SpringFactoriesLoader类作用详解

目录

• SpringFactoriesLoader类
• 介绍
• 方法
• 测试

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

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

到此这篇关于SpringFactoriesLoader类作用详解的文章就介绍到这了,更多相关SpringFactoriesLoader内容请搜索自由互联以前的文章或继续浏览下面的相关文章希望大家以后多多支持自由互联！