Spring-MVC非同步請求之Servlet非同步處理
在Servlet3.0的規範中新增了對非同步請求的支援，SpringMVC又在此基礎上對非同步請求提供了方便。
非同步請求是在處理比較耗時的業務時先將request返回，然後另起執行緒處理耗時的業務，處理完後在返回給使用者。
非同步請求可以給我們帶來很多方便，最直接的用法就是處理耗時的業務，比如，需要查詢資料庫，需要呼叫別的伺服器來處理等情況下可以先將請求返回給客戶端，然後啟用新執行緒處理耗時業務。
如果我們合適的擴充套件可以實現訂閱者模式的訊息訂閱功能，比如，當有異常情況發生時可以主動將相關資訊傳送給運維人員，還有現在的很多郵箱自動回覆都是使用這種技術。
Http協議是單向的，只能客戶端自己拉不能伺服器主動推，Servlet對非同步請求的支援並沒有修改Http,而是對Http的巧妙利用。非同步請求的核心原理主要分為兩大類，一類是輪詢，另一類是長連線。
輪詢就是定時自動發起請求檢查有沒有需要返回的資料，這種對資源浪費比較大。長連線的原理是客戶端發起請求，服務端處理並返回後並不結束連線，這樣就可以在後面再次返回給客戶端資料。
Servlet對非同步請求的支援其實採用的是長連線的方式，也就是說，非同步請求中在原始的請求返回的時候並沒有關閉連線，關閉的只是處理請求的那個縣城，只有在非同步請求全部處理完之後才會關閉連線。
Servlet3.0對非同步請求的支援
在Servlet3.0規範中使用非同步處理請求非常簡單，只需要在請求處理過程中呼叫request的startAsync返回AsyncContext。
什麼是AsyncContext在非同步請求中充當著非常重要的角色，可以稱為非同步請求上下文也可以稱為非同步請求容器。類似於ServletContext.我們多次呼叫startAsync都是返回的同一個AsyncContext。程式碼如下：
public interface AsyncContext {
String ASYNC_REQUEST_URI = "javax.servlet.async.request_uri";
String ASYNC_CONTEXT_PATH = "javax.servlet.async.context_path";
String ASYNC_PATH_INFO = "javax.servlet.async.path_info";
String ASYNC_SERVLET_PATH = "javax.servlet.async.servlet_path";
String ASYNC_QUERY_STRING = "javax.servlet.async.query_string";
ServletRequest getRequest();
ServletResponse getResponse();
boolean hasOriginalRequestAndResponse();
void dispatch();
void dispatch(String var1);
void dispatch(ServletContext var1,String var2);
void complete();
void start(Runnable var1);
void addListener(AsyncListener var1);
void addListener(AsyncListener var1,ServletRequest var2,ServletResponse var3);
<T extends AsyncListener> T createListener(Class<T> var1) throws ServletException;
void setTimeout(long var1);
long getTimeout();
}
getResponse() 用於獲取response。dispatch用於分發新地址。complete用於通知容器已經處理完了，start方法用於啟動實際處理執行緒，addListener用於新增監聽器;setTimeout方法用於修改超時時間。
Servlet3.0處理非同步請求例項
@WebServlet(
name = “WorkServlet”,urlPatterns = “/work”,asyncSupported = true
)
public class WorkServlet extends HttpServlet {
private static final long serialVersionUID =1L;
@Override
protected void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException {
this.doPost(req,resp);
}
@Override
protected void doPost(HttpServletRequest req,IOException {
//設定ContentType,關閉快取
resp.setContentType("text/plain;charset=UTF-8");
resp.setHeader("Cache-Control","private");
resp.setHeader("Pragma","no-cache");
final PrintWriter writer= resp.getWriter();
writer.println("老師檢查作業了");
writer.flush();
List<String> zuoyes=new ArrayList<String>();
for (int i = 0; i < 10; i++) {
zuoyes.add("zuoye"+i);;
}
final AsyncContext ac=req.startAsync();//開啟非同步請求
doZuoye(ac,zuoyes);
writer.println("老師佈置作業");
writer.flush();
}
private void doZuoye(final AsyncContext ac,final List<String> zuoyes) {
ac.setTimeout(1*60*60*1000L);
ac.start(new Runnable() {
@Override
public void run() {
//通過response獲得字元輸出流
try {
PrintWriter writer=ac.getResponse().getWriter();
for (String zuoye:zuoyes) {
writer.println("\""+zuoye+"\"請求處理中");
Thread.sleep(1*1000L);
writer.flush();
}
ac.complete();
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
非同步請求監聽器
在上面的程式是我們最基本的非同步請求，不過不夠完善。老師是需要思考巨集觀問題，所以在寫完作業之後需要給老師彙報哪些題難，哪些題目有問題或者自己的這次經驗總結，不過這些事不應該由做作業的學生來做，應該由專門的學習彙報員來統計分析。所以就有了監聽器。
public class TeacherListener implements AsyncListener {
final SimpleDateFormat formatter=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
@Override
public void onComplete(AsyncEvent event) throws IOException {
System.out.println("在"+formatter.format(new Date())+"工作處理完成");
