java实现异步回调返回给前端

在Java中实现异步回调并将结果返回给前端，通常是在Web应用开发中处理耗时操作时所采用的技术手段，以避免阻塞HTTP请求线程并提高用户体验。以下是一个简单的例子，说明如何通过Spring WebFlux或者Servlet 3.0及以上标准的异步API配合JSON响应的方式实现这一目标。

示例1 - 使用Spring WebFlux（非阻塞IO）

import org.springframework.web.bind.annotation.*;
import reactor.core.publisher.Mono;

@RestController
public class AsyncController {

    @GetMapping("/asyncTask")
    public Mono<String> asyncTask(@RequestParam("taskId") String taskId) {
        return doAsyncTask(taskId).map(result -> {
            // 这里假设doAsyncTask返回Mono对象，当异步任务完成时会填充结果
            return ResponseEntity.ok().body(result);
        });
    }

    private Mono<YourResultType> doAsyncTask(String taskId) {
        return Mono.create(sink -> {
            // 创建一个异步任务，在完成后回调
            someService.doSomethingAsync(taskId, new SomeCallback(sink));
        });
    }

    class SomeCallback implements AsyncCallback<YourResultType> {
        private final MonoSink<YourResultType> sink;

        public SomeCallback(MonoSink<YourResultType> sink) {
            this.sink = sink;
        }

        @Override
        public void onResult(YourResultType result) {
            // 当异步任务完成时，通过sink.success()传递结果
            sink.success(result);
        }

        @Override
        public void onError(Throwable error) {
            // 如果发生错误，通过sink.error()传递异常
            sink.error(error);
        }
    }
}

// 假设`YourResultType`是你异步任务的结果类型

示例2 - 使用Servlet 3.0异步API（阻塞IO但不阻塞HTTP线程）

import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(urlPatterns = "/asyncTask", asyncSupported = true)
public class AsyncServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        final AsyncContext asyncContext = request.startAsync();
        Thread taskRunner = new Thread(() -> {
            YourResultType result;
            try {
                // 执行异步任务
                result = someService.doSomethingAsync(request.getParameter("taskId"));
            } catch (Exception e) {
                handleError(asyncContext, e);
                return;
            }

            // 将结果写回响应并结束异步上下文
            writeResponse(asyncContext, result);
        });

        taskRunner.start();
    }

    private void writeResponse(AsyncContext asyncContext, YourResultType result) {
        HttpServletResponse response = (HttpServletResponse) asyncContext.getResponse();
        try {
            response.setContentType("application/json");
            response.getWriter().write(objectMapper.writeValueAsString(result));
            asyncContext.complete();
        } catch (IOException e) {
            handleError(asyncContext, e);
        }
    }

    private void handleError(AsyncContext asyncContext, Throwable error) {
        HttpServletResponse response = (HttpServletResponse) asyncContext.getResponse();
        try {
            response.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
            response.getWriter().write(error.getMessage());
            asyncContext.complete();
        } catch (IOException e) {
            // 处理写入错误...
        }
    }
}

在这两个示例中，核心思想都是启动一个异步任务，然后在任务完成后通过某种机制（如Reactor的MonoSink或Servlet的AsyncContext）将结果返回给前端。前端可以通过AJAX请求获取异步任务的结果，并根据返回的状态进行相应的处理，如展示数据或更新UI。