Spring PropertyPlaceholderConfigurerResolver

	private final class PropertyPlaceholderConfigurerResolver implements PlaceholderResolver {

		private final Properties props;

		private PropertyPlaceholderConfigurerResolver(Properties props) {
			this.props = props;
		}

		@Override
		@Nullable
		public String resolvePlaceholder(String placeholderName) {
			return PropertyPlaceholderConfigurer.this.resolvePlaceholder(placeholderName,
					this.props, systemPropertiesMode);
		}
	}

	@Nullable
	protected String resolvePlaceholder(String placeholder, Properties props, int systemPropertiesMode) {
		String propVal = null;
		if (systemPropertiesMode == SYSTEM_PROPERTIES_MODE_OVERRIDE) {
			propVal = resolveSystemProperty(placeholder);
		}
		if (propVal == null) {
			propVal = resolvePlaceholder(placeholder, props);
		}
		if (propVal == null && systemPropertiesMode == SYSTEM_PROPERTIES_MODE_FALLBACK) {
			propVal = resolveSystemProperty(placeholder);
		}
		return propVal;
	}

	@Nullable
	protected String resolvePlaceholder(String placeholder, Properties props) {
		return props.getProperty(placeholder);
	}

	@Nullable
	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;
		}
	}