Spring 源码解析十一：Spring 的扩展加载机制
Spring 的扩展加载机制主要有 2 个：自动加载第三方包中的类、扩展 xml 配置文件中 bean 的命名空间
1. 自动加载第三方包中的类
spring-core 提供了一个类似 Java SPI 的的扩展机制，在 META-INF/spring.factories 文件中定义需要自动加载的类，就可以自动实例化其他包指定的类，spring-boot, spring-cloud 都依赖这个机制自动加载资源。
比如 spring-boot 的扩展（一部分）
# Logging Systems
org.springframework.boot.logging.LoggingSystemFactory=\
org.springframework.boot.logging.logback.LogbackLoggingSystem.Factory,\
org.springframework.boot.logging.log4j2.Log4J2LoggingSystem.Factory,\
org.springframework.boot.logging.java.JavaLoggingSystem.Factory
# PropertySource Loaders
org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader
# ConfigData Location Resolvers
org.springframework.boot.context.config.ConfigDataLocationResolver=\
org.springframework.boot.context.config.ConfigTreeConfigDataLocationResolver,\
org.springframework.boot.context.config.StandardConfigDataLocationResolver
# ConfigData Loaders
org.springframework.boot.context.config.ConfigDataLoader=\
org.springframework.boot.context.config.ConfigTreeConfigDataLoader,\
org.springframework.boot.context.config.StandardConfigDataLoader
# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener实现这个功能的类是 SpringFactoriesLoader
public final class SpringFactoriesLoader {
// 自动加载文件地址
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
// 加载spring.factories
public static List loadFactories(Class factoryType, @Nullable ClassLoader classLoader) {
ClassLoader classLoaderToUse = classLoader;
if (classLoaderToUse == null) {
classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
}
// 加载spring.factories中的bean名字
List factoryImplementationNames = loadFactoryNames(factoryType, classLoaderToUse);
// 结果集
List result = new ArrayList<>(factoryImplementationNames.size());
for (String factoryImplementationName : factoryImplementationNames) {
// 初始化bean，并加入到结果集
result.add(instantiateFactory(factoryImplementationName, factoryType, classLoaderToUse));
}
return result;
}
// 加载spring.factories中的bean名字
public static List loadFactoryNames(Class factoryType, @Nullable ClassLoader classLoader) {
ClassLoader classLoaderToUse = classLoader;
if (classLoaderToUse == null) {
classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
}
String factoryTypeName = factoryType.getName();
// 加载spring.factories中的bean定义，并提取出名字
return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
}
// 加载spring.factories中的bean定义
private static Map> loadSpringFactories(ClassLoader classLoader) {
// 如果有缓存，直接返回缓存
Map> result = cache.get(classLoader);
if (result != null) {
return result;
}
// 结果集
result = new HashMap<>();
try {
// 加载所有包下的spring.factories，不光是主包，还有各种依赖包
Enumeration urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
// 遍历读取
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
// 把属性加载出来，以properties文件对待
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry entry : properties.entrySet()) {
// bean名字
String factoryTypeName = ((String) entry.getKey()).trim();
// bean类型，可以用逗号分隔多个
String[] factoryImplementationNames =
StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
// 遍历bean类型
for (String factoryImplementationName : factoryImplementationNames) {
// 加入到结果集
result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
.add(factoryImplementationName.trim());
}
}
}
// 去重
result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
.collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
// 加入缓存
cache.put(classLoader, result);
}
catch (IOException ex) {
// ... 代码省略
}
return result;
}
// 初始化bean
