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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