENSE-2.0
+@rem
+@rem Unless required by applicable law or agreed to in writing, software
+@rem distributed under the License is distributed on an "AS IS" BASIS,
+@rem WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+@rem See the License for the specific language governing permissions and
+@rem limitations under the License.
+@rem
+
+@if "%DEBUG%" == "" @echo off
+@rem ##########################################################################
+@rem
+@rem  Gradle startup script for Windows
+@rem
+@rem ##########################################################################
+
+@rem Set local scope for the variables with windows NT shell
+if "%OS%"=="Windows_NT" setlocal
+
+set DIRNAME=%~dp0
+if "%DIRNAME%" == "" set DIRNAME=.
+set APP_BASE_NAME=%~n0
+set APP_HOME=%DIRNAME%
+
+@rem Resolve any "." and ".." in APP_HOME to make it shorter.
+for %%i in ("%APP_HOME%") do set APP_HOME=%%~fi
+
+@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+set DEFAULT_JVM_OPTS="-Xmx64m" "-Xms64m"
+
+@rem Find java.exe
+if defined JAVA_HOME goto findJavaFromJavaHome
+
+set JAVA_EXE=java.exe
+%JAVA_EXE% -version >NUL 2>&1
+if "%ERRORLEVEL%" == "0" goto init
+
+echo.
+echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+echo.
+echo Please set the JAVA_HOME variable in your environment to match the
+echo location of your Java installation.
+
+goto fail
+
+:findJavaFromJavaHome
+set JAVA_HOME=%JAVA_HOME:"=%
+set JAVA_EXE=%JAVA_HOME%/bin/java.exe
+
+if exist "%JAVA_EXE%" goto init
+
+echo.
+echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME%
+echo.
+echo Please set the JAVA_HOME variable in your environment to match the
+echo location of your Java installation.
+
+goto fail
+
+:init
+@rem Get command-line arguments, handling Windows variants
+
+if not "%OS%" == "Windows_NT" goto win9xME_args
+
+:win9xME_args
+@rem Slurp the command line arguments.
+set CMD_LINE_ARGS=
+set _SKIP=2
+
+:win9xME_args_slurp
+if "x%~1" == "x" goto execute
+
+set CMD_LINE_ARGS=%*
+
+:execute
+@rem Setup the command line
+
+set CLASSPATH=%APP_HOME%\gradle\wrapper\gradle-wrapper.jar
+
+@rem Execute Gradle
+"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %GRADLE_OPTS% "-Dorg.gradle.appname=%APP_BASE_NAME%" -classpath "%CLASSPATH%" org.gradle.wrapper.GradleWrapperMain %CMD_LINE_ARGS%
+
+:end
+@rem End local scope for the variables with windows NT shell
+if "%ERRORLEVEL%"=="0" goto mainEnd
+
+:fail
+rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
+rem the _cmd.exe /c_ return code!
+if  not "" == "%GRADLE_EXIT_CONSOLE%" exit 1
+exit /b 1
+
+:mainEnd
+if "%OS%"=="Windows_NT" endlocal
+
+:omega

settings.gradle

+rootProject.name = 'webflux-usecases'

src/main/java/com/nisum/WebfluxUsecasesApplication.java

+package com.nisum;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class WebfluxUsecasesApplication {
+
+	public static void main(String[] args) {
+		SpringApplication.run(WebfluxUsecasesApplication.class, args);
+	}
+
+}

src/main/resources/application.properties

+

src/test/java/com/nisum/ColdAndHotPublisherTest.java

+package com.nisum;
+
+import org.junit.Test;
+import reactor.core.publisher.ConnectableFlux;
+import reactor.core.publisher.Flux;
+
+import java.time.Duration;
+
+public class ColdAndHotPublisherTest {
+
+    @Test
+    public void coldPublisherTest() throws InterruptedException {
+
+        Flux<String> stringFlux = Flux.just("A","B","C","D","E","F")
+                .delayElements(Duration.ofSeconds(1));
+
+        stringFlux.subscribe(s -> System.out.println("Subscriber 1 : " + s)); //emits the value from beginning
+
+        Thread.sleep(2000);
+
+        stringFlux.subscribe(s -> System.out.println("Subscriber 2 : " + s));//emits the value from beginning
+
+
+        Thread.sleep(4000);
+    }
+
+    @Test
+    public void hotPublisherTest() throws InterruptedException {
+
+        Flux<String> stringFlux = Flux.just("A","B","C","D","E","F")
+                .delayElements(Duration.ofSeconds(1));
+
+        ConnectableFlux<String> connectableFlux = stringFlux.publish();
+        connectableFlux.connect();
+        connectableFlux.subscribe(s -> System.out.println("Subscriber 1 : " + s));
+        Thread.sleep(3000);
+
+        connectableFlux.subscribe(s -> System.out.println("Subscriber 2 : " + s)); // does not emit the values from beginning
+        Thread.sleep(4000);
+
+    }
+}

