网络字节序和主机字节序

Little endian：将低序字节存储在起始地址，主要是x86。

Big endian：将高序字节存储在起始地址，对于大端直接按照顺序读就可以了，网络序和PPC使用大端。

但要注意的是Big Endian 和 Little Endian并不单单由CPU或OS决定，而是两者共同决定的。

看例子，对于0x01020304

内存	Little endian	Big endian
高	01	04
	02	03
	03	02
低	04	01

Java获取系统信息最优化代码

import java.util.Map.Entry;

 

public class Env {

    public static void main(String[] args) {

       for (Entry<Object, Object> e : System.getProperties().entrySet()) {

           System.out.printf("%s = %s\n", e.getKey(), e.getValue());

       }

    }

}

readyState状态详解

    大多数javascript初学者都只知道XMLHttpRequest请求时，readyState == 4代表请求的数据已经完全可用了，但是对0,1,2,3,4等值代表的具体含义，不一定完全了解了，同时，国内关于javascript方面的书籍、文章，鱼龙混杂，互相冲突，那究竟不同的readyState代表的具体含义是什么呢？查阅了最权威的w3c官方文档，终于找到了结果：
    文档The XMLHttpRequest Object，对readyState的描述如下:
    readyState of type unsigned short, readonly

The state of the object. The attribute MUSt be one of the following values:

    0 Uninitialized

The initial value.

    1 Open

The open() method has been successfully called.

    2 Sent

The UA successfully completed the request, but no data has yet been received.

    3 Receiving

Immediately before receiving the message body (if any). All HTTP headers have been received.

    4 Loaded

The data transfer has been completed.

    但是，这并不是最终的答案，我们注意到，这份文档标记为W3C Working Draft 05 April 2006，在该文档标题下方，标记有Latest version：XMLHttpRequest，标记日期为W3C Working Draft 19 November 2009，也就是说，前面的描述不是最新的，那最新的文档怎么描述的呢？两者是否相同呢？答案是否定的，内容如下：
    The XMLHttpRequest object can be in several states. The readyState attribute, on getting, must return the current state, which must be one of the following values:

    UNSENT (numeric value 0)

The object has been constructed.

    OPENED (numeric value 1)

The open() method has been successfully invoked. During this state request headers can be set using setRequestHeader() and the request can be made using the send() method.

    HEADERS_RECEIVED (numeric value 2)

All HTTP headers have been received. Several response members of the object are now available.

    LOADING (numeric value 3)

The response entity body is being received.

    DONE (numeric value 4)

The data transfer has been completed or something went wrong during the transfer (e.g. infinite redirects).

    The OPENED state has an associated send() flag that indicates whether the send() method has been invoked. It can be either true or false and has an initial value of false.

    The DONE state has an associated error flag that indicates some type of network error or abortion. It can be either true or false and has an initial value of false.

    略加分析，我们可以发现，这两份文档对readyState == 0,1,4的描述基本相同，但对于2和3的描述是完全不同的旧文档中，readyState == 2时，只是已经完成了send()操作，并没有收到任何数据，readyState == 3只是收到了HTTP headers，这样，浏览器大多数时间应该是停在3上，等待数据体和最后的数据处理了，并且readyState == 3时，是不能够处理数据的，新文档中，去除了Sent状态，2状态时所作的事情基本等同于旧文档的2和3状态，即调用了send()，并且只收到了HTTP headers，同时强化了数据接收的状态描述，将数据接收分为了接收数据内容(readyState == [2, 3))和内容处理(readyState == [3, 4))（主要包括null处理，根据Content-Type读取和处理MIME类型，字符集转换和错误处理等工作），也就是说，如果您很清楚返回值的情况的话，那么当readyState == 3时，服务器返回的数据已经完全传输完毕，您就可以自己开始处理数据了。

linux下查看cpu信息

Linux下可以在/proc/cpuinfo中看到每个cpu地详细信息。

当然对于双核地cpu，在cpuinfo中会看到两个cpu。常常会让人误以为是两个单核地cpu。

其实应该通过Physical Processor ID来区分单核和双核。而Physical Processor ID可以从cpuinfo或者dmesg中找到

例如
root@debian:~:0# dmesg | grep CPU | grep ID
CPU0: Initial APIC ID: 0, Physical Processor ID: 0
CPU1: Initial APIC ID: 1, Physical Processor ID: 0
CPU2: Initial APIC ID: 6, Physical Processor ID: 3
CPU3: Initial APIC ID: 7, Physical Processor ID: 3

