[转自] http://unmi.cc/spring-injection-system-properties-env/
在 Spring 中为 javabean 注入属性文件中的属性值一般人都知道的，可以通过 org.springframework.beans.factory.config.PropertyPlaceholderConfigurer 引入一个属性文件，然后给 bean 指定属性的时候就可以用 ${jdbc.url} 方式赋值了。比如在 Spring 中是这样的配置：
classpath:jdbc.properties
只是有时候我们需要给 bean 赋上系统属性(System.getProperties() ) 中的值或环境变量(System.getenv() ) 中的值，根据程序所处的环境产生不同的行为，这样我们无法事先在某个 properties 文件预先设定好值的。
这种需求也是不怕做不到就怕想不到，基于此，既然上面是用 PropertyPlaceholderConfigurer 来读取属性文件，那么有没有像 EnvironmentPlaceholderConfigurer 类似的东西呢，查下 api 没找到相关的，只是看下它有两个子类：
org.springframework.beans.factory.config.PreferencesPlaceholderConfigurer
org.springframework.web.context.support.ServletContextPropertyPlaceholderConfigurer
上面那两个类估计以后也是用得着，前面那个应该是从系统配置或注册表里要属性，后面的可以从 ServletContext 取相关属性值。
上面的思路应该是这样的，但实际上只要仔细阅读下 org.springframework.beans.factory.config.PropertyPlaceholderConfigurer 的 JavaDoc API 就会发现，它不光能从属性文件里取值，也能从系统属性，甚至是环境变量中取值。
Default property values can be defined via "properties", to make overriding definitions in properties files optional. A configurer will also check against system properties (e.g. "user.dir") if it cannot resolve a placeholder with any of the specified properties. This can be customized via "systemPropertiesMode".
再往下看：
public void setSystemPropertiesMode(int systemPropertiesMode)
Set how to check system properties: as fallback, as override, or never. For example, will resolve ${user.dir} to the "user.dir" system property.
The default is "fallback": If not being able to resolve a placeholder with the specified properties, a system property will be tried. "override" will check for a system property first, before trying the specified properties. "never" will not check system properties at all.---------------------------------------------------------------------
public void setSearchSystemEnvironment(boolean searchSystemEnvironment)
Set whether to search for a matching system environment variable if no matching system property has been found. Only applied when "systemPropertyMode" is active (i.e. "fallback" or "override"), right after checking JVM system properties.
Default is "true". Switch this setting off to never resolve placeholders against system environment variables. Note that it is generally recommended to pass external values in as JVM system properties: This can easily be achieved in a startup script, even for existing environment variables.NOTE: Access to environment variables does not work on the Sun VM 1.4, where the corresponding System.getenv(java.lang.String) support was disabled - before it eventually got re-enabled for the Sun VM 1.5. Please upgrade to 1.5 (or higher) if you intend to rely on the environment variable support.
说明了这个类可以从三个地方取属性值，并且可以设置覆盖关系，覆盖关系在此就不细究了。现在可以例子说明如何从系统属性或环境变量中取值了。演示一下从把系统属性 user.dir 和环境变量 COMPUTERNAME 注入到 javabean。
一. 要被注入系统属性和环境变量的 Bean
package cc.unmi.spring;
/**
* 通过 Spring 来向其中注入系统属性 workDir(${user.dir}, System.getProperty(""))
* 和环境变量 hostName(${COMPUTERNAME}, System.getenv().get("COMPUTERNAME"));
* @author Unmi
*
*/
public class Environments {
private String workDir;
private String hostName;
public String getHostName() {
return hostName;
}
public void setHostName(String hostName) {
this.hostName = hostName;
}
public String getWorkDir() {
return workDir;
}
public void setWorkDir(String workDir) {
this.workDir = workDir;
}
}二. Spring 配置文件
三. 测试客户端代码
package cc.unmi.spring;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import cc.unmi.spring.Environments;
/**
* 测试客户端
* @author Unmi
*/
public class TestClient {
public static void main(String[] args) {
BeanFactory context = new ClassPathXmlApplicationContext("applicationContext.xml");
Environments env = context.getBean(Environments.class);
System.out.println("Working directory: " + env.getWorkDir());
System.out.println("Host name: " + env.getHostName());
}
}四. 执行上面的 TestClient 后控制台下输出的结果是：
Working directory: D:\workspaces\j2ee\TestSpring
Host name: Unmi-Computer
这显示了 Spring 已成功向 Environments 注入了系统属性 user.dir 和环境变量 COMPUTERNAME。
注：你可以用 System.getProperties().list(System.out) 打印出系统属性，还不知道系统中可用哪个命令查得到其中的值：显示出来是这样的：
-- listing properties --
java.runtime.name=Java(TM) SE Runtime Environment
sun.boot.library.path=C:\Program Files\Java\jre6\bin
java.vm.version=17.1-b03
java.vm.vendor=Sun Microsystems Inc.
java.vendor.url=http://java.sun.com/
path.separator=;
java.vm.name=Java HotSpot(TM) Client VM
file.encoding.pkg=sun.io
user.country=US
...................................
环境变量可以用 System.out.print(System.getenv()) 打印出来，对应着 Windows 中的 set 命令或 Linux 下的 env 输出的值，System.getenv() 是个 Map，所以打印出这样子的：
{USERPROFILE=C:\Documents and Settings\Unmi, JAVA_HOME=C:\Program Files\Java\jdk1.6.0_20, TEMP=C:\DOCUME~1\uqiu\LOCALS~1\Temp, OS=Windows_NT, COMPUTERNAME=Unmi-Computer, .......}