本文借花獻佛，引用Tim Cull的博文“SimpleDateFormat: Performance Pig”介紹下ThreadLocal的簡單使用，同時也對SimpleDateFormat的使用有個深入的了解。
Tim Cull 寫道
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碰到一個SimpleDateFormat帶來的嚴重的性能問題，該問題主要有SimpleDateFormat引發，創建一個SimpleDateFormat實例的開銷比較昂貴，解析字符串時間時頻繁創建生命周期短暫的實例導致性能低下。即使將SimpleDateFormat定義為靜態類變量，貌似能解決這個問題，但是SimpleDateFormat是非線程安全的，同樣存在問題，如果用‘synchronized’線程同步同樣面臨問題，同步導致性能下降（線程之間序列化的獲取SimpleDateFormat實例）。
Tim Cull使用Threadlocal解決了此問題，對于每個線程SimpleDateFormat不存在影響他們之間協作的狀態，為每個線程創建一個SimpleDateFormat變量的拷貝或者叫做副本，代碼如下：
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
/**
* 使用ThreadLocal以空間換時間解決SimpleDateFormat線程安全問題。
* @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);
}
}
創建一個ThreadLocal類變量，這里創建時用了一個匿名類，覆蓋了initialValue方法，主要作用是創建時初始化實例。也可以采用下面方式創建；
//第一次調用get將返回null
private static ThreadLocal threadLocal = new ThreadLocal()；
//獲取線程的變量副本，如果不覆蓋initialValue，第一次get返回null，故需要初始化一個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;
}
我們看下我們覆蓋的initialValue方法：
protected T initialValue() {
return null;//直接返回null
}