(2)spring源码解析之前置知识点

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最近在看spring源码,但是spring的体系太庞大了,在这里记录一下阅读源码中遇到知识点

@PostConstruct

被注解的方法，在对象加载完依赖注入后执行

看个demo

package com.day01.config;

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan("com.day01")
public class SpringConfig {

}

IndexDao

package com.day01.service;

import org.springframework.stereotype.Component;

@Component
public class IndexDao {

    public IndexDao(){
        System.out.println("IndexDao  构造方法");
    }
}

IndexService

@Service
public class IndexService {
    @Autowired
    private IndexDao indexDao;


    public IndexService(){
        System.out.println("IndexService  构造方法");
    }

    @PostConstruct
    public void init(){
        System.out.println("IndexService  init方法");
    }


    public void hello(){
        System.out.println("IndexService");
    }

}

Testday01

public static void main(String[] args) {
        AnnotationConfigApplicationContext applicationContext=new AnnotationConfigApplicationContext(SpringConfig.class);
        IndexService indexService = (IndexService) applicationContext.getBean("indexService");
        indexService.hello();
    }

看结果

 可以看出在spring项目中，在一个bean的初始化过程中，方法执行先后顺序为Constructor > @Autowired > @PostConstruct

BeanPostProcessor

BeanPostProcessor是Spring框架的提供的一个扩展点,通过实现BeanPostProcessor接口，程序员就可插手bean实例化的过程

看demo

@Component
public class TestBeanPostProcessor implements BeanPostProcessor {

    /**
     * 在bean初始化之前执行
     */
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if (beanName.equals("indexService")){
            System.out.println("indexService  postProcessBeforeInitialization");
        }
        //这里也可以产生代理对象 Proxy.newProxyInstance(),也是aop实现的原理
        return bean;
    }

    /**
     * 初始化之后
     */
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if (beanName.equals("indexService")){
            System.out.println("indexService  postProcessAfterInitialization");
        }
        return bean;
    }

}

运行上文的Testday01,看下结果

 可以看出

===Spring IOC容器实例化Bean===
===调用BeanPostProcessor的postProcessBeforeInitialization方法===
===调用bean实例的初始化方法===
===调用BeanPostProcessor的postProcessAfterInitialization方法===

值得说明的是这个接口可以设置多个，会形成一个列表,那么如何确定他们的执行顺序呢?

Ordered和PriorityOrdered

Spring提供了Ordered和PriorityOrdered接口，来处理相同接口实现类的优先级问题

看个demo

TestBeanPostProcessor

@Component
public class TestBeanPostProcessor implements BeanPostProcessor , PriorityOrdered {

    /**
     * 在bean初始化之前执行
     */
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if (beanName.equals("indexService")){
            System.out.println("indexService  postProcessBeforeInitialization");
        }
        //这里也可以产生代理对象 Proxy.newProxyInstance(),也是aop实现的原理
        return bean;
    }

    /**
     * 在bean初始化之前执行
     */
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if (beanName.equals("indexService")){
            System.out.println("indexService  postProcessAfterInitialization");
        }
        return bean;
    }

    @Override
    public int getOrder() {
        return 100;//注意这里
    }
}

TestBeanPostProcessor2

@Component
public class TestBeanPostProcessor2 implements BeanPostProcessor , PriorityOrdered {

    /**
     * 在bean初始化之前执行
     */
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if (beanName.equals("indexService")){
            System.out.println("indexService  postProcessBeforeInitialization2");
        }
        //这里也可以产生代理对象 Proxy.newProxyInstance(),也是aop实现的原理
        return bean;
    }

    /**
     * 在bean初始化之前执行
     */
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if (beanName.equals("indexService")){
            System.out.println("indexService  postProcessAfterInitialization2");
        }
        return bean;
    }

    @Override
    public int getOrder() {
        return 99;
    }
}

运行test类,看结果

这段代码的逻辑：

　　1. 若对象o1是Ordered接口类型，o2是PriorityOrdered接口类型，那么o2的优先级高于o1

　　2. 若对象o1是PriorityOrdered接口类型，o2是Ordered接口类型，那么o1的优先级高于o2

　　3. 其他情况，若两者都是Ordered接口类型或两者都是PriorityOrdered接口类型，调用Ordered接口的getOrder方法得到order值，order值越大，优先级越小

若2个对象中有一个对象实现了PriorityOrdered接口，那么这个对象的优先级更高。

若2个对象都是PriorityOrdered或Ordered接口的实现类，那么比较Ordered接口的getOrder方法得到order值，值越低，优先级越高

BeanFactoryPostProcessor

spring的扩展点之一:实现该接口，可以在spring的bean创建之前修改beandefinitions属性。

例如可以把bean的scope从singleton改为prototype，也可以把property的值给修改掉。
可以同时配置多个BeanFactoryPostProcessor，并通过设置'order'属性来控制各个BeanFactoryPostProcessor的执行次序

看个demo

@Component
public class TestBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        System.out.println("===============TestBeanFactoryPostProcessor============");
        
