How to properly mock EntityManager.detach

Question

Updated

I could use some help with the proper way to mock EntityManager.detach() in Spring using Mockito. I'm not sure what to put in the doAnswer block in the test. Everything I've tried results in an NPE on the em.detach() call in the service impl. The debugger shows that both em and existing are valid, non-null objects. (Shared EntityManager proxy for target factory [javax.persistence.EntityManagerFactory#0 bean]). Full NPE stack trace added.

Service class

@Service
@Transactional
public class MyEntityServiceImpl implements MyEntityService {
  private final MyDao dao;
  
  @PersistenceContext
  private EntityManager em;
  
  public MyEntityServiceImpl(MyDao dao) {
    super();
    this.dao = dao;
  }
  public MyDto update(MyDto dto) {
    // check preconditions for null and empty
    
    MyEntity existing = dao.findByBusinessKey(dto.getBusinessKey());
    if(existing == null) {
        throw new NotFoundException();
    }
    em.detach(existing) // NPE on this line during unit test.  
    
    // business logic
    
    MyEntity saved = dao.save(existing);
  }

Unit test

@SpringJUnitConfig()
class ServiceImplTest {
  @Autowired
  private MyEntityServiceImpl service;
  
  @MockBean
  private MyDao dao;
  
  @MockBean
  EntityManagerFactory emf;
  
  @MockBean
  EntityManager em;
  
  @Test
  public void givenValidEntity_whenUpdate_thenNoException() {
    MyDto existing = MyDto.builder.businessKey("valid business key").build();
    when(dao.findByBusinessKey(any(MyDto.class)).thenReturn(existing);
    doAnswer( invocation -> {
        // what goes in here??!?!?!
    }).when(em).detach(any(MyDto.class));
    
    assertDoesNotThrow(() -> {
        service.update(existing);
    });
  }
}

Stack Trace

    org.opentest4j.AssertionFailedError: Unexpected exception thrown: java.lang.NullPointerException
    at org.junit.jupiter.api.AssertDoesNotThrow.createAssertionFailedError(AssertDoesNotThrow.java:83)
    at org.junit.jupiter.api.AssertDoesNotThrow.assertDoesNotThrow(AssertDoesNotThrow.java:54)
    at org.junit.jupiter.api.AssertDoesNotThrow.assertDoesNotThrow(AssertDoesNotThrow.java:37)
    at org.junit.jupiter.api.Assertions.assertDoesNotThrow(Assertions.java:3060)
    at MyEntityServiceImpl.givenValidEntity_whenUpdate_thenNoException(MyEntityServiceImpl.java:126)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:688)
    at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
    at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:149)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:140)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:84)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor$$Lambda$135/0x0000000075d4ab40.apply(Unknown Source)
    at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115)
    at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall$$Lambda$136/0x0000000075b80880.apply(Unknown Source)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105)
    at org.junit.jupiter.engine.execution.ExecutableInvoker$$Lambda$335/0x0000000075f81e80.apply(Unknown Source)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:104)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:210)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor$$Lambda$452/0x0000000075ba46f0.execute(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:206)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:131)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:65)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$$Lambda$215/0x0000000075d4fad0.execute(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$$Lambda$214/0x0000000075bb9a70.invoke(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$$Lambda$213/0x0000000075bb9600.execute(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService$$Lambda$219/0x0000000075d52eb0.accept(Unknown Source)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$$Lambda$215/0x0000000075d4fad0.execute(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$$Lambda$214/0x0000000075bb9a70.invoke(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$$Lambda$213/0x0000000075bb9600.execute(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService$$Lambda$219/0x0000000075d52eb0.accept(Unknown Source)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$$Lambda$215/0x0000000075d4fad0.execute(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$$Lambda$214/0x0000000075bb9a70.invoke(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask$$Lambda$213/0x0000000075bb9600.execute(Unknown Source)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:108)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:88)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.lambda$execute$0(EngineExecutionOrchestrator.java:54)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator$$Lambda$179/0x0000000075c7b430.accept(Unknown Source)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.withInterceptedStreams(EngineExecutionOrchestrator.java:67)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:52)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:96)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:84)
    at org.eclipse.jdt.internal.junit5.runner.JUnit5TestReference.run(JUnit5TestReference.java:98)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:40)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:529)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:756)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:452)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:210)
Caused by: java.lang.NullPointerException
    at java.base/java.lang.reflect.Method.invoke(Method.java:559)
    at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:311)
    at com.sun.proxy.$Proxy45.detach(Unknown Source)
    at impl.MyEntityServiceImpl.update(MyEntityServiceImpl.java:107)
    at MyEntityServiceImpl.lambda$0(MyEntityServiceImpl.java:127)
    at MyEntityServiceImpl$$Lambda$463/0x00000000765fc660.execute(Unknown Source)
    at org.junit.jupiter.api.AssertDoesNotThrow.assertDoesNotThrow(AssertDoesNotThrow.java:50)
    ... 86 more

Answer 1

The issue was the EntityManager was not being mocked. I changed from @PersistenceContext to constructor initialization and the test is green.

Fixed Service class

@Service
@Transactional
public class MyEntityServiceImpl implements MyEntityService {
  private final MyDao dao;
  
  private EntityManager em;
  
  public MyEntityServiceImpl(MyDao dao, EntityManager em) {
    super();
    this.dao = dao;
    this.em = em;
  }
  
  // other code unchanged from original question
}