SpringBoot源码解析（1.2）-自动装配原理（1）

启动类如下，在启动类中调用 org.springframework.boot.SpringApplication 类的静态run方法

@SpringBootApplication
public class SpringbootDemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringbootDemoApplication.class, args);
    }

}

如下是 SpringApplication 静态 run方法 有多个重载方法，在run方法中创建了一个SpringApplication 实例，并调用实例的run方法，如下：

public static ConfigurableApplicationContext run(Class primarySource, String... args) {
        return run(new Class[]{primarySource}, args);
    }

public static ConfigurableApplicationContext run(Class[] primarySources, String[] args) {
        return (new SpringApplication(primarySources)).run(args);
    }

public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {
        this.sources = new linkedHashSet();
        this.bannerMode = Mode.CONSOLE;
        this.logStartupInfo = true;
        this.addCommandLineProperties = true;
        this.addConversionService = true;
        this.headless = true;
        this.registerShutdownHook = true;
        this.additionalProfiles = Collections.emptySet();
        this.isCustomEnvironment = false;
        this.lazyInitialization = false;
        this.applicationContextFactory = ApplicationContextFactory.DEFAULT;
        this.applicationStartup = ApplicationStartup.DEFAULT;
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        this.primarySources = new linkedHashSet(Arrays.asList(primarySources));
        //根据引入的依赖判断当前应用类型
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
        //初始化RegistryInitializers（注册初始化器），getSpringFactoriesInstances方法在很多地方都用到需要重点关注
        this.bootstrapRegistryInitializers = new ArrayList(this.getSpringFactoriesInstances(BootstrapRegistryInitializer.class));
        this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
        //初始化监听器
        this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
        this.mainApplicationClass = this.deduceMainApplicationClass();
    }

deduceFromClasspath 方法：
deduceFromClasspath 方法内部通过使用 Class.forName 加载类的方式判断对应的类型，不同类型与Class对象对应关系如下：

REACTIVE：存在 org.springframework.web.reactive.DispatcherHandler  Class对象
NONE： （如果不存在右边其中一个类则为none） javax.servlet.Servlet, org.springframework.web.context.ConfigurableWebApplicationContext
SERVLET： 不是以上两种类型则为 SERVLET

具体逻辑可以看 org.springframework.boot.WebApplicationType

getSpringFactoriesInstances 方法：

private  Collection getSpringFactoriesInstances(Class type, Class[] parameterTypes, Object... args) {
        ClassLoader classLoader = this.getClassLoader();
        //loadFactoryNames从配置文件中根据factoryTypeName加载类名称集合
        Set names = new linkedHashSet(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        List instances = this.createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }

org.springframework.core.io.support.SpringFactoriesLoader.loadFactoryNames 方法：

根据类类型获取配置类的名称集合，例如： org.springframework.context.ApplicationContextInitializer 作为key获取：

# 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.rsocket.context.RSocketPortInfoApplicationContextInitializer,
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

public static List loadFactoryNames(Class factoryType, @Nullable ClassLoader classLoader) {
        ClassLoader classLoaderToUse = classLoader;
        if (classLoader == null) {
            classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
        }

        String factoryTypeName = factoryType.getName();
        //核心方法为 loadSpringFactories
        return (List)loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
    }

org.springframework.core.io.support.SpringFactoriesLoader.loadSpringFactories 方法：

private static Map> loadSpringFactories(ClassLoader classLoader) {
        Map> result = (Map)cache.get(classLoader);
        if (result != null) {
            return result;
        } else {
            HashMap result = new HashMap();

            try {
                //读取配置
                Enumeration urls = classLoader.getResources("meta-INF/spring.factories");

                while(urls.hasMoreElements()) {
                    URL url = (URL)urls.nextElement();
                    UrlResource resource = new UrlResource(url);
                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                    Iterator var6 = properties.entrySet().iterator();

                    while(var6.hasNext()) {
                        Entry entry = (Entry)var6.next();
                        String factoryTypeName = ((String)entry.getKey()).trim();
                        String[] factoryImplementationNames = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                        String[] var10 = factoryImplementationNames;
                        int var11 = factoryImplementationNames.length;

                        for(int var12 = 0; var12 < var11; ++var12) {
                            String factoryImplementationName = var10[var12];
                            ((List)result.computeIfAbsent(factoryTypeName, (key) -> {
                                return new ArrayList();
                            })).add(factoryImplementationName.trim());
                        }
                    }
                }

                result.replaceAll((factoryType, implementations) -> {
                    return (List)implementations.stream().distinct().collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList));
                });
                cache.put(classLoader, result);
                return result;
            } catch (IOException var14) {
                throw new IllegalArgumentException("Unable to load factories from location [meta-INF/spring.factories]", var14);
            }
        }
    }

meta-INF/spring.factories 配置有两处地方，如下：

SpringFactoriesLoader 类将工厂名对应的类名集合存放到cache中，static final Map> cache = new ConcurrentReferenceHashMap();
initializers ，listeners 初始化如下：