instance variable

–> An instance variable of a servlet belongs to the one instance of the servlet and is shared by any threads that request the servlet.

–> You can initialize an instance variable in the init method.

–> If you don’t want two or more threads to modify an instance variable at the same time, you must synchronize the access to the instance variables.

–> To synchronize access to a block of code, you can use the synchronized keyword and the this keyword.

How to limit the use of code to a single thread at  three different levels. First, you can synchronize a block of code by using the synchronized and this keywords. Second, you can synchronize an entire method. Third, you can use the SingleThreadModel interface to limit the use of  an entire servlet to a single thread. That way, you don’t have to synchronize any  of the code within the servlet.

you must remember that one thread has to  wait while another thread is using a synchronized block of code, a synchronized method, or a single-thread servlet.