Question

I’ve been trying to figure out this bug for few days. I have narrowed the issue down to a test case as follows. Again, this is a test case that causes the bug to happen. The function (at this point) is not a practical one, but just trying to figure out the bug.

Server is running:
– Tomcat 6
– OpenJDK 1.6.0_17
– CentOS 5.5

I have a simple Class file with the following method Static Method and Static variable declaration:

public static java.text.SimpleDateFormat displayDateSDF1 = new java.text.SimpleDateFormat("MM/dd/yyyy");

public static java.util.Date getSubDateMini(String inputDate)
{
    java.util.Date testObj = null;
    try
    {
        testObj = displayDateSDF1.parse("01/01/2000") ;
    }
    catch (Exception e)
    {
    }
    return testObj;
}

Testing in Tomcat:

When I run this method, I’d expect to get the same result every time, right? However, if I call this method from a JSP, I get the expected result of a Date object with value 1/1/2000 about 99.9% of the time. However, sometimes I get an unexpected data object passed back with a seemingly random date value.

To test this, I created a JSP with the following code segment in it:

for (int i=0; i<200000;i++)
{
    java.util.Date testObjLib = TestDate.getSubDateMini("") ;
    if (testObjLib!=null&&!testObjLib.toString().equalsIgnoreCase("Sat Jan 01 00:00:00 PST 2000"))
    {
            out.print("<br>"+testObjLib+"");
    }
}

Some dates that appear are as follows:

Wed Jan 01 00:00:00 PST 1

Fri Aug 01 00:00:00 PDT 2166

In 200,000 runs, I get approximately 50 bad dates which gives an error rate of ~0.025% but again it’s random. I’ve run this loop with 10 iterations and received an error. Sometimes it runs the loop with 200,000 and all dates look good.

Testing in Java:

Running this loop via a console/terminal app in CentOS with the same loop, I have not seen this error happen yet. I increased the loop to 10,000,000 and had no false results as of yet.

---

I can understand out of memory or some thrown error (which would result in a null value) but not corrupt/inconsistent data. I built up a new server from scratch with Tomcat 6 and also tried Tomcat 5.5 and both have the same results. I have not tried Tomcat 7.

Any suggestions?

Answer 1

SimpleDateFormat is not thread-safe.

This means that when it is accessed from multiple threads, unexpected results can be observed. And tomcat is serving each request from a separate thread, so whenever two requests are made at the same time, the problem would arise.

You have a number of options:

• instantiate the SimpleDateFormat on each call of the method (rather than making it static)
• if you need the format more than once per thread, use a ThreadLocal to store it.
• alternatively use joda-time, where DateTimeFormat is thread-safe.