    }
}

测试类

public static void main(String[] args) {
        AnnotationConfigApplicationContext applicationContext=new AnnotationConfigApplicationContext(SpringConfig.class);
        //AnnotationConfigApplicationContext applicationContext=new AnnotationConfigApplicationContext(IndexService.class);
        IndexService indexService = (IndexService) applicationContext.getBean("indexService");
        indexService.hello();
        IndexService indexService2 = (IndexService) applicationContext.getBean("indexService");
        System.out.println(indexService);
        System.out.println(indexService2);
    }

运行一下,看结果

 可以看到,spring管理的bean默认是单例的,我们把indexService改成prototype试一下

@Component
public class TestBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        System.out.println("===============TestBeanFactoryPostProcessor============");
        BeanDefinition indexService = beanFactory.getBeanDefinition("indexService");
        String scope = indexService.getScope();
        System.out.println(scope);//singleton
        //indexService.setScope("singleton");
        indexService.setScope("prototype");//设置为原型
        System.out.println(scope);//singleton
    }
}

看下结果

BeanDefinitionRegistryPostProcessor

BeanDefinitionRegistryPostProcessor继承了BeanFactoryPostProcessor接口,并可以注册bean到spring容器中,一共要实现以下两个方法：

void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException：
该方法的实现中，主要用来对bean定义做一些改变。

void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException：
该方法用来注册更多的bean到spring容器中，详细观察入参BeanDefinitionRegistry接口，看看这个参数能带给我们什么能力。

看个demo

public class IndexService2  {

    private ApplicationContext applicationContext;
    public IndexService2(){
        System.out.println("IndexService2  构造方法");
    }

    @PostConstruct
    public void init(){
        System.out.println("IndexService2  init方法");
    }


    public void hello(){
        System.out.println("IndexService2 hello");
    }
    
}

注意:IndexService2并没有@Component注解,说明这个类并没有交给spring管理,继续

@Component
public class MyBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {

    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
        GenericBeanDefinition genericBeanDefinition = new GenericBeanDefinition();
        genericBeanDefinition.setScope("singleton");
        genericBeanDefinition.setBeanClass(IndexService2.class);//将IndexService2交给spring管理
        registry.registerBeanDefinition("indexService2",genericBeanDefinition);
    }

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        System.out.println("===============MyBeanDefinitionRegistryPostProcessor============");
    }
}

注意上述代码中通过BeanDefinitionRegistryPostProcessor 将IndexService2手动注册交给spring管理

运行test类

public static void main(String[] args) {
        AnnotationConfigApplicationContext applicationContext=new AnnotationConfigApplicationContext(SpringConfig.class);
        //AnnotationConfigApplicationContext applicationContext=new AnnotationConfigApplicationContext(IndexService.class);
        IndexService indexService = (IndexService) applicationContext.getBean("indexService");
        indexService.hello();
        IndexService2 indexService2 = (IndexService2) applicationContext.getBean("indexService2");
        System.out.println(indexService);
        System.out.println(indexService2);
    }

看结果

ApplicationContextAware

我们可以通过注解@Autowired 很简单方便获取bean,虽然这种方法很简单方便，但是有些特殊场景用不了,比如静态方法中不能使用

 此时可以借助ApplicationContextAware获取bean

看个demo

@Service
public class TestServiceImpl {

    public String hello(){
        return "hello world";
    }

}

这里无法注入TestServiceImpl 无法使用,看demo

@Component
public class ApplicationContextUtil implements ApplicationContextAware {

    private static ApplicationContext applicationContext = null;
    @Autowired
    private TestServiceImpl testService;

    /**
     * 实现ApplicationContextAware接口, 注入Context到静态变量中.
     */
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        ApplicationContextUtil.applicationContext = applicationContext;
    }

    /**
     * 获取静态变量中的ApplicationContext.
     */
    public static ApplicationContext getApplicationContext() {
        return applicationContext;
    }

    /**
     * 从静态变量applicationContext中得到Bean, 自动转型为所赋值对象的类型.
     */
    @SuppressWarnings("unchecked")
    public static <T> T getBean(String name) {
        return (T) applicationContext.getBean(name);
    }

    /**
     * 从静态变量applicationContext中得到Bean, 自动转型为所赋值对象的类型.
     */
    public static <T> T getBean(Class<T> requiredType) {
        return applicationContext.getBean(requiredType);
    }

    /*public static String test(){
        String hello = testService.hello();
    }*/
}

看测试类

public static void main(String[] args) {
        AnnotationConfigApplicationContext applicationContext=new AnnotationConfigApplicationContext(SpringConfig.class);
        //AnnotationConfigApplicationContext applicationContext=new AnnotationConfigApplicationContext(IndexService.class);
        IndexService indexService = (IndexService) applicationContext.getBean("indexService");
        indexService.hello();
        System.out.println(indexService);
        /*IndexService2 indexService2 = (IndexService2) applicationContext.getBean("indexService2");
        System.out.println(indexService2);*/
        String testServiceImpl = ((TestServiceImpl) ApplicationContextUtil.getBean("testServiceImpl")).hello();
        System.out.println(testServiceImpl);
    }

看结果

 ok,今天就先到这里,以后有新的内容随时补充吧

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原文地址：https://www.cnblogs.com/lusaisai/p/12907402.html