Junit : How to cover CompletableFuture Code

Question

I am new to Junit and come across this problem recently. I am not able code write test cases wherever I used CompletableFuture in my code. Like below Java file

Updated

AuditService.java

@Autowired
Executor existingThreadPool;

@Override
public void auditData(List<ErrorDetails> alertList) {
    CompletableFuture.runAsync(() -> {
        if (alertList.isEmpty())
            //privateMethodCall1
        else
           //privateMethodCall2
    }, existingThreadPool);
}

I followed this link and tried below solution still getting NPE for CompletableFuture Like below error.

AuditServiceTest.java

@InjectMock
AuditService auditService;

@Mock
private CompletableFuture<Void> completableFuture = null;

@Before
public void setup() {
    MockitoAnnotations.initMocks(this);
    completableFuture = CompletableFuture.runAsync(new Runnable() {
        @Override
        public void run() {}
    },Executors.newSingleThreadExecutor());
}

@Test
public void shouldAuditData() {
    List<ErrorDetails> alertList = new ArrayList();
    auditService.auditData(alertList);
}

ERROR

java.lang.NullPointerException
    at java.util.concurrent.CompletableFuture.screenExecutor(CompletableFuture.java:415)
    at java.util.concurrent.CompletableFuture.runAsync(CompletableFuture.java:1858)
    at com.service.impl.AuditService.auditData(AuditService.java:15)
    at com.service.impl.AuditServiceTest.shouldAuditData(AuditServiceTest.java:249)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

Answer 1

in AuditService class , Executor is autowired. that is perfect setup for unit tests. what you have to do is , come up with separate configuration for test and Executor implementation should be a inline executor (you can provide your own implementation which calls runnable.run in the same calling thread). To do this you can use some implementations provided spring-test. ex: AbstractJUnit4SpringContextTests

if you dont like to go with spring-test support, now you have injected mock Executor to AuditService. so you can mock the execute method with providing custom stub.Answer and execute the runnable.run.

Mockito.doAnswer(new Answer() {
      public Object answer(InvocationOnMock invocation) {
        Object[] args = invocation.getArguments();
        ((Runnable)args[0]).run();
        return null; // void method, so return null
      }
    }).when(executor).execute(Mockito.any(Runnable.class));

Answer 2

You need to test your logic and you don't need to mock the static method CompletableFuture.runAsync(...). So your test should look like normal test with exception that you need to wait some time to be sure that asynchronous code is executed, because it is not executed in the same thread. So for the moment I will give you example that you can use with Thread.sleep() which is not good convention, in additional question you can ask how to avoid usages of Thread.sleep().

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.mockito.junit.MockitoJUnitRunner;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

@RunWith(MockitoJUnitRunner.class)
public class AuditServiceTest {

    @Mock
    Service serviceMock;

    @Test
    public void shouldAuditData() {
        AuditService auditService = new AuditService(serviceMock);
        List<Object> alertList = new ArrayList();
        auditService.auditData(alertList);
        // you can wait with Thread.sleep() 
        // because the execution is asynchronous
        Mockito.verify(serviceMock).method1();
        Mockito.verify(serviceMock, Mockito.never()).method2();
    }
}

class AuditService {

    Executor existingThreadPool = Executors.newSingleThreadExecutor();
    Service service;

    public AuditService(Service service) {
        this.service = service;
    }

    public void auditData(List<Object> alertList) {
        CompletableFuture.runAsync(() -> {
            if (alertList.isEmpty()) {
                service.method1();
            } else {
                service.method2();
            }
        }, existingThreadPool);
    }
}
class Service {
    public void method1(){};
    public void method2(){};
}