04--SpringBoot启动之初始化与扩展点加载机制

了解了SpringBoot启动类型判断之后,接着分析SpringBoot一些内部组件的初始化过程

//初始化Initializer
setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
//初始化Listener
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));

Dubbo中有通过ExtensionLoader来实现扩展点加载的机制,在SpringBoot中也有类似的实现,降低耦合

1. 扩展点加载

//扩展点加载
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
    return getSpringFactoriesInstances(type, new Class<?>[]{});
}

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
    //获取当前线程上下文类加载器
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    //获取到的扩展类名存入set集合防止重复
    Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    //创建扩展点实例
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
    //排序
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

加载扩展点类名
SpringFactoriesLoader.loadFactoryNames(type, classLoader)

//通过给定类加载器获得完全限定类名
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
        //获得factoryClass名称
        String factoryClassName = factoryClass.getName();
        //加载类并返回默认集合
        return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = cache.get(classLoader);
        if (result != null) {
            return result;
        }

        try {
            Enumeration<URL> 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<String> 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);
        }
    }

SpringBoot的扩展点存放在哪里呢,答案就在FACTORIES_RESOURCE_LOCATION静态变量中
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

image.png
classLoader会加载当前工程所有jar包(并非只有spring-boot一个工程)下/META-INF/spring.factories文件,并从中找出要被加载的类,我们来看下ApplicationContextInitializer.class类在/META-INF/spring.factories对应的信息,可以看到.factories文件的存储方式为key:val0,val1,val2...

# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
#用来报告Spring容器的一些常见的错误配置的
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
#获取应用上下文ID
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
#委托给context.initializer.classes环境属性下指定的其他初始值设定项。
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
#ApplicationContextInitializer设置服务器实际侦听Environment的端口的属性EmbeddedServletContainer。属性“local.server.port”可以直接注入使用 @Value或通过Environment。
#如果EmbeddedWebApplicationContext有一个 namespace集合，它将用于构造属性名称。例如，“管理”执行器上下文将具有属性名称“local.management.port”。属性会自动传播到任何父上下文。
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

查看debug堆栈信息

image.png
除了spring-boot工程下的/META-INF/spring.factories的ApplicationContextInitializer被加载之外,还有org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer和org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer被加载,这两个类信息位于spring-boot-autoconfigure工程的/META-INF/spring.factories下

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

至此,setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));所需要的类信息已经全部加载

2.扩展点实例化

//创建扩展点实例
List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);

//实例化扩展点,获取与参数对应的构造方法,并实例化
private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args, Set<String> names) {
    List<T> instances = new ArrayList<>(names.size());
    for (String name : names) {
        try {
            Class<?> instanceClass = ClassUtils.forName(name, classLoader);
            Assert.isAssignable(type, instanceClass);
            Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);
            T instance = (T) BeanUtils.instantiateClass(constructor, args);
            instances.add(instance);
        } catch (Throwable ex) {
            throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);
        }
    }
    return instances;
}

setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));初始化监听器的方法与上述步骤相同
至此,我们已经了解了SpringBoot的一些扩展点是如何加载并实例化的,并且完成了SpringApplication的实例化工作