1.代码

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.Map;

import org.springframework.boot.context.event.ApplicationEnvironmentPreparedEvent;
import org.springframework.boot.origin.OriginTrackedValue;
import org.springframework.context.ApplicationListener;
import org.springframework.core.env.MutablePropertySources;
import org.springframework.core.env.PropertySource;


public class ConfigureListener implements ApplicationListener<ApplicationEnvironmentPreparedEvent> {

    @Override
    public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
        Connection conn = null;
        Statement st = null;
        ResultSet rs = null;
        // 获取spring Environment
        MutablePropertySources propertySources = event.getEnvironment().getPropertySources();
        // 配置放在了application-pro或者是application-dev 中 赋值复制需要在其中赋值
        for (PropertySource<?> propertySource : propertySources) {
            boolean applicationConfig = propertySource.getName().contains("application-");
            if (!applicationConfig) {
                continue;
            }
            // 获取上文的application集合中获取数据库连接
            Map<String, OriginTrackedValue> dataBaseSource =
                (Map<String, OriginTrackedValue>)propertySource.getSource();
            String driverClass = String.valueOf(dataBaseSource.get("spring.datasource.driver-class-name").getValue());
            String url = String.valueOf(dataBaseSource.get("spring.datasource.url").getValue());
            String user = String.valueOf(dataBaseSource.get("spring.datasource.username").getValue());
            String password = String.valueOf(dataBaseSource.get("spring.datasource.password").getValue());
            // 因为在spring初始化之前 所有不能使用注解 所以需要jdbc直接连接数据库 首先建立驱动
            try {
                Class.forName("oracle.jdbc.driver.OracleDriver");
                conn = DriverManager.getConnection(url, user, password);
                // 1、获取连接对象
                // 2、创建statement类对象，用来执行SQL语句！！
                st = conn.createStatement();
                // 3、创建sql查询语句
                String sql = "select * from SYS_CONFIGURE";
                // 4、执行sql语句并且换回一个查询的结果集
                rs = st.executeQuery(sql);
                while (rs.next()) {
                    // 获取数据库中的数据
                    String item = rs.getString("ITEM");
                    String itemValue = rs.getString("ITEM_VALUE");
                    // 通过数据库中的配置 修改application集合中数据
                    Map<String, OriginTrackedValue> source =
                        (Map<String, OriginTrackedValue>)propertySource.getSource();
                    OriginTrackedValue originTrackedValue = source.get(item);
                    OriginTrackedValue newOriginTrackedValue =
                        OriginTrackedValue.of(itemValue, originTrackedValue.getOrigin());
                    source.put(item, newOriginTrackedValue);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

@SpringBootApplication
public class TestApplication {

    public static void main(String[] args) {
        SpringApplication app = new SpringApplication(TestApplication.class);
        app.addListeners(new ConfigureListener());
        app.run(args);
    }
}

2.简介

在spring boot初始化时 会读取application 放在MutablePropertySources 中 我们会读取配置MutablePropertySources 然后查询数据库 根据key查询之后 重新覆盖（因为以上操作时 bean factory还没有初始化 所以用jdbc） 

而以上拦截器会在读取application后立刻执行 不会马后炮（当spring boot初始化之后在更改MutablePropertySources ）

获取MutablePropertySources 
MutablePropertySources propertySources = event.getEnvironment().getPropertySources();