blocks|key|2021409|text|如果每个线程都有自己的实例，而你不使用静态，那么你是非常安全的:-)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2021410|没有并发问题！|2021411|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

If each thread has it's own instance, and you don't use statics, then you are pretty safe :-)

No concurrencies issues !

blocks|key|572898|text|如果来自每个线程的代码正在访问同一个类的不同实例(对象)，这不是问题；当然，除非这些对象正在访问共享对象，例如类静态对象。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|572899|正如SimpleDateFormat的文档所述：|offset|length|style|CODE|572900|572901|日期格式不同步。建议为每个线程创建单独的格式实例。如果多个线程并发访问一种格式，则必须在外部同步该格式。|blockquote|572902|572903|572904|为每个线程保留单独实例的一种方法是使用ThreadLocal对象来跟踪单独的实例。|572905|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/javase/6/docs/api/java/lang/ThreadLocal.html^0|0|2|G|0|0|0|0|0|J|B|J|B|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Y|8|@$D|Z|E|10|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|11|8|@]|9|@]|A|$]]|$1|I|3|J|5|K|7|12|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|13|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|14|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|15|8|@$D|16|E|17|F|G]]|9|@$D|18|E|19|1|1A]]|A|$]]|$1|P|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]]|Q|$R|$5|S|T|U|A|$V|W]]]]

If code from each thread is accessing different instances (objects) of the same class, that is not a problem; unless, of course, those objects are accessing shared objects, such as class static objects.

As the documentation for <code>SimpleDateFormat</code> states:

<blockquote>
 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. 
</blockquote>

One way of keeping separate instances for each thread would be to use a <a href="http://java.sun.com/javase/6/docs/api/java/lang/ThreadLocal.html" rel="nofollow noreferrer"><code>ThreadLocal</code></a> object to keep track of separate instances.

blocks|key|572923|text|关于SimpleDateFormat，正如您所指出的，它不是线程安全的，相反，您可以选择使用公共类FastDateFormat，它是线程安全的，并且可以共享。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|572924|更一般的说法是，如果你在线程之间共享对象，一定要确保它们被标记为threadsafe+--文档中应该总是标明这一点。如果它们不是threadsafe，你需要限制对它们的并发访问(例如，通过锁/同步)，或者使用多个实例(每个线程一个)。通常，如果不这样做，就有可能破坏共享数据/状态，并带来不确定的后果。|572925|entityMap|0|LINK|mutability|MUTABLE|url|http://www.isocode.cn/pku-repo/commons-lang/commons-lang/2.4/commons-lang-2.4-javadoc/org/apache/commons/lang/time/FastDateFormat.html^0|1D|E|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

With respect to SimpleDateFormat, which as you point out is not threadsafe, you can instead opt to use the commons class <a href="http://www.isocode.cn/pku-repo/commons-lang/commons-lang/2.4/commons-lang-2.4-javadoc/org/apache/commons/lang/time/FastDateFormat.html" rel="nofollow noreferrer">FastDateFormat</a> which is threadsafe and can be shared.

In more general terms, if you are sharing objects amongst threads always ensure that they are marked as threadsafe - the docs should always indicate this. If they are not threadsafe you will need to either limit concurrent access to them (e.g. via locks/synchronisation) or use multiple instances (one per thread). Generally, if you don't, you risk corrupting shared data/state with indeterminate consequences.

blocks|key|1875960|text|似乎“大多数人”都使用线程本地日期格式。类似于：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1875961|private+static+final+ThreadLocal<DateFormat>+dateFormat+=+new+ThreadLocal<DateFormat>()+{
+++++@Override+public+DateFormat+initialValue()+{
+++++++++return+new+SimpleDateFormat("fm/ts/tr",+Locale.US);
+++++}
};|code-block|syntax|javascript|1875962|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

It seems "most people" use a thread-local date format. Something like:

<pre><code>private static final ThreadLocal&lt;DateFormat&gt; dateFormat = new ThreadLocal&lt;DateFormat&gt;() {
 @Override public DateFormat initialValue() {
 return new SimpleDateFormat("fm/ts/tr", Locale.US);
 }
};
</code></pre>

the documentations always deals very explicitly with the issues of using one instance of an object with more than one thread.

However what do I need to know when there are some threads that have their own instance at the same time? Which interference could occur? How do I handle members like <code>SimpleDateFormat</code> that are quiet expensive to create?

Thanks
Mike
[;-)

Concurrecy issues with multiple instances of an object

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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文档总是非常明确地处理将对象的一个实例与多个线程一起使用的问题。但是，当一些线程同时拥有自己的实例时，我需要知道什么？可能会发生哪种干扰？我如何处理像SimpleDateFormat这样创建起来非常昂贵的成员？谢谢Mike [；-]

问一个对象的多个实例的并发问题
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