${3+12} <% request.setAttribute("mess","你好"); %>为啥报错啦

Null is a term used in computer programming to indicate the absence of a value or a reference to a nonexistent or undefined object. It is commonly represented as a value of zero or an empty string. In programming languages such as Java, C++, and Python, null is often used to signify the absence of a value in a variable or an object. It is also used to indicate the end of a data structure or a file. Null is an important concept in programming as it helps prevent errors and ensures the correct functioning of programs.