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如何理解dubbo的ExtensionLoader.getActivateExtension

如何理解dubbo的ExtensionLoader.getActivateExtension，针对这个问题，这篇文章详细介绍了相对应的分析和解答，希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。

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主要研究一下dubbo的ExtensionLoader.getActivateExtension

ExtensionLoader.getActivateExtension

dubbo-2.7.3/dubbo-common/src/main/java/org/apache/dubbo/common/extension/ExtensionLoader.java

public class ExtensionLoader {

    private static final Logger logger = LoggerFactory.getLogger(ExtensionLoader.class);

    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/";

    //......

    /**
     * This is equivalent to {@code getActivateExtension(url, url.getParameter(key).split(","), null)}
     *
     * @param url   url
     * @param key   url parameter key which used to get extension point names
     * @param group group
     * @return extension list which are activated.
     * @see #getActivateExtension(org.apache.dubbo.common.URL, String[], String)
     */
    public List getActivateExtension(URL url, String key, String group) {
        String value = url.getParameter(key);
        return getActivateExtension(url, StringUtils.isEmpty(value) ? null : COMMA_SPLIT_PATTERN.split(value), group);
    }

    /**
     * Get activate extensions.
     *
     * @param url    url
     * @param values extension point names
     * @param group  group
     * @return extension list which are activated
     * @see org.apache.dubbo.common.extension.Activate
     */
    public List getActivateExtension(URL url, String[] values, String group) {
        List exts = new ArrayList<>();
        List names = values == null ? new ArrayList<>(0) : Arrays.asList(values);
        if (!names.contains(REMOVE_VALUE_PREFIX + DEFAULT_KEY)) {
            getExtensionClasses();
            for (Map.Entry 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(REMOVE_VALUE_PREFIX + name)
                            && isActive(activateValue, url)) {
                        exts.add(ext);
                    }
                }
            }
            exts.sort(ActivateComparator.COMPARATOR);
        }
        List usrs = new ArrayList<>();
        for (int i = 0; i < names.size(); i++) {
            String name = names.get(i);
            if (!name.startsWith(REMOVE_VALUE_PREFIX)
                    && !names.contains(REMOVE_VALUE_PREFIX + name)) {
                if (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;
    }

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

    // synchronized in getExtensionClasses
    private Map> loadExtensionClasses() {
        cacheDefaultExtensionName();

        Map> 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;
    }

    private void loadDirectory(Map> extensionClasses, String dir, String type) {
        String fileName = dir + type;
        try {
            Enumeration 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);
        }
    }

    private void loadResource(Map> 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);
        }
    }

    private void loadClass(Map> 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);
                }
            }
        }
    }

    /**
     * cache Activate class which is annotated with Activate
     * 

     * for compatibility, also cache class with old alibaba Activate annotation
     */
    private void cacheActivateClass(Class clazz, String name) {
        Activate activate = clazz.getAnnotation(Activate.class);
        if (activate != null) {
            cachedActivates.put(name, 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(name, oldActivate);
            }
        }
    }
         
    /**
     * Find the extension with the given name. If the specified name is not found, then {@link IllegalStateException}
     * will be thrown.
     */
    @SuppressWarnings("unchecked")
    public T getExtension(String name) {
        if (StringUtils.isEmpty(name)) {
            throw new IllegalArgumentException("Extension name == null");
        }
        if ("true".equals(name)) {
            return getDefaultExtension();
        }
        final Holder 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;
    }

    //......
}
  • getActivateExtension方法先执行getExtensionClasses加载extension类，这里主要通过loadDirectory方法加载META-INF/dubbo/、META-INF/dubbo/internal/、META-INF/services/三个目录下的extension
  • loadDirectory方法会调用loadResource方法，loadResource方法会调用loadClass方法，loadClass方法会通过cacheActivateClass把Activate注解信息放入到cachedActivates中
  • 之后遍历cachedActivates通过getExtension方法把extension实例放入exts，然后再根据ActivateComparator.COMPARATOR进行排序，最后加载usr的extension(目前看代码names貌似一直为空)，然后合并返回
ActivateComparator
dubbo-common-2.7.3-sources.jar!/org/apache/dubbo/common/extension/support/ActivateComparator.java
public class ActivateComparator implements Comparator {

    public static final Comparator COMPARATOR = new ActivateComparator();

