SpringBoot 2.x SpringFactoriesLoader工厂加载机制解析

ゞ浴缸里的玫瑰 2022-10-06 11:58 143阅读 0赞

### 文章目录 ###

*   *   *  SpringFactoriesLoader 作用介绍
         *  源码解析
         *  SpringFactoriesLoader的流程图

### SpringFactoriesLoader 作用介绍 ###

其源码注释如下

> General purpose factory loading mechanism for internal use within the framework.  
> SpringFactoriesLoader loads and instantiates factories of a given type from "**META-INF/spring.factories" files which may be present in multiple JAR files in the classpath**. The spring.factories file **must be in Properties format**, where **the key is the fully qualified name of the interface or abstract class**, and the **value is a comma-separated list of implementation class names**. For example:  
> example.MyService=example.MyServiceImpl1,example.MyServiceImpl2  
> where example.MyService is the name of the interface, and MyServiceImpl1 and MyServiceImpl2 are two implementations.

翻译成中文就是如下四点特性

1.  框架内部使用的通用工厂加载机制
2.  classpath下多个jar包META-INF/spring.factories中读取文件并初始化类
3.  文件的内容必须是kv形式, 即必须满足Properties 格式
4.  文件的内容key是全限定名(接口或抽象类), value是接口或抽象类的实现, 多个实现用逗号进行分割.

### 源码解析 ###

SpringBoot启动分为框架运行, 框架初始化, 框架启动, 自动化装配.  
SpringFactoriesLoader工厂加载是在如下图的框架初始化的ApplicationContextInitializer中进行的.  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70]  
SpringBoot启动 框架初始化即SpringApplication的构造方法如下, 系统初始化调用了setInitializers方法. 并调用了`getSpringFactoriesInstances`方法,

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) { 
    		this.resourceLoader = resourceLoader;
    		Assert.notNull(primarySources, "PrimarySources must not be null");
    		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    		this.webApplicationType = WebApplicationType.deduceFromClasspath();
    		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    		this.mainApplicationClass = deduceMainApplicationClass();
    	}

`getSpringFactoriesInstances`方法调用了如下的`getSpringFactoriesInstances`同名方法, 方法的重载

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) { 
    		ClassLoader classLoader = getClassLoader();
    		// Use names and ensure unique to protect against duplicates
    		Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    		List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
    		AnnotationAwareOrderComparator.sort(instances);
    		return instances;
    	}

上面的方法中, 调用了`SpringFactoriesLoader.loadFactoryNames`方法,  
该方法如下:

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) { 
    		String factoryTypeName = factoryType.getName();
    		return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
    	}

该方法调用了`loadSpringFactories`方法如下

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { 
           // 先从缓存中去拿数据, 如果拿到了不为null ,那么直接返回. 
    		MultiValueMap<String, String> result = cache.get(classLoader);
    		if (result != null) { 
    			return result;
    		}
    
    		try { 
    			Enumeration<URL> urls = (classLoader != null ?
    					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
    					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
    			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 factoryTypeName = ((String) entry.getKey()).trim();
    					for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) { 
    						result.add(factoryTypeName, factoryImplementationName.trim());
    					}
    				}
    			}
    			cache.put(classLoader, result);
    			return result;
    		}
    		catch (IOException ex) { 
    			throw new IllegalArgumentException("Unable to load factories from location [" +
    					FACTORIES_RESOURCE_LOCATION + "]", ex);
    		}
    	}

上面的方法中, 有如下的这行代码.

Enumeration<URL> urls = (classLoader != null ?
    					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
    					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));

其中`FACTORIES_RESOURCE_LOCATION` 的值为如下  
`public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";`  
即classLoader类加载器会去从`META-INF/spring.factories` 中获取要加载的类资源

通过如下的断点可以看到, 加载到了`META-INF/spring.factories` 的绝对路径为`file:/D:/mycode/my-springboot2-study/target/classes/META-INF/spring.factories`  
并且构建了`Properties`对象, 其对象的内容为`spring.factories`文件中写的键值对.  
![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 1][]  
逐个遍历spring.factories文件中的key value. value值如果是用逗号分隔的, 那么就Spring提供的工具类`StringUtils.commaDelimitedListToStringArray` 去进行分隔, 分隔完成后, 把接口或抽象类作为key, 所有的实现类作为value, 放入result 缓存中去, 下次拿的时候, 直接从缓存中获取.  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 2]  
再次进行遍历 , 可以看到会从jar包中`jar:file:/D:/developsoft/jarslib/org/springframework/boot/spring-boot/2.3.12.RELEASE/spring-boot-2.3.12.RELEASE.jar!/META-INF/spring.factories`去 获取`spring.factories` 加载资源.  
![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 3][]  
从jar包中读取如下的类资源  
![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 4][]  
最终的result的值如下 , 可以看到`org.springframework.context.ApplicationContextInitializer` 的实现类中有我们在配置文件中写的`com.thc.myspringboot2study.initializer.FirstInitializer`  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 5]  
上一步return了result之后, 调用如下的这行方法  
`return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());`  
其中`getOrDefault` 方法为map接口的方法, 该方法的含义是第一个参数为key, 第二个参数为默认值. 如果该key在map中存在, 则返回该key对应的value, 如果不存在, 则返回一个空集合.  
断点获取如下的接口, 即`org.springframework.context.ApplicationContextInitializer` 的key是存在的, 即它有如下的这些实现类.  
![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 6][]  
返回到`getSpringFactoriesInstances` , 可以看到获取的names, 就是接口实现的类名.  
![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 7][]  
接着在`getSpringFactoriesInstances`方法中调用了下面一行  
`createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names)`方法, 该方法是通过反射去创建实例对象.  
createSpringFactoriesInstances 方法如下:

1.  创建实现子类大小容量的集合
2.  循环遍历这些实现类, 依次通过反射去创建实例
3.  把创建的实例, 加入到集合中去.

private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes,
    			ClassLoader classLoader, Object[] args, Set<String> names) { 
    			// 创建实现子类大小容量的集合 
    		List<T> instances = new ArrayList<>(names.size());
    		// 循环遍历这些实现类, 依次通过反射去创建实例 
    		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);
    				// 把创建的实例, 加入到集合中去. 
    				instances.add(instance);
    			}
    			catch (Throwable ex) { 
    				throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);
    			}
    		}
    		return instances;
    	}

创建完实例之后, 调用`AnnotationAwareOrderComparator.sort(instances)` 方法去进行排序.  
![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 8][]  
可以看到它是基于`OrderComparator` 去排序的.  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 9]  
经历完上面的步骤之后, 再回到SpringApplication的构造方法, 调用`setInitializers`方法

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 10]  
`setInitializers`方法 如下, 把ApplicationContextInitializer的实现类 加入到了spring容器当中.  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjI5NjY5_size_16_color_FFFFFF_t_70 11]

### SpringFactoriesLoader的流程图 ###

*  SpringBoot框架中从类路径jar包(各个jar包)中读取特定文件 spring.factories实现扩展类的载入

`loadSpringFactories`方法如下

其流程图如下:  
图片在线浏览  
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