一、前言

Springboot源码解析是一件大工程，逐行逐句的去研究代码，会很枯燥，也不容易坚持下去。
我们不追求大而全，而是试着每次去研究一个小知识点，最终聚沙成塔，这就是我们的springboot源码管中窥豹系列。

二、Initializer

上一节我们介绍了Runner,它是在项目加载完成之后执行的
有后就有前，有没有在项目加载之前执行的呢？

我们今天介绍的ApplicationContextInitializer就是在spring的bean加载之前执行的

public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {

	void initialize(C applicationContext);

}

使用很简单，实现ApplicationContextInitializer接口就可以了
它是先于普通bean加载的，所以不能用@Component的方式
具体怎么被springboot加载，往下看，我们分析源码的时候说

三、源码解析

如果对springboot源码一点都不了解的，推荐先看第一节：整体架构

1、获取ApplicationContextInitializer

我们直接先看SpringApplication的构造方法

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {

    ...

    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));

    ...
}

我们先看setInitializers方法，再看里面的getSpringFactoriesInstances方法

public void setInitializers(Collection<? extends ApplicationContextInitializer<?>> initializers) {
    this.initializers = new ArrayList<>(initializers);
}

很简单，把查询的initializers集合赋值到本地变量上
接着看getSpringFactoriesInstances方法，这个initializers集合怎么查的


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) {
    ClassLoader classLoader = getClassLoader();
    // Use names and ensure unique to protect against duplicates
    Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

(1) 获取ApplicationContextInitializer实现类的名称集合
(2) 加载成实例instances
(3) 排序，返回

我们先研究下SpringFactoriesLoader.loadFactoryNames(type, classLoader)这个方法：


public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
    String factoryTypeName = factoryType.getName();
    return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, 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()) {
                String factoryTypeName = ((String) entry.getKey()).trim();
                for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                    result.add(factoryTypeName, factoryImplementationName.trim());
                }
            }
        }
        cache.put(classLoader, result);
        return result;
    }
    catch (IOException ex) {
        throw new IllegalArgumentException("Unable to load factories from location [" +
                FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
}

注意：factoryType就是我们传入的参数 ApplicationContextInitializer.class
这里有个新的map结构：MultiValueMap<String, String>，它和Map<String, List>是一样的
我们先看一下这里：classLoader.getResources(FACTORIES_RESOURCE_LOCATION)

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

加载所有的META-INF/spring.factories，按接口名称放入MultiValueMap<String, String>，并cache
注意，这类文件不止一个，它们分布在不同的jar包里面
这么说你可能不懂，我们看一下这类文件长什么样，我们看一个系统自带的

# PropertySource Loaders
org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader

# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

# Error Reporters
org.springframework.boot.SpringBootExceptionReporter=\
org.springframework.boot.diagnostics.FailureAnalyzers

# 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

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\
org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor,\
org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor

# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanDefinitionOverrideFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoSuchMethodFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer

# FailureAnalysisReporters
org.springframework.boot.diagnostics.FailureAnalysisReporter=\
org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter

接口 = 实现类1，实现类2，实现类3
我们假如有了自己的ApplicationContextInitializer实现类，我们在resource下面新建/META-INF/spring.factories文件，按上面的格式写上就可以被加载了

org.springframework.context.ApplicationContextInitializer=\
org.my.zb.MyApplicationContextInitializer

我们把思维拉回去，讲完了怎么取的实现类名称集合
回去看 createSpringFactoriesInstances();

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;
}

(1) 获取Class
(2) 获取构造函数
(3) 利用反射新建instance对象
(4) 加入集合

至此，我们就得到了：List instances

2、执行ApplicationContextInitializer

我们看SpringApplication的run方法：

public ConfigurableApplicationContext run(String... args) {

    ...

    try {

        ...

        prepareContext(context, environment, listeners, applicationArguments, printedBanner);

        ...

    }
    catch (Throwable ex) {
        ...

    }

    ...

    return context;
}

进入到prepareContext方法：

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,
        SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {

    ...

    applyInitializers(context);

    ...

}

定位到了applyInitializers（）：

protected void applyInitializers(ConfigurableApplicationContext context) {
    for (ApplicationContextInitializer initializer : getInitializers()) {
        Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(initializer.getClass(),
                ApplicationContextInitializer.class);
        Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
        initializer.initialize(context);
    }
}

前两行判断类型
最后一行回调执行

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springboot源码解析-管中窥豹系列之Initializer（四）

一、前言

二、Initializer

三、源码解析

1、获取ApplicationContextInitializer

2、执行ApplicationContextInitializer