一、前言

• Springboot原始碼解析是一件大工程，逐行逐句的去研究程式碼，會很枯燥，也不容易堅持下去。
• 我們不追求大而全，而是試著每次去研究一個小知識點，最終聚沙成塔，這就是我們的springboot原始碼管中窺豹系列。

二、Initializer

• 上一節我們介紹了Runner,它是在專案載入完成之後執行的
• 有後就有前，有沒有在專案載入之前執行的呢？

我們今天介紹的ApplicationContextInitializer就是在spring的bean載入之前執行的

public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {

	void initialize(C applicationContext);

}
 

• 使用很簡單，實現ApplicationContextInitializer介面就可以了
• 它是先於普通bean載入的，所以不能用@Component的方式
• 具體怎麼被springboot載入，往下看，我們分析原始碼的時候說

三、原始碼解析

如果對springboot原始碼一點都不瞭解的，推薦先看第一節：整體架構

1、獲取ApplicationContextInitializer

我們直接先看SpringApplication的構造方法

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

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

我們先看setInitializers方法，再看裡面的getSpringFactoriesInstances方法

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

• 很簡單，把查詢的initializers集合賦值到本地變數上
• 接著看getSpringFactoriesInstances方法，這個initializers集合怎麼查的

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) 獲取ApplicationContextInitializer實現類的名稱集合
• (2) 載入成例項instances
• (3) 排序，返回

我們先研究下SpringFactoriesLoader.loadFactoryNames(type, classLoader)這個方法：

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就是我們傳入的引數 ApplicationContextInitializer.class
• 這裡有個新的map結構：MultiValueMap<String, String>，它和Map<String, List>是一樣的
• 我們先看一下這裡：classLoader.getResources(FACTORIES_RESOURCE_LOCATION)

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

• 載入所有的META-INF/spring.factories，按介面名稱放入MultiValueMap<String, String>，並cache
• 注意，這類檔案不止一個，它們分佈在不同的jar包裡面
• 這麼說你可能不懂，我們看一下這類檔案長什麼樣，我們看一個系統自帶的

# 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

• 介面 = 實現類1，實現類2，實現類3
• 我們假如有了自己的ApplicationContextInitializer實現類，我們在resource下面新建/META-INF/spring.factories檔案，按上面的格式寫上就可以被載入了

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

• 我們把思維拉回去，講完了怎麼取的實現類名稱集合
• 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) 獲取Class
• (2) 獲取建構函式
• (3) 利用反射新建instance物件
• (4) 加入集合

至此，我們就得到了：List instances

2、執行ApplicationContextInitializer

我們看SpringApplication的run方法：

public ConfigurableApplicationContext run(String... args) {
    
    ...

    try {
        
        ...

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

    }
    catch (Throwable ex) {
        ...

    }
    
    ...

    return context;
}

進入到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);
    }
}

• 前兩行判斷型別
• 最後一行回撥執行

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