183 lines
8.5 KiB
Markdown
183 lines
8.5 KiB
Markdown
# Spring Webflux
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## Concept
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### 核心机制
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#### reactive
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reactive代表基于“事件响应”的编程模型，
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#### back pressure
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在spring webflux中，`back pressure`为反应式编程的核心机制，用于协调生产者和消费者之间的速率差异，令系统在高负载或资源受限的情况下仍能稳定运行。
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- 同步场景：在同步场景下，阻塞式调用是一种天然的back pressure形式，调用方会阻塞并等待，直到被调用方执行完成
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- 非阻塞场景：在非阻塞的代码中，需要关注事件速率，生产者产生事件的速率不能压过消费者消费的速率
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##### reactive stream
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reactive stream为一个小型规范，定义了异步组件和back pressure交互的规范。reactive stream的主要用途是让Subscriber控制publisher产生数据的速率。
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### 编程模型
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`spring-web` module包含Spring webflux的响应式基础，包括若夏内容：
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- http抽象
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- 对支持server的reactive stream adapters
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- codecs
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- 核心WebHandler API，其与servlet api兼容
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Spring webflux提供了如下两种编程模型：
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- `Annotated Controllers`: 和spring mvc一致，基于相同的注解。`spring mvc`和`webflux controllers`都支持`reactive return type`，因此，很难区分`spring mvc`和`webflux controllers`
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- `Functional Endpoint`：基于lambda的、轻量的函数编程模型。其可以被看做是一个`支持对请求进行route和handle`的工具集合。
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### 并发模型
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#### spring mvc
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在`spring mvc`（通常为servlet应用）中，其假设当前线程可能会被阻塞（例如，远程调用等会阻塞当前线程）。为了减少处理请求时阻塞所带来的影响，servlet容器会使用包含大量线程的线程池。
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#### webflux
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对于`webflux`（通常为non-blocking server），其会假设应用并不会阻塞，故而，非阻塞的server可以使用一个线程数较少且固定的线程池（event loop workers）来处理请求。
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#### 调用阻塞api
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当想要在webflux中使用阻塞api时，可以按如下方式进行使用。`RxJava`和`Reactive`都支持`publushOn`操作，其支持在另一个线程中`继续执行`。
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## Reactive Core
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`spring-web`对于reactive web应用具有如下支持：
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- 对于server request的处理，拥有如下两种级别的支持：
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- `HttpHandler`： 基于`non-blocking I/O`和`Reactive Stream back pressure`，适配`Reactor Netty, Undertow, Tomcat, Jetty以及任一Servlet Container
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- `WebHandler`：稍高级别的、通用的web api，用于请求处理，在此基础上构建基于`annotated controllers`和`functional endpoints`的编程模型
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- 对于client端，`ClientHttpConnector`用于发送http请求，同时兼容`非阻塞io和Reactive Stream back pressure`
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- codecs用于客户端和服务端的序列化和反序列化
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### HttpHandler
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HttpHandler中只通过一个单独的方法来处理请求和相应，其对不同的http server api进行了最小化的抽象。
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下表描述了支持的api：
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<table class="tableblock frame-all grid-all stripes-odd stretch">
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<colgroup>
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<col style="width: 20%;">
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<col style="width: 40%;">
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<col style="width: 40%;">
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</colgroup>
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<thead>
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<tr>
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<th class="tableblock halign-left valign-top">Server name</th>
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<th class="tableblock halign-left valign-top">Server API used</th>
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<th class="tableblock halign-left valign-top">Reactive Streams support</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Netty</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Netty API</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock"><a href="https://github.com/reactor/reactor-netty" class="external" target="_blank">Reactor Netty</a></p></td>
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</tr>
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<tr>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Undertow</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Undertow API</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">spring-web: Undertow to Reactive Streams bridge</p></td>
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</tr>
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<tr>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Tomcat</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Servlet non-blocking I/O; Tomcat API to read and write ByteBuffers vs byte[]</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">spring-web: Servlet non-blocking I/O to Reactive Streams bridge</p></td>
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</tr>
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<tr>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Jetty</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Servlet non-blocking I/O; Jetty API to write ByteBuffers vs byte[]</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">spring-web: Servlet non-blocking I/O to Reactive Streams bridge</p></td>
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</tr>
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<tr>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Servlet container</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Servlet non-blocking I/O</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">spring-web: Servlet non-blocking I/O to Reactive Streams bridge</p></td>
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</tr>
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</tbody>
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</table>
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下表中描述了server依赖：
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<table class="tableblock frame-all grid-all stripes-odd stretch">
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<colgroup>
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<col style="width: 33.3333%;">
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<col style="width: 33.3333%;">
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<col style="width: 33.3334%;">
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</colgroup>
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<thead>
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<tr>
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<th class="tableblock halign-left valign-top">Server name</th>
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<th class="tableblock halign-left valign-top">Group id</th>
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<th class="tableblock halign-left valign-top">Artifact name</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Reactor Netty</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">io.projectreactor.netty</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">reactor-netty</p></td>
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</tr>
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<tr>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Undertow</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">io.undertow</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">undertow-core</p></td>
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</tr>
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<tr>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Tomcat</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">org.apache.tomcat.embed</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">tomcat-embed-core</p></td>
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</tr>
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<tr>
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<td class="tableblock halign-left valign-top"><p class="tableblock">Jetty</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">org.eclipse.jetty</p></td>
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<td class="tableblock halign-left valign-top"><p class="tableblock">jetty-server, jetty-servlet</p></td>
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</tr>
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</tbody>
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</table>
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#### server api adapters
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如下示例展示了如何使用`HttpHandler` Adapters来适配不同的Server Api：
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##### Reactor Netty
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```java
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HttpHandler handler = ...
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ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(handler);
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HttpServer.create().host(host).port(port).handle(adapter).bindNow();
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```
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##### Undertow
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```java
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HttpHandler handler = ...
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UndertowHttpHandlerAdapter adapter = new UndertowHttpHandlerAdapter(handler);
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Undertow server = Undertow.builder().addHttpListener(port, host).setHandler(adapter).build();
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server.start();
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```
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##### Tomcat
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```java
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HttpHandler handler = ...
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Servlet servlet = new TomcatHttpHandlerAdapter(handler);
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Tomcat server = new Tomcat();
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File base = new File(System.getProperty("java.io.tmpdir"));
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Context rootContext = server.addContext("", base.getAbsolutePath());
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Tomcat.addServlet(rootContext, "main", servlet);
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rootContext.addServletMappingDecoded("/", "main");
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server.setHost(host);
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server.setPort(port);
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server.start();
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```
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##### Jetty
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```java
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HttpHandler handler = ...
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Servlet servlet = new JettyHttpHandlerAdapter(handler);
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Server server = new Server();
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ServletContextHandler contextHandler = new ServletContextHandler(server, "");
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contextHandler.addServlet(new ServletHolder(servlet), "/");
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contextHandler.start();
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ServerConnector connector = new ServerConnector(server);
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connector.setHost(host);
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connector.setPort(port);
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server.addConnector(connector);
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server.start();
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```
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### WebHandler API
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`org.springframework.web.server` package基于`HttpHandler`构建，提供了通用的Web API。Web Api由多个`WebException`, 多个`WebFilter`, 一个`WebHandler`组件构成，组成了一个chain。
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