Java SPI机制学习与常用框架SPI案例

概念

SPI（Service Provider Interface）是JDK内置的服务提供机制，常用于框架的动态扩展，类似于可拔插机制。提供方将接口实现类配置在classpath下的指定位置，调用方读取并加载。当提供方发生变化时，接口的实现也会改变。Java生态中JDK、Dubbo、Spring等都通过SPI提供了动态扩展的能力。

样例

public interface Search {
    void search();
}

public class FileSearchImpl implements Search {
    @Override
    public void search() {
        System.out.println("file search...");
    }
}

public class DataBaseSearchImpl implements Search {
    @Override
    public void search() {
        System.out.println("db search");
    }
}



public class SpiTest {
    public static void main(String[] args) {
        ServiceLoader<Search> searches = ServiceLoader.load(Search.class);
        searches.forEach(Search::search);
    }
}

resources文件夹创建META-INF/services/wang.l1n.spi.Search文件，内容为接口实现类：

wang.l1n.spi.FileSearchImpl
wang.l1n.spi.DataBaseSearchImpl

运行结果：

load加载流程

ServiceLoder.load静态方法用于加载SPI实现类，实现逻辑如下：

1. 获取当前线程类加载器和上下文信息，调用实例化方法，重新加载SPI
2. 重新加载SPI的流程：
   1. 清空缓存providers中已实例化的SPI服务，providers是LinkedHashMap类型，用于保存已经被成功加载的SPI示例对象
      • 如果providers非空，直接返回Iterator，否则返回LazyIterator的Iterator。
   2. 创建LazyIterator懒加载迭代器，传入SPI类型和类加载器

实现代码和对应的成员变量如下：

public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

public static <S> ServiceLoader<S> load(Class<S> service,
                                            ClassLoader loader)
    {
        return new ServiceLoader<>(service, loader);
    }

private ServiceLoader(Class<S> svc, ClassLoader cl) {
        service = Objects.requireNonNull(svc, "Service interface cannot be null");
        loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
        acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
        reload();
    }

public void reload() {
        providers.clear();
        lookupIterator = new LazyIterator(service, loader);
    }

	
	private static final String PREFIX = "META-INF/services/";

    // The class or interface representing the service being loaded
    private final Class<S> service;

    // The class loader used to locate, load, and instantiate providers
    private final ClassLoader loader;

    // The access control context taken when the ServiceLoader is created
    private final AccessControlContext acc;

    // Cached providers, in instantiation order
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

    // The current lazy-lookup iterator
    private LazyIterator lookupIterator;

public Iterator<S> iterator() {
        return new Iterator<S>() {

            Iterator<Map.Entry<String,S>> knownProviders
                = providers.entrySet().iterator();

            public boolean hasNext() {
                if (knownProviders.hasNext())
                    return true;
                return lookupIterator.hasNext();
            }

            public S next() {
                if (knownProviders.hasNext())
                    return knownProviders.next().getValue();
                return lookupIterator.next();
            }

            public void remove() {
                throw new UnsupportedOperationException();
            }

        };
    }

LazyIterator加载流程

加载流程参考代码注释：

private class LazyIterator
        implements Iterator<S>
    {

        Class<S> service;
        ClassLoader loader;
        Enumeration<URL> configs = null;
        Iterator<String> pending = null;
        String nextName = null;

        private LazyIterator(Class<S> service, ClassLoader loader) {
            this.service = service;
            this.loader = loader;
        }

        private boolean hasNextService() {
            if (nextName != null) {
                return true;
            }
            if (configs == null) {
                try {
                    //获取接口的全称，拼接了META-INF/services/
                    String fullName = PREFIX + service.getName();
                    //类加载加载文件内容
                    if (loader == null)
                        configs = ClassLoader.getSystemResources(fullName);
                    else
                        configs = loader.getResources(fullName);
                } catch (IOException x) {
                    fail(service, "Error locating configuration files", x);
                }
            }
            //解析文件内容，获取SPI接口的实现类名
            while ((pending == null) || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                pending = parse(service, configs.nextElement());
            }
            nextName = pending.next();
            return true;
        }

        private S nextService() {
            if (!hasNextService())
                throw new NoSuchElementException();
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {
                //获取Class对象
                c = Class.forName(cn, false, loader);
            } catch (ClassNotFoundException x) {
                fail(service,
                     "Provider " + cn + " not found");
            }
            if (!service.isAssignableFrom(c)) {
                fail(service,
                     "Provider " + cn  + " not a subtype");
            }
            try {
                //使用newInstance创建对象，并添加到providers缓存中
                S p = service.cast(c.newInstance());
                providers.put(cn, p);
                return p;
            } catch (Throwable x) {
                fail(service,
                     "Provider " + cn + " could not be instantiated",
                     x);
            }
            throw new Error();          // This cannot happen
        }
}

JDK SPI能实现加载扩展接口的基本要求，存在几个缺点：

• 需要遍历所有class并进行实例化，需要调用某个特定实现只能循环找。
• 无法和Spring提供的上下文融合使用。
• ServiceLoader类非线程安全

常用框架SPI案例

Spring Boot

org.springframework.core.io.support.SpringFactoriesLoader#loadFactoryNames方法用于加载所有META-INF/spring.factories文件，主要流程如下：

