blocks|key|3226904|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3226905|Creating+SimpleDateFormat+is+expensive。不要使用这个，除非它很少做。如果你能接受一点阻塞，|ordered-list-item|offset|length|3226906|可以。如果不经常使用formatDate()，则使用。如果重用线程(thread+pool)，则使用|3226907|Fastest选项。使用比2更多的内存，并且具有更高的启动开销。|3226908|3226909|对于应用程序，2.和3.都是可行的选择。哪个最适合您的情况取决于您的用例。提防过早优化。只有在你认为这是一个问题的时候才去做。|3226910|对于将由第三方使用的库，我会使用选项3。|3226911|entityMap|0|LINK|mutability|MUTABLE|url|https://web.archive.org/web/20140129235842/http://www.thedwick.com/2008/04/simpledateformat-performance-pig/|1|http://download.oracle.com/javase/tutorial/essential/concurrency/pools.html^0|0|T|9|0|0|Y|B|1|0|0|0|0|0^^$0|@$1|2|3|-4|4|5|6|Y|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|Z|7|@]|8|@$D|10|E|11|1|12]]|9|$]]|$1|F|3|G|4|C|6|13|7|@]|8|@$D|14|E|15|1|16]]|9|$]]|$1|H|3|I|4|C|6|17|7|@]|8|@]|9|$]]|$1|J|3|-4|4|5|6|18|7|@]|8|@]|9|$]]|$1|K|3|L|4|5|6|19|7|@]|8|@]|9|$]]|$1|M|3|N|4|5|6|1A|7|@]|8|@]|9|$]]|$1|O|3|-4|4|5|6|1B|7|@]|8|@]|9|$]]]|P|$Q|$4|R|S|T|9|$U|V]]|W|$4|R|S|T|9|$U|X]]]]

<ol>
<li>Creating SimpleDateFormat is <a href="https://web.archive.org/web/20140129235842/http://www.thedwick.com/2008/04/simpledateformat-performance-pig/" rel="noreferrer">expensive</a>. Don't use this unless it's done seldom.</li>
<li>OK if you can live with a bit of blocking. Use if formatDate() is not used much.</li>
<li>Fastest option IF you reuse threads (<a href="http://download.oracle.com/javase/tutorial/essential/concurrency/pools.html" rel="noreferrer">thread pool</a>). Uses more memory than 2. and has higher startup overhead.</li>
</ol>

For applications both 2. and 3. are viable options. Which is best for your case depends on your use case. Beware of premature optimization. Only do it if you believe this is an issue.

For libraries that would be used by 3rd party I'd use option 3.

blocks|key|292776|text|不要使用SimpleDateFormat，而是使用joda-time的DateTimeFormatter。joda-time在解析方面有点严格，因此在SimpleDateFormat的替代品中并不是很低，但在安全性和性能方面，joda-time对并发更友好。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|292777|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Don't use SimpleDateFormat, use joda-time's DateTimeFormatter instead. It is a bit stricter in the parsing side and so isn't quite a drop in replacement for SimpleDateFormat, but joda-time is much more concurrent friendly in terms of safety and performance.

blocks|key|3226930|text|另一个选项是Commons+Lang+，但您只能将其用于日期格式化，而不能用于解析。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3226931|与Joda不同，它可以作为格式化的临时替代品。(更新:从v3.3.2开始，FastDateFormat可以生成FastDateParser，这是SimpleDateFormat的一个临时线程安全替代品)|offset|length|3226932|entityMap|0|LINK|mutability|MUTABLE|url|http://commons.apache.org/proper/commons-lang/javadocs/api-3.4/org/apache/commons/lang3/time/FastDateParser.html^0|0|1J|E|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

The other option is Commons Lang <a href="https://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/time/FastDateFormat.html" rel="noreferrer">FastDateFormat</a> but you can only use it for date formatting and not parsing. 

Unlike Joda, it can function as a drop-in replacement for formatting.
(Update: Since v3.3.2, FastDateFormat can produce a <a href="http://commons.apache.org/proper/commons-lang/javadocs/api-3.4/org/apache/commons/lang3/time/FastDateParser.html" rel="noreferrer">FastDateParser</a>, which is a drop-in thread-safe replacement for SimpleDateFormat)

blocks|key|3226939|text|我想说的是，为SimpleDateFormat创建一个简单的包装器类，它可以同步对parse()和format()的访问，并且可以用作临时替代。比你的选项#2更简单，比你的选项#3更简单。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3226940|对于Java+API设计人员来说，让SimpleDateFormat不同步似乎是一个糟糕的设计决策；我怀疑是否有人期望format()和parse()需要同步。|3226941|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|$]]

I would say, create a simple wrapper-class for SimpleDateFormat that synchronizes access to parse() and format() and can be used as a drop-in replacement. More foolproof than your option #2, less cumbersome than your option #3.

