本文轉載至:http://blog.csdn.net/zdp072/article/details/41044059
一. 爲什麼SimpleDateFormat不是線程安全的?
Java源碼如下:
/**
* Date formats are not synchronized.
* It is recommended to create separate format instances for each thread.
* If multiple threads access a format concurrently, it must be synchronized
* externally.
*/
public class SimpleDateFormat extends DateFormat {
public Date parse(String text, ParsePosition pos){
calendar.clear(); // Clears all the time fields
// other logic ...
Date parsedDate = calendar.getTime();
}
}
abstract class DateFormat{
// other logic ...
protected Calendar calendar;
public Date parse(String source) throws ParseException{
ParsePosition pos = new ParsePosition(0);
Date result = parse(source, pos);
if (pos.index == 0)
throw new ParseException("Unparseable date: \"" + source + "\"" ,
pos.errorIndex);
return result;
}
}
如果我們把SimpleDateFormat定義成static成員變量,那麼多個thread之間會共享這個sdf對象, 所以Calendar對象也會共享。
假定線程A和線程B都進入了parse(text, pos) 方法, 線程B執行到calendar.clear()後,線程A執行到calendar.getTime(), 那麼就會有問題。
二. 問題重現:
public class DateFormatTest {
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) {
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();
}
}
}
創建三個進程, 使用靜態成員變量SimpleDateFormat的parse和format方法,然後比較經過這兩個方法折騰後的值是否相等:
程序如果出現以下錯誤,說明傳入的日期就串掉了,即SimpleDateFormat不是線程安全的:
Exception in thread "Thread-0" java.lang.RuntimeException: parse failed
at cn.test.DateFormatTest$1.run(DateFormatTest.java:27)
at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.RuntimeException: Thread-0, Expected 01-Jan-1999 but got 01-Jan-2000
at cn.test.DateFormatTest$1.run(DateFormatTest.java:22)
... 1 more
三. 解決方案:
1. 解決方案a:
將SimpleDateFormat定義成局部變量:
SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
String str1 = "01-Jan-2010";
String str2 = sdf.format(sdf.parse(str1));
缺點:每調用一次方法就會創建一個SimpleDateFormat對象,方法結束又要作爲垃圾回收。
2. 解決方案b:
加一把線程同步鎖: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();
}
}
}
缺點:性能較差,每次都要等待鎖釋放後其他線程才能進入
3. 解決方案c: (推薦)
使用ThreadLocal: 每個線程都將擁有自己的SimpleDateFormat對象副本。
寫一個工具類:
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);
}
}