浅析Dubbo的SPI扩展机制

之前可以先看下Dubbo官方文档对SPI的介绍Dubbo SPI。 我们来看下官方文档是怎么说的：

SPI 全称为 Service Provider Interface，是一种服务发现机制。SPI 的本质是将接口实现类的全限定名配置在文件中，并由服务加载器读取配置文件，加载实现类。这样可以在运行时，动态为接口替换实现类。正因此特性，我们可以很容易的通过 SPI 机制为我们的程序提供拓展功能。

SPI 机制在第三方框架中也有所应用，比如 Dubbo 就是通过 SPI 机制加载所有的组件。不过，Dubbo 并未使用 Java 原生的 SPI 机制，而是对其进行了增强，使其能够更好的满足需求。

从官方示例说起

Dubbo SPI 的相关逻辑被封装在了 ExtensionLoader 类中，通过 ExtensionLoader，我们可以加载指定的实现类。Dubbo SPI 所需的配置文件需放置在 META-INF/dubbo 路径下，文件名是扩展接口的全限定名。例如com.alibaba.dubbo.rpc.Filter的文件名就是com.alibaba.dubbo.rpc.Filter。

注意：这里的配置文件是放在你自己的 jar 包内，不是 dubbo 本身的 jar 包内，Dubbo 会全 ClassPath 扫描所有 jar 包内同名的这个文件，然后进行合并。

官方例子的配置内容如下。

optimusPrime = org.apache.spi.OptimusPrime
bumblebee = org.apache.spi.Bumblebee
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以下是示例代码:

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();
    }
}
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实现分析

获取ExtensionLoad

public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
    if (type == null) {
        throw new IllegalArgumentException("Extension type == null");
    }
    // 必须是接口
    if (!type.isInterface()) {
        throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
    }
    // 必须包含@SPI注释
    if (!withExtensionAnnotation(type)) {
        throw new IllegalArgumentException("Extension type(" + type +
                ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
    }

    // 获得接口对应的扩展加载器
    ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    if (loader == null) {
        EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
        loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    }
    return loader;
}
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这里的EXTENSION_LOADERS是一个静态变量。

/**
 * 扩展加载器集合
 */
private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>();
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注意到withExtensionAnnotation方法，每个要扩展的接口都必须要包含@SPI。

private static <T> boolean withExtensionAnnotation(Class<T> type) {
    return type.isAnnotationPresent(SPI.class);
}
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我们看下ExtensionLoader的构造函数。

private ExtensionLoader(Class<?> type) {
    this.type = type;
    objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
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此时这里的type是上面例子传入的Robot.class。

获取拓展对象

public T getExtension(String name) {
    if (name == null || name.length() == 0) {
        throw new IllegalArgumentException("Extension name == null");
    }
    // 查找 默认的 扩展对象
    if ("true".equals(name)) {
        return getDefaultExtension();
    }
    Holder<Object> holder = cachedInstances.get(name);
    if (holder == null) {
        cachedInstances.putIfAbsent(name, new Holder<Object>());
        holder = cachedInstances.get(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;
}
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可以看到如果如果name是true,则返回默认的扩展对象。 检查缓存是否命中，没有的话就调用createExtension方法创建扩展对象。

查找默认的扩展对象

public T getDefaultExtension() {
    getExtensionClasses();
    if (null == cachedDefaultName || cachedDefaultName.length() == 0
            || "true".equals(cachedDefaultName)) {
        return null;
    }
    return getExtension(cachedDefaultName);
}
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先调用getExtensionClasses方法获取所有的扩展实现类，再获取cachedDefaultName对应的扩展对象。那cachedDefaultName的值又是哪里来的呢？其实就是SPI注解的value。在loadExtensionClasses方法中有这段代码。