src/test/java/com/nisum/CustomException.java

+package com.nisum;
+
+public class CustomException extends Throwable {
+
+    private String message;
+
+    @Override
+    public String getMessage() {
+        return message;
+    }
+
+    public void setMessage(String message) {
+        this.message = message;
+    }
+
+    public CustomException(Throwable e) {
+        this.message = e.getMessage();
+    }
+}

src/test/java/com/nisum/FluxAndMonoBackPressureTest.java

+package com.nisum;
+
+import org.junit.Test;
+import org.reactivestreams.Subscription;
+import reactor.core.publisher.BaseSubscriber;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+public class FluxAndMonoBackPressureTest {
+
+    @Test
+    public void backPressureTest() {
+
+        Flux<Integer> finiteFlux = Flux.range(1, 10)
+                .log();
+
+        StepVerifier.create(finiteFlux)
+                .expectSubscription()
+                .thenRequest(1)
+                .expectNext(1)
+                .thenRequest(1)
+                .expectNext(2)
+                .thenCancel()
+                .verify();
+
+    }
+
+    @Test
+    public void backPressure() {
+
+        Flux<Integer> finiteFlux = Flux.range(1, 10)
+                .log();
+
+        finiteFlux.subscribe((element) -> System.out.println("Element is : " + element)
+                , (e) -> System.err.println("Exception is : " + e)
+                , () -> System.out.println("Done")
+                , (subscription -> subscription.request(2)));
+
+    }
+
+    @Test
+    public void backPressure_cancel() {
+
+        Flux<Integer> finiteFlux = Flux.range(1, 10)
+                .log();
+
+        finiteFlux.subscribe((element) -> System.out.println("Element is : " + element)
+                , (e) -> System.err.println("Exception is : " + e)
+                , () -> System.out.println("Done")
+                , (subscription -> subscription.cancel()));
+
+    }
+
+    @Test
+    public void customized_backPressure() {
+
+        Flux<Integer> finiteFlux = Flux.range(1, 10)
+                .log();
+
+        finiteFlux.subscribe(new BaseSubscriber<Integer>() {
+            @Override
+            protected void hookOnNext(Integer value) {
+                request(1);
+                System.out.println("Value received is : " + value);
+                if(value == 4){
+                    cancel();
+                }
+
+            }
+        });
+
+    }
+
+}

