一、定义NACOS的地址环境变量配置。

SPRING_CLOUD_NACOS_CONFIG_SERVER-ADDR=127.0.0.2:8848

bootStrap的配置

 最终发现，环境变量会覆盖配置文件 的配置。

@ConfigurationProperties(NacosConfigProperties.PREFIX)
public class NacosConfigProperties {

	/**
	 * Prefix of {@link NacosConfigProperties}.
	 */
	public static final String PREFIX = "spring.cloud.nacos.config";

	/**
	 * COMMAS , .
	 */
	public static final String COMMAS = ",";

	/**
	 * SEPARATOR , .
	 */
	public static final String SEPARATOR = "[,]";

	private static final Pattern PATTERN = Pattern.compile("-(\\w)");

	private static final Logger log = LoggerFactory
			.getLogger(NacosConfigProperties.class);

	@Autowired
	@JsonIgnore
	private Environment environment;

	@PostConstruct
	public void init() {
		this.overrideFromEnv();
	}

	private void overrideFromEnv() {
		if (StringUtils.isEmpty(this.getServerAddr())) {
			String serverAddr = environment
					.resolvePlaceholders("${spring.cloud.nacos.config.server-addr:}");
			if (StringUtils.isEmpty(serverAddr)) {
				serverAddr = environment.resolvePlaceholders(
						"${spring.cloud.nacos.server-addr:localhost:8848}");
			}
			this.setServerAddr(serverAddr);
		}
		if (StringUtils.isEmpty(this.getUsername())) {
			this.setUsername(
					environment.resolvePlaceholders("${spring.cloud.nacos.username:}"));
		}
		if (StringUtils.isEmpty(this.getPassword())) {
			this.setPassword(
					environment.resolvePlaceholders("${spring.cloud.nacos.password:}"));
		}
	}

	/**
	 * nacos config server address.
	 */
	private String serverAddr;

	/**
	 * the nacos authentication username.
	 */
	private String username;

	/**
	 * the nacos authentication password.
	 */
	private String password;

	/**
	 * encode for nacos config content.
	 */
	private String encode;

	/**
	 * nacos config group, group is config data meta info.
	 */
	private String group = "DEFAULT_GROUP";

	/**
	 * nacos config dataId prefix.
	 */
	private String prefix;

	/**
	 * the suffix of nacos config dataId, also the file extension of config content.
	 */
	private String fileExtension = "properties";

	/**
	 * timeout for get config from nacos.
	 */
	private int timeout = 3000;

	/**
	 * nacos maximum number of tolerable server reconnection errors.
	 */
	private String maxRetry;

	/**
	 * nacos get config long poll timeout.
	 */
	private String configLongPollTimeout;

	/**
	 * nacos get config failure retry time.
	 */
	private String configRetryTime;

	/**
	 * If you want to pull it yourself when the program starts to get the configuration
	 * for the first time, and the registered Listener is used for future configuration
	 * updates, you can keep the original code unchanged, just add the system parameter:
	 * enableRemoteSyncConfig = "true" ( But there is network overhead); therefore we
	 * recommend that you use {@link ConfigService#getConfigAndSignListener} directly.
	 */
	private boolean enableRemoteSyncConfig = false;

	/**
	 * endpoint for Nacos, the domain name of a service, through which the server address
	 * can be dynamically obtained.
	 */
	private String endpoint;

	/**
	 * namespace, separation configuration of different environments.
	 */
	private String namespace;

	/**
	 * access key for namespace.
	 */
	private String accessKey;

	/**
	 * secret key for namespace.
	 */
	private String secretKey;

	/**
	 * context path for nacos config server.
	 */
	private String contextPath;

	/**
	 * nacos config cluster name.
	 */
	private String clusterName;

	/**
	 * nacos config dataId name.
	 */
	private String name;

	/**
	 * a set of shared configurations .e.g:
	 * spring.cloud.nacos.config.shared-configs[0]=xxx .
	 */
	private List<Config> sharedConfigs;

	/**
	 * a set of extensional configurations .e.g:
	 * spring.cloud.nacos.config.extension-configs[0]=xxx .
	 */
	private List<Config> extensionConfigs;

	/**
	 * the master switch for refresh configuration, it default opened(true).
	 */
	private boolean refreshEnabled = true;

	// todo sts support

	}

}

二、调试流程

 1.最终会从org.springframework.boot.context.properties.bind.Binder,是这个类的findProperty方法，会将一个配置类BEAN的每个属性都会循环寻找环境变量，配置文件等，是否能找到合适的属性定义，然后进行属性注入。

private <T> Object bindObject(ConfigurationPropertyName name, Bindable<T> target, BindHandler handler,
			Context context, boolean allowRecursiveBinding) {
		ConfigurationProperty property = findProperty(name, context);
		if (property == null && context.depth != 0 && containsNoDescendantOf(context.getSources(), name)) {
			return null;
		}
		AggregateBinder<?> aggregateBinder = getAggregateBinder(target, context);
		if (aggregateBinder != null) {
			return bindAggregate(name, target, handler, context, aggregateBinder);
		}
		if (property != null) {
			try {
				return bindProperty(target, context, property);
			}
			catch (ConverterNotFoundException ex) {
				// We might still be able to bind it using the recursive binders
				Object instance = bindDataObject(name, target, handler, context, allowRecursiveBinding);
				if (instance != null) {
					return instance;
				}
				throw ex;
			}
		}
		return bindDataObject(name, target, handler, context, allowRecursiveBinding);
	}

从下面可以看到，环境变量的配置覆盖了bootStrap的配置。

2.具体读取属性配置的实现类org.springframework.boot.context.properties.source.

SpringIterableConfigurationPropertySource

这里将.换成_ ，最终将读取

SystemEnvironmentPropertySource.SPRING_CLOUD_NACOS_CONFIG_SERVER-ADDR

3. 

4.最终读取到这个值，并绑定到配置对象的属性中。

 5.由于环境变量的优先级高于配置文件，所以会优先读取环境变量的配置，然后读取完就返回了，不会再去配置文件属性列表中寻找。