import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;
/**
 * @Author yanjun.hou
 * @Date 2021/12/2 11:39
 * 用于解决高并发下 simpleDateFormat 出现转换异常的问题
 */
public class LocalDateUitl {
    /** 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 = ThreadLocal.withInitial(() -> {
//                            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);
    }
}