blocks|key|318196|text|看看这里：Faster+new+String(bytes,+cs/csn)+and+String.getBytes(cs/csn)|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|318197|entityMap|0|LINK|mutability|MUTABLE|url|http://blogs.oracle.com/xuemingshen/entry/faster_new_string_bytes_cs^0|5|1O|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Take a look here: <a href="http://blogs.oracle.com/xuemingshen/entry/faster_new_string_bytes_cs" rel="nofollow noreferrer">Faster new String(bytes, cs/csn) and String.getBytes(cs/csn)</a>

blocks|key|3276553|text|您想要做的是延迟byte[]数组的处理，直到log4j确定它实际上想要记录消息。这样，您可以在调试级别记录它，例如，在测试时，然后在生产期间禁用它。例如，您可以：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3276554|final+byte[]+myArray+=+...;
Logger.getLogger(MyClass.class).debug(new+Object()+{
++++@Override+public+String+toString()+{
++++++++return+new+String(myArray);
++++}
});|code-block|syntax|javascript|3276555|现在，除非实际记录数据，否则不会造成速度损失，因为直到log4j决定实际记录消息时，才会调用toString方法！|3276556|现在我不确定你所说的“明显的表示”是什么意思，所以我假设你的意思是通过将字节重新解释为默认的字符编码来转换为字符串。现在，如果您处理的是二进制数据，这显然是毫无价值的。在这种情况下，我建议使用[Arrays.toString(byte[])](http://java.sun.com/j2se/1.5.0/docs/api/java/util/Arrays.html#toString(byte[]%2529)创建如下所示的格式化字符串|offset|length|3276557|[54,+23,+65,+...]|3276558|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/j2se/1.5.0/docs/api/java/util/Arrays.html#toString(byte[]%2529^0|0|0|0|3E|28|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|Z|8|@]|9|@$K|10|L|11|1|12]]|A|$]]|$1|M|3|N|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

What you want to do is delay processing of the byte[] array until log4j decides that it actually wants to log the message. This way you could log it at DEBUG level, for example, while testing and then disable it during production. For example, you could:

<pre><code>final byte[] myArray = ...;
Logger.getLogger(MyClass.class).debug(new Object() {
 @Override public String toString() {
 return new String(myArray);
 }
});
</code></pre>

Now you don't pay the speed penalty unless you actually log the data, because the toString method isn't called until log4j decides it'll actually log the message!

Now I'm not sure what you mean by "the obvious representation" so I've assumed that you mean convert to a String by reinterpreting the bytes as the default character encoding. Now if you are dealing with binary data, this is obviously worthless. In that case I'd suggest using <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Arrays.html#toString(byte[])" rel="noreferrer">Arrays.toString(byte[])</a> to create a formatted string along the lines of

<pre><code>[54, 23, 65, ...]
</code></pre>

blocks|key|318214|text|ASCII码是少数几种不需要算术或查表就可以与UTF16相互转换的编码之一，因此可以手动转换：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|318215|String+convert(byte[]+data)+{
++++StringBuilder+sb+=+new+StringBuilder(data.length);
++++for+(int+i+=+0;+i+<+data.length;+%2B%2B+i)+{
++++++++if+(data[i]+<+0)+throw+new+IllegalArgumentException();
++++++++sb.append((char)+data[i]);
++++}
++++return+sb.toString();
}|code-block|syntax|javascript|318216|但要确保它真的是ASCII，否则你将以垃圾告终。|318217|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|$]]

ASCII is one of the few encodings that can be converted to/from UTF16 with no arithmetic or table lookups so it's possible to convert manually:

<pre><code>String convert(byte[] data) {
 StringBuilder sb = new StringBuilder(data.length);
 for (int i = 0; i &lt; data.length; ++ i) {
 if (data[i] &lt; 0) throw new IllegalArgumentException();
 sb.append((char) data[i]);
 }
 return sb.toString();
}
</code></pre>

But make sure it really is ASCII, or you'll end up with garbage.

blocks|key|317230|text|如果您的数据实际上是ASCII码(即7位数据)，那么您应该使用new+String(data,+"US-ASCII")而不是依赖于平台默认编码。这可能比试图将其解释为您的平台默认编码(可能是UTF-8，这需要更多自省)更快。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|CODE|entityRanges|data|317231|您还可以通过缓存Charset实例并调用new+String(data,+charset)来避免每次命中字符集查找，从而加快速度。|317232|话虽如此:我已经很长时间没有在生产环境中看到真实的ASCII数据了|317233|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/javase/6/docs/api/java/lang/String.html#String(byte[],%2520java.lang.String)|1|http://java.sun.com/javase/6/docs/api/java/nio/charset/Charset.html|2|http://java.sun.com/javase/6/docs/api/java/lang/String.html#String(byte[],%2520java.nio.charset.Charset)^0|0|34|V|S|V|S|0|0|0|1T|8|7|K|P|8|7|1|K|P|2|0|0|X|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|D]]|E|@$9|11|A|12|1|13]]|F|$]]|$1|G|3|H|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|D]|$9|19|A|1A|B|D]]|E|@$9|1B|A|1C|1|1D]|$9|1E|A|1F|1|1G]]|F|$]]|$1|I|3|J|5|6|7|1H|8|@$9|1I|A|1J|B|C]]|E|@]|F|$]]|$1|K|3|-4|5|6|7|1K|8|@]|E|@]|F|$]]]|L|$M|$5|N|O|P|F|$Q|R]]|S|$5|N|O|P|F|$Q|T]]|U|$5|N|O|P|F|$Q|V]]]]

