dubbo 自定义filter 实现公用过滤器处理

　　Dubbo的Filter机制，是专门为服务提供方和服务消费方调用过程进行拦截设计的，每次远程方法执行，该拦截都会被执行。这样就为开发者提供了非常方便的扩展性，比如为dubbo接口实现ip白名单功能、监控功能等等。

怎样添加filter？

1. 基于xml配置文件配置filter

　　1、自定义Filter，必须继承com.alibaba.dubbo.rpc.Filter接口
　　2、在resources目录下添加纯文本文件META-INF/dubbo/com.alibaba.dubbo.rpc.Filter，内容写成 xxx=xxx.xxx.xxxFilter
　　3、在dubbo配置xml中添加 <dubbo:provider filter="xxxFilter" /> 或者 <dubbo:consumer filter="xxxFilter" /> 使Filter生效。

　　使用xml配置有两个问题： 1. 显示配置，影响业务开发； 2. 配置麻烦，万一忘记添加则不生效；

 

　　有没有一种更简单的方法，可以公用的增加一个filter？

　　有的，dubbo提供了一个注解扫描： @Activate

 

2. 基于 @Activate 注解增加filter 

@Activate 定义如下

/**
 * Activate. This annotation is useful for automatically activate certain extensions with the given criteria,
 * for examples: <code>@Activate</code> can be used to load certain <code>Filter</code> extension when there are
 * multiple implementations.
 * <ol>
 * <li>{@link Activate#group()} specifies group criteria. Framework SPI defines the valid group values.
 * <li>{@link Activate#value()} specifies parameter key in {@link URL} criteria.
 * </ol>
 * SPI provider can call {@link ExtensionLoader#getActivateExtension(URL, String, String)} to find out all activated
 * extensions with the given criteria.
 *
 * @see SPI
 * @see URL
 * @see ExtensionLoader
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
public @interface Activate {
    /**
     * Activate the current extension when one of the groups matches. The group passed into
     * {@link ExtensionLoader#getActivateExtension(URL, String, String)} will be used for matching.
     *
     * @return group names to match
     * @see ExtensionLoader#getActivateExtension(URL, String, String)
     */
    String[] group() default {};

    /**
     * Activate the current extension when the specified keys appear in the URL's parameters.
     * <p>
     * For example, given <code>@Activate("cache, validation")</code>, the current extension will be return only when
     * there's either <code>cache</code> or <code>validation</code> key appeared in the URL's parameters.
     * </p>
     *
     * @return URL parameter keys
     * @see ExtensionLoader#getActivateExtension(URL, String)
     * @see ExtensionLoader#getActivateExtension(URL, String, String)
     */
    String[] value() default {};

    /**
     * Relative ordering info, optional
     *
     * @return extension list which should be put before the current one
     */
    String[] before() default {};

    /**
     * Relative ordering info, optional
     *
     * @return extension list which should be put after the current one
     */
    String[] after() default {};

    /**
     * Absolute ordering info, optional
     *
     * @return absolute ordering info
     */
    int order() default 0;
}

　　注解的意思也就是，自动扫描 extentions ， filter 只是其中的一个！

操作步骤：

　　1. 添加 Filter 实现如下：

@Activate(group = { Constants.PROVIDER })
public class ProviderHelloFilter implements Filter {
    private static final Logger logger = LoggerFactory.getLogger(ProviderHelloFilter.class);
    private final TraceConf conf  =   TraceConfLoader.loadAppProperties();

    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        logger.info("hello ok!");
        return invoker.invoke(invocation);

    }

}

 

　　2. 基于dubbo 的 spi 机制，添加一个配置文件: META-INF/(services|dubbo|dubbo/Interal)/com.alibaba.dubbo.rpc.Filter, 内容如下

helloFilter=com.xxx.ProviderHelloFilter

 

　　具体加载逻辑如下：

    // 初次加载所有
    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;
    }

    // synchronized in getExtensionClasses
    private Map<String, Class<?>> loadExtensionClasses() {
        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);
        loadDirectory(extensionClasses, DUBBO_DIRECTORY);
        loadDirectory(extensionClasses, SERVICES_DIRECTORY);
        return extensionClasses;
    }

 自动扫描 @Activate 注解实现如下：（com.alibaba.dubbo.common.extension.ExtensionLoader）

    /**
     * Get activate extensions.
     *
     * @param url    url
     * @param values extension point names
     * @param group  group
     * @return extension list which are activated
     * @see com.alibaba.dubbo.common.extension.Activate
     */
    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);
        if (!names.contains(Constants.REMOVE_VALUE_PREFIX + Constants.DEFAULT_KEY)) {
            getExtensionClasses();
            for (Map.Entry<String, Activate> entry : cachedActivates.entrySet()) {
                String name = entry.getKey();
                Activate activate = entry.getValue();
                if (isMatchGroup(group, activate.group())) {
                    T ext = getExtension(name);
                    if (!names.contains(name)
                            && !names.contains(Constants.REMOVE_VALUE_PREFIX + name)
                            && isActive(activate, url)) {
                        exts.add(ext);
                    }
                }
            }
            Collections.sort(exts, ActivateComparator.COMPARATOR);
        }
        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)) {
                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;
    }

 

　　而这个扫描的动作，则会在第一次调用远程服务或者在暴露远程服务时，进行操作！