private Map<String, Class<?>> loadExtensionClasses() {
    // 通过@SPI注解,获得默认的扩展实现类名
    final SPI defaultAnnotation = type.getAnnotation(SPI.class);
    if (defaultAnnotation != null) {
        String value = defaultAnnotation.value();
        if ((value = value.trim()).length() > 0) {
            String[] names = NAME_SEPARATOR.split(value);
            if (names.length > 1) {
                throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()
                        + ": " + Arrays.toString(names));
            }
            if (names.length == 1) {
                cachedDefaultName = names[0];
            }
        }
    }
    // 省略方法代码
    return extensionClasses;
}
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创建扩展对象

private T createExtension(String name) {
    // 获得扩展名对应的扩展实现类
    Class<?> clazz = getExtensionClasses().get(name);
    if (clazz == null) {
        throw findException(name);
    }
    try {
        // 从缓存中，获取扩展对象
        T instance = (T) EXTENSION_INSTANCES.get(clazz);
        if (instance == null) {
            // 当缓存不存在时，创建扩展对象，并添加到缓存中
            EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
            instance = (T) EXTENSION_INSTANCES.get(clazz);
            }
            // 注入依赖的属性
            injectExtension(instance);
            // 创建Wrapper扩展对象
            Set<Class<?>> wrapperClasses = cachedWrapperClasses;
            if (wrapperClasses != null && !wrapperClasses.isEmpty()) {
                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 + ")  could not be instantiated: " + t.getMessage(), t);
        }
    }
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createExtension方法包括以下的几个步骤:

1. 通过 getExtensionClasses 获取所有的拓展类
2. 通过反射创建拓展对象
3. 向拓展对象中注入依赖
4. 将拓展对象包裹在相应的 Wrapper 对象中

获取所有的扩展类

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;
}
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对cachedClasses进行双重检查，若缓存的扩展实现类集合为空，则调用loadExtensionClasses方法加载扩展实现类集合。

private Map<String, Class<?>> loadExtensionClasses() {
    // 通过@SPI注解,获得默认的扩展实现类名
    final SPI defaultAnnotation = type.getAnnotation(SPI.class);
    if (defaultAnnotation != null) {
        String value = defaultAnnotation.value();
        if ((value = value.trim()).length() > 0) {
            String[] names = NAME_SEPARATOR.split(value);
            if (names.length > 1) {
                throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()
                        + ": " + Arrays.toString(names));
            }
            if (names.length == 1) {
                cachedDefaultName = names[0];
            }
        }
    }

    Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
    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;
}
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加载的配置文件路径见下面。

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/";
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从配置文件中，加载扩展实现类数组。

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 when load extension class(interface: " +
                    type + ", description file: " + fileName + ").", t);
        }
    }
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private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader, java.net.URL resourceURL) {
    try {
        BufferedReader reader = new BufferedReader(new InputStreamReader(resourceURL.openStream(), "utf-8"));
        try {
            String line;
            while ((line = reader.readLine()) != null) {
                // 跳过当前被注释调的情况，例如 #spring=xxxxxxxx
                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);
                    }
                }
            }
        } finally {
            reader.close();
        }
    } catch (Throwable t) {
        logger.error("Exception when load extension class(interface: " +
                type + ", class file: " + resourceURL + ") in " + resourceURL, t);
    }
}
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private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, Class<?> clazz, String name) throws NoSuchMethodException {
    // 判断扩展实现，是否实现扩展接口
    if (!type.isAssignableFrom(clazz)) {
        throw new IllegalStateException("Error when load extension class(interface: " +
                type + ", class line: " + clazz.getName() + "), class "
                + clazz.getName() + "is not subtype of interface.");
    }
    // 缓存自适应扩展对象的类别 'cachedAdaptiveClass'
    if (clazz.isAnnotationPresent(Adaptive.class)) {
        if (cachedAdaptiveClass == null) {
            cachedAdaptiveClass = clazz;
        } else if (!cachedAdaptiveClass.equals(clazz)) {
            throw new IllegalStateException("More than 1 adaptive class found: "
                    + cachedAdaptiveClass.getClass().getName()
                    + ", " + clazz.getClass().getName());
        }
        // 缓存拓展Wrapper实现类到'cachedWrapperClasses'
    } else if (isWrapperClass(clazz)) {
        Set<Class<?>> wrappers = cachedWrapperClasses;
        if (wrappers == null) {
            cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
            wrappers = cachedWrapperClasses;
        }
        wrappers.add(clazz);
    } else {
        // 缓存扩展实现类'extensionClasses'
        clazz.getConstructor();
        if (name == null || name.length() == 0) {
            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 (names != null && names.length > 0) {
            Activate activate = clazz.getAnnotation(Activate.class);
            if (activate != null) {
                cachedActivates.put(names[0], activate);
            } else {
                // support com.alibaba.dubbo.common.extension.Activate
                com.alibaba.dubbo.common.extension.Activate oldActivate = clazz.getAnnotation(com.alibaba.dubbo.common.extension.Activate.class);
                if (oldActivate != null) {
                    cachedActivates.put(names[0], oldActivate);
                }
            }
            for (String n : names) {
                if (!cachedNames.containsKey(clazz)) {
                    cachedNames.put(clazz, n);
                }
                Class<?> c = extensionClasses.get(n);
                if (c == null) {
                    extensionClasses.put(n, clazz);
                } else if (c != clazz) {
                    throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
                }
            }
        }
    }
}
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向扩展对象中注入依赖