Seems like making SimpleDateFormat unsynchronized was a poor design decision on the part of the Java API designers; I doubt anyone expects format() and parse() to need to be synchronized.

blocks|key|292031|text|假设你的应用程序只有一个线程。那么为什么要同步访问SimpleDataFormat变量呢？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|292032|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Imagine your application has one thread. Why would you synchronize access to SimpleDataFormat variable then?

blocks|key|3226960|text|Commons+Lang+3.x现在既有FastDateFormat，也有FastDateParser。它是线程安全的，而且比SimpleDateFormat更快。它还使用与SimpleDateFormat相同的格式/解析模式规范。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3226961|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Commons Lang 3.x now has FastDateParser as well as FastDateFormat. It is thread safe and faster than SimpleDateFormat. It also uses the same format/parse pattern specifications as SimpleDateFormat.

blocks|key|3226980|text|另一种选择是将实例放在线程安全队列中：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3226981|import+java.util.concurrent.ArrayBlockingQueue;
private+static+final+int+DATE_FORMAT_QUEUE_LEN+=+4;
private+static+final+String+DATE_PATTERN+=+"yyyy-MM-dd+HH:mm:ss";
private+ArrayBlockingQueue<SimpleDateFormat>+dateFormatQueue+=+new+ArrayBlockingQueue<SimpleDateFormat>(DATE_FORMAT_QUEUE_LEN);
//+thread-safe+date+time+formatting
public+String+format(Date+date)+{
++++SimpleDateFormat+fmt+=+dateFormatQueue.poll();
++++if+(fmt+==+null)+{
++++++++fmt+=+new+SimpleDateFormat(DATE_PATTERN);
++++}
++++String+text+=+fmt.format(date);
++++dateFormatQueue.offer(fmt);
++++return+text;
}
public+Date+parse(String+text)+throws+ParseException+{
++++SimpleDateFormat+fmt+=+dateFormatQueue.poll();
++++if+(fmt+==+null)+{
++++++++fmt+=+new+SimpleDateFormat(DATE_PATTERN);
++++}
++++Date+date+=+null;
++++try+{
++++++++date+=+fmt.parse(text);
++++}+finally+{
++++++++dateFormatQueue.offer(fmt);
++++}
++++return+date;
}|code-block|syntax|javascript|3226982|dateFormatQueue的大小应该接近可以同时调用此函数的估计线程数。在最坏的情况下，实际使用所有实例的线程数超过此数量时，会创建一些无法返回到dateFormatQueue的SimpleDateFormat实例，因为它已满。这不会产生错误，它只会导致创建一些只使用一次的SimpleDateFormat的惩罚。|3226983|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Another option is to keep instances in a thread-safe queue:

<pre><code>import java.util.concurrent.ArrayBlockingQueue;
private static final int DATE_FORMAT_QUEUE_LEN = 4;
private static final String DATE_PATTERN = "yyyy-MM-dd HH:mm:ss";
private ArrayBlockingQueue&lt;SimpleDateFormat&gt; dateFormatQueue = new ArrayBlockingQueue&lt;SimpleDateFormat&gt;(DATE_FORMAT_QUEUE_LEN);
// thread-safe date time formatting
public String format(Date date) {
 SimpleDateFormat fmt = dateFormatQueue.poll();
 if (fmt == null) {
 fmt = new SimpleDateFormat(DATE_PATTERN);
 }
 String text = fmt.format(date);
 dateFormatQueue.offer(fmt);
 return text;
}
public Date parse(String text) throws ParseException {
 SimpleDateFormat fmt = dateFormatQueue.poll();
 if (fmt == null) {
 fmt = new SimpleDateFormat(DATE_PATTERN);
 }
 Date date = null;
 try {
 date = fmt.parse(text);
 } finally {
 dateFormatQueue.offer(fmt);
 }
 return date;
}
</code></pre>

The size of dateFormatQueue should be something close to the estimated number of threads which can routinely call this function at the same time.
In the worst case where more threads than this number do actually use all the instances concurrently, some SimpleDateFormat instances will be created which cannot be returned to dateFormatQueue because it is full. This will not generate an error, it will just incur the penalty of creating some SimpleDateFormat which are used only once.

