SpringMVC @SessionAttributes 使用详解以及源码分析

原文地址：https://www.cnblogs.com/daxin/p/SessionAttributes.html

@sessionattributes

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface SessionAttributes {
String[] value() default {};
Class[] types() default {};
}

 @sessionattributes注解应用到Controller上面，可以将Model中的属性同步到session当中。

先看一个最基本的方法

@Controller
@RequestMapping("/Demo.do")
@SessionAttributes(value={"attr1","attr2"})
public class Demo {

@RequestMapping(params="method=index")
public ModelAndView index() {
ModelAndView mav = new ModelAndView("index.jsp");
mav.addObject("attr1", "attr1Value");
mav.addObject("attr2", "attr2Value");
return mav;
}

@RequestMapping(params="method=index2")
public ModelAndView index2(@ModelAttribute("attr1")String attr1, @ModelAttribute("attr2")String attr2) {
ModelAndView mav = new ModelAndView("success.jsp");
return mav;
}
}

index方法返回一个ModelAndView 其中包括视图index.jsp 和 两个键值放入model当中，在没有加入@sessionattributes注解的时候，放入model当中的键值是request级别的。

现在因为在Controller上面标记了@SessionAttributes(value={"attr1","attr2"}) 那么model中的attr1,attr2会同步到session中，这样当你访问index 然后在去访问index2的时候也会获取这俩个属性的值。

当需要清除session当中的值得时候，我们只需要在controller的方法中传入一个SessionStatus的类型对象 通过调用setComplete方法就可以清除了。

 

@RequestMapping(params="method=index3")
　　public ModelAndView index4(SessionStatus status) {
　　ModelAndView mav = new ModelAndView("success.jsp");
　　status.setComplete();
　　return mav;
}

 

 

 

 

下面就直接分析代码来看Spring是如可封装的。

首先我们需要看2个类 DefaultSessionAttributeStore和SessionAttributesHandler

DefaultSessionAttributeStore这个类是用来往WebRequest存取数据的工具类，WebRequest是Spring包装的HttpServletRequest，大家理解为普通的HttpServletRequest就行了。

public class DefaultSessionAttributeStore implements SessionAttributeStore {

private String attributeNamePrefix = "";

public void setAttributeNamePrefix(String attributeNamePrefix) {
this.attributeNamePrefix = (attributeNamePrefix != null ? attributeNamePrefix : "");
}

public void storeAttribute(WebRequest request, String attributeName, Object attributeValue) {
Assert.notNull(request, "WebRequest must not be null");
Assert.notNull(attributeName, "Attribute name must not be null");
Assert.notNull(attributeValue, "Attribute value must not be null");
String storeAttributeName = getAttributeNameInSession(request, attributeName);
request.setAttribute(storeAttributeName, attributeValue, WebRequest.SCOPE_SESSION);
}

public Object retrieveAttribute(WebRequest request, String attributeName) {
Assert.notNull(request, "WebRequest must not be null");
Assert.notNull(attributeName, "Attribute name must not be null");
String storeAttributeName = getAttributeNameInSession(request, attributeName);
return request.getAttribute(storeAttributeName, WebRequest.SCOPE_SESSION);
}

public void cleanupAttribute(WebRequest request, String attributeName) {
Assert.notNull(request, "WebRequest must not be null");
Assert.notNull(attributeName, "Attribute name must not be null");
String storeAttributeName = getAttributeNameInSession(request, attributeName);
request.removeAttribute(storeAttributeName, WebRequest.SCOPE_SESSION);
}

protected String getAttributeNameInSession(WebRequest request, String attributeName) {
return this.attributeNamePrefix + attributeName;
}

}

 

