Environment 与 PropertySource:属性解析顺序与占位符

  PropertySource 的顺序(先找前面的):[systemProperties, systemEnvironment, class path resource [app.properties]]
  app.name = from -D system property(系统属性、环境变量、app.properties 里都可能有)
  only.file = only in file;MY_NAME(环境变量)用 my.name 取 = 来自环境变量;用 MY_NAME 取 = 来自环境变量
  addFirst 一个 MapPropertySource 之后 app.name = 运行时 addFirst 加的
  再用 addLast 加一个同名键:app.name = from -D system property(addLast 排在最后,赢不了);late.only = 晚到的

  resolvePlaceholders("${nope}") = ${nope}(宽松,保持原样)
  resolveRequiredPlaceholders("${nope}") → PlaceholderResolutionException: Could not resolve placeholder 'nope' in value "${nope}"
  ${nope:默认} → 默认;${who} → spring;嵌套默认值 greeting → hello spring
  嵌套占位符:url=${url.${region}},region=cn → ${url} = https://cn.example

  nopspc:@Value("${missing.key}") = [${missing.key}];带默认值的 = [默认值];@Value("${app.name}") = [from app.properties];SpEL #{systemProperties['user.language']} = [en]
  pspc:启动失败 PlaceholderResolutionException: Could not resolve placeholder 'missing.key' in value "${missing.key}"

  字符串 → Boolean:{true=true, yes=true, on=true, 1=true, false=false, no=false, off=false, 0=false, TRUE=true, abc=ConversionFailedException}
  port 作为 Integer = 8080;不存在的键 getProperty("x", Integer.class) = null;带默认值 = 9090;getRequiredProperty 不存在的键 → IllegalStateException: Required key 'x' not found

  default:title = [中文标题] 字符数 12(期望“中文标题”4 个字符)
  utf8:title = [中文标题] 字符数 4(期望“中文标题”4 个字符)

  ignore:启动成功
  noignore:FileNotFoundException: class path resource [nope.properties] cannot be opened because it does not exist