spring 配置文件的bean自动注入失败的解决方法是什么？
一个Spring注入问题，首先看一个普通Spring Bean，
public class Foo {
@Autowired
Bar bar;
public void doSomething(){
bar.doSomething();
}
}
Spring配置一：
<bean id="bar" class="com.test.Bar"></bean>
<bean id="foo" class="com.test.Foo"></bean>
单元测试：
@Test
public void test_doSomthing(){
ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext-test.xml");
Foo foo = ctx.getBean(Foo.class);
foo.doSomething();
}
执行上述测试方法，报错
java.lang.NullPointerException
at com.test.Foo.doSomething(Foo.java:15)
at com.test.FooTest.test_doSomthing(FooTest.java:13)
即foo bean中的bar并未注入。
Spring配置二：
<context:component-scan base-package="com.test"></context:component-scan>
当改成配置二后执行上述单元测试方法便能成功通过。经分析日志及查看源代码，发现使用配置二时供装载了6个bean，如下所示：
DEBUG org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:216) Loaded 6 bean definitions from location pattern [applicationContext-test.xml]
DEBUG org.springframework.context.support.AbstractApplicationContext.obtainFreshBeanFactory(AbstractApplicationContext.java:530) Bean factory for org.springframework.context.support.ClassPathXmlApplicationContext@3c4e80d3: org.springframework.beans.factory.support.DefaultListableBeanFactory@14cc51c8: defining beans [bar,foo,org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor]; root of factory hierarchy
而使用配置一时只有两个bean，如下所示：
DEBUG org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:216) Loaded 2 bean definitions from location pattern [applicationContext-test.xml]
DEBUG org.springframework.context.support.AbstractApplicationContext.obtainFreshBeanFactory(AbstractApplicationContext.java:530) Bean factory for org.springframework.context.support.ClassPathXmlApplicationContext@45ebbb93: org.springframework.beans.factory.support.DefaultListableBeanFactory@18481697: defining beans [bar,foo]; root of factory hierarchy
配置二执行单元测试通过的原因似乎就在于多出的这几个bean。是不是只要有context：component-scan元素在自动就会有这几个bean的产生？验证此假设
在配置一中添加一个无实际意义的context：component-scan元素，如下所示：
<context:component-scan base-package="com.nonexist"></context:component-scan>
这时执行单元测试能通过，同配置二一样也会装载6个bean。那么这6个bean中到底哪个对注入bar到Foo中起了作用呢？
经过断点调试发现是AutowiredAnnotationBeanPostProcessor bean起了作用，见输出日志：
2015-04-25 20:23:09 DEBUG org.springframework.beans.factory.annotation.InjectionMetadata.<init>(InjectionMetadata.java:60) Found injected element on class [com.test.Foo]: AutowiredFieldElement for com.test.Bar com.test.Foo.bar
2015-04-25 20:23:09 DEBUG org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:85) Processing injected method of bean 'foo': AutowiredFieldElement for com.test.Bar com.test.Foo.bar
2015-04-25 20:23:09 DEBUG org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:245) Returning cached instance of singleton bean 'bar'
2015-04-25 20:23:09 DEBUG org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.registerDependentBeans(AutowiredAnnotationBeanPostProcessor.java:424) Autowiring by type from bean name 'foo' to bean named 'bar'
2015-04-25 20:23:09 DEBUG org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458) Finished creating instance of bean 'foo'
那么直接在配置一种显式添加AutowiredAnnotationBeanPostProcessor bean呢？如下所示：
<bean id="bar" class="com.tcl.account.service.test.Bar"></bean>
<bean id="foo" class="com.tcl.account.service.test.Foo"></bean>
<bean id="autowiredAnnotationBeanPostProcessor" class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"></bean>
测试会不会通过？会通过。见日志：
DEBUG org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:216) Loaded 3 bean definitions from location pattern [applicationContext-test.xml]
DEBUG org.springframework.context.support.AbstractApplicationContext.obtainFreshBeanFactory(AbstractApplicationContext.java:530) Bean factory for org.springframework.context.support.ClassPathXmlApplicationContext@7767d3c1: org.springframework.beans.factory.support.DefaultListableBeanFactory@1924ed52: defining beans [bar,foo,autowiredAnnotationBeanPostProcessor]; root of factory hierarchy
DEBUG org.springframework.beans.factory.annotation.InjectionMetadata.<init>(InjectionMetadata.java:60) Found injected element on class [com.test.Foo]: AutowiredFieldElement for com.test.Bar com.test.Foo.bar
DEBUG org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:85) Processing injected method of bean 'foo': AutowiredFieldElement for com.test.Bar com.test.Foo.bar
DEBUG org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:245) Returning cached instance of singleton bean 'bar'
DEBUG org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.registerDependentBeans(AutowiredAnnotationBeanPostProcessor.java:424) Autowiring by type from bean name 'foo' to bean named 'bar'
DEBUG org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458) Finished creating instance of bean 'foo'
那么为什么在配置文件中添加了context: componet-scan元素后就会自动添加那另外4个bean呢？经过断点调试发现Spring隐式装载的4个bean是在如下方法中加载的：
Set<BeanDefinitionHolder> org.springframework.context.annotation.AnnotationConfigUtils.registerAnnotationConfigProcessors(BeanDefinitionRegistry registry, Object source)
其调用链如下所示：
补充一：
若仍用配置一，但单元测试改成如下形式也可以测试通过。
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = { "classpath:/applicationContext-test.xml" })
public class FooTest2 {
@Autowired
private Foo foo;
@Test
public void test_doSomthing(){
foo.doSomething();
}
}
当然一点都不意外，这种方式也会隐式加载那4个bean，见日志：
DEBUG org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:216) Loaded 2 bean definitions from location pattern [classpath:/applicationContext-test.xml]
INFO org.springframework.context.support.AbstractApplicationContext.prepareRefresh(AbstractApplicationContext.java:500) Refreshing org.springframework.context.support.GenericApplicationContext@51f3336e: startup date [Sun Apr 26 17:27:35 CST 2015]; root of context hierarchy
DEBUG org.springframework.context.support.AbstractApplicationContext.obtainFreshBeanFactory(AbstractApplicationContext.java:530) Bean factory for org.springframework.context.support.GenericApplicationContext@51f3336e: org.springframework.beans.factory.support.DefaultListableBeanFactory@4f9d1352: defining beans [bar,foo,org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor]; root of factory hierarchy
补充二，若使用的是@Value,效果同@Autowired，即也需要隐式加载AutowiredAnnotationBeanPostProcessor bean。
public class Foo {
@Value("${bar}")
String bar;
public void doSomething(){
System.out.println(bar);
}
}
补充三：
若使用配置一，@PostConstruct标注的方法也不会被执行，但此时需要隐式加载的Spring bean是：org.springframework.context.annotation.CommonAnnotationBeanPostProcessor
补充四：
在配置一中添加如下配置
<context:annotation-config/>
也会隐式加载那4个bean
