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SpringFactoriesLoader类作用详解

作者:沫洺

SpringFactoriesLoader可以加载jar包下META-INF下的spring.factories,把相关接口的实现按照key,value的形式加载到内存,一个接口的多个实现可以按照","进行分割

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

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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