SimpleDateFormat线程不安全及解决方案

我就是我 2021-09-29 00:36 550阅读 0赞

原文链接：[https://www.cnblogs.com/yangzhenlong/p/8385061.html][https_www.cnblogs.com_yangzhenlong_p_8385061.html]

我看完后，照着里面的代码敲了一遍，只不过我换成了单元测试。这里记录下

线程不安全示例

@Test
    public void test1() {
    
    	SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    	String dateStr = "1111-11-11 11:11:11";
    	ExecutorService executorService = Executors.newFixedThreadPool(100);
    	for (int i = 0; i < 100; i++) {
    		executorService.submit(new Runnable() {
    			@Override
    			public void run() {
    				try {
    					// 多个线程操作同一个sdf对象
    					System.out.println(sdf.format(sdf.parse(dateStr)) + "---"
    							+ Thread.currentThread().getName());
    				} catch (ParseException e) {
    					System.out.println("--------------> error," + e.getMessage());
    				}
    			}
    		});
    	}
    	executorService.shutdown();
    }

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA5OTk4MDk_size_16_color_FFFFFF_t_70][]

解决方案：

1. 为每个实例创建一个单独的SimpleDataFormat对象

@Test
    public void test2() {
    
    	String dateStr = "1111-11-11 11:11:11";
    	ExecutorService executorService = Executors.newFixedThreadPool(100);
    	for (int i = 0; i < 100; i++) {
    		executorService.submit(new Runnable() {
    			// 为每个线程创建自己的sdf对象
    			SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    			@Override
    			public void run() {
    				try {
    					// 多个线程操作同一个sdf对象
    					System.out.println(sdf.format(sdf.parse(dateStr)) + "---"
    							+ Thread.currentThread().getName());
    				} catch (ParseException e) {
    					System.out.println("--------------> error," + e.getMessage());
    				}
    			}
    		});
    	}
    	executorService.shutdown();
    }

缺点：每次new一个实例，都会new一个format对象，虚拟机内存消耗大，垃圾回收频繁

2. 给SimpleDateFormat对象加锁，使用Lock或者synchronized修饰

@Test
    public void test3() {
    
    	SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    	String dateStr = "1111-11-11 11:11:11";
    	ExecutorService executorService = Executors.newFixedThreadPool(100);
    	for (int i = 0; i < 100; i++) {
    		executorService.submit(new Runnable() {
    			@Override
    			public void run() {
    				// 加同步锁
    				synchronized (sdf) {
    					try {
    						// 多个线程操作同一个sdf对象
    						System.out.println(sdf.format(sdf.parse(dateStr)) + "---"
    								+ Thread.currentThread().getName());
    					} catch (ParseException e) {
    						System.out.println("--------------> error," + e.getMessage());
    					}
    				}
    			}
    		});
    	}
    	executorService.shutdown();
    }

缺点：性能差，其他线程要等待锁释放

3. 使用ThreadLocal为每个线程创建一个SimpleDateFormat对象副本，有线程隔离性，各自的副本对象也不会被其他线程影响

public static void main(String[] args) {
    
    	//SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    	//初始化threadLocal并设置值
    	ThreadLocal<SimpleDateFormat> threadLocal = new ThreadLocal<SimpleDateFormat>(){
    		@Override
    		protected SimpleDateFormat initialValue() {
    			return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    		}
    	};
    	String dateStr = "1111-11-11 11:11:11";
    	ExecutorService executorService = Executors.newFixedThreadPool(100);
    	for(int i = 0; i < 100; i++) {
    		executorService.submit(new Runnable() {
    			@Override
    			public void run() {
    				try {
    					System.out.println(threadLocal.get().format(threadLocal.get().parse(dateStr)) + "---" + Thread.currentThread().getName());
    				} catch (ParseException e) {
    					System.out.println("--------------> error, " + e.getMessage());
    				}
    			}
    		});
    
    	}
    	executorService.shutdown();
    	//清理threadLocal，生产环境不清理容易导致内存溢出
    	threadLocal.remove();
    }

这里没用单元测试来写，因为用单元测试写里面的信息基本上不打印（原因暂时没找到，有遇到过类似情况的可以交流下），写main方法没问题

另外，这里重写了ThreadLocal的initialValue方法，给ThreadLocal提供初始值；这里也可以采用手动设置的方式提供初始值，代码如下：

public class SimpleDateFormatTest {
    
        public static void main(String[] args) {
            String dateStr = "1111-11-11 11:11:11";
            ExecutorService executorService = Executors.newFixedThreadPool(100);
            for(int i = 0; i < 100; i++) {
                executorService.submit(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            System.out.println(getTime().format(getTime().parse(dateStr)) + "---" + Thread.currentThread().getName());
                        } catch (ParseException e) {
                            System.out.println("--------------> error, " + e.getMessage());
                        }
                    }
                });
    
            }
            executorService.shutdown();
        }
    
        private static ThreadLocal<SimpleDateFormat> local = new ThreadLocal<SimpleDateFormat>();
        private static SimpleDateFormat getTime() {
            SimpleDateFormat time = local.get();
            if (time == null) {
                time = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
                local.set(time);
            }
            return time;
        }
    }

参考链接：[https://www.cnblogs.com/yangzhenlong/p/8385061.html][https_www.cnblogs.com_yangzhenlong_p_8385061.html]

[https://www.cnblogs.com/hongmoshui/p/10432297.html?tdsourcetag=s\_pctim\_aiomsg][https_www.cnblogs.com_hongmoshui_p_10432297.html_tdsourcetag_s_pctim_aiomsg]

[https_www.cnblogs.com_yangzhenlong_p_8385061.html]: https://www.cnblogs.com/yangzhenlong/p/8385061.html
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA5OTk4MDk_size_16_color_FFFFFF_t_70]: /images/20210923/53b5372e83c44f3893fde01efff99208.png
[https_www.cnblogs.com_hongmoshui_p_10432297.html_tdsourcetag_s_pctim_aiomsg]: https://www.cnblogs.com/hongmoshui/p/10432297.html?tdsourcetag=s_pctim_aiomsg