使用Mockito注入Mock Spring Repository时出现空指针异常

The Problem #

I'm trying to test my DefaultRuleTester class, which has an @Autowired RuleChainRepository field. Every time I run the test, I get a NullPointerException pointing to that autowired field. I've tried using MockitoJUnitRunner and MockitoAnnotations.initMocks(this), but neither has fixed the issue.

Code Snippets #

Under Test: DefaultRuleTester

@Component 
public class DefaultRuleTester implements RuleTester { 
    private static final Logger LOG = LogManager.getLogger(DefaultRuleTester.class); 
    @Autowired 
    RuleChainRepository ruleChainRepository; 

    public DefaultRuleTester() { } 

    @Override 
    public List<ResultChain> test(Entity entity) { 
        if (entity == null) return Collections.emptyList(); 
        try { 
            return selectAndTestAllRuleChains(entity); 
        } catch (InvalidClassException e) { 
            LOG.error("Unable to select RuleChains from database.", e); 
            return Collections.emptyList(); 
        } 
    } 

    protected List<ResultChain> selectAndTestAllRuleChains(Entity entity) throws InvalidClassException { 
        Collection<RuleChain> ruleChains = selectRuleChainsFromConnection(); 
        return testRuleChains(entity, ruleChains); 
    } 

    private Collection selectRuleChainsFromConnection() throws InvalidClassException { 
        Collection<RuleChain> ruleChains = ruleChainRepository.findAll(); 
        if (ruleChains == null) return Collections.emptyList(); 
        return ruleChains; 
    } 

    protected List<ResultChain> testRuleChains(Entity entity, Collection<RuleChain> ruleChains) { 
        // business logic 
    } 
}

Repository Definition: RuleChainRepository

public interface RuleChainRepository extends JpaRepository<RuleChain, Long> { }

Test Class: DefaultRuleTesterTest

@RunWith(MockitoJUnitRunner.class) 
public class DefaultRuleTesterTest { 
    private final Incident entity = new Incident(); 

    @InjectMocks 
    private DefaultRuleTester tester; 

    @Mock 
    private RuleChainRepository ruleChainRepository; 

    @Before 
    public void setUp() { 
        tester = new DefaultRuleTester(); 
    } 

    @Test 
    public void whenEntityTested_shouldReturnGivenResultChains() { 
        Mockito.when(ruleChainRepository.findAll()).thenReturn(singletonList(new RuleChainSpy())); 
        List<ResultChain> resultChains = tester.test(entity); 
        assert(1, resultChains.size()); 
    } 
}

Error Stack Trace #

java.lang.NullPointerException 
at com.remsdaq.resque.officersubscription.entity_controller.DefaultRuleTester.selectRuleChainsFromConnection(DefaultRuleTester.java:30) 
at com.remsdaq.resque.officersubscription.entity_controller.DefaultRuleTester.selectAndTestAllRuleChains(DefaultRuleTester.java:25) 
at com.remsdaq.resque.officersubscription.entity_controller.DefaultRuleTester.test(DefaultRuleTester.java:17) 
at com.remsdaq.resque.officersubscription.entity_controller.DefaultRuleTesterTest.whenTested_shouldReturnGivenResultChains(DefaultRuleTesterTest.java:20) 
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.mockito.internal.runners.DefaultInternalRunner$1.run(DefaultInternalRunner.java:79) 
at org.mockito.internal.runners.DefaultInternalRunner.run(DefaultInternalRunner.java:85) 
at org.mockito.internal.runners.StrictRunner.run(StrictRunner.java:39) 
at org.mockito.junit.MockitoJUnitRunner.run(MockitoJUnitRunner.java:163) 
at org.junit.runner.JUnitCore.run(JUnitCore.java:137) 
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68) 
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47) 
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242) 
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

The Fix #

Your issue is in the setUp method! You're manually creating a new DefaultRuleTester instance, which overwrites the one that Mockito already created and injected with your mock RuleChainRepository.

Step-by-Step Fix #

1. Remove the manual instance creation in setUp: Delete the line tester = new DefaultRuleTester();. Mockito will automatically create the tester instance and inject the mock repository for you.
2. Fix the assertion syntax: Replace assert(1, resultChains.size()); with assertEquals(1, resultChains.size()); — the original syntax is invalid for Java's assert statement.

Here's the corrected test class:

@RunWith(MockitoJUnitRunner.class) 
public class DefaultRuleTesterTest { 
    private final Incident entity = new Incident(); 

    @InjectMocks 
    private DefaultRuleTester tester; 

    @Mock 
    private RuleChainRepository ruleChainRepository; 

    // Removed problematic setUp method

    @Test 
    public void whenEntityTested_shouldReturnGivenResultChains() { 
        Mockito.when(ruleChainRepository.findAll()).thenReturn(Collections.singletonList(new RuleChainSpy())); 
        List<ResultChain> resultChains = tester.test(entity); 
        assertEquals(1, resultChains.size()); 
    } 
}

Alternative Approach (If You Prefer Manual Instance Creation) #

If you want to keep creating the instance manually, you'll need to inject the mock yourself using reflection (since Mockito can't inject into manually created instances automatically):

public class DefaultRuleTesterTest { 
    private final Incident entity = new Incident(); 

    private DefaultRuleTester tester; 

    @Mock 
    private RuleChainRepository ruleChainRepository; 

    @Before 
    public void setUp() { 
        MockitoAnnotations.initMocks(this);
        tester = new DefaultRuleTester();
        // Manually inject the mock using Spring's ReflectionTestUtils
        ReflectionTestUtils.setField(tester, "ruleChainRepository", ruleChainRepository);
    } 

    // Test method remains the same
}

The first approach (letting Mockito handle instance creation) is cleaner and recommended.

内容的提问来源于stack exchange，提问作者Chris