SimpleDateFormat线程不安全演示，DateTimeFormatter和ThreadLocal替换方案示例

偏执的太偏执、 2023-07-02 12:25 16阅读 0赞

# 问题演示： #

### DateUtil错误示例 ###

public class DateUtil {
        private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    
        public static Date dateStrFormat(String dateStr) {
            Date date = null;
            try {
                date = sdf.parse(dateStr);
            } catch (ParseException e) {
                e.printStackTrace();
            }
            return date;
        }
    }

### 运行示例 ###

public static void main(String[] args) {
            ExecutorService service = Executors.newFixedThreadPool(10);
    
            for (int i = 0; i < 20; i++) {
                service.execute(()->{
                    System.out.println(DateUtil.dateStrFormat("2019-12-26 12:02:12"));
                });
            }
            service.shutdown();
    
        }

### 运行结果：多线程下出现异常信息 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA3NzIyMzA_size_16_color_FFFFFF_t_70][]

原因：SimpleDateFormat 为线程不安全类，多线程下，会出现异常。

# 解决方案： #

思路：当多个线程同时读写同一共享变量时存在并发问题，如果不共享不就没有并发问题了。

### 方案1：改造，直接每次new一个SimpleDateFormat  ###

代码示例

public class DateUtil {
        public static Date dateStrFormat(String dateStr) {
            Date date = null;
            try {
                date =  new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse(dateStr);
            } catch (ParseException e) {
                e.printStackTrace();
            }
            return date;
        }
    }

### 方案2：用ThreadLocal，为每个线程单独存储自己的变量值 ###

public class DateUtil {
        private static ThreadLocal<DateFormat> threadLocalSdf = ThreadLocal.withInitial(
                ()-> new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
        public static Date dateStrFormat(String dateStr) {
            Date date = null;
            try {
                date = threadLocalSdf.get().parse(dateStr);
            } catch (ParseException e) {
                e.printStackTrace();
            }
            return date;
        }
    }

### 方案3：用线程安全类DateTimeFormatter示例 ###

public class DateUtil {
        private static final DateTimeFormatter formatter =  DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        public static Date dateStrFormat(String dateStr) {
            Date date = null;
            try {
                date = Date.from(LocalDateTime.parse(dateStr,formatter).atZone(ZoneId.systemDefault()).toInstant());
            } catch (DateTimeParseException e) {
                e.printStackTrace();
            }
            return date;
        }
    }

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA3NzIyMzA_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20200130100009575.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA3NzIyMzA=,size_16,color_FFFFFF,t_70