为什么80%的码农都做不了架构师？>>> 
spring boot 如何加载META-INF/spring.factories，依赖spring-boot版本为[2.0.2.RELEASE]
通过main主入口，加载SpringApplication，源码如下：
public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
this.webApplicationType = deduceWebApplicationType();
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
this.mainApplicationClass = deduceMainApplicationClass();
}
通过上面，可以提取以下几点信息：
- 应用类型 deduceWebApplicationType 识别
- setInitializers 初始化
- setListeners 初始化
- deduceMainApplicationClass 主入口识别
由于本次主要介绍META-INF/spring.factories解析，下面任意将逐步进行阅读
- 第一步
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
- 第二步
private Collection getSpringFactoriesInstances(Class type,
Class[] parameterTypes, Object... args) {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
// Use names and ensure unique to protect against duplicates
Set names = new LinkedHashSet<>(
SpringFactoriesLoader.loadFactoryNames(type, classLoader));
List instances = createSpringFactoriesInstances(type, parameterTypes,
classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
- 第三步重点解析，根据传入Class类型进行全包扫描是否包含 META-INF/spring.factories文件
Set names = new LinkedHashSet<>(
SpringFactoriesLoader.loadFactoryNames(type, classLoader));
private static Map> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap result = cache.get(classLoader);
if (result != null) {
return result;
}
try {
Enumeration urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
result = new LinkedMultiValueMap<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry entry : properties.entrySet()) {
List factoryClassNames = Arrays.asList(
StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
result.addAll((String) entry.getKey(), factoryClassNames);
}
}
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
第四步，你可以选择debug，假如当前pom中只依赖spring-boot-starter，展开依赖包你将看到配置META-INF/spring.factories 目录的jar包有spring-boot、spring-boot-autoconfigure、spring-bean 等三个项目包，
org.springframework.boot
spring-boot-starter
第五步，以[ApplicationContextInitializer] 为例，你将看到分别在spring-boot、spring-boot-autoconfigure中发现
# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer
# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener
第六步，最终你将看到names set集合中包含了上面6个实现类，在下面具体代码中进行实例化和排序
private Collection getSpringFactoriesInstances(Class type,
Class[] parameterTypes, Object... args) {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
// Use names and ensure unique to protect against duplicates
Set names = new LinkedHashSet<>(
SpringFactoriesLoader.loadFactoryNames(type, classLoader));
List instances = createSpringFactoriesInstances(type, parameterTypes,
classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}