ApplicationContext、Environment原来可以这样获取

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1.前言

日常开发过程中你可能需要根据名称从IOC容器中获取Bean对象、可能需要根据类型从IOC容器中获取一组Bean对象、也可能需要根据key读取配置文件中的value。

从IOC容器中获取Bean你需要使用ApplicationContext，从配置文件中读取配置你需要使用Environment，那么ApplicationContext和Environment你应该如何获取呢？

2.ApplicationContext

2.1 实现ApplicationContextAware接口

xxxAware接口有一种赋能作用，就是你想拥有什么能力，只需要实现对应的接口即可。比如想拥有获取容器内容能力，只需要实现ApplicationContextAware接口

@Component
public class RegisterEventPublisher implements ApplicationContextAware {

    private ApplicationContext applicationContext;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
}
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2.2 监听ApplicationContextInitializedEvent事件

想拥有获取容器内容能力，除了实现ApplicationContextAware接口之外，你还可以监听ApplicationContextInitializedEvent事件

public class ApplicationContextInitializedEventListener implements ApplicationListener<ApplicationContextInitializedEvent> {

    @Override
    public void onApplicationEvent(ApplicationContextInitializedEvent event) {
        ApplicationContextUtils.setApplicationContext(event.getApplicationContext());
    }
}
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3.Environment

3.1 实现EnvironmentAware接口

同2.1章节一致，想要拥有获取配置能力，只需要实现EnvironmentAware接口

@Component
public class xxxx implements EnvironmentAware {

    private Environment environment;

    @Override
    public void setEnvironment(Environment environment) {
        this.environment = environment;
    }
}
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3.2 监听ApplicationEnvironmentPreparedEvent事件

想要拥有获取配置能力，除了实现EnvironmentAware接口之外，你还可以监听ApplicationEnvironmentPreparedEvent事件

public class ApplicationEnvironmentListener implements ApplicationListener<ApplicationEnvironmentPreparedEvent> {

    @Override
    public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
        ApplicationContextUtils.setEnvironment(event.getEnvironment());
    }
}
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由于ApplicationEnvironmentPreparedEvent事件是在容器刷新之前发布的，因此通过@Component注解标注不会生效，需要在spring.factories中指定

org.springframework.context.ApplicationListener= \
  com.boot.example.listener.ApplicationEnvironmentListener,\
  com.boot.example.listener.ApplicationContextInitializedEventListener
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4.封装

通过上面介绍你已经知道了如何获取ApplicationContext和Environment。实现接口的方式实际使用过程中会有些繁琐，每个想用到的地方必须满足两个条件

• 实现xxxAware接口
• 本身是一个Bean也就是要被@Component注解标注

鉴于此，我们可以选择使用监听器方式封装一个工具类，这样可以达到随用随取的作用

4.1 工具类封装

public class ApplicationContextUtils {

    private static ApplicationContext applicationContext;

    private static Environment environment;

    private ApplicationContextUtils() {
    }

    public static void setApplicationContext(ApplicationContext applicationContext) {
        ApplicationContextUtils.applicationContext = applicationContext;
    }

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

    public static void setEnvironment(Environment environment) {
        ApplicationContextUtils.environment = environment;
    }

    public static Environment getEnvironment() {
        return ApplicationContextUtils.environment;
    }
}
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4.2 监听ApplicationContextInitializedEvent事件

public class ApplicationContextInitializedEventListener implements ApplicationListener<ApplicationContextInitializedEvent> {

    @Override
    public void onApplicationEvent(ApplicationContextInitializedEvent event) {
        ApplicationContextUtils.setApplicationContext(event.getApplicationContext());
    }
}
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4.3 监听ApplicationEnvironmentPreparedEvent事件

public class ApplicationEnvironmentListener implements ApplicationListener<ApplicationEnvironmentPreparedEvent> {

    @Override
    public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
        ApplicationContextUtils.setEnvironment(event.getEnvironment());
    }
}
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