springframework
https://docs.spring.io/spring-framework/docs/current/javadoc-api/index.html?constant-values.html
Apache Commons
http://commons.apache.org/proper/commons-lang/javadocs/api-3.9/index.html?constant-values.html
Apache HttpComponents
https://hc.apache.org/httpcomponents-core-ga/httpcore/apidocs/index.html?constant-values.html
