SpringFactoriesLoader类作用详解
作者:沫洺
SpringFactoriesLoader类
介绍
SpringFactoriesLoader类的主要作用是通过类路径下的META-INF/spring.factories文件获取工厂类接口的实现类,初始化并保存在缓存中,以供Springboot启动过程中各个阶段的调用。Spring的自动化配置功能,也与此息息相关。
SpringFactoriesLoader 工厂加载机制是 Spring 内部提供的一个约定俗成的加载方式,只需要在模块的 META-INF/spring.factories 文件中,以 Properties 类型(即 key-value 形式)配置,就可以将相应的实现类注入 Spirng 容器中。
Properties类型格式:
key:value
- key:是全限定名(抽象类|接口)
- value:是实现类,多个实现类通过逗号进行分割
spring boot类路径下: META-INFO/spring.factories
方法
返回值 | 方法 | 描述 |
<T>List<T> | loadFactories(Class<T> factoryType,@Nullable ClassLoader classLoader) | 静态方法 根据接口获取其实现类的实例 该方法返回的是实现类对象列表 |
List<String> | loadFactoryNames(Class<?>) factoryType,@Nullable ClassLoader classLoader) | 公共静态方法 根据接口获取其实现类的名称 该方法返回的是实现类的类名的列表 |
public final class SpringFactoriesLoader { //文件位置,可以存在多个JAR文件中 public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"; //用来缓存MultiValueMap对象 private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>(); private SpringFactoriesLoader() { } /** * 根据给定的类型加载并实例化工厂的实现类 */ public static <T> List<T> loadFactories(Class<T> factoryType, @Nullable ClassLoader classLoader) { Assert.notNull(factoryType, "'factoryType' must not be null"); //获取类加载器 ClassLoader classLoaderToUse = classLoader; if (classLoaderToUse == null) { classLoaderToUse = SpringFactoriesLoader.class.getClassLoader(); } //加载类的全限定名 List<String> factoryImplementationNames = loadFactoryNames(factoryType, classLoaderToUse); if (logger.isTraceEnabled()) { logger.trace("Loaded [" + factoryType.getName() + "] names: " + factoryImplementationNames); } //创建一个存放对象的List List<T> result = new ArrayList<>(factoryImplementationNames.size()); for (String factoryImplementationName : factoryImplementationNames) { //实例化Bean,并将Bean放入到List集合中 result.add(instantiateFactory(factoryImplementationName, factoryType, classLoaderToUse)); } //对List中的Bean进行排序 AnnotationAwareOrderComparator.sort(result); return result; } /** * 根据给定的类型加载类路径的全限定名 */ public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) { //获取名称 String factoryTypeName = factoryType.getName(); //加载并获取所有META-INF/spring.factories中的value return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList()); } private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { //根据类加载器从缓存中获取,如果缓存中存在,就直接返回,如果不存在就去加载 MultiValueMap<String, String> result = cache.get(classLoader); if (result != null) { return result; } try { //获取所有JAR及classpath路径下的META-INF/spring.factories的路径 Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) : ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION)); result = new LinkedMultiValueMap<>(); //遍历所有的META-INF/spring.factories的路径 while (urls.hasMoreElements()) { URL url = urls.nextElement(); UrlResource resource = new UrlResource(url); //将META-INF/spring.factories中的key value加载为Prpperties对象 Properties properties = PropertiesLoaderUtils.loadProperties(resource); for (Map.Entry<?, ?> entry : properties.entrySet()) { //key名称 String factoryTypeName = ((String) entry.getKey()).trim(); for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) { //以factoryTypeName为key,value为值放入map集合中 result.add(factoryTypeName, factoryImplementationName.trim()); } } } //放入到缓存中 cache.put(classLoader, result); return result; } catch (IOException ex) { throw new IllegalArgumentException("Unable to load factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex); } } //实例化Bean对象 @SuppressWarnings("unchecked") private static <T> T instantiateFactory(String factoryImplementationName, Class<T> factoryType, ClassLoader classLoader) { try { Class<?> factoryImplementationClass = ClassUtils.forName(factoryImplementationName, classLoader); if (!factoryType.isAssignableFrom(factoryImplementationClass)) { throw new IllegalArgumentException( "Class [" + factoryImplementationName + "] is not assignable to factory type [" + factoryType.getName() + "]"); } return (T) ReflectionUtils.accessibleConstructor(factoryImplementationClass).newInstance(); } catch (Throwable ex) { throw new IllegalArgumentException( "Unable to instantiate factory class [" + factoryImplementationName + "] for factory type [" + factoryType.getName() + "]", ex); } } }
测试
resources下新建META-INF/spring.factories
com.moming.service.IStudentService=\
com.moming.service.impl.StudentServiceImpl1,\
com.moming.service.impl.StudentServiceImpl2
创建业务层接口及实现类
service/IStudentService
service/impl/StudentServiceImpl1
service/impl/StudentServiceImpl2
测试
@SpringBootApplication public class App{ public static void main(String[] args) { //SpringApplication.run(App.class, args); List<String> names = SpringFactoriesLoader.loadFactoryNames(IStudentService.class, ClassUtils.getDefaultClassLoader()); for (String name : names) { System.out.println(name); } System.out.println("==================================="); List<IStudentService> iStudentServices = SpringFactoriesLoader.loadFactories(IStudentService.class, ClassUtils.getDefaultClassLoader()); for (IStudentService iStudentService : iStudentServices) { System.out.println(iStudentService); System.out.println(iStudentService.game()); } } }
通过以上可以证明,SpringFactoriesLoader会寻找jar包中配置META-INF下的spring.factories配置文件相应Key的value,并根据需要实例化
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