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获取FlashMap的3种方法

前言

FlashMap 是传递重定向参数的时候要用到的一个类。

getAttributes(笨重, 不推荐)

在源码 DispatcherServletdoService 方法中注入了 FlashMap

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// org.springframework.web.servlet.DispatcherServlet
public class DispatcherServlet extends FrameworkServlet {
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
if (inputFlashMap != null) {
request.setAttribute("org.springframework.web.servlet.DispatcherServlet.INPUT_FLASH_MAP", Collections.unmodifiableMap(inputFlashMap));
}
request.setAttribute("org.springframework.web.servlet.DispatcherServlet.OUTPUT_FLASH_MAP", new FlashMap());
request.setAttribute("org.springframework.web.servlet.DispatcherServlet.FLASH_MAP_MANAGER", this.flashMapManager);
}
}

所以我们可以使用 getAttributes 获取

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@Controller
public class DemoController {
@GetMapping("/redirect")
public String redirect() {
FlashMap redirectAttributes = (FlashMap) ((ServletRequestAttributes) (RequestContextHolder.getRequestAttributes()))
.getRequest().getAttribute("org.springframework.web.servlet.DispatcherServlet.OUTPUT_FLASH_MAP");
redirectAttributes.put("key", "value");
return "redirect:/demo";
}
@GetMapping("/demo")
public String demo(Model model) {
return "view.jsp";
}
}

通过 RequestContextUtils 获取

Spring早已将上面的获取代码封装到 RequestContextUtils

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@Controller
public class DemoController {
@GetMapping("/redirect")
public String redirect(HttpServletRequest request) {
FlashMap redirectAttributes = RequestContextUtils.getOutputFlashMap(request);
redirectAttributes.put("key", "value");
return "redirect:/demo";
}
@GetMapping("/demo")
public String demo(Model model) {
return "view.jsp";
}
}

// org.springframework.web.servlet.support.RequestContextUtils
public abstract class RequestContextUtils {
public static FlashMap getOutputFlashMap(HttpServletRequest request) {
return (FlashMap) request.getAttribute(DispatcherServlet.OUTPUT_FLASH_MAP_ATTRIBUTE);
}
}

Spring MVC 在 Controller 注入形参

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@Controller
public class DemoController {
@GetMapping("/redirect")
public String redirect(RedirectAttributes redirectAttributes) {
// 和上面一样, 保存到 OUTPUT_FLASH_MAP 中
redirectAttributes.addFlashAttribute("key1", "value2");
// 不保存到 flashMap 中, 拼接到 url 中
redirectAttributes.addAttribute("key2", "value2");
return "redirect:/demo";
}
@GetMapping("/demo")
public String demo(Model model) {
return "view.jsp";
}
}
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