线程不安全验证：
/** * SimpleDateFormat线程安全测试 * 〈功能详细描述〉 * * @author 17090889 * @see [相关类/方法]（可选） * @since [产品/模块版本] （可选） */public class SimpleDateFormatTest { private SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); ThreadPoolExecutor poolExecutor = new ThreadPoolExecutor(10, 100, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<>(1000), new MyThreadFactory("SimpleDateFormatTest")); public void test() { while (true) { poolExecutor.execute(new Runnable() { @Override public void run() { String dateString = simpleDateFormat.format(new Date()); try { Date parseDate = simpleDateFormat.parse(dateString); String dateString2 = simpleDateFormat.format(parseDate); System.out.println(dateString.equals(dateString2)); } catch (ParseException e) { e.printStackTrace(); } } }); } } 输出：
true
falsetruetruefalse出现了false，说明线程不安全
1、format方法
public StringBuffer format(Date date, StringBuffer toAppendTo, FieldPosition pos) { pos.beginIndex = pos.endIndex = 0; return format(date, toAppendTo, pos.getFieldDelegate()); } // Called from Format after creating a FieldDelegate private StringBuffer format(Date date, StringBuffer toAppendTo, FieldDelegate delegate) { // Convert input date to time field list calendar.setTime(date); boolean useDateFormatSymbols = useDateFormatSymbols(); for (int i = 0; i < compiledPattern.length; ) { int tag = compiledPattern[i] >>> 8; int count = compiledPattern[i++] & 0xff; if (count == 255) { count = compiledPattern[i++] << 16; count |= compiledPattern[i++]; } switch (tag) { case TAG_QUOTE_ASCII_CHAR: toAppendTo.append((char)count); break; case TAG_QUOTE_CHARS: toAppendTo.append(compiledPattern, i, count); i += count; break; default: subFormat(tag, count, delegate, toAppendTo, useDateFormatSymbols); break; } } return toAppendTo; } protected Calendar calendar;
可以看到，多个线程之间共享变量calendar，并修改calendar，因此在多线程环境下，线程是不安全的。
解决方案：
1、将SimpleDateFormat定义成局部变量
2、 加一把线程同步锁：synchronized(lock)
3、使用ThreadLocal，每个线程都拥有自己的SimpleDateFormat对象副本
如：
/** * SimpleDateFormat线程安全测试 * 〈功能详细描述〉 * * @author 17090889 * @see [相关类/方法]（可选） * @since [产品/模块版本] （可选） */public class SimpleDateFormatTest { ThreadLocal local = new ThreadLocal<>(); // private SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); ThreadPoolExecutor poolExecutor = new ThreadPoolExecutor(10, 100, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<>(1000), new MyThreadFactory("SimpleDateFormatTest")); public void test() { while (true) { poolExecutor.execute(new Runnable() { @Override public void run() { SimpleDateFormat simpleDateFormat = local.get(); if (simpleDateFormat == null) { simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); } String dateString = simpleDateFormat.format(new Date()); try { Date parseDate = simpleDateFormat.parse(dateString); String dateString2 = simpleDateFormat.format(parseDate); System.out.println(dateString.equals(dateString2)); } catch (ParseException e) { e.printStackTrace(); } finally { local.remove(); } } }); } }}
4、使用DateTimeFormatter代替SimpleDateFormat