Could not resolve placeholder ‘XXX‘ in string value “${XXX}

启动springboot项目时遇到Could not resolve placeholder XXX in string value "${XXX}";

错误一：target目录中没有springboot的核心配置文件

解决一：在pom.xml中写入<resource>标签，使配置文件能被解析。

        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>
                        **/*.yml
                    </include>
                </includes>
                <filtering>true</filtering>
            </resource>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>
                        **/*.yml
                    </include>
                </includes>
                <filtering>true</filtering>
            </resource>
        </resources>

错误二：配置文件中自定义属性无法被解析

解决二：在启动类中将PropertySourcesPlaceholderConfigurer放到IOC容器中

@Bean
    public static PropertySourcesPlaceholderConfigurer placeholderConfigurer() {

        PropertySourcesPlaceholderConfigurer c = new PropertySourcesPlaceholderConfigurer();

        c.setIgnoreUnresolvablePlaceholders(true);

        return c;
    }

然后我又发现以上问题出在，resources文件夹不知道怎么了变成了普通文件夹，改成资源文件夹就OK了