“Java DateFormat不是线程安全的”这会导致什么? [英] "Java DateFormat is not threadsafe" what does this leads to?
问题描述
每个人都警告Java DateFormat不是线程安全的,理论上我理解这个概念。
但是我无法想象我们可能遇到的实际问题这个。比如,我在类中有一个DateFormat字段,并且在多线程环境中的类中的不同方法(格式化日期)中使用相同的字段。
这是否会原因:
- 格式异常等任何异常
- 数据差异
- 任何其他问题?
另外,请解释原因。
让我们试一试。
这是一个程序,其中多个线程使用共享 SimpleDateFormat
。
计划:
public static void main(String [] args)throws Exception {
final DateFormat format = new SimpleDateFormat(yyyyMMdd);
Callable< Date> task = new Callable< Date>(){
public Date call()throws Exception {
return format.parse(20101022);
}
};
//包含5个主题的池
ExecutorService exec = Executors.newFixedThreadPool(5);
列表<未来<日期>> results = new ArrayList< Future< Date>>();
//执行10个日期转换
for(int i = 0; i< 10; i ++){
results.add(exec.submit(task));
}
exec.shutdown();
//查看结果
for(Future< Date> result:results){
System.out.println(result.get());
}
}
运行几次,你会看到:<这是一些例子:
1。
引起:java.lang.NumberFormatException:对于输入字符串:
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
at java.lang.Long.parseLong(Long.java:431)
at java.lang.Long.parseLong (Long.java:468)
at java.text.DigitList.getLong(DigitList.java:177)
at java.text.DecimalFormat.parse(DecimalFormat.java:1298)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)
2。
引起:java.lang.NumberFormatException:对于输入字符串:。10201E.102014E4
at sun.misc.FloatingDecimal.readJavaFormatString (FloatingDecimal.java:1224)
at java.lang.Double.parseDouble(Double.java:510)
at java.text .DigitList.getDouble(DigitList.java:151)
at java.text.DecimalFormat.parse(DecimalFormat.java:1303)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)
3。
引起:java.lang.NumberFormatException:sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1084)的多个点
java.lang.Double.parseDouble的
(Double.java :510)
at java.text.DigitList.getDouble(DigitList.java:151)
at java.text.DecimalFormat.parse(DecimalFormat.java:1303)
at java.text。 SimpleDateFormat.subParse(SimpleDateFormat.java:1936)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1312)
结果错误:
周六10月22日00:00:00 BST 2011
Thu Jan 22 00:00:00 GMT 1970
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
周四10月22日00:00:00 GMT 1970
星期五10月22日00:00:00 BST 2010
星期五10月22日00:00:00 BST 2010
星期五10月22日00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
正确结果:
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
在多线程环境中安全使用DateFormats的另一种方法是使用 ThreadLocal
变量来保存 DateFormat
object,这意味着每个线程都有自己的副本,不需要等待其他线程释放它。具体如下:
public class DateFormatTest {
private static final ThreadLocal< DateFormat> df = new ThreadLocal< DateFormat>(){
@Override
protected DateFormat initialValue(){
return new SimpleDateFormat(yyyyMMdd);
}
};
public Date convert(String source)throws ParseException {
Date d = df.get()。parse(source);
返回d;
}
}
这是一个很好的提供更多详情。
Everybody cautions regarding Java DateFormat not being thread safe and I understand the concept theoretically.
But I'm not able to visualize what actual issues we can face due to this. Say, I've a DateFormat field in a class and the same is used in different methods in the class (formatting dates) in a multi-threaded environment.
Will this cause:
- any exception like format exception
- discrepancy in data
- any other issue?
Also, please explain why.
Let's try it out.
Here is a program in which multiple threads use a shared SimpleDateFormat
.
Program:
public static void main(String[] args) throws Exception {
final DateFormat format = new SimpleDateFormat("yyyyMMdd");
Callable<Date> task = new Callable<Date>(){
public Date call() throws Exception {
return format.parse("20101022");
}
};
//pool with 5 threads
ExecutorService exec = Executors.newFixedThreadPool(5);
List<Future<Date>> results = new ArrayList<Future<Date>>();
//perform 10 date conversions
for(int i = 0 ; i < 10 ; i++){
results.add(exec.submit(task));
}
exec.shutdown();
//look at the results
for(Future<Date> result : results){
System.out.println(result.get());
}
}
Run this a few times and you will see:
Exceptions:
Here are a few examples:
1.
Caused by: java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
at java.lang.Long.parseLong(Long.java:431)
at java.lang.Long.parseLong(Long.java:468)
at java.text.DigitList.getLong(DigitList.java:177)
at java.text.DecimalFormat.parse(DecimalFormat.java:1298)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)
2.
Caused by: java.lang.NumberFormatException: For input string: ".10201E.102014E4"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1224)
at java.lang.Double.parseDouble(Double.java:510)
at java.text.DigitList.getDouble(DigitList.java:151)
at java.text.DecimalFormat.parse(DecimalFormat.java:1303)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)
3.
Caused by: java.lang.NumberFormatException: multiple points
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1084)
at java.lang.Double.parseDouble(Double.java:510)
at java.text.DigitList.getDouble(DigitList.java:151)
at java.text.DecimalFormat.parse(DecimalFormat.java:1303)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1936)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1312)
Incorrect Results:
Sat Oct 22 00:00:00 BST 2011
Thu Jan 22 00:00:00 GMT 1970
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Thu Oct 22 00:00:00 GMT 1970
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Correct Results:
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Another approach to safely use DateFormats in a multi-threaded environment is to use a ThreadLocal
variable to hold the DateFormat
object, which means that each thread will have its own copy and doesn't need to wait for other threads to release it. This is how:
public class DateFormatTest {
private static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>(){
@Override
protected DateFormat initialValue() {
return new SimpleDateFormat("yyyyMMdd");
}
};
public Date convert(String source) throws ParseException{
Date d = df.get().parse(source);
return d;
}
}
Here is a good post with more details.
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