Spring读取系统属性的方法就是先读system.getProperty() 然后再读 System.getenv()

这是整个调用的Stacktrace

Thread [main] (Suspended)	
	owns: Object  (id=22)	
	PropertyPlaceholderConfigurer.resolveSystemProperty(String) line: 358	
	PropertyPlaceholderConfigurer.resolvePlaceholder(String, Properties, int) line: 324	
	PropertyPlaceholderConfigurer$PropertyPlaceholderConfigurerResolver.resolvePlaceholder(String) line: 418	
	PropertyPlaceholderHelper.parseStringValue(String, PlaceholderResolver, Set<String>) line: 146	
	PropertyPlaceholderHelper.replacePlaceholders(String, PropertyPlaceholderHelper$PlaceholderResolver) line: 125	
	PropertyPlaceholderConfigurer$PlaceholderResolvingStringValueResolver.resolveStringValue(String) line: 403	
	BeanDefinitionVisitor.resolveStringValue(String) line: 281	
	BeanDefinitionVisitor.resolveValue(Object) line: 202	
	BeanDefinitionVisitor.visitPropertyValues(MutablePropertyValues) line: 140	
	BeanDefinitionVisitor.visitBeanDefinition(BeanDefinition) line: 81	
	PropertyPlaceholderConfigurer.processProperties(ConfigurableListableBeanFactory, Properties) line: 284	
	PropertyPlaceholderConfigurer(PropertyResourceConfigurer).postProcessBeanFactory(ConfigurableListableBeanFactory) line: 75	
	FileSystemXmlApplicationContext(AbstractApplicationContext).invokeBeanFactoryPostProcessors(Collection<BeanFactoryPostProcessor>, ConfigurableListableBeanFactory) line: 663	
	FileSystemXmlApplicationContext(AbstractApplicationContext).invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory) line: 638	
	FileSystemXmlApplicationContext(AbstractApplicationContext).refresh() line: 407	
	FileSystemXmlApplicationContext.<init>(String[], boolean, ApplicationContext) line: 140	
	FileSystemXmlApplicationContext.<init>(String) line: 84	
	Main.main(String[]) line: 14	
	

这是方法内部， 一清二楚啊。。

	/**
	 * Resolve the given key as JVM system property, and optionally also as
	 * system environment variable if no matching system property has been found.
	 * @param key the placeholder to resolve as system property key
	 * @return the system property value, or <code>null</code> if not found
	 * @see #setSearchSystemEnvironment
	 * @see java.lang.System#getProperty(String)
	 * @see java.lang.System#getenv(String)
	 */
	protected String resolveSystemProperty(String key) {
		try {
			String value = System.getProperty(key);
			if (value == null && this.searchSystemEnvironment) {
				value = System.getenv(key);
			}
			return value;
		}
		catch (Throwable ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Could not access system property '" + key + "': " + ex);
			}
			return null;
		}
	}