@PropertySource属性源注解的原理

我们都知道spring默认读取bootstrap、application的配置文件，如果想让spring读取我们自定义的配置文件呢，例如我们的工程名my.propertie配置文件，其实可以通过@PropertySource注解实现，用起来很简单，通过注解指定配置文件的类路径即可。

java 复制代码

@PropertySource("classpath:my.properties")
@Component
public class PropertySourceTest {
}

会用以后开始看它的原理吧，还是在扫描bean阶段，发现bean上存在PropertySources或PropertySource注解时，就会进入这个注解的处理逻辑。

kotlin 复制代码

		for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), PropertySources.class,
				org.springframework.context.annotation.PropertySource.class)) {
			if (this.environment instanceof ConfigurableEnvironment) {
				processPropertySource(propertySource);
			}
			else {
				logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
						"]. Reason: Environment must implement ConfigurableEnvironment");
			}
		}

先处理@PropertySource注解，获取属性文件的类路径，一般为classpath:my.properties格式，然后读取生成属性源的工厂，用来根据属性文件Resource创建属性源PropertySource，之后再加入sping的环境配置的属性源头，加入的过程可以参加我之前写的一文看懂spring配置原理

java 复制代码

	private void processPropertySource(AnnotationAttributes propertySource) throws IOException {
		String name = propertySource.getString("name");
		if (!StringUtils.hasLength(name)) {
			name = null;
		}
		String encoding = propertySource.getString("encoding");
		if (!StringUtils.hasLength(encoding)) {
			encoding = null;
		}
        //获取属性源的类路径
		String[] locations = propertySource.getStringArray("value");
		Assert.isTrue(locations.length > 0, "At least one @PropertySource(value) location is required");
		boolean ignoreResourceNotFound = propertySource.getBoolean("ignoreResourceNotFound");
        //生成属性源的工厂方法
		Class<? extends PropertySourceFactory> factoryClass = propertySource.getClass("factory");
		PropertySourceFactory factory = (factoryClass == PropertySourceFactory.class ?
				DEFAULT_PROPERTY_SOURCE_FACTORY : BeanUtils.instantiateClass(factoryClass));

		for (String location : locations) {
			try {
                //根据路径加载Resource
				String resolvedLocation = this.environment.resolveRequiredPlaceholders(location);
				Resource resource = this.resourceLoader.getResource(resolvedLocation);
                //添加到环境配置的属性源供spring使用
				addPropertySource(factory.createPropertySource(name, new EncodedResource(resource, encoding)));
			}
			catch (IllegalArgumentException | FileNotFoundException | UnknownHostException | SocketException ex) {
				// Placeholders not resolvable or resource not found when trying to open it
				if (ignoreResourceNotFound) {
					if (logger.isInfoEnabled()) {
						logger.info("Properties location [" + location + "] not resolvable: " + ex.getMessage());
					}
				}
				else {
					throw ex;
				}
			}
		}
	}

如果发现环境中已经存在同名属性源了，就合并之后再保存到spring的环境配置中。否则如果是第一次处理@PropertySource，则添加到最后，没有处理过添加到上次处理过的属性源之前，即保存到配置中的属性源和加载顺序相反的，原因是spring默认读取最后加载的配置文件，即最新的配置文件。

java 复制代码

	private void addPropertySource(PropertySource<?> propertySource) {
		String name = propertySource.getName();
		MutablePropertySources propertySources = ((ConfigurableEnvironment) this.environment).getPropertySources();
        //如果已有同名属性源
		if (this.propertySourceNames.contains(name)) {
			// We've already added a version, we need to extend it
			PropertySource<?> existing = propertySources.get(name);
			if (existing != null) {
				PropertySource<?> newSource = (propertySource instanceof ResourcePropertySource ?
						((ResourcePropertySource) propertySource).withResourceName() : propertySource);
                //同名属性源为CompositePropertySource直接合并
				if (existing instanceof CompositePropertySource) {
					((CompositePropertySource) existing).addFirstPropertySource(newSource);
				}
                //转化成CompositePropertySource再合并
				else {
					if (existing instanceof ResourcePropertySource) {
						existing = ((ResourcePropertySource) existing).withResourceName();
					}
					CompositePropertySource composite = new CompositePropertySource(name);
					composite.addPropertySource(newSource);
					composite.addPropertySource(existing);
					propertySources.replace(name, composite);
				}
				return;
			}
		}
        //之前没有通过`@PropertySource`注解添加属性源
		if (this.propertySourceNames.isEmpty()) {
			propertySources.addLast(propertySource);
		}
        //通过`@PropertySource`注解添加属性源了，就往前放
		else {
			String firstProcessed = this.propertySourceNames.get(this.propertySourceNames.size() - 1);
			propertySources.addBefore(firstProcessed, propertySource);
		}
		this.propertySourceNames.add(name);
	}

总结下，spring处理@PropertySource注解，会根据注解读取配置文件，保存到spring的环境配置中，为了保证能读取到最新的配置，spring会将后加载的属性源放到前面。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