如何解决‘Cannot subclass final class java.lang.reflect.Method’异常？

问题场景 #

测试代码：

@InjectMocks
ReportAssembler reportAssembler;

private ReportingReturnResponse reportingReturnResponse;
private ReturnReport returnReport;
private StaticPathLinkBuilder staticPathLinkBuilder;

@Test
public void testMergeReturnCompletesSuccessfully() {
    ReportingResource reportResource = reportAssembler.mergeReturn(returnReport, this.reportingResource);
    assertNotNull(reportResource.getReturnResource());
}

被调用的业务代码：

public ReportingResource mergeReturn(ReturnReport returnReport, ReportingResource reportingResource) {
    Link link = staticPathLinkBuilder.linkTo(
        methodOn(ReturnController.class).getReturnById(
            returnReport.getAccount().getId(),
            returnReport.getReturn().getId())
    ).withSelfRel();

    return reportingResource;
}

public LinkBuilder linkTo(Object dummyInvocation) {
    LastInvocationAware lastInvocationAware = (LastInvocationAware) dummyInvocation;

    MethodInvocation methodInvocation = lastInvocationAware.getLastInvocation();

    StaticPathLinkBuilder staticPathLinkBuilder = getThis();
    return staticPathLinkBuilder.linkTo(methodInvocation.getMethod(), methodInvocation.getArguments());
}

运行测试时，return staticPathLinkBuilder.linkTo(methodInvocation.getMethod(), methodInvocation.getArguments())抛出异常：

org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class java.lang.reflect.Method: Common causes of this problem include using a final class or a non-visible class
    ...
Caused by: java.lang.IllegalArgumentException: Cannot subclass final class java.lang.reflect.Method
    ...

原因分析 #

异常核心原因：

• java.lang.reflect.Method是final类，CGLIB无法为final类生成代理子类；
• 测试中staticPathLinkBuilder是Mockito创建的Mock对象，且配置了调用真实方法（比如@Mock(answer = Answers.CALLS_REAL_METHODS)），导致执行到linkTo(methodInvocation.getMethod(), methodInvocation.getArguments())时，Spring HATEOAS内部尝试用CGLIB代理Method对象，触发了final类不能被代理的限制。

解决方案 #

方案1：MocklinkTo方法的返回值，跳过真实逻辑 #

直接MockStaticPathLinkBuilder的linkTo方法，返回预先构造的LinkBuilder或Link对象，避免进入触发异常的真实代码：

@ExtendWith(MockitoExtension.class)
class ReportingAssemblerReturnTest {
    @InjectMocks
    ReportAssembler reportAssembler;

    @Mock
    private StaticPathLinkBuilder staticPathLinkBuilder;

    private ReturnReport returnReport;
    private ReportingResource reportingResource;

    @BeforeEach
    void setUp() {
        // 初始化测试数据
        returnReport = new ReturnReport();
        Account account = new Account();
        account.setId(1L);
        ReturnEntity returnEntity = new ReturnEntity();
        returnEntity.setId(2L);
        returnReport.setAccount(account);
        returnReport.setReturn(returnEntity);
        
        reportingResource = new ReportingResource();
        
        // Mock linkTo方法的行为，返回模拟的LinkBuilder
        LinkBuilder mockLinkBuilder = Mockito.mock(LinkBuilder.class);
        Link mockLink = Link.of("/accounts/1/returns/2");
        when(mockLinkBuilder.withSelfRel()).thenReturn(mockLink);
        when(staticPathLinkBuilder.linkTo(any(Object.class))).thenReturn(mockLinkBuilder);
    }

    @Test
    public void testMergeReturnCompletesSuccessfully() {
        ReportingResource result = reportAssembler.mergeReturn(returnReport, reportingResource);
        assertNotNull(result.getReturnResource());
    }
}

方案2：改用Spring HATEOAS测试工具构造Link #

如果需要构造真实的Link，可使用Spring HATEOAS的MockMvcLinkBuilder替代业务中的StaticPathLinkBuilder，或在测试中直接构造Link对象：

// 在测试中直接构造符合要求的Link并mock返回
when(staticPathLinkBuilder.linkTo(any(Object.class))).thenReturn(
    MockMvcLinkBuilder.linkTo(MockMvcLinkBuilder.methodOn(ReturnController.class).getReturnById(1L, 2L))
);

方案3：调整Mock配置，禁止调用真实方法 #

如果不需要调用StaticPathLinkBuilder的真实方法，移除CALLS_REAL_METHODS配置，让Mockito默认返回null或自定义mock行为，避免触发异常逻辑。

内容的提问来源于stack exchange，提问作者Peter Penzov