}
@Override
public void onTimeout(AsyncEvent event) throws IOException {
System.out.println("在"+formatter.format(new Date())+"工作超時");
}
@Override
public void onError(AsyncEvent event) throws IOException {
System.out.println("在"+formatter.format(new Date())+"工作處理錯誤");
}
@Override
public void onStartAsync(AsyncEvent event) throws IOException {
System.out.println("在"+formatter.format(new Date())+"工作處理開始");
}
}
所有程式碼具體參照github地址
https://github.com/lzggsimida123/ServletAsync
補充：SpringMVC對Servlet3非同步請求的支援
SpringMVC對Servlet3非同步請求的支援有兩種方式，分別是通過處理器方法返回Callable和DeferredResult。
按照Servlet3的規範，支援非同步請求時需要配置對應的Servlet和Filter支援非同步請求，為了使SpringMVC支援非同步請求的處理，需要在定義DispatcherServlet時配置其支援非同步請求，在DispatcherServlet之前定義的Filter也需要配置支援非同步請求。
<servlet>
<servlet-name>springmvc</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/applicationContext-mvc.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
<!-- 啟用非同步支援 -->
<async-supported>true</async-supported>
</servlet>
<servlet-mapping>
<servlet-name>springmvc</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
返回Callable
當處理器的返回方法是Callable型別時會預設發起非同步請求，並使用一個TaskExecutor來呼叫返回的Callable，之後的處理就跟正常的SpringMVC請求是一樣的。Callable的返回結果也跟正常請求SpringMVC的一樣，可以返回Model、ModelAndView、String、Object等，也可以結合@ResponseBody使用，具體可以參考CallableMethodReturnValueHandler的handleReturnValue()。
@RequestMapping("/callable")
public Callable<String> forCallable(Model model) throws Exception {
return () -> {
TimeUnit.SECONDS.sleep(1);//睡眠1秒，模仿某些業務操作
model.addAttribute("a","aaaaaaa");
return "async_request_callable";
};
}
如果需要針對於單個Callable請求指定超時時間，我們可以把Callable用一個WebAsyncTask包裹起來。然後還可以指定超時回撥和正常處理完成的回撥。
@RequestMapping("/callable/timeout")
public WebAsyncTask<String> forCallableWithTimeout(Model model) throws Exception {
long timeout = 5 * 1000L;
WebAsyncTask<String> asyncTask = new WebAsyncTask<>(timeout,() -> {
TimeUnit.MILLISECONDS.sleep(timeout + 10);
model.addAttribute("a","aaaaaaa");
return "async_request_callable";
});
asyncTask.onTimeout(() -> {
System.out.println("響應超時回撥");
return "async_request_callable_timeout";
});
asyncTask.onCompletion(() -> {
System.out.println("響應callable呼叫完成的回撥");
});
return asyncTask;
}
返回DeferredResult
使用DeferredResult的返回結果的程式設計通常是在處理器方法中建立一個DeferredResult例項，把它儲存起來後再進行返回，比如儲存到一個佇列中，然後在另外的一個執行緒中會從這個佇列中拿到相應的DeferredResult物件進行相應的業務處理後會往DeferredResult中設定對應的返回值。返回了DeferredResult後SpringMVC將建立一個DeferredResultHandler用於監聽DeferredResult，一旦DeferredResult中設定了返回值後，DeferredResultHandler就將對返回值進行處理。DeferredResult的處理過程見DeferredResultMethodReturnValueHandler的handleReturnValue()。
@RequestMapping("/deferredresult")
public DeferredResult<String> forDeferredResult() throws Exception {
DeferredResult<String> result = new DeferredResult<>();
new Thread(() -> {
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
result.setResult("async_request_deferredresult");
}).start();
return result;
}
對於DeferredResult也是可以單獨指定超時時間和超時後的回撥的，它的超時時間可以直接通過建構函式傳遞，單位是毫秒。
@RequestMapping("/deferredresult/timeout")
public DeferredResult<String> forDeferredResultWithTimeout() throws Exception {
DeferredResult<String> result = new DeferredResult<>(10 * 1000);
new Thread(() -> {
try {
TimeUnit.SECONDS.sleep(31);
} catch (InterruptedException e) {
e.printStackTrace();
}
result.setResult("async_request_deferredresult");
}).start();
result.onTimeout(() -> {
System.out.println("響應超時回撥函式");
});
result.onCompletion(() -> {
System.out.println("響應完成的回撥函式");
});
return result;
