了解了spring mvc web容器中一个http请求的全过程，能给我们提升多少武力值

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继上一篇文章什么，这年头还有人不知道404 - 掘金 (juejin.cn)后，有些同学发现，学了之后有啥用，有什么实际场景可以用到吗？程序员就是这样，不习惯于纸上谈兵，给一个场景show me code才是最实在的，好了，不扯淡了，回归正文吧！

一、场景

有这么一个场景，大家看看怎么来实现，在咱们使用sentinel（熔断限流器）alibaba/Sentinel: A powerful flow control component enabling reliability, resilience and monitoring for microservices. (面向云原生微服务的高可用流控防护组件) (github.com)时，需要在dashboard展示和编辑各种各样的数据，比如展示某个应用下集群机器列表、展示实时监控数据、规则展示、规则编辑等等。

dashboard展示图如下：

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二、需求拆解

看到这个场景后，我们能想到的就是这些数据从哪里来？又流向哪里？清楚这个后，才能制定具体的事实施方案。

1. 这些需要展示的数据从哪里来？

   客户端

2. 在dashboard上编辑规则后，这些数据流向哪里？

   客户端

三、需求实现

那么在清楚需求之后，总结起来就是一句话，客户端有数据需要传到dashborad，同样dashborad也有数据需要传到客户端。那么如何实现呢？

1. dashboard 如何知道某个app下某个接口的通讯 ip + port
2. dashboard 如何接受客户端的请求
3. 同样，客户端如何接受dashboard的请求（这是本文讲解的重点）

sentinel 的实现逻辑如下：

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根据上图，如果换做我们，那估计就是分别在客户端和dashboard上开几个接口就ok了，那么sentinel 是这么做的吗？是，也不是。我们拿dashboard从客户端读/写数据为例，在早期的sentinel版本中，并没有在客户端使用web容器开启http接口，因为它觉得使用web容器的方式太重了。不信，你看sentinel官方给出的解释

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使用web容器太过于重要级我理解有两层含义，第一就是web框架本身就比较重，其次就是有些客户端并不是使用的spring或者spring mvc 框架，为了减小依赖，sentinel提供了比较原生的实现方式。从图中可以看出，sentinel 专门写了一个transport模块用来通信，早期的transport中包含sentinel-transport-simple-http 和 sentinel-transport-netty-http两个模块，sentinel-transport-simple-http 使用的是jdk原生的socket 而sentinel-transport-netty-http采用的netty来实现http server。那么怎么实现的呢？可以简单看看，以 sentinel-transport-simple-http 模块为例，其大概得执行过程是：

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可以简单看看代码：

// HttpEventTask 类
        public void run() {
        if (socket == null) {
            return;
        }

        PrintWriter printWriter = null;
        InputStream inputStream = null;
        try {
            long start = System.currentTimeMillis();
            inputStream = new BufferedInputStream(socket.getInputStream());
            OutputStream outputStream = socket.getOutputStream();

            printWriter = new PrintWriter(
                new OutputStreamWriter(outputStream, Charset.forName(SentinelConfig.charset())));

            String firstLine = readLine(inputStream);
            CommandCenterLog.info("[SimpleHttpCommandCenter] Socket income: " + firstLine
                + ", addr: " + socket.getInetAddress());
            CommandRequest request = processQueryString(firstLine);

            if (firstLine.length() > 4 && StringUtil.equalsIgnoreCase("POST", firstLine.substring(0, 4))) {
                // Deal with post method
                processPostRequest(inputStream, request);
            }

            // Validate the target command.
            String commandName = HttpCommandUtils.getTarget(request);
            if (StringUtil.isBlank(commandName)) {
                writeResponse(printWriter, StatusCode.BAD_REQUEST, INVALID_COMMAND_MESSAGE);
                return;
            }

            // Find the matching command handler.
            CommandHandler<?> commandHandler = SimpleHttpCommandCenter.getHandler(commandName);
            if (commandHandler != null) {
                CommandResponse<?> response = commandHandler.handle(request);
                handleResponse(response, printWriter);
            } else {
                // No matching command handler.
                writeResponse(printWriter, StatusCode.BAD_REQUEST, "Unknown command `" + commandName + '`');
            }

