Spring：spring-web中DeferredResult的实现原理

﹏ヽ暗。殇╰゛Y 2024-03-16 13:39 4阅读 0赞

spring-web中DeferredResult是采用异步servlet实现的，以下是通过servlet的代码模拟DeferredResult的实现。

import java.io.IOException;
    import java.util.concurrent.Executors;
    
    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;
    
    @WebServlet(value = "/testComplete", asyncSupported = true)
    public class IssueServlet extends HttpServlet {
    
    	private static final long serialVersionUID = -2504425408734161943L;
    
    	private static final String CTX_RESULT = "CTX_RESULT";
    
    	// true: sync; false: async
    	private boolean syncHandling = true;
    
    	@Override
    	protected void doGet(HttpServletRequest request, HttpServletResponse response)
    			throws ServletException, IOException {
    		doPost(request, response);
    	}
    
    	@Override
    	protected void doPost(HttpServletRequest request, HttpServletResponse response)
    			throws ServletException, IOException {
    
    		String result = (String) request.getAttribute(CTX_RESULT);
    
    		if (result != null) {
    			response.getWriter().write(result);
    
    			return;
    		}
    
    		AsyncContext asyncContext = request.startAsync();
    
    		boolean isAsyncStarted01 = request.isAsyncStarted();
    		System.out.println("isAsyncStarted01:" + isAsyncStarted01);
    
    		// set return result mock DeferredResult.setResult(...);
    		setResult(request, asyncContext);
    
    		boolean isAsyncStarted02 = request.isAsyncStarted();
    		System.out.println("isAsyncStarted02:" + isAsyncStarted02);
    
    		if (isAsyncStarted02) {
    
    			return;
    		} else {
    			String ctxPath = request.getContextPath();
    			String url = request.getRequestURI();
    
    			System.out.println(ctxPath + "," + url);
    
    			request.getRequestDispatcher(ctxPath + url).forward(request, response);
    		}
    	}
    
    	public void setResult(HttpServletRequest request, AsyncContext asyncContext) {
    		// (1) set result before doPost return,
    		if (syncHandling) {
    			// make result
    			String ret = "this is a test";
    
    			request.setAttribute(CTX_RESULT, ret);
    
    			asyncContext.dispatch();   // 
    		} else {
    			// (2) set result after doPost return,
    			Executors.newSingleThreadExecutor().execute(new Runnable() {
    
    				public void run() {
    					try {
    						Thread.sleep(5); // mock time-consuming
    					} catch (InterruptedException e) {
    					}
    
    					// make result
    					String ret = "this is a test";
    
    					request.setAttribute(CTX_RESULT, ret);
    
    					asyncContext.dispatch();
    				}
    			});
    		}
    	}
    
    }

其中，setResult(...)是模拟DeferredResult.setResult(...)的操作，主要是将生成的结果存入request中。

生成的结果存入request中的操作可以是在当前线程中完成，也可以在其它线程中完成，但最重要的是要在结果存入request后调用asyncContext.dispatch()。

asyncContext.dispatch()主要有2个作用：

1.  通知servlet容器，结果已存入request中，可以去获取结果了
2.  通知servlet容器，再次调用当前url就可以获取到结果

于是，servlet容器会再次调用此url的servlet的doPost方法获取结果。