关于SimpleDateFormat线程不安全的解决办法

在需要的时候创建实例

public static String format(Date date) throws Exception {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    return sdf.format(date);
}

public static Date parse(String strDate) throws Exception {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    return sdf.parse(strDate);
}

缺点：每次调用方法都要创建和销毁sdf对象，效率较低。

synchronized

private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

public static String format(Date date) throws Exception {
    synchronized(sdf){
        return sdf.format(date);
    }
}

public static Date parse(String strDate) throws Exception {
    synchronized(sdf){
        return sdf.parse(strDate);    
    }
}

缺点：并发量大时线程阻塞，对性能有影响。

ThreadLocal

ThreadLocal可以让每个线程都得到一个sdf对象

private static ThreadLocal threadLocal = new ThreadLocal() {
    @Override
    protected DateFormat initialValue() {
        return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    }
};

public static Date parse(String strDate) throws Exception {
    return threadLocal.get().parse(strDate);
}

public static String format(Date date) throws Exception {
    return threadLocal.get().format(date);
}

基于JDK1.8的DateTimeFormatter(线程安全的类)

private static final DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

public static String format(LocalDateTime date) throws Exception {
    return dtf.format(date);
}

public static LocalDateTime parse(String strDate) {
    return LocalDateTime.parse(strDate, dtf);
}