Exception: “java.lang.IllegalStateException: Cannot call sendError() after the response has been committed”

The error message “java.lang.IllegalStateException: Cannot call sendError() after the response has been committed” indicates that your code is attempting to call the sendError() method on the response object after the response has already been committed. Once the response is committed, you cannot modify the response headers or status. Here are some common reasons why this error might occur and suggestions on how to address them:

1. Response Already Committed:
   • Ensure that you are not attempting to modify the response after it has been sent to the client. Once any content has been written to the response stream or the response headers have been sent, you cannot make changes.
2. Exception Handling:
   • If you are calling sendError() as part of exception handling, make sure that it is not being called after content has already been written to the response. Also, check if you are handling exceptions properly, and if necessary, use a try-catch block to prevent exceptions from reaching the point where sendError() is called.
3. Check for Filters and Interceptors:
   • If you have filters or interceptors in your application, they might be modifying the response before your code reaches the point where sendError() is called. Ensure that any filters or interceptors are not interfering with the response.
4. Response Headers Modification:
   • Check if you are modifying response headers after the response has been committed. Any attempt to modify headers after committing the response can result in this error.
5. Concurrency Issues:
   • If your application is multi-threaded, ensure that there are no concurrency issues where one thread is trying to modify the response while another has already committed it.

Lets understand this with an example, Suppose you have a Spring MVC controller method that handles a request, and you want to send an error response if a certain condition is not met. In this example, if the condition is not met, the controller method throws a custom exception (CustomException) to trigger the exception handling block. The exception handling block catches the exception and sends an error response using ResponseEntity.

@RestController
@RequestMapping("/api")
public class CustomController {

    @GetMapping("/security")
    public ResponseEntity<String> exampleMethod() {
        try {
            
            if (!paasword.ispresent) {
                // Send an error response
                throw new CustomException("Condition not met");
            }

            // Normal processing if the condition is met
            return ResponseEntity.ok("Success");
        } catch (CustomException e) {
            // Handle the exception and send an error response
            return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("Bad Request: " + e.getMessage());
        }
    }
}

A common mistake that might lead to the “Cannot call sendError() after the response has been committed” error is if you try to modify the response headers or body after an error has occurred.

} catch (CustomException e) {
    // Attempt to modify the response after it has been committed
    response.setHeader("Custom-Header", "Value");
    return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("Bad Request: " + e.getMessage());
}

To avoid the error, ensure that any modifications to the response (headers, status, body) are done before sending the response. In the example, you can modify the response without causing issues:

} catch (CustomException e) {
    // Modify the response without issues
    HttpHeaders headers = new HttpHeaders();
    headers.add("Custom-Header", "Value");
    return ResponseEntity.status(HttpStatus.BAD_REQUEST).headers(headers).body("Bad Request: " + e.getMessage());
}