DateFormat of the format () method thread unsafe problem analysis

Recently saw "Detective Theater: Heap Memory Mysterious Overflow event" https://my.oschina.net/u/2368090/blog/1628720, so I also want to test to understand dateformat multithreading security issues.

First look at the phenomenon: do a multithreaded use of the same Simplydateformat test example:

Import Java.text.simpledateformat;import Java.util.date;public class Dateformattest {    static SimpleDateFormat SDF = new SimpleDateFormat ("Yyyy-mm-dd");    public static void Main (string[] args) throws exception{        Date start = Sdf.parse ("2018-03-01");        Date end = Sdf.parse ("2018-03-03");        Final Date other =sdf.parse ("2018-03-07");        for (int i=0, i<100; i++) {            new Thread (new Runnable () {public                void run () {                    sdf.format (other);                }            } ). Start ();        }        System.out.println ("Start:" + Sdf.format (start));        System.out.println ("End:" + sdf.format (end));}    }

(This code in the virtual function quickly reproduce the problem, on the Mac to quickly and continuously click on the run will be reproduced--)

Expected Result:

The results of thread safety occur:

After reading the phenomenon to find the reason:

1.DateFormat (parent Class) defines a global calendar:

The Simplydateformat format () method uses the global calendar in the case of unsafe threads:

The approximate order in which the problem is generated may be this:

DateFormat of the format () method thread unsafe problem analysis