Spring会为每一个Controller初始化一个SessionAttributesHandler实例,用来记录@SessionAttributes(value={"attr1","attr2"})里面的属性信息，当需要同步model的值时，会先判断是否在SessionAttributes当中定义。

public class SessionAttributesHandler {

private final Set<String> attributeNames = new HashSet<String>();

private final Set<Class<?>> attributeTypes = new HashSet<Class<?>>();

private final Set<String> knownAttributeNames = Collections.synchronizedSet(new HashSet<String>(4));

private final SessionAttributeStore sessionAttributeStore;

public SessionAttributesHandler(Class<?> handlerType, SessionAttributeStore sessionAttributeStore) {
Assert.notNull(sessionAttributeStore, "SessionAttributeStore may not be null.");
this.sessionAttributeStore = sessionAttributeStore;

SessionAttributes annotation = AnnotationUtils.findAnnotation(handlerType, SessionAttributes.class);
if (annotation != null) {
this.attributeNames.addAll(Arrays.asList(annotation.value()));
this.attributeTypes.addAll(Arrays.<Class<?>>asList(annotation.types()));
}

this.knownAttributeNames.addAll(this.attributeNames);
}

public boolean hasSessionAttributes() {
return ((this.attributeNames.size() > 0) || (this.attributeTypes.size() > 0));
}

public boolean isHandlerSessionAttribute(String attributeName, Class<?> attributeType) {
Assert.notNull(attributeName, "Attribute name must not be null");
if (this.attributeNames.contains(attributeName) || this.attributeTypes.contains(attributeType)) {
this.knownAttributeNames.add(attributeName);
return true;
}
else {
return false;
}
}

public void storeAttributes(WebRequest request, Map<String, ?> attributes) {
for (String name : attributes.keySet()) {
Object value = attributes.get(name);
Class<?> attrType = (value != null) ? value.getClass() : null;

if (isHandlerSessionAttribute(name, attrType)) {
this.sessionAttributeStore.storeAttribute(request, name, value);
}
}
}

public Map<String, Object> retrieveAttributes(WebRequest request) {
Map<String, Object> attributes = new HashMap<String, Object>();
for (String name : this.knownAttributeNames) {
Object value = this.sessionAttributeStore.retrieveAttribute(request, name);
if (value != null) {
attributes.put(name, value);
}
}
return attributes;
}

public void cleanupAttributes(WebRequest request) {
for (String attributeName : this.knownAttributeNames) {
this.sessionAttributeStore.cleanupAttribute(request, attributeName);
}
}

Object retrieveAttribute(WebRequest request, String attributeName) {
return this.sessionAttributeStore.retrieveAttribute(request, attributeName);
}

}

 

 

当我们访问controller中的一个方法时，会调用RequestMappingHandlerAdapter类当中的invokeHandleMethod的方法。

private ModelAndView invokeHandleMethod(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

ServletWebRequest webRequest = new ServletWebRequest(request, response);

WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);

ModelAndViewContainer mavContainer = new ModelAndViewContainer();
mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);
mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

AsyncWebRequest asyncWebRequest = AsyncWebUtils.createAsyncWebRequest(request, response);
asyncWebRequest.setTimeout(this.asyncRequestTimeout);

final WebAsyncManager asyncManager = AsyncWebUtils.getAsyncManager(request);
asyncManager.setTaskExecutor(this.taskExecutor);
asyncManager.setAsyncWebRequest(asyncWebRequest);

if (asyncManager.hasConcurrentResult()) {
Object result = asyncManager.getConcurrentResult();
mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
asyncManager.clearConcurrentResult();

if (logger.isDebugEnabled()) {
logger.debug("Found concurrent result value [" + result + "]");
}
requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
}

requestMappingMethod.invokeAndHandle(webRequest, mavContainer);

if (asyncManager.isConcurrentHandlingStarted()) {
return null;
}

return getModelAndView(mavContainer, modelFactory, webRequest);
}

 

我们看这行代码

modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);

跟进去看源码

public void initModel(NativeWebRequest request, ModelAndViewContainer mavContainer, HandlerMethod handlerMethod)
throws Exception {