            long cost = System.currentTimeMillis() - start;
            CommandCenterLog.info("[SimpleHttpCommandCenter] Deal a socket task: " + firstLine
                + ", address: " + socket.getInetAddress() + ", time cost: " + cost + " ms");
        } catch (RequestException e) {
            writeResponse(printWriter, e.getStatusCode(), e.getMessage());
        } catch (Throwable e) {
            CommandCenterLog.warn("[SimpleHttpCommandCenter] CommandCenter error", e);
            try {
                if (printWriter != null) {
                    String errorMessage = SERVER_ERROR_MESSAGE;
                    e.printStackTrace();
                    if (!writtenHead) {
                        writeResponse(printWriter, StatusCode.INTERNAL_SERVER_ERROR, errorMessage);
                    } else {
                        printWriter.println(errorMessage);
                    }
                    printWriter.flush();
                }
            } catch (Exception e1) {
                CommandCenterLog.warn("Failed to write error response", e1);
            }
        } finally {
            closeResource(inputStream);
            closeResource(printWriter);
            closeResource(socket);
        }
    }

CommandHandler<?> commandHandler = SimpleHttpCommandCenter.getHandler(commandName); 这行代码就是根据commandName 获取 CommandHandler，CommandHandler 是一个顶层接口，其实现类上定义了一个@CommandMapping，该注解中有个name字段，用来定义command路径，这里有点类似 @RequestMapping的味道，具体代码如下：

@CommandMapping(name = "tree", desc = "get metrics in tree mode, use id to specify detailed tree root")
public class FetchTreeCommandHandler implements CommandHandler<String> {

    @Override
    public CommandResponse<String> handle(CommandRequest request) {
        String id = request.getParam("id");

        StringBuilder sb = new StringBuilder();

        DefaultNode start = Constants.ROOT;

        if (id == null) {
            visitTree(0, start, sb);
        } else {
            boolean exactly = false;
            for (Node n : start.getChildList()) {
                DefaultNode dn = (DefaultNode)n;
                if (dn.getId().getName().equals(id)) {
                    visitTree(0, dn, sb);
                    exactly = true;
                    break;
                }
            }

            if (!exactly) {
                for (Node n : start.getChildList()) {
                    DefaultNode dn = (DefaultNode)n;
                    if (dn.getId().getName().contains(id)) {
                        visitTree(0, dn, sb);
                    }
                }
            }
        }
        sb.append("\r\n\r\n");
        sb.append(
            "t:threadNum  pq:passQps  bq:blockQps  tq:totalQps  rt:averageRt  prq: passRequestQps 1mp:1m-pass "
                + "1mb:1m-block 1mt:1m-total").append("\r\n");
        return CommandResponse.ofSuccess(sb.toString());
    }

    private void visitTree(int level, DefaultNode node, /*@NonNull*/ StringBuilder sb) {
        for (int i = 0; i < level; ++i) {
            sb.append("-");
        }
        if (!(node instanceof EntranceNode)) {
            sb.append(String.format("%s(t:%s pq:%s bq:%s tq:%s rt:%s prq:%s 1mp:%s 1mb:%s 1mt:%s)",
                node.getId().getShowName(), node.curThreadNum(), node.passQps(),
                node.blockQps(), node.totalQps(), node.avgRt(), node.successQps(),
                node.totalRequest() - node.blockRequest(), node.blockRequest(),
                node.totalRequest())).append("\n");
        } else {
            sb.append(String.format("EntranceNode: %s(t:%s pq:%s bq:%s tq:%s rt:%s prq:%s 1mp:%s 1mb:%s 1mt:%s)",
                node.getId().getShowName(), node.curThreadNum(), node.passQps(),
                node.blockQps(), node.totalQps(), node.avgRt(), node.successQps(),
                node.totalRequest() - node.blockRequest(), node.blockRequest(),
                node.totalRequest())).append("\n");
        }
        for (Node n : node.getChildList()) {
            DefaultNode dn = (DefaultNode)n;
            visitTree(level + 1, dn, sb);
        }
    }
}

这样我们请求接口http://localhost:10000/tree?type=root时，其返回结果如下：

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同样，sentinel-transport-netty-http 也是类似的逻辑！这样看来一切安好。

四、spring-mvc 模式通信兼容

直到有一天有人提出以下问题：

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总的来看就是现在和dashboard交互的端口需要和sprinboot web 应用共用一个端口。那现在有个难题。由于已经存在 sentinel-transport-simple-http和sentinel-transport-netty-http 模块，底层设计采用的是 CommandHandler 来适配各类请求，那么如果是以web容器来执行mvc模式的请求该如何兼容呢？

