前言

不建议写这么奇葩的代码！！！
这就有点像考试喜欢出的试题，有一堆overload和override的代码，选择题选择调用的是哪个。
不建议写这种让人看着费劲的代码。

问题引出

言归正传，如果有一个这样的配置类，@Bean注解了相同name = "cupcake"的bean：

public class BeanOverrideConfig {
    @Bean(name = "cupcake")
    public Cupcake cupcake1() {
        Cupcake cupcake = new Cupcake();
        cupcake.setName("Cupcake1");
        return cupcake;
    }
    
    @Bean(name = "cupcake")
    public Cupcake cupcake2() {
        Cupcake cupcake = new Cupcake();
        cupcake.setName("Cupcake2");
        return cupcake;
    }
    
}

下面这个测试类能通过测试吗？注意最后一行代码Assert.assertEquals("Cupcake1", cupcake.getName());：

public class BeanOverrideTest {
    private ApplicationContext ctx = null;
    
    @Before
    public void setUp() {
        ctx = new AnnotationConfigApplicationContext(BeanOverrideConfig.class);
    }
    
    @Test
    public void testGetBean() {
        Cupcake cupcake = ctx.getBean(Cupcake.class);
        Assert.assertNotNull(cupcake);
        Assert.assertEquals("Cupcake1", cupcake.getName());
    }
    
}

结果

测试通过！

原因

Spring对configuration class的加载
加载BeanDefinition的过程中有一步：

org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForBeanMethod(BeanMethod)

在这个方法中会判断现在的beanName在现有的beanDefinitionMap中是否已存在，然后决定是否覆盖。是否覆盖的策略如下org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.isOverriddenByExistingDefinition(BeanMethod, String)：

// Is the existing bean definition one that was created from a configuration class?
// -> allow the current bean method to override, since both are at second-pass level.
// However, if the bean method is an overloaded case on the same configuration class,
// preserve the existing bean definition.
if (existingBeanDef instanceof ConfigurationClassBeanDefinition) {
    ConfigurationClassBeanDefinition ccbd = (ConfigurationClassBeanDefinition) existingBeanDef;
    return ccbd.getMetadata().getClassName().equals(
            beanMethod.getConfigurationClass().getMetadata().getClassName());
}

源码里说的很清楚了，如果来自不同层级的bean method，允许覆盖，如果是the same configuration class，preserve the existing bean definition(同一configuration class的overload，保留先前的)。
回到我们的测试类，即保留方法public Cupcake cupcake1()对应的bean definition，最后测试的时候getName就返回Cupcake1。

深入

如果是下面这种配置和测试：

public class BeanOverrideConfig1 {
    @Bean(name = "cupcake")
    public Cupcake cupcake1() {
        Cupcake cupcake = new Cupcake();
        cupcake.setName("Cupcake1");
        return cupcake;
    }
    
}

public class BeanOverrideConfig2 {
    
    @Bean(name = "cupcake")
    public Cupcake cupcake2() {
        Cupcake cupcake = new Cupcake();
        cupcake.setName("Cupcake2");
        return cupcake;
    }
    
}

public class BeanOverrideTest {
    private ApplicationContext ctx = null;
    
    @Before
    public void setUp() {
        ctx = new AnnotationConfigApplicationContext(BeanOverrideConfig1.class, BeanOverrideConfig2.class);
    }
    
    @Test
    public void testOverride() {
        Cupcake cupcake = ctx.getBean(Cupcake.class);
        Assert.assertNotNull(cupcake);
        Assert.assertEquals("Cupcake2", cupcake.getName());
    }
    
}

很显然测试能通过，即会覆盖。

@Import呢？

如果是这种情况呢？

@Import(BeanOverrideConfig2.class)
public class BeanOverrideConfig1 {
    @Bean(name = "cupcake")
    public Cupcake cupcake1() {
        Cupcake cupcake = new Cupcake();
        cupcake.setName("Cupcake1");
        return cupcake;
    }
    
}

public class BeanOverrideConfig2 {
    
    @Bean(name = "cupcake")
    public Cupcake cupcake2() {
        Cupcake cupcake = new Cupcake();
        cupcake.setName("Cupcake2");
        return cupcake;
    }
    
}

public class BeanOverrideTest {
    private ApplicationContext ctx = null;
    
    @Before
    public void setUp() {
        ctx = new AnnotationConfigApplicationContext(BeanOverrideConfig1.class);
    }
    
    @Test
    public void testOverride() {
        Cupcake cupcake = ctx.getBean(Cupcake.class);
        Assert.assertNotNull(cupcake);
        Assert.assertEquals("Cupcake1", cupcake.getName());
    }
    
}

测试通过，这种@Import的情况也没认为是同一配置类，不会覆盖。