private static T instantiateFactory(String factoryImplementationName, Class factoryType, ClassLoader classLoader) {
try {
// 取出class
Class factoryImplementationClass = ClassUtils.forName(factoryImplementationName, classLoader);
// factoryImplementationClass不是factoryType的子类
if (!factoryType.isAssignableFrom(factoryImplementationClass)) {
// 报错
}
// 调用newInstance实例化
return (T) ReflectionUtils.accessibleConstructor(factoryImplementationClass).newInstance();
}
catch (Throwable ex) {
// ... 代码省略
}
}
} 然后在 CachedIntrospectionResults
静态加载
public final class CachedIntrospectionResults {
private static final List beanInfoFactories = SpringFactoriesLoader.loadFactories(
BeanInfoFactory.class, CachedIntrospectionResults.class.getClassLoader());
} 2. 扩展 xml 配置文件中 bean 的命名空间
spring-beans 提供了基于 XML 配置的、第三方对 bean 的命令空间扩展机制，主要是在META-INF/spring.handlers, META-INF/spring.schemas 文件中定义需要扩展的命令空间，
如
比如 spring-beans 下的扩展
http\://www.springframework.org/schema/c=org.springframework.beans.factory.xml.SimpleConstructorNamespaceHandler
http\://www.springframework.org/schema/p=org.springframework.beans.factory.xml.SimplePropertyNamespaceHandler
http\://www.springframework.org/schema/util=org.springframework.beans.factory.xml.UtilNamespaceHandler实现这个功能的类是 DefaultNamespaceHandlerResolver
public class DefaultNamespaceHandlerResolver implements NamespaceHandlerResolver {
// 自动加载文件地址
public static final String DEFAULT_HANDLER_MAPPINGS_LOCATION = "META-INF/spring.handlers";
// 默认加载DEFAULT_HANDLER_MAPPINGS_LOCATION
public DefaultNamespaceHandlerResolver() {
this(null, DEFAULT_HANDLER_MAPPINGS_LOCATION);
}
// 默认加载DEFAULT_HANDLER_MAPPINGS_LOCATION
public DefaultNamespaceHandlerResolver(@Nullable ClassLoader classLoader) {
this(classLoader, DEFAULT_HANDLER_MAPPINGS_LOCATION);
}
public DefaultNamespaceHandlerResolver(@Nullable ClassLoader classLoader, String handlerMappingsLocation) {
this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
this.handlerMappingsLocation = handlerMappingsLocation;
}
// 解析命名空间
@Override
public NamespaceHandler resolve(String namespaceUri) {
// 获取命令空间映射
Map handlerMappings = getHandlerMappings();
// 获取处理器
Object handlerOrClassName = handlerMappings.get(namespaceUri);
// 没有，返回null
if (handlerOrClassName == null) {
return null;
}
// NamespaceHandler，返回NamespaceHandler
else if (handlerOrClassName instanceof NamespaceHandler) {
return (NamespaceHandler) handlerOrClassName;
}
else {
String className = (String) handlerOrClassName;
try {
// 当做类加载
Class handlerClass = ClassUtils.forName(className, this.classLoader);
// 初始化类，并调用init方法
NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
namespaceHandler.init();
// 载入缓存
handlerMappings.put(namespaceUri, namespaceHandler);
return namespaceHandler;
}
catch (ClassNotFoundException ex) {
// ... 代码省略
}
// ... 代码省略
}
}
// 获取命令空间映射
private Map getHandlerMappings() {
Map handlerMappings = this.handlerMappings;
// 如果已经加载过了，就不加载了
if (handlerMappings == null) {
synchronized (this) {
handlerMappings = this.handlerMappings;
if (handlerMappings == null) {
try {
// 加载所有包下的spring.handlers，不光是主包，还有各种依赖包
// 把属性加载出来，以properties文件对待
Properties mappings =
PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);
handlerMappings = new ConcurrentHashMap<>(mappings.size());
CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);
// 赋值给handlerMappings
this.handlerMappings = handlerMappings;
}
catch (IOException ex) {
// ... 代码省略
}
}
}
}
return handlerMappings;
}
} 因为 DefaultNamespaceHandlerResolver 是默认的命名空间解析器，所以从一开始就会被初始化，所以也就会自动加载 spring.handlers
后续
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