众所周知，spring配置属性源有四个：命令行参数、Java系统属性、操作系统环境变量、配置文件，现在看下spring如何获取到这四个属性源。
private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,ApplicationArguments applicationArguments) {//创建环境接口ConfigurableEnvironment environment = getOrCreateEnvironment();//封装命令行参数configureEnvironment(environment, applicationArguments.getSourceArgs());ConfigurationPropertySources.attach(environment);//发布环境准备好事件listeners.environmentPrepared(environment);bindToSpringApplication(environment);if (!this.isCustomEnvironment) {environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,deduceEnvironmentClass());}ConfigurationPropertySources.attach(environment);return environment;}创建的ConfigurableEnvironment接口默认为StandardServletEnvironment,所有配置均保存在private final MutablePropertySources propertySources = new MutablePropertySources();，StandardServletEnvironment继承的父类AbstractEnvironment构造方法中会添加Java系统属性、操作系统环境变量到内置属性源，此时会将Java系统属性和操作系统环境变量添加到属性源MutablePropertySources。
public AbstractEnvironment() {customizePropertySources(this.propertySources);}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()));}再就是封装命令行参数到属性源，使用SimpleCommandLinePropertySource进行封装后添加进来，参数格式为--开头则被解析成key、value格式保存下来，不然直接保存。
protected void configurePropertySources(ConfigurableEnvironment environment, String[] args) {MutablePropertySources sources = environment.getPropertySources();if (this.defaultProperties != null && !this.defaultProperties.isEmpty()) {sources.addLast(new MapPropertySource("defaultProperties", this.defaultProperties));}if (this.addCommandLineProperties && args.length > 0) {String name = CommandLinePropertySource.COMMAND_LINE_PROPERTY_SOURCE_NAME;if (sources.contains(name)) {PropertySource<?> source = sources.get(name);CompositePropertySource composite = new CompositePropertySource(name);composite.addPropertySource(new SimpleCommandLinePropertySource("springApplicationCommandLineArgs", args));composite.addPropertySource(source);sources.replace(name, composite);}else {sources.addFirst(new SimpleCommandLinePropertySource(args));}}}public CommandLineArgs parse(String... args) {CommandLineArgs commandLineArgs = new CommandLineArgs();for (String arg : args) {if (arg.startsWith("--")) {String optionText = arg.substring(2);String optionName;String optionValue = null;int indexOfEqualsSign = optionText.indexOf('=');if (indexOfEqualsSign > -1) {optionName = optionText.substring(0, indexOfEqualsSign);optionValue = optionText.substring(indexOfEqualsSign + 1);}else {optionName = optionText;}if (optionName.isEmpty()) {throw new IllegalArgumentException("Invalid argument syntax: " + arg);}commandLineArgs.addOptionArg(optionName, optionValue);}else {commandLineArgs.addNonOptionArg(arg);}}return commandLineArgs;}之后就是读取配置文件，这一步有点复杂，之前已经讲过spring会读取事件监听器，现在发布ApplicationEnvironmentPreparedEvent后，会找到支持的ConfigFileApplicationListener读取配置文件，现在看下这个过程。ConfigFileApplicationListener会从spring.factories找到所有EnvironmentPostProcessor处理配置，我们只关心读取配置文件的EnvironmentPostProcessor即ConfigFileApplicationListener自身即可。
private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {List<EnvironmentPostProcessor> postProcessors = loadPostProcessors();postProcessors.add(this);AnnotationAwareOrderComparator.sort(postProcessors);for (EnvironmentPostProcessor postProcessor : postProcessors) {postProcessor.postProcessEnvironment(event.getEnvironment(), event.getSpringApplication());}}//加载配置文件void load() {FilteredPropertySource.apply(this.environment, DEFAULT_PROPERTIES, LOAD_FILTERED_PROPERTY,(defaultProperties) -> {this.profiles = new LinkedList<>();this.processedProfiles = new LinkedList<>();this.activatedProfiles = false;this.loaded = new LinkedHashMap<>();initializeProfiles();while (!this.profiles.isEmpty()) {Profile profile = this.profiles.poll();if (isDefaultProfile(profile)) {addProfileToEnvironment(profile.getName());}load(profile, this::getPositiveProfileFilter,addToLoaded(MutablePropertySources::addLast, false));this.processedProfiles.add(profile);}load(null, this::getNegativeProfileFilter, addToLoaded(MutablePropertySources::addFirst, true));addLoadedPropertySources();applyActiveProfiles(defaultProperties);});}可以看到共分成两部，首先读取环境配置，再根据环境配置。读取环境配置时，首先添加null，意思是没有任何环境后缀的配置文件，然后从已有的属性源即Java属性和环境变量中获取spring.profiles.active和spring.profiles.include的配置，如果没有的话就是default。
