聊聊dubbo的Filter

@SPI
public interface Filter {
    /**
     * Does not need to override/implement this method.
     */
    Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException;
​
    /**
     * Filter itself should only be response for passing invocation, all callbacks has been placed into {@link Listener}
     *
     * @param appResponse
     * @param invoker
     * @param invocation
     * @return
     */
    @Deprecated
    default Result onResponse(Result appResponse, Invoker<?> invoker, Invocation invocation) {
        return appResponse;
    }
​
    interface Listener {
​
        void onResponse(Result appResponse, Invoker<?> invoker, Invocation invocation);
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        void onError(Throwable t, Invoker<?> invoker, Invocation invocation);
    }
​
}

public class ProtocolFilterWrapper implements Protocol {
​
    private final Protocol protocol;
​
    public ProtocolFilterWrapper(Protocol protocol) {
        if (protocol == null) {
            throw new IllegalArgumentException("protocol == null");
        }
        this.protocol = protocol;
    }
​
​
​
    private static <T> Invoker<T> buildInvokerChain(final Invoker<T> invoker, String key, String group) {
        Invoker<T> last = invoker;
        List<Filter> filters = ExtensionLoader.getExtensionLoader(Filter.class).getActivateExtension(invoker.getUrl(), key, group);
​
        if (!filters.isEmpty()) {
            for (int i = filters.size() - 1; i >= 0; i--) {
                final Filter filter = filters.get(i);
                final Invoker<T> next = last;
                last = new Invoker<T>() {
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                    @Override
                    public Class<T> getInterface() {
                        return invoker.getInterface();
                    }
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                    @Override
                    public URL getUrl() {
                        return invoker.getUrl();
                    }
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                    @Override
                    public boolean isAvailable() {
                        return invoker.isAvailable();
                    }
​
                    @Override
                    public Result invoke(Invocation invocation) throws RpcException {
                        Result asyncResult;
                        try {
                            asyncResult = filter.invoke(next, invocation);
                        } catch (Exception e) {
                            // onError callback
                            if (filter instanceof ListenableFilter) {
                                Filter.Listener listener = ((ListenableFilter) filter).listener();
                                if (listener != null) {
                                    listener.onError(e, invoker, invocation);
                                }
                            }
                            throw e;
                        }
                        return asyncResult;
                    }
​
                    @Override
                    public void destroy() {
                        invoker.destroy();
                    }
​
                    @Override
                    public String toString() {
                        return invoker.toString();
                    }
                };
            }
        }
​
        return new CallbackRegistrationInvoker<>(last, filters);
    }
​
    @Override
    public int getDefaultPort() {
        return protocol.getDefaultPort();
    }
​
    @Override
    public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
        if (REGISTRY_PROTOCOL.equals(invoker.getUrl().getProtocol())) {
            return protocol.export(invoker);
        }
        return protocol.export(buildInvokerChain(invoker, SERVICE_FILTER_KEY, CommonConstants.PROVIDER));
    }
​
    @Override
    public <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException {
        if (REGISTRY_PROTOCOL.equals(url.getProtocol())) {
            return protocol.refer(type, url);
        }
        return buildInvokerChain(protocol.refer(type, url), REFERENCE_FILTER_KEY, CommonConstants.CONSUMER);
    }
​
    @Override
    public void destroy() {
        protocol.destroy();
    }
​
    static class CallbackRegistrationInvoker<T> implements Invoker<T> {
​
        private final Invoker<T> filterInvoker;
        private final List<Filter> filters;
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        public CallbackRegistrationInvoker(Invoker<T> filterInvoker, List<Filter> filters) {
            this.filterInvoker = filterInvoker;
            this.filters = filters;
        }
​
        @Override
        public Result invoke(Invocation invocation) throws RpcException {
            Result asyncResult = filterInvoker.invoke(invocation);
​
            asyncResult.thenApplyWithContext(r -> {
                for (int i = filters.size() - 1; i >= 0; i--) {
                    Filter filter = filters.get(i);
                    // onResponse callback
                    if (filter instanceof ListenableFilter) {
                        Filter.Listener listener = ((ListenableFilter) filter).listener();
                        if (listener != null) {
                            listener.onResponse(r, filterInvoker, invocation);
                        }
                    } else {
                        filter.onResponse(r, filterInvoker, invocation);
                    }
                }
                return r;
            });
​
            return asyncResult;
        }
​
        @Override
        public Class<T> getInterface() {
            return filterInvoker.getInterface();
        }
​
        @Override
        public URL getUrl() {
            return filterInvoker.getUrl();
        }
​
        @Override
        public boolean isAvailable() {
            return filterInvoker.isAvailable();
        }
​
        @Override
        public void destroy() {
            filterInvoker.destroy();
        }
    }
}