}
配置
可以通過<mvc:annotation-driven/>的子元素<mvc:async-support/>來定義處理非同步請求預設的超時時間和需要使用的TaskExecutor。如果不指定預設超時時間則預設會使用容器的非同步請求超時時間，如果不指定需要使用的TaskExecutor，則預設會使用一個SimpleAsyncTaskExecutor。在下面的配置中我們就配置了預設的超時時間是15秒，且處理非同步請求的TaskExecutor是bean容器中名為asyncTaskExecutor的TaskExecutor。
<mvc:annotation-driven> <mvc:async-support default-timeout="15000" task-executor="asyncTaskExecutor"/> </mvc:annotation-driven>
攔截器
返回Callable型別的請求可以通過實現CallableProcessingInterceptor介面自定義一個攔截器來攔截，也可以通過繼承CallableProcessingInterceptorAdapter抽象類來定義攔截器，這樣就只需要選擇自己感興趣的方法進行實現。CallableProcessingInterceptor介面定義如下：
public interface CallableProcessingInterceptor {
static final Object RESULT_NONE = new Object();
static final Object RESPONSE_HANDLED = new Object();
/**
* Invoked <em>before</em> the start of concurrent handling in the original
* thread in which the {@code Callable} is submitted for concurrent handling.
*
* <p>
* This is useful for capturing the state of the current thread just prior to
* invoking the {@link Callable}. Once the state is captured,it can then be
* transferred to the new {@link Thread} in
* {@link #preProcess(NativeWebRequest,Callable)}. Capturing the state of
* Spring Security's SecurityContextHolder and migrating it to the new Thread
* is a concrete example of where this is useful.
* </p>
*
* @param request the current request
* @param task the task for the current async request
* @throws Exception in case of errors
*/
<T> void beforeConcurrentHandling(NativeWebRequest request,Callable<T> task) throws Exception;
/**
* Invoked <em>after</em> the start of concurrent handling in the async
* thread in which the {@code Callable} is executed and <em>before</em> the
* actual invocation of the {@code Callable}.
*
* @param request the current request
* @param task the task for the current async request
* @throws Exception in case of errors
*/
<T> void preProcess(NativeWebRequest request,Callable<T> task) throws Exception;
/**
* Invoked <em>after</em> the {@code Callable} has produced a result in the
* async thread in which the {@code Callable} is executed. This method may
* be invoked later than {@code afterTimeout} or {@code afterCompletion}
* depending on when the {@code Callable} finishes processing.
*
* @param request the current request
* @param task the task for the current async request
* @param concurrentResult the result of concurrent processing,which could
* be a {@link Throwable} if the {@code Callable} raised an exception
* @throws Exception in case of errors
*/
<T> void postProcess(NativeWebRequest request,Callable<T> task,Object concurrentResult) throws Exception;
/**
* Invoked from a container thread when the async request times out before
* the {@code Callable} task completes. Implementations may return a value,* including an {@link Exception},to use instead of the value the
* {@link Callable} did not return in time.
*
* @param request the current request
* @param task the task for the current async request
* @return a concurrent result value; if the value is anything other than
* {@link #RESULT_NONE} or {@link #RESPONSE_HANDLED},concurrent processing
* is resumed and subsequent interceptors are not invoked
* @throws Exception in case of errors
*/
<T> Object handleTimeout(NativeWebRequest request,Callable<T> task) throws Exception;
/**
* Invoked from a container thread when async processing completes for any
* reason including timeout or network error.
