SpringContextHolder 作用

以靜態變量保存Spring ApplicationContext, 可在任何代碼任何地方任何時候取出ApplicaitonContext.
使用方式。在啟動類里添加Bean

package com.yl;

import com.yl.utils.SpringContextHolder;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.scheduling.annotation.EnableAsync;
import org.springframework.transaction.annotation.EnableTransactionManagement;

@SpringBootApplication
@EnableTransactionManagement
public class MainApplication {

    public static void main(String[] args) {
        SpringApplication.run(MainApplication.class, args);
    }

    @Bean
    public SpringContextHolder springContextHolder() {
        return new SpringContextHolder();
    }
}

package com.yl.utils;

import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.DisposableBean;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;

/**
 * @author
 * @date
 */
@Slf4j
public class SpringContextHolder implements ApplicationContextAware, DisposableBean {

    private static ApplicationContext applicationContext = null;

    /**
     * 取得存儲在靜態變量中的ApplicationContext.
     */
    public static ApplicationContext getApplicationContext() {
        assertContextInjected();
        return applicationContext;
    }

    /**
     * 從靜態變量applicationContext中取得Bean, 自動轉型為所賦值對象的類型.
     */
    public static <T> T getBean(String name) {
        assertContextInjected();
        return (T) applicationContext.getBean(name);
    }

    /**
     * 從靜態變量applicationContext中取得Bean, 自動轉型為所賦值對象的類型.
     */
    public static <T> T getBean(Class<T> requiredType) {
        assertContextInjected();
        return applicationContext.getBean(requiredType);
    }

    /**
     * 檢查ApplicationContext不為空.
     */
    private static void assertContextInjected() {
        if (applicationContext == null) {
            throw new IllegalStateException("applicaitonContext屬性未注入, 請在applicationContext" +
                    ".xml中定義SpringContextHolder或在SpringBoot啟動類中注冊SpringContextHolder.");
        }
    }

    /**
     * 清除SpringContextHolder中的ApplicationContext為Null.
     */
    public static void clearHolder() {
        log.debug("清除SpringContextHolder中的ApplicationContext:"
                + applicationContext);
        applicationContext = null;
    }

    @Override
    public void destroy() throws Exception {
        SpringContextHolder.clearHolder();
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        if (SpringContextHolder.applicationContext != null) {
            log.warn("SpringContextHolder中的ApplicationContext被覆蓋, 原有ApplicationContext為:" + SpringContextHolder.applicationContext);
        }
        SpringContextHolder.applicationContext = applicationContext;
    }
}