Why Passing HttpServletRequest to Async Threads Causes Bugs in Spring Boot

When a HttpServletRequest is passed to a new thread, Tomcat may recycle the request after the original servlet method returns, causing parameters to become unavailable and leading to unexpected null values or missing query parameters.

The author reproduces the problem with a simple Spring Boot controller that starts a new thread and calls request.getParameter(...) . The first request works, but the second request fails because the request object has been recycled and the internal flag didQueryParameters is already true , so Tomcat skips parsing the query string.

Debugging Tomcat’s org.apache.catalina.connector.Request and org.apache.tomcat.util.http.Parameters reveals that Tomcat reuses request objects via the recycle() method, and parameter parsing occurs only once per request unless asynchronous processing is enabled.

The Servlet specification states that a request is valid only within the servlet’s service method unless startAsync() is invoked. When startAsync() is used, the request remains alive until AsyncContext.complete() is called.

Correct usage is to start asynchronous processing with request.startAsync() , perform work in another thread, and finish with asyncContext.complete() . This prevents premature recycling of the request.

Key takeaway: avoid passing HttpServletRequest to ordinary threads; use the servlet async API for safe asynchronous operations.