private void initializeProfiles() {// The default profile for these purposes is represented as null. We add it// first so that it is processed first and has lowest priority.this.profiles.add(null);Binder binder = Binder.get(this.environment);Set<Profile> activatedViaProperty = getProfiles(binder, ACTIVE_PROFILES_PROPERTY);Set<Profile> includedViaProperty = getProfiles(binder, INCLUDE_PROFILES_PROPERTY);List<Profile> otherActiveProfiles = getOtherActiveProfiles(activatedViaProperty, includedViaProperty);this.profiles.addAll(otherActiveProfiles);// Any pre-existing active profiles set via property sources (e.g.// System properties) take precedence over those added in config files.this.profiles.addAll(includedViaProperty);addActiveProfiles(activatedViaProperty);if (this.profiles.size() == 1) { // only has null profilefor (String defaultProfileName : this.environment.getDefaultProfiles()) {Profile defaultProfile = new Profile(defaultProfileName, true);this.profiles.add(defaultProfile);}}}读取到环境配置后，要加载配置文件了，此时从保存环境Profile的队列中取出来，再进行加载，可以看到null的环境配置加载了两次，由于存在spring.config.activate.on-profile=dev的配置项，所以第一次加载时无法处理，需要在加载完读取到环境配置时再进行处理。
void load() {FilteredPropertySource.apply(this.environment, DEFAULT_PROPERTIES, LOAD_FILTERED_PROPERTY,(defaultProperties) -> {//待处理的环境配置this.profiles = new LinkedList<>();//标记处理过的环境配置this.processedProfiles = new LinkedList<>();this.activatedProfiles = false;//已完成加载的配置this.loaded = new LinkedHashMap<>();initializeProfiles();while (!this.profiles.isEmpty()) {Profile profile = this.profiles.poll();if (isDefaultProfile(profile)) {addProfileToEnvironment(profile.getName());}load(profile, this::getPositiveProfileFilter,addToLoaded(MutablePropertySources::addLast, false));this.processedProfiles.add(profile);}load(null, this::getNegativeProfileFilter, addToLoaded(MutablePropertySources::addFirst, true));addLoadedPropertySources();applyActiveProfiles(defaultProperties);});}默认读取配置文件的路径为private static final String DEFAULT_SEARCH_LOCATIONS = "classpath:/,classpath:/config/,file:./,file:./config/*/,file:./config/";，现在spring会到这些路径下根据环境读取配置文件。
private void load(Profile profile, DocumentFilterFactory filterFactory, DocumentConsumer consumer) {getSearchLocations().forEach((location) -> {boolean isDirectory = location.endsWith("/");Set<String> names = isDirectory ? getSearchNames() : NO_SEARCH_NAMES;names.forEach((name) -> load(location, name, profile, filterFactory, consumer));});}PropertySourceLoader读取器是由spring.factories文件配置的，默认有两个：PropertiesPropertySourceLoader、YamlPropertySourceLoader。
private void load(String location, String name, Profile profile, DocumentFilterFactory filterFactory,DocumentConsumer consumer) {if (!StringUtils.hasText(name)) {for (PropertySourceLoader loader : this.propertySourceLoaders) {if (canLoadFileExtension(loader, location)) {load(loader, location, profile, filterFactory.getDocumentFilter(profile), consumer);return;}}throw new IllegalStateException("File extension of config file location '" + location+ "' is not known to any PropertySourceLoader. If the location is meant to reference "+ "a directory, it must end in '/'");}Set<String> processed = new HashSet<>();for (PropertySourceLoader loader : this.propertySourceLoaders) {for (String fileExtension : loader.getFileExtensions()) {if (processed.add(fileExtension)) {loadForFileExtension(loader, location + name, "." + fileExtension, profile, filterFactory,consumer);}}}}现在根据配置的环境拼接成配置文件路径后，就开始加载配置文件，过程不复杂，检查文件是否存在，存在的话就loadDocuments。
private void load(PropertySourceLoader loader, String location, Profile profile, DocumentFilter filter,DocumentConsumer consumer) {Resource[] resources = getResources(location);for (Resource resource : resources) {try {if (resource == null || !resource.exists()) {if (this.logger.isTraceEnabled()) {StringBuilder description = getDescription("Skipped missing config ", location, resource,profile);this.logger.trace(description);}continue;}if (!StringUtils.hasText(StringUtils.getFilenameExtension(resource.getFilename()))) {if (this.logger.isTraceEnabled()) {StringBuilder description = getDescription("Skipped empty config extension ", location,resource, profile);this.logger.trace(description);}continue;}String name = "applicationConfig: [" + getLocationName(location, resource) + "]";List<Document> documents = loadDocuments(loader, name, resource);if (CollectionUtils.isEmpty(documents)) {if (this.logger.isTraceEnabled()) {StringBuilder description = getDescription("Skipped unloaded config ", location, resource,profile);this.logger.trace(description);}continue;}List<Document> loaded = new ArrayList<>();for (Document document : documents) {if (filter.match(document)) {addActiveProfiles(document.getActiveProfiles());addIncludedProfiles(document.getIncludeProfiles());loaded.add(document);}}Collections.reverse(loaded);if (!loaded.isEmpty()) {loaded.forEach((document) -> consumer.accept(profile, document));if (this.logger.isDebugEnabled()) {StringBuilder description = getDescription("Loaded config file ", location, resource,profile);this.logger.debug(description);}}}catch (Exception ex) {StringBuilder description = getDescription("Failed to load property source from ", location,resource, profile);throw new IllegalStateException(description.toString(), ex);}}}还是缓存的思想，优化从缓存中获取，没有再load()。
private List<Document> loadDocuments(PropertySourceLoader loader, String name, Resource resource)throws IOException {DocumentsCacheKey cacheKey = new DocumentsCacheKey(loader, resource);List<Document> documents = this.loadDocumentsCache.get(cacheKey);if (documents == null) {List<PropertySource<?>> loaded = loader.load(name, resource);documents = asDocuments(loaded);this.loadDocumentsCache.put(cacheKey, documents);}return documents;}一路看到底吧，以properties文件为例，读取时其实就是字符串处理，读取key时读取到'='或':'为止，读取value时读取到空格符、制表符、换行符为止，然后读取完配置文件中所有配置，至此读取完配置文件，读取完用OriginTrackedMapPropertySource封装。
Map<String, OriginTrackedValue> load(boolean expandLists) throws IOException {try (CharacterReader reader = new CharacterReader(this.resource)) {Map<String, OriginTrackedValue> result = new LinkedHashMap<>();StringBuilder buffer = new StringBuilder();while (reader.read()) {String key = loadKey(buffer, reader).trim();if (expandLists && key.endsWith("[]")) {key = key.substring(0, key.length() - 2);int index = 0;do {OriginTrackedValue value = loadValue(buffer, reader, true);put(result, key + "[" + (index++) + "]", value);if (!reader.isEndOfLine()) {reader.read();}}while (!reader.isEndOfLine());}else {OriginTrackedValue value = loadValue(buffer, reader, false);put(result, key, value);}}return result;}}//读取keyprivate String loadKey(StringBuilder buffer, CharacterReader reader) throws IOException {buffer.setLength(0);boolean previousWhitespace = false;while (!reader.isEndOfLine()) {if (reader.isPropertyDelimiter()) {reader.read();return buffer.toString();}if (!reader.isWhiteSpace() && previousWhitespace) {return buffer.toString();}previousWhitespace = reader.isWhiteSpace();buffer.append(reader.getCharacter());reader.read();}return buffer.toString();}//读取valueprivate OriginTrackedValue loadValue(StringBuilder buffer, CharacterReader reader, boolean splitLists)throws IOException {buffer.setLength(0);while (reader.isWhiteSpace() && !reader.isEndOfLine()) {reader.read();}Location location = reader.getLocation();while (!reader.isEndOfLine() && !(splitLists && reader.isListDelimiter())) {buffer.append(reader.getCharacter());reader.read();}Origin origin = new TextResourceOrigin(this.resource, location);return OriginTrackedValue.of(buffer.toString(), origin);}接下来要将加载的配置文件和已有的属性源都保存到MutablePropertySources中,可以看到配置文件是放在最后的，所以配置文件的配置优先级最低。
private void addLoadedPropertySources() {MutablePropertySources destination = this.environment.getPropertySources();List<MutablePropertySources> loaded = new ArrayList<>(this.loaded.values());Collections.reverse(loaded);String lastAdded = null;Set<String> added = new HashSet<>();for (MutablePropertySources sources : loaded) {for (PropertySource<?> source : sources) {//每个配置文件只添加一次if (added.add(source.getName())) {addLoadedPropertySource(destination, lastAdded, source);//添加到上个配置文件之后lastAdded = source.getName();}}}}private void addLoadedPropertySource(MutablePropertySources destination, String lastAdded,PropertySource<?> source) {if (lastAdded == null) {if (destination.contains(DEFAULT_PROPERTIES)) {destination.addBefore(DEFAULT_PROPERTIES, source);}else {destination.addLast(source);}}else {destination.addAfter(lastAdded, source);}}总结下，springboot加载配置时会先加载Java属性和操作系统环境变量，再封装命令行参数添加到属性源开头，最后根据环境读取配置文件，优先级依次为命令行参数->Java属性->操作系统环境变量->配置文件。有不对的地方请大神指出，欢迎大家一起讨论交流，共同进步，更多请关注微信公众号 葡萄开源
