SpringFactories机制

SpringFactories是spring提供的SPI机制，它读取META-INF/文件中配置的接口实现类名称，然后在程序中读取这些配置文件并实例化。
在日常工作中，我们可能需要实现一些SDK或者Spring Boot Starter给被人使用，这个使用我们就可以使用Factories机制。Factories机制可以让SDK或者Starter的使用只需要很少或者不需要进行配置，只需要在服务中引入我们的jar包。

springboot的配置文件如下

# PropertySource Loaders
=\
,\


# Run Listeners
=\


# Error Reporters
=\


# Application Context Initializers
=\
,\
,\
,\


# Application Listeners
=\
,\
,\
,\
,\
,\
,\
,\
,\

SpringFactoriesLoader

在这个类中主要的方法.

• loadFactories 根据接口类获取其实现类的实例，这个方法返回的是对象列表。
• instantiateFactory根据类创建实例对象。
• loadFactoryNames 根据接口获取其接口类的名称，这个方法返回的是类名的列表。

loadFactories

public static <T> List<T> loadFactories(Class<T> factoryClass, @Nullable ClassLoader classLoader) {
    Assert.notNull(factoryClass, "'factoryClass' must not be null");
   
    ClassLoader classLoaderToUse = classLoader;
    if (classLoaderToUse == null) {
        classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
    }
    // 调用loadFactoryNames获取接口的实现类
    List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse);
    if (logger.isTraceEnabled()) {
        logger.trace("Loaded [" + factoryClass.getName() + "] names: " + factoryNames);
    }
    // 遍历 factoryNames 数组，创建实现类的对象
    List<T> result = new ArrayList<>(factoryNames.size());
    for (String factoryName : factoryNames) {
        result.add(instantiateFactory(factoryName, factoryClass, classLoaderToUse));
    }
    // 排序
    AnnotationAwareOrderComparator.sort(result);
    return result;
}

private static <T> T instantiateFactory(String instanceClassName, Class<T> factoryClass, ClassLoader classLoader) {
    try {
        
        Class<?> instanceClass = ClassUtils.forName(instanceClassName, classLoader);
        // 是否实现了指定接口
        if (!factoryClass.isAssignableFrom(instanceClass)) {
            throw new IllegalArgumentException("Class [" + instanceClassName + "] is not assignable to [" + factoryClass.getName() + "]");
        }
        // 创建对象
        return (T) ReflectionUtils.accessibleConstructor(instanceClass).newInstance();
    } catch (Throwable ex) {
        throw new IllegalArgumentException("Unable to instantiate factory class: " + factoryClass.getName(), ex);
    }
}

loadFactoryNames

在这个方法中会遍历整个ClassLoader中所有jar包下的文件。通过Properties解析所有接口的实现类名称。

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    try {
        Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
        List<String> result = new ArrayList<String>();
        while (urls.hasMoreElements()) {
            URL url = urls.nextElement();
            Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
            String factoryClassNames = properties.getProperty(factoryClassName);
            result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
        }
        return result;
    }
    catch (IOException ex) {
        throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
}