SprintBoot中如何构造Bean&原理分析

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原文链接：https://my.oschina.net/qiangzigege/blog/905985

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本文介绍了Spring Boot中两种获取Bean的方式：通过ApplicationContext手动获取和使用@Autowired注解自动注入，并详细解析了这两种方式的工作原理。

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MySpringBoot打算构造加入Bean的功能，代码完成，下面解释操作&原理。

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获取Bean有2种方法，

1)拿到Web容器里的ApplicationContext，调用getBean(Class)方法

2)直接在类里使用Autowired注解

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下面就来解释这2点

1)拿到ApplicationContext,代码调用getBean

写上1个类

package helper;

import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.stereotype.Component;

import controller.UserExample;

//使用方法
//在任何类中调用
//YourClass aaa=(YourClass) BeanContextHelper.getApplicationContext().getBean(YourClass.class);

@Component
public class BeanContextHelper implements ApplicationContextAware {

	private static ApplicationContext applicationContext = null;

	@Override
	public void setApplicationContext(ApplicationContext context) throws BeansException {
		if (null == applicationContext) {
			applicationContext = context;
		}
		System.out.println("setApplicationContext invoked...");
	}

	public static ApplicationContext getApplicationContext() {
		return applicationContext;
	}
}

这个类所在的package必须在boot main类的scan路径内。

然后setApplicationContext的方法会被调用，我们看一下调用栈

at helper.BeanContextHelper.setApplicationContext(BeanContextHelper.java:21)
	at org.springframework.context.support.ApplicationContextAwareProcessor.invokeAwareInterfaces(ApplicationContextAwareProcessor.java:121)
	at org.springframework.context.support.ApplicationContextAwareProcessor.postProcessBeforeInitialization(ApplicationContextAwareProcessor.java:97)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization(AbstractAutowireCapableBeanFactory.java:409)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1620)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:555)
	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:761)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:866)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542)
	at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:122)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:737)
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:370)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:314)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1162)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1151)
	at server.Boot.main(Boot.java:13)

为什么会触发到这个函数呢？

private void invokeAwareInterfaces(Object bean) {
		if (bean instanceof Aware) {
			if (bean instanceof EnvironmentAware) {
				((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
			}
			if (bean instanceof EmbeddedValueResolverAware) {
				((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
			}
			if (bean instanceof ResourceLoaderAware) {
				((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
			}
			if (bean instanceof ApplicationEventPublisherAware) {
				((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
			}
			if (bean instanceof MessageSourceAware) {
				((MessageSourceAware) bean).setMessageSource(this.applicationContext);
			}
			if (bean instanceof ApplicationContextAware) {
                //原因在这里
				((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
			}
		}
	}

其实也就是，对于单例的bean,web启动时，预先构造，然后做一些初始化操作。

因为上面这个类implements ApplicationContextAware接口，所以会执行最后1行代码。

所以，如果你在任何类里面写上

FirstBean bean = (FirstBean) BeanContextHelper.getApplicationContext().getBean(FirstBean.class);
System.out.println(bean);

就会拿到这个Bean的单例对象。

2)通过@Autowired注解

这个是自动装配，让我们看如何实现的。

假设我们有这么1个Controller类,代码如下：

package controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

import bean.FirstBean;

@RestController
@RequestMapping("/user")
public class UserExample {
	
	public UserExample() {
		System.out.println("user example . bean --->"+bean);
		new Exception().printStackTrace();
	}

	@SuppressWarnings("unused")
	@Autowired
	private FirstBean bean;

	@RequestMapping(value = "/hi", method = { RequestMethod.GET })
	public String index() {
		return "{\"hello\":\"world\"}";
	}
}

我们看到有1个Autowired注解，什么时候发挥作用的呢？

想找到这个很简单，注入一个prototype的类，打印这个prototype的栈就可以了。

尝试一把，确实可以，注入属性的位置是

at bean.ProtoTypeBean.<init>(ProtoTypeBean.java:10)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
	at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:142)
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:89)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1147)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1099)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:513)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:325)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
	at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:208)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1138)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1066)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:585)
	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:1264)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)
	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:761)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:866)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542)
	at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:122)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:737)
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:370)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:314)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1162)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1151)
	at server.Boot.main(Boot.java:13)

这样就找到了注入属性的位置，属性就是这样注入的。

转载于:https://my.oschina.net/qiangzigege/blog/905985