src/test/java/com/nisum/FluxAndMonoCombineTest.java

+package com.nisum;
+
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+import reactor.test.scheduler.VirtualTimeScheduler;
+
+import java.time.Duration;
+
+public class FluxAndMonoCombineTest {
+
+    @Test
+    public void combineUsingMerge(){
+
+        Flux<String> flux1 = Flux.just("A","B","C");
+        Flux<String> flux2 = Flux.just("D","E","F");
+
+        Flux<String> mergedFlux = Flux.merge(flux1,flux2);
+
+        StepVerifier.create(mergedFlux.log())
+                .expectSubscription()
+                .expectNext("A","B","C","D","E","F")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void combineUsingMerge_withDelay(){
+
+        Flux<String> flux1 = Flux.just("A","B","C").delayElements(Duration.ofSeconds(1));
+        Flux<String> flux2 = Flux.just("D","E","F").delayElements(Duration.ofSeconds(1));;
+
+        Flux<String> mergedFlux = Flux.merge(flux1,flux2);
+
+        StepVerifier.create(mergedFlux.log())
+                .expectSubscription()
+                .expectNextCount(6)
+                //.expectNext("A","B","C","D","E","F")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void combineUsingConcat(){
+
+        Flux<String> flux1 = Flux.just("A","B","C");
+        Flux<String> flux2 = Flux.just("D","E","F");
+
+        Flux<String> mergedFlux = Flux.concat(flux1,flux2);
+
+        StepVerifier.create(mergedFlux.log())
+                .expectSubscription()
+                .expectNext("A","B","C","D","E","F")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void combineUsingConcat_withDelay(){
+
+        VirtualTimeScheduler.getOrSet();
+
+        Flux<String> flux1 = Flux.just("A","B","C").delayElements(Duration.ofSeconds(1));
+        Flux<String> flux2 = Flux.just("D","E","F").delayElements(Duration.ofSeconds(1));
+
+        Flux<String> mergedFlux = Flux.concat(flux1,flux2);
+
+        StepVerifier.withVirtualTime(()->mergedFlux.log())
+                .expectSubscription()
+                .thenAwait(Duration.ofSeconds(6))
+                .expectNextCount(6)
+                .verifyComplete();
+
+
+      /*  StepVerifier.create(mergedFlux.log())
+                .expectSubscription()
+                .expectNext("A","B","C","D","E","F")
+                .verifyComplete();*/
+
+    }
+
+    @Test
+    public void combineUsingZip(){
+
+        Flux<String> flux1 = Flux.just("A","B","C");
+        Flux<String> flux2 = Flux.just("D","E","F");
+
+        Flux<String> mergedFlux = Flux.zip(flux1,flux2, (t1,t2) -> {
+            return t1.concat(t2); // AD, BE, CF
+        }); //A,D : B,E : C:F
+
+        StepVerifier.create(mergedFlux.log())
+                .expectSubscription()
+                .expectNext("AD", "BE", "CF")
+                .verifyComplete();
+
+    }
+
+
+}

src/test/java/com/nisum/FluxAndMonoErrorTest.java

+package com.nisum;
+
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+import java.time.Duration;
+
+public class FluxAndMonoErrorTest {
+
+    @Test
+    public void fluxErrorHandling() {
+
+        Flux<String> stringFlux = Flux.just("A", "B", "C")
+                .concatWith(Flux.error(new RuntimeException("Exception Occurred")))
+                .concatWith(Flux.just("D"))
+                .onErrorResume((e) -> {    // this block gets executed
+                    System.out.println("Exception is : " + e);
+                    return Flux.just("default", "default1");
+                });
+
+        StepVerifier.create(stringFlux.log())
+                .expectSubscription()
+                .expectNext("A", "B", "C")
+                //.expectError(RuntimeException.class)
+                //.verify();
+                .expectNext("default", "default1")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void fluxErrorHandling_OnErrorReturn() {
+
+        Flux<String> stringFlux = Flux.just("A", "B", "C")
+                .concatWith(Flux.error(new RuntimeException("Exception Occurred")))
+                .concatWith(Flux.just("D"))
+                .onErrorReturn("default");
+
+        StepVerifier.create(stringFlux.log())
+                .expectSubscription()
+                .expectNext("A", "B", "C")
+                .expectNext("default")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void fluxErrorHandling_OnErrorMap() {
+
+        Flux<String> stringFlux = Flux.just("A", "B", "C")
+                .concatWith(Flux.error(new RuntimeException("Exception Occurred")))
+                .concatWith(Flux.just("D"))
+                .onErrorMap((e) -> new CustomException(e));
+
+        StepVerifier.create(stringFlux.log())
+                .expectSubscription()
+                .expectNext("A", "B", "C")
+                .expectError(CustomException.class)
+                .verify();
+
+    }
+
+    @Test
+    public void fluxErrorHandling_OnErrorMap_withRetry() {
+
+        Flux<String> stringFlux = Flux.just("A", "B", "C")
+                .concatWith(Flux.error(new RuntimeException("Exception Occurred")))
+                .concatWith(Flux.just("D"))
+                .onErrorMap((e) -> new CustomException(e))
+                .retry(2);
+
+        StepVerifier.create(stringFlux.log())
+                .expectSubscription()
+                .expectNext("A", "B", "C")
+                .expectNext("A", "B", "C")
+                .expectNext("A", "B", "C")
+                .expectError(CustomException.class)
+                .verify();
+
+    }
+
+    @Test
+    public void fluxErrorHandling_OnErrorMap_withRetryBackoff() {
+
+        Flux<String> stringFlux = Flux.just("A", "B", "C")
+                .concatWith(Flux.error(new RuntimeException("Exception Occurred")))
+                .concatWith(Flux.just("D"))
+                .onErrorMap((e) -> new CustomException(e))
+                .retryBackoff(2, Duration.ofSeconds(5));
+
+        StepVerifier.create(stringFlux.log())
+                .expectSubscription()
+                .expectNext("A", "B", "C")
+                .expectNext("A", "B", "C")
+                .expectNext("A", "B", "C")
+                .expectError(IllegalStateException.class)
+                .verify();
+
+    }
+
+
+}

