本文借花献佛，引用Tim Cull的博文“SimpleDateFormat: Performance Pig”介绍下ThreadLocal的简单使用，同时也对SimpleDateFormat的使用有个深入的了解。
Tim Cull 写道
Just yesterday I came across this problem “in the wild” for the third time in my career so far: an application with performance problems creating tons of java.text.SimpleDateFormat instances. So, I have to get this out there: creating a new instance of SimpleDateFormat is incredibly expensive and should be minimized. In the case that prompted this post, I was using JProfiler to profile this code that parses a CSV file and discovered that 50% of the time it took to suck in the file and make 55,000 objects out of it was spent solely in the constructor of SimpleDateFormat. It created and then threw away a new one every time it had to parse a date. Whew!
“Great,” you think, “I’ll just create one, static instance, slap it in a field in a DateUtils helper class and life will be good.”
Well, more precisely, life will be good about 97% of the time. A few days after you roll that code into production you’ll discover the second cool fact that’s good to know: SimpleDateFormat is not thread safe. Your code will work just fine most of the time and all of your regression tests will probably pass, but once your system gets under a production load you’ll see the occasional exception.
“Fine,” you think, “I’ll just slap a ’synchronized’ around my use of that one, static instance.”
Ok, fine, you could do that and you’d be more or less ok, but the problem is that you’ve now taken a very common operation (date formatting and parsing) and crammed all of your otherwise-lovely, super-parallel application through a single pipe to get it done.
大致意思：Tim Cull碰到一个SimpleDateFormat带来的严重的性能问题，该问题主要有SimpleDateFormat引发，创建一个SimpleDateFormat实例的开销比较昂贵，解析字符串时间时频繁创建生命周期短暂的实例导致性能低下。即使将SimpleDateFormat定义为静态类变量，貌似能解决这个问题，但是SimpleDateFormat是非线程安全的，同样存在问题，如果用‘synchronized’线程同步同样面临问题，同步导致性能下降（线程之间序列化的获取SimpleDateFormat实例）。
Tim Cull使用Threadlocal解决了此问题，对于每个线程SimpleDateFormat不存在影响他们之间协作的状态，为每个线程创建一个SimpleDateFormat变量的拷贝或者叫做副本，代码如下：
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
/**
* 使用ThreadLocal以空间换时间解决SimpleDateFormat线程安全问题。
* @author
*
*/
public class DateUtil {
private static final String DATE_FORMAT = "yyyy-MM-dd HH:mm:ss";
@SuppressWarnings("rawtypes")
private static ThreadLocal threadLocal = new ThreadLocal() {
protected synchronized Object initialValue() {
return new SimpleDateFormat(DATE_FORMAT);
}
};
public static DateFormat getDateFormat() {
return (DateFormat) threadLocal.get();
}
public static Date parse(String textDate) throws ParseException {
return getDateFormat().parse(textDate);
}
}
创建一个ThreadLocal类变量，这里创建时用了一个匿名类，覆盖了initialValue方法，主要作用是创建时初始化实例。也可以采用下面方式创建；
//第一次调用get将返回null
private static ThreadLocal threadLocal = new ThreadLocal()；
//获取线程的变量副本，如果不覆盖initialValue，第一次get返回null，故需要初始化一个SimpleDateFormat，并set到threadLocal中
public static DateFormat getDateFormat()
{
DateFormat df = (DateFormat) threadLocal.get();
if(df==null){
df = new SimpleDateFormat(DATE_FORMAT)
threadLocal.set(df);
}
return df;
}
我们看下我们覆盖的initialValue方法：
protected T initialValue() {
return null;//直接返回null
}