private T injectExtension(T instance) {
    try {
        if (objectFactory != null) {
            for (Method method : instance.getClass().getMethods()) {
                if (method.getName().startsWith("set")
                        && method.getParameterTypes().length == 1
                        && Modifier.isPublic(method.getModifiers())) {
                    /**
                     * Check {@link DisableInject} to see if we need auto injection for this property
                     */
                    if (method.getAnnotation(DisableInject.class) != null) {
                        continue;
                    }
                    Class<?> pt = method.getParameterTypes()[0];
                    try {
                        String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
                        Object object = objectFactory.getExtension(pt, property);
                        if (object != null) {
                            method.invoke(instance, object);
                        }
                    } catch (Exception e) {
                        logger.error("fail to inject via method " + method.getName()
                                + " of interface " + type.getName() + ": " + e.getMessage(), e);
                    }
                }
            }
        }
    } catch (Exception e) {
        logger.error(e.getMessage(), e);
    }
    return instance;
}
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遍历查找set方法，反射注入依赖。

扩展点自适应

参考官方文档的自适应扩展机制。

有关Adaptive注解还可参考 Dubbo SPI之Adaptive详解 。

在类上加Adaptive注解优先级最高。

private Class<?> getAdaptiveExtensionClass() {
    getExtensionClasses();
    if (cachedAdaptiveClass != null) {
        return cachedAdaptiveClass;
    }
    return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
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在loadClass方法中可以看到带有Adaptive注解的类对象会被赋值给cachedAdaptiveClass。

// 缓存自适应扩展对象的类别 'cachedAdaptiveClass'
if (clazz.isAnnotationPresent(Adaptive.class)) {
    if (cachedAdaptiveClass == null) {
        cachedAdaptiveClass = clazz;
    } else if (!cachedAdaptiveClass.equals(clazz)) {
        throw new IllegalStateException("More than 1 adaptive class found: "
                + cachedAdaptiveClass.getClass().getName()
                + ", " + clazz.getClass().getName());
    }
    // 缓存拓展Wrapper实现类到'cachedWrapperClasses'
} 
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SPI注解的value值是是默认值，在上面我们知道被赋值给了cachedDefaultName，如果通过url获取不到取哪个类作为Adaptive类的话就用这个默认值。

