spring-boot解析spring.factories文件

启动SpringBoot自动装配的工厂类方法实现
复制代码
    /**
     * 解析 spring.factories 文件
     *
     * @return 读取到的所有数据
     */
    private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader){
        try {
            Enumeration<URL> urls = (classLoader != null ?
                    classLoader.getResources("META-INF/spring.factories") :
                    ClassLoader.getSystemResources("META-INF/spring.factories"));
            MultiValueMap<String, String> 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 factoryClassName = ((String) entry.getKey()).trim();
                    for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                        result.add(factoryClassName, factoryName.trim());
                    }
                }
            }
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", ex);
        }
    }

    /**
     * 反射实例化
     */
    private static <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;
    }

    public static void main(String[] args) throws IOException {
        ClassLoader classLoader = ClassUtils.getDefaultClassLoader();
        // 解析 spring.factories 文件
        Map<String, List<String>> maps = loadSpringFactories(classLoader);
        // 获取指定的类 ApplicationContextInitializer 配置
        String className = ApplicationContextInitializer.class.getName();
        List<String> values = maps.getOrDefault(className,Collections.emptyList());
        // 反射实例化类
        Set<String> names = new LinkedHashSet<>(values);
        List<ApplicationContextInitializer> instances = createSpringFactoriesInstances(ApplicationContextInitializer.class, new Class<?>[]{}, classLoader, args, names);
        for (ApplicationContextInitializer instance : instances) {
            System.out.println(instance.toString());
        }
    }