System Environment 指的是操作系统的环境变量，而 System Properties 指的是java 程序jvm的系统变量

System Environment

// 获取全部的环境变量
Map<String, String> systemEnvironment = System.getenv();
// 获取某个环境变量 比如：PATH
String path = System.getenv("PATH");

System Properties

// 获取所有的环境变量
Properties properties = System.getProperties();
// 获取指定的环境变量 如java.class.path
String property = System.getProperty("java.class.path");

设置jvm的系统变量有两种方式：

通过java -Dkey=value 参数设置，如果value有空格，需要用双引号括起来
在代码里通过System.setProperty(key, value)进行设置

Spring Environment

在Spring通过EnvironmentCapable接口中的**getEnvironment()**方法可以获取当前环境变量。

如上图所示，ApplicationContext实现了EnvironmentCapable接口，具体的实现方法是在AbstractApplicationContext中。

关于Environment接口类图如下图所示：

Environment创建

getEnvironment()

AbstractApplicationContext类

public ConfigurableEnvironment getEnvironment() {
	if (this.environment == null) {
		this.environment = createEnvironment();
	}
	return this.environment;
}

createEnvironment()

AbstractApplicationContext类

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protected ConfigurableEnvironment createEnvironment() {
	return new StandardEnvironment();
}

因为StandardEnvironment继承自AbstractEnvironment，将调用父类的构造方法。

AbstractEnvironment()

AbstractEnvironment类

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public AbstractEnvironment() {
	this(new MutablePropertySources());
}

AbstractEnvironment(MutablePropertySources propertySources)

AbstractEnvironment类

protected AbstractEnvironment(MutablePropertySources propertySources) {
	this.propertySources = propertySources;
	this.propertyResolver = createPropertyResolver(propertySources);
	customizePropertySources(propertySources);
}

customizePropertySources(MutablePropertySources propertySources)

StandardEnvironment类

@Override
protected void customizePropertySources(MutablePropertySources propertySources) {
	propertySources.addLast(
			new PropertiesPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties()));
	propertySources.addLast(
			new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
}

优先级结论

在调用getProperty()方法时从propertySources集合按照添加顺序获取值，可以看出来SYSTEM_PROPERTIES优先级高于SYSTEM_ENVIRONMENT

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System Environment 和 System Properties