    @Override
    public int compare(Object o1, Object o2) {
        if (o1 == null && o2 == null) {
            return 0;
        }
        if (o1 == null) {
            return -1;
        }
        if (o2 == null) {
            return 1;
        }
        if (o1.equals(o2)) {
            return 0;
        }

        Class inf = findSpi(o1.getClass());

        ActivateInfo a1 = parseActivate(o1.getClass());
        ActivateInfo a2 = parseActivate(o2.getClass());

        if ((a1.applicableToCompare() || a2.applicableToCompare()) && inf != null) {
            ExtensionLoader extensionLoader = ExtensionLoader.getExtensionLoader(inf);
            if (a1.applicableToCompare()) {
                String n2 = extensionLoader.getExtensionName(o2.getClass());
                if (a1.isLess(n2)) {
                    return -1;
                }

                if (a1.isMore(n2)) {
                    return 1;
                }
            }

            if (a2.applicableToCompare()) {
                String n1 = extensionLoader.getExtensionName(o1.getClass());
                if (a2.isLess(n1)) {
                    return 1;
                }

                if (a2.isMore(n1)) {
                    return -1;
                }
            }
        }
        int n1 = a1 == null ? 0 : a1.order;
        int n2 = a2 == null ? 0 : a2.order;
        // never return 0 even if n1 equals n2, otherwise, o1 and o2 will override each other in collection like HashSet
        return n1 > n2 ? 1 : -1;
    }

    private Class findSpi(Class clazz) {
        if (clazz.getInterfaces().length <= 0) {
            return null;
        }

        for (Class intf : clazz.getInterfaces()) {
            if (intf.isAnnotationPresent(SPI.class)) {
                return intf;
            } else {
                Class result = findSpi(intf);
                if (result != null) {
                    return result;
                }
            }
        }

        return null;
    }

    private ActivateInfo parseActivate(Class clazz) {
        ActivateInfo info = new ActivateInfo();
        if (clazz.isAnnotationPresent(Activate.class)) {
            Activate activate = clazz.getAnnotation(Activate.class);
            info.before = activate.before();
            info.after = activate.after();
            info.order = activate.order();
        } else {
            com.alibaba.dubbo.common.extension.Activate activate = clazz.getAnnotation(
                    com.alibaba.dubbo.common.extension.Activate.class);
            info.before = activate.before();
            info.after = activate.after();
            info.order = activate.order();
        }
        return info;
    }

    private static class ActivateInfo {
        private String[] before;
        private String[] after;
        private int order;

        private boolean applicableToCompare() {
            return ArrayUtils.isNotEmpty(before) || ArrayUtils.isNotEmpty(after);
        }

        private boolean isLess(String name) {
            return Arrays.asList(before).contains(name);
        }

        private boolean isMore(String name) {
            return Arrays.asList(after).contains(name);
        }
    }
}
  • ActivateComparator首先通过parseActivate解析注解信息到ActivateInfo，然后对于有配置before或after的则根据其值进行升序排序，否则则通过order(没有指定默认为0)来排序，大于返回1，否则返回-1
小结
  • getActivateExtension方法先执行getExtensionClasses加载extension类，这里主要通过loadDirectory方法加载META-INF/dubbo/、META-INF/dubbo/internal/、META-INF/services/三个目录下的extension
  • loadDirectory方法会调用loadResource方法，loadResource方法会调用loadClass方法，loadClass方法会通过cacheActivateClass把Activate注解信息放入到cachedActivates中
  • 之后遍历cachedActivates通过getExtension方法把extension实例放入exts，然后再根据ActivateComparator.COMPARATOR进行排序，最后加载usr的extension(目前看代码names貌似一直为空)，然后合并返回
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