簡單的java程序 dde prope project als 建立連接 boot property var

不論是一個簡單的Java程序或者是基於Spring或者Spring Boot框架實現的程序，都存在外部化配置信息的需求，例如一個抽獎程序需要制定隨機器的種子值，或者與數據庫建立連接的url/username/password，這些配置信息你都不希望直接固定寫入程序中，因此需要一種方式能夠外部化制定這些配置信息，在代碼的對這些數據的使用點讀取對應的配置來實現動態化程序的行為。

1. 環境變量(System.getEnv)

Environment variables are set in the OS, e.g. in Linux export HOME=/Users/myusername or on Windows SET WINDIR=C:\Windows

etc, and, unlike properties, may not be set at runtime.
To get a specific environment variable you can use System.getenv(String name).

2. Java系統屬性(System.getProperties)

System properties are set on the Java command line using the -Dpropertyname=value syntax. They can also be added at runtime using System.setProperty(String key, String value)

or via the various System.getProperties().load() methods.
To get a specific system property you can use System.getProperty(String key) or System.getProperty(String key, String def). As it happens, Java chose to expose OS variables as properties (just as Java exposes current directory and "other stuff" as properties)

Different from Environment Variables: https://www.baeldung.com/java-system-get-property-vs-system-getenv

3. spring的外部化配置策略

spring.config.location, 只對spring boot有效，查找配置文件的詳細地址。

Property and Enviroment

application.properties/.yml

Common sub project configuration

above spring 3.1 and below spring 3.1 , propertysourceplaceholder propertyplaceholder <context: place-holder> @PropertySource

Spring/Spring Boot的外部化配置