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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