• 搜索classpath下所有META-INF/spring.factories配置文件
• 解析文件，获取文件中对应的全限定类名

代码注释如下：

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

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

private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
		
    	//读取缓存
    	Map<String, List<String>> result = cache.get(classLoader);
		if (result != null) {
			return result;
		}

		result = new HashMap<>();
		try {
            //获取文件路径
			Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
            //遍历所有路径
			while (urls.hasMoreElements()) {
				URL url = urls.nextElement();
				UrlResource resource = new UrlResource(url);
                解析获取Properties对象
				Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                //解析properties，存放到map中
				for (Map.Entry<?, ?> entry : properties.entrySet()) {
					String factoryTypeName = ((String) entry.getKey()).trim();
					String[] factoryImplementationNames =
							StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
					for (String factoryImplementationName : factoryImplementationNames) {
						result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
								.add(factoryImplementationName.trim());
					}
				}
			}

			// Replace all lists with unmodifiable lists containing unique elements
			result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
					.collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
			cache.put(classLoader, result);
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
		return result;
	}

Dubbo

Dubbo并未使用Java SPI，基于SPI机制实现了了功能更强的ExtensionLoader，核心模块位于：org.apache.dubbo.common.extension.ExtensionLoader 配置文件需要放在META-INF/dubbo下，以如下配置为例：

optimusPrime = org.apache.spi.OptimusPrime
bumblebee = org.apache.spi.Bumblebee

public class DubboSPITest {

    @Test
    public void sayHello() throws Exception {
        ExtensionLoader<Robot> extensionLoader = 
            ExtensionLoader.getExtensionLoader(Robot.class);
        Robot optimusPrime = extensionLoader.getExtension("optimusPrime");
        optimusPrime.sayHello();
        Robot bumblebee = extensionLoader.getExtension("bumblebee");
        bumblebee.sayHello();
    }
}

下面简要一下Dubbo SPI的核心类ExtensionLoader的实现。

核心成员变量

• LoadingStrategy[] strategies：配置文件加载策略。
• ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS：每个扩展接口对应一个ExtensionLoader实例，该集合缓存了所有Loader实例。
• ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES：该Map缓存了扩展实现类与对应实例对象的映射关系

LoadStrategy

Subbo在加载SPI实现类时存在优先级，当前存在三个实现：

配置文件目录有以下三个位置： 1. META-INF/dubbo：由DubboLoadingStrategy负责，用户自定义SPI配置文件 2. META-INF/services：由ServicesLoadingStrategy负责，兼容JDK SPI 3. META-INF/dubbo/internal：由DubboInternalLoadingStrategy负责，Dubbo内部使用的SPI配置文件

核心实现

Dubbo SPI的核心实现方法为：org.apache.dubbo.common.extension.ExtensionLoader#getExtension

public T getExtension(String name) {
        if (StringUtils.isEmpty(name)) {
            throw new IllegalArgumentException("Extension name == null");
        }
        if ("true".equals(name)) {
	        //获取默认的扩展实现类
            return getDefaultExtension();
        }
        //从cachedInstances<name, Holder<>>
        final Holder<Object> holder = getOrCreateHolder(name);
        Object instance = holder.get();
        //double check
        if (instance == null) {
            synchronized (holder) {
                instance = holder.get();
                if (instance == null) {
	                //若未创建实例，则创建并set
                    instance = createExtension(name);
                    holder.set(instanc  e);
                }
            }
        }
        return (T) instance;
    }

在createExtension()方法中实现了SPI配置文件的查找已经对应实现类的实例化，同时还实现了自动装配以及自动Wrapper包装等功能：

1. getExtensionClasses()会获取cachedClass缓存，根据扩展名获取对应的实现类，如果未初始化，则调用loadExtensionClasses()加载实现类。
2. 自动装配扩展对象的属性（injectExtension()方法）
3. 自动包装扩展对象

//扩展实现类的加载过程
private T createExtension(String name) {
        Class<?> clazz = getExtensionClasses().get(name); // 1
        if (clazz == null) {
            throw findException(name);
        }
        try {
            T instance = (T) EXTENSION_INSTANCES.get(clazz);
            if (instance == null) {
                EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
                instance = (T) EXTENSION_INSTANCES.get(clazz);
            }
            injectExtension(instance); // 2
        //3
            Set<Class<?>> wrapperClasses = cachedWrapperClasses;
            if (CollectionUtils.isNotEmpty(wrapperClasses)) {
                for (Class<?> wrapperClass : wrapperClasses) {
                    instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
                }
            }
            initExtension(instance);
            return instance;
        } catch (Throwable t) {
            throw new IllegalStateException("Extension instance (name: " + name + ", class: " +
                    type + ") couldn't be instantiated: " + t.getMessage(), t);
        }
    }

getExtensionClasses()实现：

private Map<String, Class<?>> getExtensionClasses() {  
    Map<String, Class<?>> classes = cachedClasses.get();  
    if (classes == null) {  
        synchronized (cachedClasses) {  
            classes = cachedClasses.get();  
            if (classes == null) {  
                classes = loadExtensionClasses();  
                cachedClasses.set(classes);  
            }  
        }  
    }  
    return classes;  
}

private Map<String, Class<?>> loadExtensionClasses() {
        cacheDefaultExtensionName();

        Map<String, Class<?>> extensionClasses = new HashMap<>();

        for (LoadingStrategy strategy : strategies) {
            loadDirectory(extensionClasses, strategy.directory(), type.getName(), strategy.preferExtensionClassLoader(), strategy.overridden(), strategy.excludedPackages());
            loadDirectory(extensionClasses, strategy.directory(), type.getName().replace("org.apache", "com.alibaba"), strategy.preferExtensionClassLoader(), strategy.overridden(), strategy.excludedPackages());
        }

        return extensionClasses;
    }