原文：https://www.jianshu.com/p/b212afa16f1f
1.SimpleDateFormat为什么不是线程安全的？
- 如果我们把SimpleDateFormat定义成static成员变量，那么多个thread之间会共享这个SimpleDateFormat对象， 所以Calendar对象也会共享。
public static SimpleDateFormat formater = new SimpleDateFormat(
"yyyy-MM-dd HH:mm:ss");
System.out.println(formater.format(new Date())+" Exception made...");
- DateFormat.java
public final String format(Date date)
{
return format(date, new StringBuffer(),
DontCareFieldPosition.INSTANCE).toString();
}
public abstract StringBuffer format(Date date, StringBuffer toAppendTo,
FieldPosition fieldPosition);
- SimpleDateFormat.java
@Override
public StringBuffer format(Date date, StringBuffer toAppendTo,
FieldPosition pos)
{
// 如此轻易地使用内部变量，肯定不能线程安全
// 线程都对pos进行写操作，必然会影响其他线程的读操作
pos.beginIndex = pos.endIndex = 0;
return format(date, toAppendTo, pos.getFieldDelegate());
}
private StringBuffer format(Date date, StringBuffer toAppendTo,
FieldDelegate delegate) {
// Convert input date to time field list
// 这里已经彻底毁坏线程的安全性
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;
}
1.1 复现错误
public class DateFormatTest {
private static SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
private static String date[] = { "01-Jan-1999", "09-Jan-2000", "08-Jan-2001" , "07-Jan-2002" , "06-Jan-2003" , "05-Jan-2004" , "04-Jan-2005" , "03-Jan-2006" , "02-Jan-2007" };
public static void main(String[] args) {
for (int i = 0; i < date.length; i++) {
final int temp = i;
new Thread(new Runnable() {
@Override
public void run() {
try {
while (true) {
String str1 = date[temp];
String str2 = sdf.format(sdf.parse(str1));
System.out.println(Thread.currentThread().getName() + ", " + str1 + "," + str2);
if(!str1.equals(str2)){
throw new RuntimeException(Thread.currentThread().getName()
+ ", Expected " + str1 + " but got " + str2);
}
}
} catch (Exception e) {
throw new RuntimeException("parse failed", e);
}
}
}).start();
}
}
}
2.SimpleDateFormat线程不安全的解决方法
2.1 将SimpleDateFormat定义成局部变量：
SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
String str1 = "01-Jan-2010";
String str2 = sdf.format(sdf.parse(str1));
缺点：每调用一次方法就会创建一个SimpleDateFormat对象，方法结束又要作为垃圾回收。
2.2 加一把线程同步锁：synchronized(lock)
public class SyncDateFormatTest {
private static SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
private static String date[] = { "01-Jan-1999", "01-Jan-2000", "01-Jan-2001" };
public static void main(String[] args) {
for (int i = 0; i < date.length; i++) {
final int temp = i;
new Thread(new Runnable() {
@Override
public void run() {
try {
while (true) {
synchronized (sdf) {
String str1 = date[temp];
Date date = sdf.parse(str1);
String str2 = sdf.format(date);
System.out.println(Thread.currentThread().getName() + ", " + str1 + "," + str2);
if(!str1.equals(str2)){
throw new RuntimeException(Thread.currentThread().getName()
+ ", Expected " + str1 + " but got " + str2);
}
}
}
} catch (Exception e) {
throw new RuntimeException("parse failed", e);
}
}
}).start();
}
}
}
缺点：性能较差，每次都要等待锁释放后其他线程才能进入
2.3 使用ThreadLocal
每个线程都将拥有自己的SimpleDateFormat对象副本。
public class DateUtil {
private static ThreadLocal<SimpleDateFormat> local = new ThreadLocal<SimpleDateFormat>();
public static Date parse(String str) throws Exception {
SimpleDateFormat sdf = local.get();
if (sdf == null) {
sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
local.set(sdf);
}
return sdf.parse(str);
}
public static String format(Date date) throws Exception {
SimpleDateFormat sdf = local.get();
if (sdf == null) {
sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
local.set(sdf);
}
return sdf.format(date);
}
}
public class ThreadLocalDateFormatTest {
private static String date[] = { "01-Jan-1999", "01-Jan-2000", "01-Jan-2001" };
public static void main(String[] args) {
for (int i = 0; i < date.length; i++) {
final int temp = i;
new Thread(new Runnable() {
@Override
public void run() {
try {
while (true) {
String str1 = date[temp];
Date date = DateUtil.parse(str1);
String str2 = DateUtil.format(date);
System.out.println(str1 + "," + str2);
if(!str1.equals(str2)){
throw new RuntimeException(Thread.currentThread().getName()
+ ", Expected " + str1 + " but got " + str2);
}
}
} catch (Exception e) {
throw new RuntimeException("parse failed", e);
}
}
}).start();
}
}
}
3.使用DateTimeFormatter代替SimpleDateFormat
代码参考DateTimeFormatterTest.java
jdk1.8中新增了 LocalDate 与 LocalDateTime等类来解决日期处理方法，同时引入了一个新的类DateTimeFormatter来解决日期格式化问题。
LocalDateTime,DateTimeFormatter两个类都没有线程问题，只要你自己不把它们创建为共享变量就没有线程问题。
可以使用Instant代替 Date，LocalDateTime代替 Calendar，DateTimeFormatter 代替 SimpleDateFormat。
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy MM dd");
LocalDate date = LocalDate.parse("2017 06 17", formatter);
System.out.println(formatter.format(date));
自测类
import java.text.SimpleDateFormat; import java.time.LocalDate; import java.time.format.DateTimeFormatter; import java.util.Locale; public class DateFormatTest { private static SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy", Locale.US); private static String date[] = { "01-01-1999", "09-01-2000", "08-01-2001" , "07-01-2002" , "06-01-2003" , "05-01-2004" , "04-01-2005" , "03-01-2006" , "02-01-2007" }; private static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy"); public static void main(String[] args) { for (int i = 0; i < date.length; i++) { final int temp = i; new Thread(new Runnable() { @Override public void run() { try { while (true) { String str1 = date[temp]; //线程安全 LocalDate date = LocalDate.parse(str1, formatter); String str2 = formatter.format(date); //线程不安全 // String str2 = sdf.format(sdf.parse(str1)); System.out.println(Thread.currentThread().getName() + ", " + str1 + "," + str2); if(!str1.equals(str2)){ throw new RuntimeException(Thread.currentThread().getName() + ", Expected " + str1 + " but got " + str2); } } } catch (Exception e) { throw new RuntimeException("parse failed", e); } } }).start(); } } }