If your data is in fact ASCII (i.e. 7-bit data), then you should be using <a href="http://java.sun.com/javase/6/docs/api/java/lang/String.html#String(byte%5B%5D,%20java.lang.String)" rel="noreferrer"><code>new String(data, "US-ASCII")</code></a> instead of depending on the platform default encoding. This may be faster than trying to interpret it as your platform default encoding (which could be UTF-8, which requires more introspection).

You could also speed this up by avoiding the Charset-Lookup hit each time, by caching the <a href="http://java.sun.com/javase/6/docs/api/java/nio/charset/Charset.html" rel="noreferrer"><code>Charset</code></a> instance and calling <a href="http://java.sun.com/javase/6/docs/api/java/lang/String.html#String(byte%5B%5D,%20java.nio.charset.Charset)" rel="noreferrer"><code>new String(data, charset)</code></a> instead.

Having said that: it's been a very, very long time since I've seen real ASCII data in production environment

blocks|key|3276656|text|性能减半？这个字节数组有多大？例如，如果是1MB，那么除了将字节“转换”为字符(应该足够快)之外，当然还有更多的因素需要考虑。将+1MB的数据写入日志文件，而不是“仅仅”100字节(+byte[].toString()可能会生成)，这显然需要一些时间。磁盘文件系统不如RAM内存快。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|CODE|entityRanges|data|3276657|您需要更改字节数组的字符串表示形式。可能包含一些更敏感的信息，例如与其关联的名称(文件名？)、其长度等。毕竟，这个字节数组到底代表了什么？|3276658|编辑：我不记得在你的问题中看到过“大约30us”的短语，也许你在提问后的5分钟内编辑了它，但这实际上是微优化，在一般情况下它肯定不会导致“性能减半”。除非你每秒写一百万次(尽管如此，你为什么要这样做呢？你不是在过度使用“日志记录”这个现象吗？)|3276659|entityMap^0|1R|1|2K|H|0|0|0|2|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|D]]|E|@]|F|$]]|$1|G|3|H|5|6|7|R|8|@]|E|@]|F|$]]|$1|I|3|J|5|6|7|S|8|@$9|T|A|U|B|C]]|E|@]|F|$]]|$1|K|3|-4|5|6|7|V|8|@]|E|@]|F|$]]]|L|$]]

Halved performance? How large is this byte array? If it's for example 1MB, then there are certainly more factors to take into account than just "converting" from bytes to chars (which is supposed to be fast enough though). Writing 1MB of data instead of "just" 100bytes (which the <code>byte[].toString()</code> may generate) to a log file is obviously going to take some time. The disk file system is not as fast as RAM memory.

You'll need to change the string representation of the byte array. Maybe with some more sensitive information, e.g. the name associated with it (filename?), its length and so on. After all, what does that byte array actually represent?

Edit: I can't remember to have seen the "approximately 30us" phrase in your question, maybe you edited it in within 5 minutes after asking, but this is actually microoptimization and it should certainly not cause "halved performance" in general. Unless you write them a million times per second (still then, why would you want to do that? aren't you overusing the phenomenon "logging"?).

I am trying to pass a byte[] containing ASCII characters to log4j, to be logged into a file using the obvious representation. When I simply pass in the byt[] it is of course treated as an object and the logs are pretty useless. When I try to convert them to strings using <code>new String(byte[] data)</code>, the performance of my application is halved.

How can I efficiently pass them in, without incurring the approximately 30us time penalty of converting them to strings.

Also, why does it take so long to convert them?

Thanks.

Edit

I should add that I am optmising for latency here - and yes, 30us does make a difference! Also, these arrays vary from ~100 all the way up to a few thousand bytes.

Convert ASCII byte[] to String

日志文件

文件系统

我正在尝试向log4j传递一个包含ASCII字符的byte[]，以便使用明显的表示形式登录到一个文件中。当我只是简单地传入byt[]时，它当然会被当作一个对象来处理，而日志是非常无用的。当我尝试使用new String(byte[] data)将它们转换为字符串时，我的应用程序的性能降低了一半。我如何才能有效地传递它们...

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