什么是SPI

SPI全称为Service Provider Interface，是一种动态替换发现的机制，一种解耦非常优秀的思想，SPI可以很灵活的让接口和实现分离，让api提供者只提供接口，第三方来实现，然后可以使用配置文件的方式来实现替换或者扩展，在框架中比较常见，提高框架的可扩展性。

简单来说SPI是一种非常优秀的设计思想，它的核心就是解耦、方便扩展。

Java SPI机制–ServiceLoader

ServiceLoader是Java提供的一种简单的SPI机制的实现，Java的SPI实现约定了以下两件事：

• 文件必须放在META-INF/services/目录底下
• 文件名必须为接口的全限定名，内容为接口实现的全限定名

这样就能够通过ServiceLoader加载到文件中接口的实现。

demo展示

接口类

public interface LoadBalance {
}

实现类，这里引用了google的auto-service，可以在编译的时候生成配置文件com.charles.spi.LoadBalance，内容是com.charles.spi.RandomLoadBalance。

import com.google.auto.service.AutoService;

@AutoService(LoadBalance.class)
public class RandomLoadBalance implements LoadBalance {

    public RandomLoadBalance() {
        System.out.println("random init ...");
    }
}

测试效果

public class SpiTest {

    public static void main(String[] args) {
        ServiceLoader<LoadBalance> serviceLoader = ServiceLoader.load(LoadBalance.class);
        Iterator<LoadBalance> iterator = serviceLoader.iterator();
        while (iterator.hasNext()){
            LoadBalance next = iterator.next();
            System.out.println("获取到负载均衡策略:" + next);
        }
    }
}

源码解析

ServiceLoader#load()，初始化了LazyIterator

    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);
    }

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

ServiceLoader.LazyIterator#next，迭代器获取，根据接口名称获取配置文件，解析每一行数据，使用自定义的类加载器加载。

        public S next() {
            if (acc == null) {
                return nextService();
            } else {
                PrivilegedAction<S> action = new PrivilegedAction<S>() {
                    public S run() { return nextService(); }
                };
                return AccessController.doPrivileged(action, acc);
            }
        }

        private S nextService() {
            if (!hasNextService())
                throw new NoSuchElementException();
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {
                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 {
                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
        }

        private boolean hasNextService() {
            if (nextName != null) {
                return true;
            }
            if (configs == null) {
                try {
                    String fullName = PREFIX + service.getName();
                    if (loader == null)
                        configs = ClassLoader.getSystemResources(fullName);
                    else
                        configs = loader.getResources(fullName);
                } catch (IOException x) {
                    fail(service, "Error locating configuration files", x);
                }
            }
            while ((pending == null) || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                pending = parse(service, configs.nextElement());
            }
            nextName = pending.next();
            return true;
        }

    private Iterator<String> parse(Class<?> service, URL u)
        throws ServiceConfigurationError
    {
        InputStream in = null;
        BufferedReader r = null;
        ArrayList<String> names = new ArrayList<>();
        try {
            in = u.openStream();
            r = new BufferedReader(new InputStreamReader(in, "utf-8"));
            int lc = 1;
            while ((lc = parseLine(service, u, r, lc, names)) >= 0);
        } catch (IOException x) {
            fail(service, "Error reading configuration file", x);
        } finally {
            try {
                if (r != null) r.close();
                if (in != null) in.close();
            } catch (IOException y) {
                fail(service, "Error closing configuration file", y);
            }
        }
        return names.iterator();
    }

优缺点

• 浪费资源，会对每一个实现类进行实例化
• 无法区分具体的实现，做不到按需加载

Spring SPI机制–SpringFactoriesLoader

pring的SPI机制的约定如下：

• 配置文件必须在META-INF/目录下，文件名必须为spring.factories
• 文件内容为键值对，一个键可以有多个值，只需要用逗号分割就行，同时键值都需要是类的全限定名，键和值可以没有任何类与类之间的关系，当然也可以有实现的关系。

demo展示

在META-INF/目录下创建spring.factories文件，LoadBalance为键，RandomLoadBalance为值。

com.charles.spi.LoadBalance=com.charles.spi.RandomLoadBalance

测试效果

public class SpringSpiTest {

    public static void main(String[] args) {
        List<LoadBalance> loadBalances = SpringFactoriesLoader.loadFactories(LoadBalance.class, null);
//        List<String> factoryNames = SpringFactoriesLoader.loadFactoryNames(LoadBalance.class, null);
        System.out.println(loadBalances);
    }
}

源码解析

SpringFactoriesLoader#loadFactories，获取factoryType的类名。加载META-INF/spring.factories目录下的文件，用Properties来接收数据，用map做缓存，避免重复加载。获取到指定接口的全部实现类的类名称，调用SpringFactoriesLoader#instantiateFactory进行实例化。

	public static <T> List<T> loadFactories(Class<T> factoryType, @Nullable ClassLoader classLoader) {
		Assert.notNull(factoryType, "'factoryType' must not be null");
		ClassLoader classLoaderToUse = classLoader;
		if (classLoaderToUse == null) {
			classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
		}
		List<String> factoryImplementationNames = loadFactoryNames(factoryType, classLoaderToUse);
		if (logger.isTraceEnabled()) {
			logger.trace("Loaded [" + factoryType.getName() + "] names: " + factoryImplementationNames);
		}
		List<T> result = new ArrayList<>(factoryImplementationNames.size());
		for (String factoryImplementationName : factoryImplementationNames) {
			result.add(instantiateFactory(factoryImplementationName, factoryType, classLoaderToUse));
		}
		AnnotationAwareOrderComparator.sort(result);
		return result;
	}