src/test/java/com/nisum/FluxAndMonoFactoryTest.java

+package com.nisum;
+
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.function.Supplier;
+
+public class FluxAndMonoFactoryTest {
+
+    List<String> names = Arrays.asList("adam","anna","jack","jenny");
+
+
+    @Test
+    public void fluxUsingIterable(){
+
+        Flux<String> namesFlux = Flux.fromIterable(names)
+                .log();
+
+        StepVerifier.create(namesFlux)
+                .expectNext("adam","anna","jack","jenny")
+                .verifyComplete();
+    }
+
+    @Test
+    public void fluxUsingArray(){
+
+        String[] names = new String[]{"adam","anna","jack","jenny"};
+
+        Flux<String> namesFlux = Flux.fromArray(names);
+        StepVerifier.create(namesFlux)
+                .expectNext("adam","anna","jack","jenny")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void fluxUsingStream(){
+
+        Flux<String> namesFlux = Flux.fromStream(names.stream());
+
+        StepVerifier.create(namesFlux)
+                .expectNext("adam","anna","jack","jenny")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void monoUsingJustOrEmpty(){
+
+        Mono<String> mono = Mono.justOrEmpty(null); //Mono.Empty();
+
+        StepVerifier.create(mono.log())
+                .verifyComplete();
+    }
+
+    @Test
+    public void monoUsingSupplier(){
+
+        Supplier<String> stringSupplier = () -> "adam";
+
+        Mono<String> stringMono = Mono.fromSupplier(stringSupplier);
+
+        System.out.println(stringSupplier.get());
+
+        StepVerifier.create(stringMono.log())
+                .expectNext("adam")
+                .verifyComplete();
+    }
+
+    @Test
+    public void fluxUsingRange(){
+
+       Flux<Integer> integerFlux = Flux.range(1,5).log();
+
+       StepVerifier.create(integerFlux)
+               .expectNext(1,2,3,4,5)
+               .verifyComplete();
+    }
+
+
+
+
+}

src/test/java/com/nisum/FluxAndMonoFilterTest.java

+package com.nisum;
+
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+import java.util.Arrays;
+import java.util.List;
+
+public class FluxAndMonoFilterTest {
+
+    List<String> names = Arrays.asList("adam","anna","jack","jenny");
+
+    @Test
+    public void filterTest(){
+
+        Flux<String> namesFlux = Flux.fromIterable(names) // adam, anna, jack,jenny
+                .filter(s->s.startsWith("a"))
+                .log(); //adam,anna
+
+        StepVerifier.create(namesFlux)
+                .expectNext("adam","anna")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void filterTestLength(){
+
+        Flux<String> namesFlux = Flux.fromIterable(names) // adam, anna, jack,jenny
+                .filter(s->s.length() >4)
+                .log(); //jenny
+
+        StepVerifier.create(namesFlux)
+                .expectNext("jenny")
+                .verifyComplete();
+
+    }
+
+}

