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。

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