[java] Why is Java's SimpleDateFormat not thread-safe?

Here’s an example defines a SimpleDateFormat object as a static field. When two or more threads access “someMethod” concurrently with different dates, they can mess with each other’s results.

    public class SimpleDateFormatExample {
         private static final SimpleDateFormat simpleFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");

         public String someMethod(Date date) {
            return simpleFormat.format(date);
         }
    }

You can create a service like below and use jmeter to simulate concurrent users using the same SimpleDateFormat object formatting different dates and their results will be messed up.

public class FormattedTimeHandler extends AbstractHandler {

private static final String OUTPUT_TIME_FORMAT = "yyyy-MM-dd HH:mm:ss.SSS";
private static final String INPUT_TIME_FORMAT = "yyyy-MM-ddHH:mm:ss";
private static final SimpleDateFormat simpleFormat = new SimpleDateFormat(OUTPUT_TIME_FORMAT);
// apache commons lang3 FastDateFormat is threadsafe
private static final FastDateFormat fastFormat = FastDateFormat.getInstance(OUTPUT_TIME_FORMAT);

public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response)
        throws IOException, ServletException {

    response.setContentType("text/html;charset=utf-8");
    response.setStatus(HttpServletResponse.SC_OK);
    baseRequest.setHandled(true);

    final String inputTime = request.getParameter("time");
    Date date = LocalDateTime.parse(inputTime, DateTimeFormat.forPattern(INPUT_TIME_FORMAT)).toDate();

    final String method = request.getParameter("method");
    if ("SimpleDateFormat".equalsIgnoreCase(method)) {
        // use SimpleDateFormat as a static constant field, not thread safe
        response.getWriter().println(simpleFormat.format(date));
    } else if ("FastDateFormat".equalsIgnoreCase(method)) {
        // use apache commons lang3 FastDateFormat, thread safe
        response.getWriter().println(fastFormat.format(date));
    } else {
        // create new SimpleDateFormat instance when formatting date, thread safe
        response.getWriter().println(new SimpleDateFormat(OUTPUT_TIME_FORMAT).format(date));
    }
}

public static void main(String[] args) throws Exception {
    // embedded jetty configuration, running on port 8090. change it as needed.
    Server server = new Server(8090);
    server.setHandler(new FormattedTimeHandler());

    server.start();
    server.join();
}

}

The code and jmeter script can be downloaded here .

Examples related to java

Under what circumstances can I call findViewById with an Options Menu / Action Bar item? How much should a function trust another function How to implement a simple scenario the OO way Two constructors How do I get some variable from another class in Java? this in equals method How to split a string in two and store it in a field How to do perspective fixing? String index out of range: 4 My eclipse won't open, i download the bundle pack it keeps saying error log

Examples related to thread-safety

Are these methods thread safe? apache server reached MaxClients setting, consider raising the MaxClients setting How can I make a JUnit test wait? How to stop a thread created by implementing runnable interface? What Makes a Method Thread-safe? What are the rules? Android - Best and safe way to stop thread Why is Java's SimpleDateFormat not thread-safe? What is thread Safe in java? How does lock work exactly? Thread-safe List<T> property

Examples related to simpledateformat

How to convert an Instant to a date format? Get Date Object In UTC format in Java How to format a java.sql.Timestamp(yyyy-MM-dd HH:mm:ss.S) to a date(yyyy-MM-dd HH:mm:ss) How to convert date to string and to date again? Java Convert GMT/UTC to Local time doesn't work as expected Java SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'") gives timezone as IST SimpleDateFormat parse loses timezone java.text.ParseException: Unparseable date Java format yyyy-MM-dd'T'HH:mm:ss.SSSz to yyyy-mm-dd HH:mm:ss SimpleDateFormat returns 24-hour date: how to get 12-hour date?