src/test/java/com/nisum/FluxAndMonoTest.java

+package com.nisum;
+
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+
+public class FluxAndMonoTest {
+
+    @Test
+    public void fluxTest() {
+
+        Flux<String> stringFlux = Flux.just("Spring", "Spring Boot", "Reactive Spring")
+                /*.concatWith(Flux.error(new RuntimeException("Exception Occurred")))*/
+                .concatWith(Flux.just("After Error"))
+                .log();
+
+        stringFlux
+                .subscribe(System.out::println,
+                        (e) -> System.err.println("Exception is " + e)
+                        , () -> System.out.println("Completed"));
+    }
+
+    @Test
+    public void fluxTestElements_WithoutError() {
+
+        Flux<String> stringFlux = Flux.just("Spring", "Spring Boot", "Reactive Spring")
+                .log();
+
+        StepVerifier.create(stringFlux)
+                .expectNext("Spring")
+                .expectNext("Spring Boot")
+                .expectNext("Reactive Spring")
+                .verifyComplete();
+
+
+    }
+
+    @Test
+    public void fluxTestElements_WithError() {
+
+        Flux<String> stringFlux = Flux.just("Spring", "Spring Boot", "Reactive Spring")
+                .concatWith(Flux.error(new RuntimeException("Exception Occurred")))
+                .log();
+
+        StepVerifier.create(stringFlux)
+                .expectNext("Spring")
+                .expectNext("Spring Boot")
+                .expectNext("Reactive Spring")
+                //.expectError(RuntimeException.class)
+                .expectErrorMessage("Exception Occurred")
+                .verify();
+
+
+    }
+
+    @Test
+    public void fluxTestElements_WithError1() {
+
+        Flux<String> stringFlux = Flux.just("Spring", "Spring Boot", "Reactive Spring")
+                .concatWith(Flux.error(new RuntimeException("Exception Occurred")))
+                .log();
+
+        StepVerifier.create(stringFlux)
+                .expectNext("Spring","Spring Boot","Reactive Spring")
+                .expectErrorMessage("Exception Occurred")
+                .verify();
+
+
+    }
+
+    @Test
+    public void fluxTestElementsCount_WithError() {
+
+        Flux<String> stringFlux = Flux.just("Spring", "Spring Boot", "Reactive Spring")
+                .concatWith(Flux.error(new RuntimeException("Exception Occurred")))
+                .log();
+
+        StepVerifier.create(stringFlux)
+                .expectNextCount(3)
+                .expectErrorMessage("Exception Occurred")
+                .verify();
+
+
+    }
+
+    @Test
+    public void monoTest(){
+
+        Mono<String>  stringMono = Mono.just("Spring");
+
+        StepVerifier.create(stringMono.log())
+                .expectNext("Spring")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void monoTest_Error(){
+
+
+        StepVerifier.create(Mono.error(new RuntimeException("Exception Occurred")).log())
+                .expectError(RuntimeException.class)
+                .verify();
+
+    }
+
+
+}

src/test/java/com/nisum/FluxAndMonoTransformTest.java

+package com.nisum;
+
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+import java.util.Arrays;
+import java.util.List;
+
+import static reactor.core.scheduler.Schedulers.parallel;
+
+public class FluxAndMonoTransformTest {
+
+    List<String> names = Arrays.asList("adam", "anna", "jack", "jenny");
+
+    @Test
+    public void transformUsingMap() {
+
+        Flux<String> namesFlux = Flux.fromIterable(names)
+                .map(s -> s.toUpperCase()) //ADAM, ANNA, JACK, JENNY
+                .log();
+
+        StepVerifier.create(namesFlux)
+                .expectNext("ADAM", "ANNA", "JACK", "JENNY")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void transformUsingMap_Length() {
+
+        Flux<Integer> namesFlux = Flux.fromIterable(names)
+                .map(s -> s.length()) //ADAM, ANNA, JACK, JENNY
+                .log();
+
+        StepVerifier.create(namesFlux)
+                .expectNext(4,4,4,5)
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void transformUsingMap_Length_repeat() {
+
+        Flux<Integer> namesFlux = Flux.fromIterable(names)
+                .map(s -> s.length()) //ADAM, ANNA, JACK, JENNY
+                .repeat(1)
+                .log();
+
+        StepVerifier.create(namesFlux)
+                .expectNext(4,4,4,5,4,4,4,5)
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void transformUsingMap_Filter() {
+
+        Flux<String> namesFlux = Flux.fromIterable(names)
+                .filter(s -> s.length()>4)
+                .map(s -> s.toUpperCase()) // JENNY
+                .log();
+
+        StepVerifier.create(namesFlux)
+                .expectNext("JENNY")
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void tranformUsingFlatMap(){
+
+        Flux<String> stringFlux = Flux.fromIterable(Arrays.asList("A","B","C","D","E","F")) // A, B, C, D, E, F
+                .flatMap(s -> {
+
+                    return Flux.fromIterable(convertToList(s)); // A -> List[A, newValue] , B -> List[B, newValue]
+                })//db or external service call that returns a flux -> s -> Flux<String>
+                .log();
+
+        StepVerifier.create(stringFlux)
+                .expectNextCount(12)
+                .verifyComplete();
+    }
+
+    private List<String> convertToList(String s)  {
+
+        try {
+            Thread.sleep(1000);
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+        return Arrays.asList(s, "newValue");
+    }
+
+    @Test
+    public void tranformUsingFlatMap_usingparallel(){
+
+        Flux<String> stringFlux = Flux.fromIterable(Arrays.asList("A","B","C","D","E","F")) // Flux<String>
+                .window(2) //Flux<Flux<String> -> (A,B), (C,D), (E,F)
+                .flatMap((s) ->
+                    s.map(this::convertToList).subscribeOn(parallel())) // Flux<List<String>
+                    .flatMap(s -> Flux.fromIterable(s)) //Flux<String>
+                .log();
+
+        StepVerifier.create(stringFlux)
+                .expectNextCount(12)
+                .verifyComplete();
+    }
+
+    @Test
+    public void tranformUsingFlatMap_parallel_maintain_order(){
+
+        Flux<String> stringFlux = Flux.fromIterable(Arrays.asList("A","B","C","D","E","F")) // Flux<String>
+                .window(2) //Flux<Flux<String> -> (A,B), (C,D), (E,F)
+               /* .concatMap((s) ->
+                        s.map(this::convertToList).`(parallel())) */// Flux<List<String>
+                .flatMapSequential((s) ->
+                        s.map(this::convertToList).subscribeOn(parallel()))
+                .flatMap(s -> Flux.fromIterable(s)) //Flux<String>
+                .log();
+
+        StepVerifier.create(stringFlux)
+                .expectNextCount(12)
+                .verifyComplete();
+    }
+
+}

