springMVC源码分析--FlashMap和FlashMapManager重定向数据保存

在上一篇博客springMVC源码分析--页面跳转RedirectView（三）中我们看到了在RedirectView跳转时会将跳转之前的请求中的参数保存到fFlashMap中，然后通过FlashManager保存起来。

protected void renderMergedOutputModel(Map<String, Object> model, HttpServletRequest request,
            HttpServletResponse response) throws IOException {
        //创建跳转链接
        String targetUrl = createTargetUrl(model, request);
        targetUrl = updateTargetUrl(targetUrl, model, request, response);
        //获取原请求所携带的数据
        FlashMap flashMap = RequestContextUtils.getOutputFlashMap(request);
        if (!CollectionUtils.isEmpty(flashMap)) {
            UriComponents uriComponents = UriComponentsBuilder.fromUriString(targetUrl).build();
            flashMap.setTargetRequestPath(uriComponents.getPath());
            flashMap.addTargetRequestParams(uriComponents.getQueryParams());
            FlashMapManager flashMapManager = RequestContextUtils.getFlashMapManager(request);
            if (flashMapManager == null) {
                throw new IllegalStateException("FlashMapManager not found despite output FlashMap having been set");
            }
            //将数据保存起来，作为跳转之后请求的数据使用
            flashMapManager.saveOutputFlashMap(flashMap, request, response);
        }
        //重定向操作
        sendRedirect(request, response, targetUrl, this.http10Compatible);
    }  

接下来我们分别认识一下FlashMap和FalshMapManager

FlashMapManager是一个接口，定义了保存FlashMap和获取FlashMap的方法。

public interface FlashMapManager {

	//从session中获取flashMap
	FlashMap retrieveAndUpdate(HttpServletRequest request, HttpServletResponse response);

	//将falshMap保存到session中
	void saveOutputFlashMap(FlashMap flashMap, HttpServletRequest request, HttpServletResponse response);

}

两个实现方法都在AbstractFlashMapManager抽象方法中：

saveOutputFlashMap方法实现如下

@Override
	public final void saveOutputFlashMap(FlashMap flashMap, HttpServletRequest request, HttpServletResponse response) {
		if (CollectionUtils.isEmpty(flashMap)) {
			return;
		}
		String path = decodeAndNormalizePath(flashMap.getTargetRequestPath(), request);
		flashMap.setTargetRequestPath(path);
		if (logger.isDebugEnabled()) {
			logger.debug("Saving FlashMap=" + flashMap);
		}
		flashMap.startExpirationPeriod(getFlashMapTimeout());

		Object mutex = getFlashMapsMutex(request);
		if (mutex != null) {
			synchronized (mutex) {
				List<FlashMap> allFlashMaps = retrieveFlashMaps(request);
				allFlashMaps = (allFlashMaps != null ? allFlashMaps : new CopyOnWriteArrayList<FlashMap>());
				allFlashMaps.add(flashMap);
				//flashMap是在子类SessionFlashManager中
				updateFlashMaps(allFlashMaps, request, response);
			}
		}
		else {
			List<FlashMap> allFlashMaps = retrieveFlashMaps(request);
			allFlashMaps = (allFlashMaps != null ? allFlashMaps : new LinkedList<FlashMap>());
			allFlashMaps.add(flashMap);
		    //flashMap是在子类SessionFlashManager中
			updateFlashMaps(allFlashMaps, request, response);
		}
	}

updateFlashMaps方法的实现机制就是将数据保存的请求的Session中，这样跳转后的请求可以从Session中获取数据，并且所有的实现都保存在同一个Session中，为了防止不同重定向请求的数据相互影响，这边有锁进行处理控制。