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江湖中不缺好手，时隔半年后，有人提出，在不改变底层设计的情况下，只需要实现HandlerAdapter 和 HandlerMapping 即可，看到这里是不是觉得很熟悉，HandlerAdapter 和 HandlerMapping不就是大名鼎鼎的处理器适配器和处理器映射器吗？咱们回顾下，用大白话说HandlerMapping的作用就是根据url路径找handler， HandlerAdapter就是对handler进行装饰，忽略底层细节，对上层提供统一的调用方法来进行handler处理。那么sentinel是怎么做的呢？我们看看

public class SentinelApiHandlerAdapter implements HandlerAdapter, Ordered {

    private int order = Ordered.LOWEST_PRECEDENCE;

    public void setOrder(int order) {
        this.order = order;
    }

    @Override
    public int getOrder() {
        return order;
    }

    @Override
    public boolean supports(Object handler) {
        return handler instanceof SentinelApiHandler;
    }

    @Override
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        SentinelApiHandler sentinelApiHandler = (SentinelApiHandler) handler;
        // 调用底层的CommandHandler接口
        sentinelApiHandler.handle(request, response);
        return null;
    }

    @Override
    public long getLastModified(HttpServletRequest request, Object handler) {
        return -1;
    }
}

public class SentinelApiHandlerMapping extends AbstractHandlerMapping implements ApplicationListener {

    private static final String SPRING_BOOT_WEB_SERVER_INITIALIZED_EVENT_CLASS = "org.springframework.boot.web.context.WebServerInitializedEvent";
    private static Class webServerInitializedEventClass;

    static {
        try {
            webServerInitializedEventClass = ClassUtils.forName(SPRING_BOOT_WEB_SERVER_INITIALIZED_EVENT_CLASS, null);
            RecordLog.info("[SentinelApiHandlerMapping] class {} is present, this is a spring-boot app, we can auto detect port", SPRING_BOOT_WEB_SERVER_INITIALIZED_EVENT_CLASS);
        } catch (ClassNotFoundException e) {
            RecordLog.info("[SentinelApiHandlerMapping] class {} is not present, this is not a spring-boot app, we can not auto detect port", SPRING_BOOT_WEB_SERVER_INITIALIZED_EVENT_CLASS);
        }
    }

    final static Map<String, CommandHandler> handlerMap = new ConcurrentHashMap<>();

    private boolean ignoreInterceptor = true;

    public SentinelApiHandlerMapping() {
        setOrder(Ordered.LOWEST_PRECEDENCE - 10);
    }

    @Override
    protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
        String commandName = request.getRequestURI();
        if (commandName.startsWith("/")) {
            commandName = commandName.substring(1);
        }
        // 获取底层CommandHandler
        CommandHandler commandHandler = handlerMap.get(commandName);
        return commandHandler != null ? new SentinelApiHandler(commandHandler) : null;
    }

    @Override
    protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
        return ignoreInterceptor ? new HandlerExecutionChain(handler) : super.getHandlerExecutionChain(handler, request);
    }

    public void setIgnoreInterceptor(boolean ignoreInterceptor) {
        this.ignoreInterceptor = ignoreInterceptor;
    }

    public static void registerCommand(String commandName, CommandHandler handler) {
        if (StringUtil.isEmpty(commandName) || handler == null) {
            return;
        }

        if (handlerMap.containsKey(commandName)) {
            CommandCenterLog.warn("[SentinelApiHandlerMapping] Register failed (duplicate command): " + commandName);
            return;
        }

        handlerMap.put(commandName, handler);
    }

    public static void registerCommands(Map<String, CommandHandler> handlerMap) {
        if (handlerMap != null) {
            for (Map.Entry<String, CommandHandler> e : handlerMap.entrySet()) {
                registerCommand(e.getKey(), e.getValue());
            }
        }
    }

    @Override
    public void onApplicationEvent(ApplicationEvent applicationEvent) {
        if (webServerInitializedEventClass != null && webServerInitializedEventClass.isAssignableFrom(applicationEvent.getClass())) {
            Integer port = null;
            try {
                BeanWrapper beanWrapper = new BeanWrapperImpl(applicationEvent);
                port = (Integer) beanWrapper.getPropertyValue("webServer.port");
            } catch (Exception e) {
                RecordLog.warn("[SentinelApiHandlerMapping] resolve port from event " + applicationEvent + " fail", e);
            }
            if (port != null && TransportConfig.getPort() == null) {
                RecordLog.info("[SentinelApiHandlerMapping] resolve port {} from event {}", port, applicationEvent);
                TransportConfig.setRuntimePort(port);
            }
        }
    }
}

后来sentinel官方也采用了这种方法做了升级，sentinel 1.8.2 升级说明如下：

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好了，看到这里，你是否对spring mvc web容器下的http请求过程有了更深的理解呢？

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