src/test/java/com/nisum/FluxAndMonoWithTimeTest.java

+package com.nisum;
+
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+import java.time.Duration;
+
+public class FluxAndMonoWithTimeTest {
+
+    @Test
+    public void infiniteSequence() throws InterruptedException {
+
+        Flux<Long> infiniteFlux = Flux.interval(Duration.ofMillis(100))
+                .log(); // starts from 0 --> ......
+
+        infiniteFlux
+                .subscribe((element) -> System.out.println("Value is : " + element));
+
+        Thread.sleep(3000);
+
+    }
+
+
+    @Test
+    public void infiniteSequenceTest() throws InterruptedException {
+
+        Flux<Long> finiteFlux = Flux.interval(Duration.ofMillis(100))
+                .take(3)
+                .log();
+
+        StepVerifier.create(finiteFlux)
+                .expectSubscription()
+                .expectNext(0L, 1L,2L)
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void infiniteSequenceMap() throws InterruptedException {
+
+        Flux<Integer> finiteFlux = Flux.interval(Duration.ofMillis(100))
+                .map(l -> new Integer(l.intValue()))
+                .take(3)
+                .log();
+
+        StepVerifier.create(finiteFlux)
+                .expectSubscription()
+                .expectNext(0, 1,2)
+                .verifyComplete();
+
+    }
+
+    @Test
+    public void infiniteSequenceMap_withDelay() throws InterruptedException {
+
+        Flux<Integer> finiteFlux = Flux.interval(Duration.ofMillis(100))
+                .delayElements(Duration.ofSeconds(1))
+                .map(l -> new Integer(l.intValue()))
+                .take(3)
+                .log();
+
+        StepVerifier.create(finiteFlux)
+                .expectSubscription()
+                .expectNext(0, 1,2)
+                .verifyComplete();
+
+    }
+
+}

src/test/java/com/nisum/VirtualTimeTest.java

+package com.nisum;
+
+import org.junit.Test;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+import reactor.test.scheduler.VirtualTimeScheduler;
+
+import java.time.Duration;
+
+public class VirtualTimeTest {
+
+    @Test
+    public void testingWithoutVirtualTime(){
+
+        Flux<Long> longFlux = Flux.interval(Duration.ofSeconds(1))
+                .take(3);
+
+        StepVerifier.create(longFlux.log())
+                .expectSubscription()
+                .expectNext(0l,1l,2l)
+                .verifyComplete();
+    }
+
+    @Test
+    public void testingWithVirtualTime(){
+
+        VirtualTimeScheduler.getOrSet();
+
+        Flux<Long> longFlux = Flux.interval(Duration.ofSeconds(1))
+                .take(3);
+
+        StepVerifier.withVirtualTime(() -> longFlux.log())
+                .expectSubscription()
+                .thenAwait(Duration.ofSeconds(3))
+                .expectNext(0l,1l,2l)
+                .verifyComplete();
+
+    }
+}

src/test/java/com/nisum/WebfluxUsecasesApplicationTests.java

+package com.nisum;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.test.context.SpringBootTest;
+
+@SpringBootTest
+class WebfluxUsecasesApplicationTests {
+
+	@Test
+	void contextLoads() {
+	}
+
+}