标签：
【问题】
publicclassProveNotSafe {
staticSimpleDateFormat df = newSimpleDateFormat("dd-MMM-yyyy", Locale.US);
staticString testdata[] = { "01-Jan-1999", "14-Feb-2001", "31-Dec-2007"};
publicstaticvoidmain(String[] args) {
Runnable r[] = newRunnable[testdata.length];
for(inti = 0; i < r.length; i++) {
finalinti2 = i;
r[i] = newRunnable() {
publicvoidrun() {
try{
for(intj = 0; j < 1000; j++) {
String str = testdata[i2];
String str2 = null;
/* synchronized(df) */{
Date d = df.parse(str);
str2 = df.format(d);
System.out.println("i: "+ i2 + "\tj: "+ j + "\tThreadID: "
+ Thread.currentThread().getId() + "\tThreadName: "
+ Thread.currentThread().getName() + "\t"+ str + "\t"+ str2);
}
if(!str.equals(str2)) {
thrownewRuntimeException("date conversion failed after "+ j
+ " iterations. Expected "+ str + " but got "+ str2);
}
}
} catch(ParseException e) {
thrownewRuntimeException("parse failed");
}
}
};
newThread(r[i]).start();
}
}
}
i: 2 j: 0 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 1 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 2 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 3 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 4 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 5 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 6 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 7 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 8 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 9 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 10 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 11 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 12 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 13 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 14 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 15 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 16 ThreadID: 10 ThreadName: Thread-2 31-Dec-200731-Dec-2007i: 2 j: 17 ThreadID: 10 ThreadName: Thread-2 31-Dec-200711-Jan-1999i: 0 j: 0 ThreadID: 8ThreadName: Thread-0 01-Jan-199911-Jan-1999Exception in thread "Thread-2"i: 1j: 0 ThreadID: 9ThreadName: Thread-1 14-Feb-200111-Jan-2001Exception in thread "Thread-0"java.lang.RuntimeException: date conversion failed after 0iterations. Expected 01-Jan-1999but got 11-Jan-1999 at test.date.ProveNotSafe$1.run(ProveNotSafe.java:30) at java.lang.Thread.run(Thread.java:619)Exception in thread "Thread-1"java.lang.RuntimeException: date conversion failed after 0iterations. Expected 14-Feb-2001but got 11-Jan-2001 at test.date.ProveNotSafe$1.run(ProveNotSafe.java:30) at java.lang.Thread.run(Thread.java:619)java.lang.RuntimeException: date conversion failed after 17iterations. Expected 31-Dec-2007but got 11-Jan-1999 at test.date.ProveNotSafe$1.run(ProveNotSafe.java:30) at java.lang.Thread.run(Thread.java:619)
SimpleDateFormat和DateFormat类不是线程安全的。我们之所以忽视线程安全的问题，是因为从SimpleDateFormat和DateFormat类提供给我们的接口上来看，实在让人看不出它与线程安全有何相干。只是在JDK文档的最下面有如下说明：
SimpleDateFormat中的日期格式不是同步的。推荐（建议）为每个线程创建独立的格式实例。如果多个线程同时访问一个格式，则它必须保持外部同步。
JDK原始文档如下：
Synchronization：
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.
下面我们通过看JDK源码来看看为什么SimpleDateFormat和DateFormat类不是线程安全的真正原因：
SimpleDateFormat继承了DateFormat,在DateFormat中定义了一个protected属性的 Calendar类的对象：calendar。只是因为Calendar累的概念复杂，牵扯到时区与本地化等等，Jdk的实现中使用了成员变量来传递参数，这就造成在多线程的时候会出现错误。
// 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);
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);
break;
}
}
return toAppendTo;
}
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
2.同步代码块 synchronized(code)privatestatic SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
publicstatic String formatDate(Date date)throws ParseException{
synchronized(sdf){
return sdf.format(date);
}
}
publicstatic Date parse(String strDate) throws ParseException{
synchronized(sdf){
return sdf.parse(strDate);
}
}
3.使用ThreadLocal:privatestatic ThreadLocal<SimpleDateFormate> threadLocal = new ThreadLocal<SimpleDateFormate>();
【JAVA】JDK-SimpleDataFormat 线程不安全！
标签：
原文地址：http://www.cnblogs.com/liuyongcn/p/5338870.html
