elasticSearch源码分析之插件机制

    前面已经部署了elasticSearch源码，并了解了ES组件（Modules）的组织方式，可以大致的认为ES是由一个一个的组件构成的，本文讨论ES的插件机制，插件机制只是它的一个组件，具体在(PluginsModule)。
   Modules里面有一个方法处理预处理组件：

/**
 * 处理预处理组件
 * @param modules
 */
public static void processModules(Iterable<Module> modules) {
  for (Module module : modules) {
     if (module instanceof PreProcessModule) {
        for (Module module1 : modules) {
           ((PreProcessModule) module).processModule(module1);
        }
     }
  }
}

插件调用的层次为：Modules->PluginsModule->PluginsService->XXPlugin；都是调用void processModule(Module module)方法。
区别在于
1：PluginsModule实现的是PreProcessModule接口。
2：PluginsService是自己本来的方法。
3：XXPlugin实现的是Plugin接口。
虽然他们都有processModule方法，但不是实现的同一个类。

接下来我们可以看一下插件服务具体实现的类PluginsService，具体做了什么？
查看代码：

/**
 * Constructs a new PluginService
 * @param settings The settings of the system
 * @param environment The environment of the system
 */
public PluginsService(Settings settings, Environment environment) {
  super(settings);
  this.environment = environment;
  this.checkLucene = componentSettings.getAsBoolean("check_lucene", true);
  this.esPluginPropertiesFile = componentSettings.get(ES_PLUGIN_PROPERTIES_FILE_KEY, ES_PLUGIN_PROPERTIES);
  this.loadClasspathPlugins = componentSettings.getAsBoolean(LOAD_PLUGIN_FROM_CLASSPATH, true);
  ImmutableList.Builder<Tuple<PluginInfo, Plugin>> tupleBuilder = ImmutableList.builder();
  // first we load all the default plugins from the settings
  String[] defaultPluginsClasses = settings.getAsArray("plugin.types");
  for (String pluginClass : defaultPluginsClasses) {
      Plugin plugin = loadPlugin(pluginClass, settings);
      PluginInfo pluginInfo = new PluginInfo(plugin.name(), plugin.description(), hasSite(plugin.name()), true, PluginInfo.VERSION_NOT_AVAILABLE);
      if (logger.isTraceEnabled()) {
          logger.trace("plugin loaded from settings [{}]", pluginInfo);
      }
      tupleBuilder.add(new Tuple<>(pluginInfo, plugin));
  }
  // now, find all the ones that are in the classpath
        loadPluginsIntoClassLoader();
        if (loadClasspathPlugins) {
            tupleBuilder.addAll(loadPluginsFromClasspath(settings));
        }
        this.plugins = tupleBuilder.build();

        // We need to build a List of jvm and site plugins for checking mandatory plugins
        Map<String, Plugin> jvmPlugins = Maps.newHashMap();
        List<String> sitePlugins = Lists.newArrayList();

        for (Tuple<PluginInfo, Plugin> tuple : this.plugins) {
            jvmPlugins.put(tuple.v2().name(), tuple.v2());
            if (tuple.v1().isSite()) {
                sitePlugins.add(tuple.v1().getName());
            }
        }

        // we load site plugins
        ImmutableList<Tuple<PluginInfo, Plugin>> tuples = loadSitePlugins();
        for (Tuple<PluginInfo, Plugin> tuple : tuples) {
            sitePlugins.add(tuple.v1().getName());
        }

        // Checking expected plugins
        String[] mandatoryPlugins = settings.getAsArray("plugin.mandatory", null);
        if (mandatoryPlugins != null) {
            Set<String> missingPlugins = Sets.newHashSet();
            for (String mandatoryPlugin : mandatoryPlugins) {
                if (!jvmPlugins.containsKey(mandatoryPlugin) && !sitePlugins.contains(mandatoryPlugin) && !missingPlugins.contains(mandatoryPlugin)) {
                    missingPlugins.add(mandatoryPlugin);
                }
            }
            if (!missingPlugins.isEmpty()) {
                throw new ElasticsearchException("Missing mandatory plugins [" + Strings.collectionToDelimitedString(missingPlugins, ", ") + "]");
            }
        }

        logger.info("loaded {}, sites {}", jvmPlugins.keySet(), sitePlugins);

        MapBuilder<Plugin, List<OnModuleReference>> onModuleReferences = MapBuilder.newMapBuilder();
        for (Plugin plugin : jvmPlugins.values()) {
            List<OnModuleReference> list = Lists.newArrayList();
            for (Method method : plugin.getClass().getDeclaredMethods()) {
                if (!method.getName().equals("onModule")) {
                    continue;
                }
                if (method.getParameterTypes().length == 0 || method.getParameterTypes().length > 1) {
                    logger.warn("Plugin: {} implementing onModule with no parameters or more than one parameter", plugin.name());
                    continue;
                }
                Class moduleClass = method.getParameterTypes()[0];
                if (!Module.class.isAssignableFrom(moduleClass)) {
                    logger.warn("Plugin: {} implementing onModule by the type is not of Module type {}", plugin.name(), moduleClass);
                    continue;
                }
                method.setAccessible(true);
                list.add(new OnModuleReference(moduleClass, method));
            }
            if (!list.isEmpty()) {
                onModuleReferences.put(plugin, list);
            }
        }
    this.onModuleReferences = onModuleReferences.immutableMap();

    this.refreshInterval = componentSettings.getAsTime("info_refresh_interval", TimeValue.timeValueSeconds(10));
    }

可以看出其功能是注册ES根目录/[plugins]文件夹下的所有插件。