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](https://my.oschina.net/bean) 注解了相同name = "cupcake"的bean：

1public class BeanOverrideConfig {
2    @Bean(name = "cupcake")
3    public Cupcake cupcake1() {
4        Cupcake cupcake = new Cupcake();
5        cupcake.setName("Cupcake1");
6        return cupcake;
7    }
8    
9    @Bean(name = "cupcake")
10    public Cupcake cupcake2() {
11        Cupcake cupcake = new Cupcake();
12        cupcake.setName("Cupcake2");
13        return cupcake;
14    }
15    
16}

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());：

1public class BeanOverrideTest {
2    private ApplicationContext ctx = null;
3    
4    @Before
5    public void setUp() {
6        ctx = new AnnotationConfigApplicationContext(BeanOverrideConfig.class);
7    }
8    
9    @Test
10    public void testGetBean() {
11        Cupcake cupcake = ctx.getBean(Cupcake.class);
12        Assert.assertNotNull(cupcake);
13        Assert.assertEquals("Cupcake1", cupcake.getName());
14    }
15    
16}

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)：

1// Is the existing bean definition one that was created from a configuration class?
2// -> allow the current bean method to override, since both are at second-pass level.
3// However, if the bean method is an overloaded case on the same configuration class,
4// preserve the existing bean definition.
5if (existingBeanDef instanceof ConfigurationClassBeanDefinition) {
6    ConfigurationClassBeanDefinition ccbd = (ConfigurationClassBeanDefinition) existingBeanDef;
7    return ccbd.getMetadata().getClassName().equals(
8            beanMethod.getConfigurationClass().getMetadata().getClassName());
9}

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：

1public class BeanOverrideConfig1 {
2    @Bean(name = "cupcake")
3    public Cupcake cupcake1() {
4        Cupcake cupcake = new Cupcake();
5        cupcake.setName("Cupcake1");
6        return cupcake;
7    }
8    
9}
10
11public class BeanOverrideConfig2 {
12    
13    @Bean(name = "cupcake")
14    public Cupcake cupcake2() {
15        Cupcake cupcake = new Cupcake();
16        cupcake.setName("Cupcake2");
17        return cupcake;
18    }
19    
20}
21
22public class BeanOverrideTest {
23    private ApplicationContext ctx = null;
24    
25    @Before
26    public void setUp() {
27        ctx = new AnnotationConfigApplicationContext(BeanOverrideConfig1.class, BeanOverrideConfig2.class);
28    }
29    
30    @Test
31    public void testOverride() {
32        Cupcake cupcake = ctx.getBean(Cupcake.class);
33        Assert.assertNotNull(cupcake);
34        Assert.assertEquals("Cupcake2", cupcake.getName());
35    }
36    
37}

很显然测试能通过，即会覆盖。

@Import呢？

如果是这种情况呢？

1@Import(BeanOverrideConfig2.class)
2public class BeanOverrideConfig1 {
3    @Bean(name = "cupcake")
4    public Cupcake cupcake1() {
5        Cupcake cupcake = new Cupcake();
6        cupcake.setName("Cupcake1");
7        return cupcake;
8    }
9    
10}
11
12public class BeanOverrideConfig2 {
13    
14    @Bean(name = "cupcake")
15    public Cupcake cupcake2() {
16        Cupcake cupcake = new Cupcake();
17        cupcake.setName("Cupcake2");
18        return cupcake;
19    }
20    
21}
22
23public class BeanOverrideTest {
24    private ApplicationContext ctx = null;
25    
26    @Before
27    public void setUp() {
28        ctx = new AnnotationConfigApplicationContext(BeanOverrideConfig1.class);
29    }
30    
31    @Test
32    public void testOverride() {
33        Cupcake cupcake = ctx.getBean(Cupcake.class);
34        Assert.assertNotNull(cupcake);
35        Assert.assertEquals("Cupcake1", cupcake.getName());
36    }
37    
38}

测试通过，这种@Import的情况也没认为是同一配置类，不会覆盖。