	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) {
		Map<String, List<String>> result = cache.get(classLoader);
		if (result != null) {
			return result;
		}

		result = new HashMap<>();
		try {
			Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
			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();
					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;
	}

优缺点

• Spring的SPI机制对Java的SPI机制对进行了一些简化，Java的SPI每个接口都需要对应的文件，而Spring的SPI机制只需要一个spring.factories文件。
• Java的SPI机制文件内容必须为接口的实现类，而Spring的SPI并不要求键值对必须有什么关系，更加灵活。

Dubbo SPI机制–ExtensionLoader

ExtensionLoader是dubbo的SPI机制的实现类。每一个接口都会有一个自己的ExtensionLoader实例对象，这点跟Java的SPI机制是一样的。

同样地，Dubbo的SPI机制也做了以下几点约定：

• 接口必须要加@SPI注解
• 配置文件可以放在META-INF/services/、META-INF/dubbo/internal/ 、META-INF/dubbo/ 、META-INF/dubbo/external/这四个目录底下，文件名也是接口的全限定名
• 内容为键值对，键为短名称（可以理解为spring中Bean的名称），值为实现类的全限定名

demo展示

引入依赖

        <dependency>
            <groupId>org.apache.dubbo</groupId>
            <artifactId>dubbo</artifactId>
            <version>2.7.4.1</version>
        </dependency>

接口类

@SPI
public interface LoadBalance {
}

实现类

public class RandomLoadBalance implements LoadBalance {

    public RandomLoadBalance() {
        System.out.println("random init ...");
    }
}

实现类

@Adaptive
public class RoundLoadBalance implements LoadBalance{
}

在META-INF\dubbo创建文件名为com.charles.spi.LoadBalance的文件

random=com.charles.spi.RandomLoadBalance
round=com.charles.spi.RoundLoadBalance

测试效果

public class DubboSpiTest {

    public static void main(String[] args) {
        ExtensionLoader<LoadBalance> extensionLoader = ExtensionLoader.getExtensionLoader(LoadBalance.class);
        LoadBalance random = extensionLoader.getExtension("random");
        LoadBalance adaptiveExtension = extensionLoader.getAdaptiveExtension();
        System.out.println(random);
        System.out.println(adaptiveExtension);
    }
}

通过ExtensionLoader的getExtension方法，传入短名称，这样就可以精确地找到短名称对的实现类。Dubbo的SPI机制解决了前面提到的无法获取指定实现类的问题。

源码解析

ExtensionLoader#getExtension，根据名称获取实现类名，进行初始化。

    public T getExtension(String name) {
        if (StringUtils.isEmpty(name)) {
            throw new IllegalArgumentException("Extension name == null");
        }
        if ("true".equals(name)) {
            return getDefaultExtension();
        }
        final Holder<Object> holder = getOrCreateHolder(name);
        Object instance = holder.get();
        if (instance == null) {
            synchronized (holder) {
                instance = holder.get();
                if (instance == null) {
                    instance = createExtension(name);
                    holder.set(instance);
                }
            }
        }
        return (T) instance;
    }

    private T createExtension(String name) {
        Class<?> clazz = getExtensionClasses().get(name);
        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);
            Set<Class<?>> wrapperClasses = cachedWrapperClasses;
            if (CollectionUtils.isNotEmpty(wrapperClasses)) {
                for (Class<?> wrapperClass : wrapperClasses) {
                    instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
                }
            }
            return instance;
        } catch (Throwable t) {
            throw new IllegalStateException("Extension instance (name: " + name + ", class: " +
                    type + ") couldn't be instantiated: " + t.getMessage(), t);
        }
    }

    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;
    }

ExtensionLoader#loadExtensionClasses，加载指定目录下面的文件。

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

    private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";

    private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";

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

        Map<String, Class<?>> extensionClasses = new HashMap<>();
        loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName());
        loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
        loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName());
        loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
        loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName());
        loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
        return extensionClasses;
    }

ExtensionLoader#loadDirectory，加载指定目录下的文件（META-INF/dubbo/com.charles.spi.LoadBalance）

    private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir, String type) {
        String fileName = dir + type;
        try {
            Enumeration<java.net.URL> urls;
            ClassLoader classLoader = findClassLoader();
            if (classLoader != null) {
                urls = classLoader.getResources(fileName);
            } else {
                urls = ClassLoader.getSystemResources(fileName);
            }
            if (urls != null) {
                while (urls.hasMoreElements()) {
                    java.net.URL resourceURL = urls.nextElement();
                    loadResource(extensionClasses, classLoader, resourceURL);
                }
            }
        } catch (Throwable t) {
            logger.error("Exception occurred when loading extension class (interface: " +
                    type + ", description file: " + fileName + ").", t);
        }
    }

