从一个异常探索autowired 的原理。

首先环境是这样的：

public class Boss {    @Autowired    private Car car;}//@Component 加上这个注释，上面的Boss 的Autowired car就会失败，出现下面的异常public class Car {    private String brand;    private double price;}

xml 是这样的：

 

测试类：

import com.baobaotao.Boss;import org.junit.Test;import org.junit.runner.RunWith;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.test.context.ContextConfiguration;import org.springframework.test.context.junit4.AbstractJUnit4SpringContextTests;import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = {
   "classpath:/beans.xml"})public class AutowiredlTest extends        AbstractJUnit4SpringContextTests {    @Autowired    Boss boss;    @Test    public void aaa() {        System.out.println("boss = " + boss);    }}

 

结果是出现异常：

D:\soft\java\jdk1.7\bin\java -Didea.launcher.port=7533 "-Didea.launcher.bin.path=D:\Program Files (x86)\JetBrains\IntelliJ IDEA Community Edition 2016.1.2\bin" -Dfile.encoding=UTF-8 -classpath "D:\soft\java\jdk1.7\jre\lib\charsets.jar;D:\soft\java\jdk1.7\jre\lib\deploy.jar;D:\soft\java\jdk1.7\jre\lib\ext\access-bridge-64.jar;D:\soft\java\jdk1.7\jre\lib\ext\dnsns.jar;D:\soft\java\jdk1.7\jre\lib\ext\jaccess.jar;D:\soft\java\jdk1.7\jre\lib\ext\localedata.jar;D:\soft\java\jdk1.7\jre\lib\ext\sunec.jar;D:\soft\java\jdk1.7\jre\lib\ext\sunjce_provider.jar;D:\soft\java\jdk1.7\jre\lib\ext\sunmscapi.jar;D:\soft\java\jdk1.7\jre\lib\ext\zipfs.jar;D:\soft\java\jdk1.7\jre\lib\javaws.jar;D:\soft\java\jdk1.7\jre\lib\jce.jar;D:\soft\java\jdk1.7\jre\lib\jfr.jar;D:\soft\java\jdk1.7\jre\lib\jfxrt.jar;D:\soft\java\jdk1.7\jre\lib\jsse.jar;D:\soft\java\jdk1.7\jre\lib\management-agent.jar;D:\soft\java\jdk1.7\jre\lib\plugin.jar;D:\soft\java\jdk1.7\jre\lib\resources.jar;D:\soft\java\jdk1.7\jre\lib\rt.jar;D:\code\ws\spring\hz\spring-learn\target\test-classes;D:\code\ws\spring\hz\spring-learn\target\classes;C:\Users\lkms\.m2\repository\junit\junit\4.12\junit-4.12.jar;C:\Users\lkms\.m2\repository\org\hamcrest\hamcrest-core\1.3\hamcrest-core-1.3.jar;C:\Users\lkms\.m2\repository\org\springframework\spring-context\4.3.5.RELEASE\spring-context-4.3.5.RELEASE.jar;C:\Users\lkms\.m2\repository\org\springframework\spring-aop\4.3.5.RELEASE\spring-aop-4.3.5.RELEASE.jar;C:\Users\lkms\.m2\repository\org\springframework\spring-beans\4.3.5.RELEASE\spring-beans-4.3.5.RELEASE.jar;C:\Users\lkms\.m2\repository\org\springframework\spring-core\4.3.5.RELEASE\spring-core-4.3.5.RELEASE.jar;C:\Users\lkms\.m2\repository\commons-logging\commons-logging\1.2\commons-logging-1.2.jar;C:\Users\lkms\.m2\repository\org\springframework\spring-expression\4.3.5.RELEASE\spring-expression-4.3.5.RELEASE.jar;C:\Users\lkms\.m2\repository\org\springframework\spring-web\4.3.5.RELEASE\spring-web-4.3.5.RELEASE.jar;C:\Users\lkms\.m2\repository\org\springframework\spring-test\4.3.5.RELEASE\spring-test-4.3.5.RELEASE.jar;D:\Program Files (x86)\JetBrains\IntelliJ IDEA Community Edition 2016.1.2\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain AnnoIoCTest十一月 11, 2017 2:21:34 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefreshINFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@7a277bd2: startup date [Sat Nov 11 14:21:34 CST 2017]; root of context hierarchy十一月 11, 2017 2:21:34 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitionsINFO: Loading XML bean definitions from class path resource [beans.xml]十一月 11, 2017 2:21:35 下午 org.springframework.context.support.ClassPathXmlApplicationContext refreshWARNING: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'boss': Unsatisfied dependency expressed through field 'car'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.baobaotao.Car' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'boss': Unsatisfied dependency expressed through field 'car'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.baobaotao.Car' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:588)    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:366)    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1225)    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:552)    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:759)    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:866)    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542)    at org.springframework.context.support.ClassPathXmlApplicationContext.
     
      (ClassPathXmlApplicationContext.java:139)    at org.springframework.context.support.ClassPathXmlApplicationContext.
      
       (ClassPathXmlApplicationContext.java:93)    at AnnoIoCTest.main(AnnoIoCTest.java:8)    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)    at java.lang.reflect.Method.invoke(Method.java:601)    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.baobaotao.Car' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}    at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1474)    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1102)    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1064)    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:585)    ... 20 moreProcess finished with exit code 1
      
     

 

spring 的启动过程中会读取配置的xml，注册所有的 beanDefinition，这个是准备过程。 准备完后是 beanfactory 的refresh ，这个时候会 进行注解的处理， 也就是 BeanPostProcessor。 这其中就包含了对 bean 中包含的各种注解的 解析， 比如 Autowired 注解等。 AutowiredAnnotationBeanPostProcessor 的 postProcessPropertyValues 是继承于InstantiationAwareBeanPostProcessor  。 是对它的实现。 如果我们仔细观察这个错误堆栈， 也许我们会从中发现很多很多的细节。

 

参考：

http://blog.csdn.net/mack415858775/article/details/47721909 写得非常详细，非常好！