@Override
	protected void updateFlashMaps(List<FlashMap> flashMaps, HttpServletRequest request, HttpServletResponse response) {
		WebUtils.setSessionAttribute(request, FLASH_MAPS_SESSION_ATTRIBUTE, (!flashMaps.isEmpty() ? flashMaps : null));
	}

retrieveAndUpdate获取FlashMap的实现如下：

@Override
	public final FlashMap retrieveAndUpdate(HttpServletRequest request, HttpServletResponse response) {
		List<FlashMap> allFlashMaps = retrieveFlashMaps(request);
		........
	}

retrieveFlashMaps中的实现机制就是从Session中获取数据

@Override
	@SuppressWarnings("unchecked")
	protected List<FlashMap> retrieveFlashMaps(HttpServletRequest request) {
		HttpSession session = request.getSession(false);
		return (session != null ? (List<FlashMap>) session.getAttribute(FLASH_MAPS_SESSION_ATTRIBUTE) : null);
	}

当重定向的请求在浏览器中重定向之后会进入的DispatcherServlet的doService方法

@Override
	protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {

		........

		//从Session中获取保存的FlashMap中的值
		FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
		if (inputFlashMap != null) {
			request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
		}
		//将值保存到request中。这样就不需要通过浏览器跳转组装链接来传递参数了
		request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
		request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

		.......

	}

FlashMap简单来说就是一个HashMap，用于数据保存。

public final class FlashMap extends HashMap<String, Object> implements Comparable<FlashMap> {

	private String targetRequestPath;

	private final MultiValueMap<String, String> targetRequestParams = new LinkedMultiValueMap<String, String>(4);

	private long expirationTime = -1;

	public void setTargetRequestPath(String path) {
		this.targetRequestPath = path;
	}
	public String getTargetRequestPath() {
		return this.targetRequestPath;
	}
	public FlashMap addTargetRequestParams(MultiValueMap<String, String> params) {
		if (params != null) {
			for (String key : params.keySet()) {
				for (String value : params.get(key)) {
					addTargetRequestParam(key, value);
				}
			}
		}
		return this;
	}

	public FlashMap addTargetRequestParam(String name, String value) {
		if (StringUtils.hasText(name) && StringUtils.hasText(value)) {
			this.targetRequestParams.add(name, value);
		}
		return this;
	}

	public MultiValueMap<String, String> getTargetRequestParams() {
		return this.targetRequestParams;
	}

	public void startExpirationPeriod(int timeToLive) {
		this.expirationTime = System.currentTimeMillis() + timeToLive * 1000;
	}

	public void setExpirationTime(long expirationTime) {
		this.expirationTime = expirationTime;
	}

	public long getExpirationTime() {
		return this.expirationTime;
	}

	public boolean isExpired() {
		return (this.expirationTime != -1 && System.currentTimeMillis() > this.expirationTime);
	}

	@Override
	public int compareTo(FlashMap other) {
		int thisUrlPath = (this.targetRequestPath != null ? 1 : 0);
		int otherUrlPath = (other.targetRequestPath != null ? 1 : 0);
		if (thisUrlPath != otherUrlPath) {
			return otherUrlPath - thisUrlPath;
		}
		else {
			return other.targetRequestParams.size() - this.targetRequestParams.size();
		}
	}

	@Override
	public boolean equals(Object other) {
		if (this == other) {
			return true;
		}
		if (!(other instanceof FlashMap)) {
			return false;
		}
		FlashMap otherFlashMap = (FlashMap) other;
		return (super.equals(otherFlashMap) &&
				ObjectUtils.nullSafeEquals(this.targetRequestPath, otherFlashMap.targetRequestPath) &&
				this.targetRequestParams.equals(otherFlashMap.targetRequestParams));
	}

	@Override
	public int hashCode() {
		int result = super.hashCode();
		result = 31 * result + ObjectUtils.nullSafeHashCode(this.targetRequestPath);
		result = 31 * result + this.targetRequestParams.hashCode();
		return result;
	}

	@Override
	public String toString() {
		return "FlashMap [attributes=" + super.toString() + ", targetRequestPath=" +
				this.targetRequestPath + ", targetRequestParams=" + this.targetRequestParams + "]";
	}

}