blocks|key|3012887|text|如果您使用的是Java8，则可能需要使用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3012888|3012889|这个类是不可变的，并且是线程安全的。|blockquote|3012890|3012891|3012892|例如：|3012893|DateTimeFormatter+formatter+=+DateTimeFormatter.ofPattern("yyyy-MM-dd");
String+str+=+new+java.util.Date().toInstant()
+++++++++++++++++++++++++++++++++.atZone(ZoneId.systemDefault())
+++++++++++++++++++++++++++++++++.format(formatter);|code-block|syntax|javascript|3012894|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|R|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|S|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|T|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|U|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|V|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|W|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|P|$]]

If you are using Java 8, you may want to use <a href="http://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html"><code>java.time.format.DateTimeFormatter</code></a>:

<blockquote>
 This class is immutable and thread-safe.
</blockquote>

e.g.:

<pre><code>DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
String str = new java.util.Date().toInstant()
 .atZone(ZoneId.systemDefault())
 .format(formatter);
</code></pre>

blocks|key|3012965|text|我只是用选项3实现了这一点，但做了一些代码更改：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3012966|3012967|ThreadLocal通常是静态的|unordered-list-item|3012968|覆盖initialValue()而不是测试(get()+==+null)|3012969|你可能想要设置语言环境和时区，除非你真的想要缺省设置(缺省设置很容易出错)|3012970|3012971|私有静态最终返回tl+=+ThreadLocal()+{+@Override+protected+SimpleDateFormat+initialValue()+{+SimpleDateFormat+sdf+=+new+SimpleDateFormat("yyyy-MM-hh"，Locale.US)；sdf.setTimeZone(TimeZone.getTimeZone("America/Los_Angeles"))；return+sdf；}+}；公共字符串formatDate(tl.get+d)+{+return+tl.get().format(d)；}|3012972|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|P|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|Q|8|@]|9|@]|A|$]]|$1|F|3|G|5|E|7|R|8|@]|9|@]|A|$]]|$1|H|3|I|5|E|7|S|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|N|$]]

I just implemented this with Option 3, but made a few code changes:

<ul>
<li>ThreadLocal should usually be static</li>
<li>Seems cleaner to override initialValue() rather than test if (get() == null)</li>
<li>You may want to set locale and time zone unless you really want the default settings (defaults are very error prone with Java)

<pre><code>private static final ThreadLocal&lt;SimpleDateFormat&gt; tl = new ThreadLocal&lt;SimpleDateFormat&gt;() {
 @Override
 protected SimpleDateFormat initialValue() {
 SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-hh", Locale.US);
 sdf.setTimeZone(TimeZone.getTimeZone("America/Los_Angeles"));
 return sdf;
 }
};
public String formatDate(Date d) {
 return tl.get().format(d);
}
</code></pre></li>
</ul>

The javadoc for SimpleDateFormat states that SimpleDateFormat is not synchronized.

<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>

But what is the best approach to using an instance of SimpleDateFormat in a multi threaded environment. Here are a few options I have thought of, I have used options 1 and 2 in the past but I am curious to know if there are any better alternatives or which of these options would offer the best performance and concurrency.

Option 1: Create local instances when required

<pre><code>public String formatDate(Date d) {
 SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
 return sdf.format(d);
}
</code></pre>

Option 2: Create an instance of SimpleDateFormat as a class variable but synchronize access to it.

<pre><code>private SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
public String formatDate(Date d) {
 synchronized(sdf) {
 return sdf.format(d);
 }
}
</code></pre>

Option 3: Create a ThreadLocal to store a different instance of SimpleDateFormat for each thread.

<pre><code>private ThreadLocal&lt;SimpleDateFormat&gt; tl = new ThreadLocal&lt;SimpleDateFormat&gt;();
public String formatDate(Date d) {
 SimpleDateFormat sdf = tl.get();
 if(sdf == null) {
 sdf = new SimpleDateFormat("yyyy-MM-hh");
 tl.set(sdf);
 }
 return sdf.format(d);
}
</code></pre>

Synchronizing access to SimpleDateFormat

Java

SimpleDateFormat的javadoc声明SimpleDateFormat不同步。“日期格式不同步。建议为每个线程创建单独的格式实例。如果多个线程同时访问一种格式，则必须外部同步。”但是，在多线程环境中使用SimpleDateFormat实例的最佳方法是什么呢？下面是我想过的几个选项，我过去用过选项1和选项2...

问同步对SimpleDateFormat的访问
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