JUC-ThreadLocal

落日映苍穹つ 2022-09-15 12:24 107阅读 0赞

*  **ThreadLocal用完后要remove**

**必须回收自定义的ThreadLocal变量，尤其在线程池场景下，线程经常会被复用，如果不清理自定义的ThreadLocal变量，可能会影响后续业务逻辑和造成内存泄漏等问题。尽量在代理中使用try-finally块进行回收。**

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16][]

--------------------

/**
     * @author jl
     * @since 2021/10/11 19:51
     * SimpleDateFormat线程安全问题示例
     */
    public class SimpleDateFormatDemo {
        public static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
    
        public static Date parse(String StringDateFormat) throws ParseException {
            return sdf.parse(StringDateFormat);
        }
    
        public static void main(String[] args) {
            for (int i = 0; i < 3; i++) {
                new Thread(()->{
                    try {
                        System.out.println(SimpleDateFormatDemo.parse("2021-11-11 11:11:11"));
                    } catch (ParseException e) {
                        e.printStackTrace();
                    }
                }).start();
            }
        }
    }

异常1

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 1][]

异常2

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 2][] 异常3

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 3][]

异常4

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_9_color_FFFFFF_t_70_g_se_x_16][]

** 原因：**

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 4][]

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 5][]

** cal.clear();会清除别的线程设置的值**

解决：

1、加锁，影响性能，不推荐，有其他方式也能解决

2、**使用ThreadLocal<SimpleDateFormat>**

public class SimpleDateFormatDemo {
        public static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
    
        public static Date parse(String StringDateFormat) throws ParseException {
            return sdf.parse(StringDateFormat);
        }
    
        public static final ThreadLocal<SimpleDateFormat> thread = ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd"));
    
        public static Date parseByThreadLocal(String stringDateFormat) throws ParseException {
            return thread.get().parse(stringDateFormat);
        }
    
        public static void main(String[] args) {
            for (int i = 0; i < 30; i++) {
                new Thread(()->{
                    try {
    //                    System.out.println(SimpleDateFormatDemo.parse("2021-11-11 11:11:11"));
                        System.out.println(SimpleDateFormatDemo.parseByThreadLocal("2021-11-11 11:11:11"));
                    } catch (ParseException e) {
                        e.printStackTrace();
                    } finally {
                        thread.remove();
                    }
                }).start();
            }
        }
    }

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JUC-ThreadLocal

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