SimpleDateFormat 的文档中其实已经指明了这种情况。

Date formats are not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

示例代码：

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;

public class SimpleDateFormatMultithread {
    private static final SimpleDateFormat fmt = new SimpleDateFormat("yyyyMMdd");

    public static void bench() throws Exception {

        Callable<Date> task = new Callable<Date>() {
            public Date call() throws Exception {
                return fmt.parse("20101022");
            }
        };

        // pool with 5 threads         ExecutorService exec = Executors.newFixedThreadPool(5);
        List<Future<Date>> results = new ArrayList<Future<Date>>();

        // perform 10 date conversions         for (int i = 0; i < 10; i++) {
            results.add(exec.submit(task));
        }
        exec.shutdown();

        // look at the results         for (Future<Date> result : results) {
            System.out.println(result.get());
        }
    }

    public static void main(String[] args) throws Exception {
        for (int i = 0; i < 10; i++) {
            bench();
        }
    }
}

多运行几次会出现以下几种问题：

Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NumberFormatException: For input string: ""
	at java.util.concurrent.FutureTask.report(FutureTask.java:122)
	at java.util.concurrent.FutureTask.get(FutureTask.java:192)

Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NumberFormatException: multiple points
	at java.util.concurrent.FutureTask.report(FutureTask.java:122)
	at java.util.concurrent.FutureTask.get(FutureTask.java:192)

Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NumberFormatException: For input string: "E120"
	at java.util.concurrent.FutureTask.report(FutureTask.java:122)
	at java.util.concurrent.FutureTask.get(FutureTask.java:192)

处理方法：

1. 局部化
2. 使用ThreadLocal

参考：

“Java DateFormat is not threadsafe” what does this leads to?