Java 常用工具类(4) : 线程安全的日期工具类

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

/**
* 线程安全的日期工具类
*/
public class DateUtil {

/** 锁对象 */
private static final Object lockObj = new Object();

/** 存放不同的日期模板格式的sdf的Map */
private static Map<String, ThreadLocal<SimpleDateFormat>> sdfMap = new HashMap<String, ThreadLocal<SimpleDateFormat>>();

/**
* 返回一个ThreadLocal的sdf,每个线程只会new一次sdf
*
* @param pattern
* @return
*/
private static SimpleDateFormat getSdf(final String pattern) {
ThreadLocal<SimpleDateFormat> tl = sdfMap.get(pattern);

// 此处的双重判断和同步是为了防止sdfMap这个单例被多次put重复的sdf
if (tl == null) {
synchronized (lockObj) {
tl = sdfMap.get(pattern);
if (tl == null) {
// 只有Map中还没有这个pattern的sdf才会生成新的sdf并放入map
System.out.println("put new sdf of pattern " + pattern + " to map");

// 这里是关键,使用ThreadLocal<SimpleDateFormat>替代原来直接new SimpleDateFormat
tl = new ThreadLocal<SimpleDateFormat>() {

@Override
protected SimpleDateFormat initialValue() {
//System.out.println("thread: " + Thread.currentThread() + " init pattern: " + pattern);
return new SimpleDateFormat(pattern);
}
};
sdfMap.put(pattern, tl);
}
}
}

return tl.get();
}

/**
* 是用ThreadLocal<SimpleDateFormat>来获取SimpleDateFormat,这样每个线程只会有一个SimpleDateFormat
*
* @param date
* @param pattern
* @return
*/
public static String format(Date date, String pattern) {
return getSdf(pattern).format(date);
}

public static Date parse(String dateStr, String pattern) throws ParseException {
return getSdf(pattern).parse(dateStr);
}

}