Map<String, ?> attributesInSession = this.sessionAttributesHandler.retrieveAttributes(request);
mavContainer.mergeAttributes(attributesInSession);

invokeModelAttributeMethods(request, mavContainer);

for (String name : findSessionAttributeArguments(handlerMethod)) {
if (!mavContainer.containsAttribute(name)) {
Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
if (value == null) {
throw new HttpSessionRequiredException("Expected session attribute '" + name + "'");
}
mavContainer.addAttribute(name, value);
}
}
}

 

其中这两句是关键的地方,把session的值取出来全部放入到ModelAndViewContainer当中去，这样我就可以再controller当中的方法中获取了，当然直接从session中拿也是可以的。

Map<String, ?> attributesInSession = this.sessionAttributesHandler.retrieveAttributes(request);
mavContainer.mergeAttributes(attributesInSession);

 

在controller中获取sessionAttributes的只有两种方式。

一、public ModelAndView index2(@ModelAttribute("attr1")String attr1, @ModelAttribute("attr2")String attr2) 

二、public ModelAndView index2(ModelMap map)

这俩种方式的区别是，如果使用@ModelAttribute属性获取值，并且@SessionAttributes注解当中还设置了该属性，当属性为null时会跑出异常，因为这几行代码。

for (String name : findSessionAttributeArguments(handlerMethod)) {
if (!mavContainer.containsAttribute(name)) {
Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
if (value == null) {
throw new HttpSessionRequiredException("Expected session attribute '" + name + "'");
}
mavContainer.addAttribute(name, value);
}
}

 

 

以上分析的都是取值的过程，那么Spring是如何将Model中的数据同步到session当中的呢。

 

我们看上面invokeHandleMethod方法中最后一句话 return getModelAndView(mavContainer, modelFactory, webRequest); 我们跟进去看源码。

private ModelAndView getModelAndView(ModelAndViewContainer mavContainer,
ModelFactory modelFactory, NativeWebRequest webRequest) throws Exception {

modelFactory.updateModel(webRequest, mavContainer);

if (mavContainer.isRequestHandled()) {
return null;
}
ModelMap model = mavContainer.getModel();
ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model);
if (!mavContainer.isViewReference()) {
mav.setView((View) mavContainer.getView());
}
if (model instanceof RedirectAttributes) {
Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);
RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
}
return mav;
}

 

有一行代码是modelFactory.updateModel(webRequest, mavContainer); 我们在跟进去看源码。

我们看到首先判断一下ModelAndViewContainer中的SessionStatus是否是完成状态，如果是TRUE就清除session中的值，反之将ModelAndViewContainer中的model值设置进去。

public void updateModel(NativeWebRequest request, ModelAndViewContainer mavContainer) throws Exception {

if (mavContainer.getSessionStatus().isComplete()){
this.sessionAttributesHandler.cleanupAttributes(request);
}
else {
this.sessionAttributesHandler.storeAttributes(request, mavContainer.getModel());
}

if (!mavContainer.isRequestHandled()) {
updateBindingResult(request, mavContainer.getModel());
}
}

 

 

看到这里可能还有一个疑问，就是我们的数据时设置到ModelAndView里面的,那么这些数据时如何跑到ModelAndViewContainer当中去的呢，其实是在方法执行完成之后Spring帮我们做的，

具体细节查看 ModelAndViewMethodReturnValueHandler方法中的最后一行代码mavContainer.addAllAttributes(mav.getModel());

public void handleReturnValue(
Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws Exception {

if (returnValue == null) {
mavContainer.setRequestHandled(true);
return;
}

ModelAndView mav = (ModelAndView) returnValue;
if (mav.isReference()) {
String viewName = mav.getViewName();
mavContainer.setViewName(viewName);
if (viewName != null && viewName.startsWith("redirect:")) {
mavContainer.setRedirectModelScenario(true);
}
}
else {
View view = mav.getView();
mavContainer.setView(view);
if (view instanceof SmartView) {
if (((SmartView) view).isRedirectView()) {
mavContainer.setRedirectModelScenario(true);
}
}
}
mavContainer.addAllAttributes(mav.getModel());
}