ExtensionLoader#loadResource，读取文件。

    private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader, java.net.URL resourceURL) {
        try {
            try (BufferedReader reader = new BufferedReader(new InputStreamReader(resourceURL.openStream(), StandardCharsets.UTF_8))) {
                String line;
                while ((line = reader.readLine()) != null) {
                    final int ci = line.indexOf('#');
                    if (ci >= 0) {
                        line = line.substring(0, ci);
                    }
                    line = line.trim();
                    if (line.length() > 0) {
                        try {
                            String name = null;
                            int i = line.indexOf('=');
                            if (i > 0) {
                                name = line.substring(0, i).trim();
                                line = line.substring(i + 1).trim();
                            }
                            if (line.length() > 0) {
                                loadClass(extensionClasses, resourceURL, Class.forName(line, true, classLoader), name);
                            }
                        } catch (Throwable t) {
                            IllegalStateException e = new IllegalStateException("Failed to load extension class (interface: " + type + ", class line: " + line + ") in " + resourceURL + ", cause: " + t.getMessage(), t);
                            exceptions.put(line, e);
                        }
                    }
                }
            }
        } catch (Throwable t) {
            logger.error("Exception occurred when loading extension class (interface: " +
                    type + ", class file: " + resourceURL + ") in " + resourceURL, t);
        }
    }

ExtensionLoader#loadClass，加载类

    private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, Class<?> clazz, String name) throws NoSuchMethodException {
        if (!type.isAssignableFrom(clazz)) {
            throw new IllegalStateException("Error occurred when loading extension class (interface: " +
                    type + ", class line: " + clazz.getName() + "), class "
                    + clazz.getName() + " is not subtype of interface.");
        }
        if (clazz.isAnnotationPresent(Adaptive.class)) {
            cacheAdaptiveClass(clazz);
        } else if (isWrapperClass(clazz)) {
            cacheWrapperClass(clazz);
        } else {
            clazz.getConstructor();
            if (StringUtils.isEmpty(name)) {
                name = findAnnotationName(clazz);
                if (name.length() == 0) {
                    throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + resourceURL);
                }
            }

            String[] names = NAME_SEPARATOR.split(name);
            if (ArrayUtils.isNotEmpty(names)) {
                cacheActivateClass(clazz, names[0]);
                for (String n : names) {
                    cacheName(clazz, n);
                    saveInExtensionClass(extensionClasses, clazz, n);
                }
            }
        }
    }

ExtensionLoader#cacheAdaptiveClass，判断是否Adaptive注解，将类赋值给cachedAdaptiveClass。

    private void cacheAdaptiveClass(Class<?> clazz) {
        if (cachedAdaptiveClass == null) {
            cachedAdaptiveClass = clazz;
        } else if (!cachedAdaptiveClass.equals(clazz)) {
            throw new IllegalStateException("More than 1 adaptive class found: "
                    + cachedAdaptiveClass.getName()
                    + ", " + clazz.getName());
        }
    }

ExtensionLoader#saveInExtensionClass，保存结果在extensionClasses。

    private void saveInExtensionClass(Map<String, Class<?>> extensionClasses, Class<?> clazz, String name) {
        Class<?> c = extensionClasses.get(name);
        if (c == null) {
            extensionClasses.put(name, clazz);
        } else if (c != clazz) {
            String duplicateMsg = "Duplicate extension " + type.getName() + " name " + name + " on " + c.getName() + " and " + clazz.getName();
            logger.error(duplicateMsg);
            throw new IllegalStateException(duplicateMsg);
        }
    }

获取自适应实现类，如果没有，进行创建。

    public T getAdaptiveExtension() {
        Object instance = cachedAdaptiveInstance.get();
        if (instance == null) {
            if (createAdaptiveInstanceError != null) {
                throw new IllegalStateException("Failed to create adaptive instance: " +
                        createAdaptiveInstanceError.toString(),
                        createAdaptiveInstanceError);
            }

            synchronized (cachedAdaptiveInstance) {
                instance = cachedAdaptiveInstance.get();
                if (instance == null) {
                    try {
                        instance = createAdaptiveExtension();
                        cachedAdaptiveInstance.set(instance);
                    } catch (Throwable t) {
                        createAdaptiveInstanceError = t;
                        throw new IllegalStateException("Failed to create adaptive instance: " + t.toString(), t);
                    }
                }
            }
        }

        return (T) instance;
    }

    private T createAdaptiveExtension() {
        try {
            return injectExtension((T) getAdaptiveExtensionClass().newInstance());
        } catch (Exception e) {
            throw new IllegalStateException("Can't create adaptive extension " + type + ", cause: " + e.getMessage(), e);
        }
    }

ExtensionLoader#getAdaptiveExtensionClass，执行getExtensionClasses进行类加载，获取cachedAdaptiveClass

  private Class<?> getAdaptiveExtensionClass() {
        getExtensionClasses();
        if (cachedAdaptiveClass != null) {
            return cachedAdaptiveClass;
        }
        return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }