英文:
How does Spring interpolate ${x} inside a string?
问题
我有一个在Spring项目中的Java类，看起来像这样：
@Component
public class X {
private static final ApplicationContext CTX = new FileSystemXmlApplicationContext("file:${PATH}/ApplicationContext.xml");
...
}
我正在寻找关于如何在字符串参数中插值${PATH}的参考文档。PATH 作为系统属性传递（例如 java -DPATH=...），所以我认为它是从那里获取的，但我找不到描述这个机制的解释。它是否是与Spring相关的功能，类似于@Value中使用的语法？
英文:
I have a java class in a Spring project that looks (edited) like:
@Component
public class X
{
private static final ApplicationContext CTX = new FileSystemXmlApplicationContext("file:${PATH}/ApplicationContext.xml");
...
I am looking for the reference explaining how the ${PATH} is interpolated in the string parameter. The PATH is passed as a system property ( java -DPATH=...) so I assume it takes it from there but I can't find an explanation describing the mechanism. Is it a Spring related feature similar to the syntax used in @Value?
答案1
得分: 2
configLocations（类型为String）传递给FileSystemXmlApplicationContext构造函数之一，将由从AbstractRefreshableConfigApplicationContext类继承的resolvePath()方法进行处理。
resolvePath()文档说明：
> 解析给定的路径，如果需要，用相应的环境属性值替换占位符。应用于配置位置。
>
> 另请参阅：
> PropertyResolver.resolveRequiredPlaceholders(String)
resolveRequiredPlaceholders()文档说明：
> 解析给定文本中的${...}占位符，将其替换为由getProperty(java.lang.String)解析的相应属性值。无默认值的无法解析的占位符将被忽略并保持不变。
声明getProperty()方法的PropertyResolver实际上是一个StandardEnvironment。
StandardEnvironment文档说明：
> Environment实现，适用于“标准”（即非Web）应用程序。
>
> 除了ConfigurableEnvironment的常规功能，如属性解析和与配置文件相关的操作外，此实现还配置了两个默认的属性源，按以下顺序进行搜索：
>
> - 系统属性
> - 系统环境变量
英文:
configLocations (type String) passed to one of the FileSystemXmlApplicationContext constructors are processed by the resolvePath() method inherited from the AbstractRefreshableConfigApplicationContext class.
resolvePath() documentation says:
> Resolve the given path, replacing placeholders with corresponding environment property values if necessary. Applied to config locations.
>
> See Also:
> PropertyResolver.resolveRequiredPlaceholders(String)
resolveRequiredPlaceholders() documentation says:
> Resolve ${...} placeholders in the given text, replacing them with corresponding property values as resolved by getProperty(java.lang.String). Unresolvable placeholders with no default value are ignored and passed through unchanged.
The PropertyResolver declaring that getProperty() method is actually a StandardEnvironment.
StandardEnvironment documentation says:
> Environment implementation suitable for use in 'standard' (i.e. non-web) applications.
>
> In addition to the usual functions of a ConfigurableEnvironment such as property resolution and profile-related operations, this implementation configures two default property sources, to be searched in the following order:
>
> - system properties
> - system environment variables
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