String defaultExtName = cachedDefaultName;
String getNameCode = null;
for (int i = value.length - 1; i >= 0; --i) {
    if (i == value.length - 1) {
        if (null != defaultExtName) {
            if (!"protocol".equals(value[i])) {
                if (hasInvocation) {
                    getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                } else {
                    getNameCode = String.format("url.getParameter(\"%s\", \"%s\")", value[i], defaultExtName);
                }
            } else {
                getNameCode = String.format("( url.getProtocol() == null ? \"%s\" : url.getProtocol() )", defaultExtName);
            }
        } else {
            if (!"protocol".equals(value[i])) {
                if (hasInvocation) {
                    getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                } else {
                    getNameCode = String.format("url.getParameter(\"%s\")", value[i]);
                }
            } else {
                getNameCode = "url.getProtocol()";
            }
        }
    } else {
        if (!"protocol".equals(value[i])) {
            if (hasInvocation) {
                getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
            } else {
                getNameCode = String.format("url.getParameter(\"%s\", %s)", value[i], getNameCode);
            }
        } else {
            getNameCode = String.format("url.getProtocol() == null ? (%s) : url.getProtocol()", getNameCode);
        }
    }
}
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扩展点自动激活

对于一些集合类扩展点，比如：Filter，可以同时加载多个实现，此时可以用自动激活来简化配置。

import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.rpc.Filter;
 
@Activate // 无条件自动激活
public class XxxFilter implements Filter {
    // ...
}
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或者

import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.rpc.Filter;
 
@Activate("xxx") // 当配置了xxx参数，并且参数为有效值时激活，比如配了cache="lru"，自动激活CacheFilter。
public class XxxFilter implements Filter {
    // ...
}
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或者

import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.rpc.Filter;
 
@Activate(group = "provider", value = "xxx") // 只对提供方激活，group可选"provider"或"consumer"
public class XxxFilter implements Filter {
    // ...
}
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或者

import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.rpc.Filter;
 
@Activate(group = "provider", value = "xxx", order=1) // order表示优先级，低的 排序 优先级高
public class XxxFilter implements Filter {
    // ...
}
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我们来看下实现

public List<T> getActivateExtension(URL url, String key, String group) {
    String value = url.getParameter(key);
    return getActivateExtension(url, value == null || value.length() == 0 ? null : Constants.COMMA_SPLIT_PATTERN.split(value), group);
}
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public List<T> getActivateExtension(URL url, String[] values, String group) {
    List<T> exts = new ArrayList<T>();
    List<String> names = values == null ? new ArrayList<String>(0) : Arrays.asList(values);
    // 处理自动激活的扩展对象们
    // 判断不存在配置 "-name"
    if (!names.contains(Constants.REMOVE_VALUE_PREFIX + Constants.DEFAULT_KEY)) {
        // 获得扩展实现类数组
        getExtensionClasses();
        for (Map.Entry<String, Object> entry : cachedActivates.entrySet()) {
            String name = entry.getKey();
            Object activate = entry.getValue();

            String[] activateGroup, activateValue;

            if (activate instanceof Activate) {
                activateGroup = ((Activate) activate).group();
                activateValue = ((Activate) activate).value();
            } else if (activate instanceof com.alibaba.dubbo.common.extension.Activate) {
                activateGroup = ((com.alibaba.dubbo.common.extension.Activate) activate).group();
                activateValue = ((com.alibaba.dubbo.common.extension.Activate) activate).value();
            } else {
                continue;
            }
            if (isMatchGroup(group, activateGroup)) {
                T ext = getExtension(name);
                if (!names.contains(name)
                        && !names.contains(Constants.REMOVE_VALUE_PREFIX + name)
                        && isActive(activateValue, url)) {
                    exts.add(ext);
                }
            }
        }
        Collections.sort(exts, ActivateComparator.COMPARATOR);
    }
    // 处理自定义配置的扩展对象们
    List<T> usrs = new ArrayList<T>();
    for (int i = 0; i < names.size(); i++) {
        String name = names.get(i);
        if (!name.startsWith(Constants.REMOVE_VALUE_PREFIX)
                && !names.contains(Constants.REMOVE_VALUE_PREFIX + name)) {
            // 将配置的自定义在自动激活的扩展对象们前面
            if (Constants.DEFAULT_KEY.equals(name)) {
                if (!usrs.isEmpty()) {
                    exts.addAll(0, usrs);
                    usrs.clear();
                }
            } else {
                T ext = getExtension(name);
                usrs.add(ext);
            }
        }
    }
    if (!usrs.isEmpty()) {
        exts.addAll(usrs);
    }
    return exts;
}
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