usually message number is stored in a file
suppose it is 1000
servlet A read num=1000
servlet B read num=1000
servlet A increase num=1001
servlet B increase num=1001
servelt A write 1001
servelt B write 1001 !
the num should be 1002!
suppose it is 1000
servlet A read num=1000
servlet B read num=1000
servlet A increase num=1001
servlet B increase num=1001
servelt A write 1001
servelt B write 1001 !
the num should be 1002!