*
* @param request the current request
* @param task the task for the current async request
* @throws Exception in case of errors
*/
<T> void afterCompletion(NativeWebRequest request,Callable<T> task) throws Exception;
}
它的配置是通過<mvc:callable-interceptors/>配置的。
<mvc:annotation-driven>
<mvc:async-support default-timeout="15000" task-executor="asyncTaskExecutor">
<mvc:callable-interceptors>
<bean class="YourCallableProcessingInterceptor"/>
</mvc:callable-interceptors>
</mvc:async-support>
</mvc:annotation-driven>
返回DeferredResult的也可以進行攔截，這需要我們實現DeferredResultProcessingInterceptor介面或者繼承自DeferredResultProcessingInterceptorAdapter。DeferredResultProcessingInterceptor介面定義如下：
public interface DeferredResultProcessingInterceptor {
/**
* Invoked immediately before the start of concurrent handling,in the same
* thread that started it. This method may be used to capture state just prior
* to the start of concurrent processing with the given {@code DeferredResult}.
*
* @param request the current request
* @param deferredResult the DeferredResult for the current request
* @throws Exception in case of errors
*/
<T> void beforeConcurrentHandling(NativeWebRequest request,DeferredResult<T> deferredResult) throws Exception;
/**
* Invoked immediately after the start of concurrent handling,in the same
* thread that started it. This method may be used to detect the start of
* concurrent processing with the given {@code DeferredResult}.
*
* <p>The {@code DeferredResult} may have already been set,for example at
* the time of its creation or by another thread.
*
* @param request the current request
* @param deferredResult the DeferredResult for the current request
* @throws Exception in case of errors
*/
<T> void preProcess(NativeWebRequest request,DeferredResult<T> deferredResult) throws Exception;
/**
* Invoked after a {@code DeferredResult} has been set,via
* {@link DeferredResult#setResult(Object)} or
* {@link DeferredResult#setErrorResult(Object)},and is also ready to
* handle the concurrent result.
*
* <p>This method may also be invoked after a timeout when the
* {@code DeferredResult} was created with a constructor accepting a default
* timeout result.
*
* @param request the current request
* @param deferredResult the DeferredResult for the current request
* @param concurrentResult the result to which the {@code DeferredResult}
* @throws Exception in case of errors
*/
<T> void postProcess(NativeWebRequest request,DeferredResult<T> deferredResult,Object concurrentResult) throws Exception;
/**
* Invoked from a container thread when an async request times out before
* the {@code DeferredResult} has been set. Implementations may invoke
* {@link DeferredResult#setResult(Object) setResult} or
* {@link DeferredResult#setErrorResult(Object) setErrorResult} to resume processing.
*
* @param request the current request
* @param deferredResult the DeferredResult for the current request; if the
* {@code DeferredResult} is set,then concurrent processing is resumed and
* subsequent interceptors are not invoked
* @return {@code true} if processing should continue,or {@code false} if
* other interceptors should not be invoked
* @throws Exception in case of errors
*/
<T> boolean handleTimeout(NativeWebRequest request,DeferredResult<T> deferredResult) throws Exception;
/**
* Invoked from a container thread when an async request completed for any
* reason including timeout and network error. This method is useful for
* detecting that a {@code DeferredResult} instance is no longer usable.
*
* @param request the current request
* @param deferredResult the DeferredResult for the current request
* @throws Exception in case of errors
*/
<T> void afterCompletion(NativeWebRequest request,DeferredResult<T> deferredResult) throws Exception;
}
自定義的DeferredResultProcessingInterceptor是通過<mvc:deferred-result-interceptors>配置的。
<mvc:annotation-driven>
<mvc:async-support default-timeout="15000" task-executor="asyncTaskExecutor">
<mvc:deferred-result-interceptors>
<bean class="YourDeferredResultProcessingInterceptor"/>
</mvc:deferred-result-interceptors>
</mvc:async-support>
</mvc:annotation-driven>
當發起非同步請求時，SpringMVC傳統的HandlerInterceptor的postHandle()和afterCompletion()不會執行，但是等非同步請求結束後它們還是會執行的。如果需要在非同步處理完成之後做一些事情，也可以選擇實現AsyncHandlerInterceptor介面的afterConcurrentHandlingStarted()，AsyncHandlerInterceptor介面繼承了HandlerInterceptor。
（注：本文是基於Spring4.1.0所寫）
以上為個人經驗，希望能給大家一個參考，也希望大家多多支援我們。如有錯誤或未考慮完全的地方，望不吝賜教。