可以看到，这台机器有两个双核地CPU，ID分别是0和3。

Mime Types By File Extension

Extension	Type/sub-type
	application/octet-stream
323	text/h323
acx	application/internet-property-stream
ai	application/postscript
aif	audio/x-aiff
aifc	audio/x-aiff
aiff	audio/x-aiff
asf	video/x-ms-asf
asr	video/x-ms-asf
asx	video/x-ms-asf
au	audio/basic
avi	video/x-msvideo
axs	application/olescript
bas	text/plain
bcpio	application/x-bcpio
bin	application/octet-stream
bmp	image/bmp
c	text/plain
cat	application/vnd.ms-pkiseccat
cdf	application/x-cdf
cer	application/x-x509-ca-cert
class	application/octet-stream
clp	application/x-msclip
cmx	image/x-cmx
cod	image/cis-cod
cpio	application/x-cpio
crd	application/x-mscardfile
crl	application/pkix-crl
crt	application/x-x509-ca-cert
csh	application/x-csh
css	text/css
dcr	application/x-director
der	application/x-x509-ca-cert
dir	application/x-director
dll	application/x-msdownload
dms	application/octet-stream
doc	application/msword
dot	application/msword
dvi	application/x-dvi
dxr	application/x-director
eps	application/postscript
etx	text/x-setext
evy	application/envoy
exe	application/octet-stream
fif	application/fractals
flr	x-world/x-vrml
gif	image/gif
gtar	application/x-gtar
gz	application/x-gzip
h	text/plain
hdf	application/x-hdf
hlp	application/winhlp
hqx	application/mac-binhex40
hta	application/hta
htc	text/x-component
htm	text/html
html	text/html
htt	text/webviewhtml
ico	image/x-icon
ief	image/ief
iii	application/x-iphone
ins	application/x-internet-signup
isp	application/x-internet-signup
jfif	image/pipeg
jpe	image/jpeg
jpeg	image/jpeg
jpg	image/jpeg
js	application/x-javascript
latex	application/x-latex
lha	application/octet-stream
lsf	video/x-la-asf
lsx	video/x-la-asf
lzh	application/octet-stream
m13	application/x-msmediaview
m14	application/x-msmediaview
m3u	audio/x-mpegurl
man	application/x-troff-man
mdb	application/x-msaccess
me	application/x-troff-me
mht	message/rfc822
mhtml	message/rfc822
mid	audio/mid
mny	application/x-msmoney
mov	video/quicktime
movie	video/x-sgi-movie
mp2	video/mpeg
mp3	audio/mpeg
mpa	video/mpeg
mpe	video/mpeg
mpeg	video/mpeg
mpg	video/mpeg
mpp	application/vnd.ms-project
mpv2	video/mpeg
ms	application/x-troff-ms
mvb	application/x-msmediaview
nws	message/rfc822
oda	application/oda
p10	application/pkcs10
p12	application/x-pkcs12
p7b	application/x-pkcs7-certificates
p7c	application/x-pkcs7-mime
p7m	application/x-pkcs7-mime
p7r	application/x-pkcs7-certreqresp
p7s	application/x-pkcs7-signature
pbm	image/x-portable-bitmap
pdf	application/pdf
pfx	application/x-pkcs12
pgm	image/x-portable-graymap
pko	application/ynd.ms-pkipko
pma	application/x-perfmon
pmc	application/x-perfmon
pml	application/x-perfmon
pmr	application/x-perfmon
pmw	application/x-perfmon
pnm	image/x-portable-anymap
pot,	application/vnd.ms-powerpoint
ppm	image/x-portable-pixmap
pps	application/vnd.ms-powerpoint
ppt	application/vnd.ms-powerpoint
prf	application/pics-rules
ps	application/postscript
pub	application/x-mspublisher
qt	video/quicktime
ra	audio/x-pn-realaudio
ram	audio/x-pn-realaudio
ras	image/x-cmu-raster
rgb	image/x-rgb
rmi	audio/mid
roff	application/x-troff
rtf	application/rtf
rtx	text/richtext
scd	application/x-msschedule
sct	text/scriptlet
setpay	application/set-payment-initiation
setreg	application/set-registration-initiation
sh	application/x-sh
shar	application/x-shar
sit	application/x-stuffit
snd	audio/basic
spc	application/x-pkcs7-certificates
spl	application/futuresplash
src	application/x-wais-source
sst	application/vnd.ms-pkicertstore
stl	application/vnd.ms-pkistl
stm	text/html
svg	image/svg+xml
sv4cpio	application/x-sv4cpio
sv4crc	application/x-sv4crc
swf	application/x-shockwave-flash
t	application/x-troff
tar	application/x-tar
tcl	application/x-tcl
tex	application/x-tex
texi	application/x-texinfo
texinfo	application/x-texinfo
tgz	application/x-compressed
tif	image/tiff
tiff	image/tiff
tr	application/x-troff
trm	application/x-msterminal
tsv	text/tab-separated-values
txt	text/plain
uls	text/iuls
ustar	application/x-ustar
vcf	text/x-vcard
vrml	x-world/x-vrml
wav	audio/x-wav
wcm	application/vnd.ms-works
wdb	application/vnd.ms-works
wks	application/vnd.ms-works
wmf	application/x-msmetafile
wps	application/vnd.ms-works
wri	application/x-mswrite
wrl	x-world/x-vrml
wrz	x-world/x-vrml
xaf	x-world/x-vrml
xbm	image/x-xbitmap
xla	application/vnd.ms-excel
xlc	application/vnd.ms-excel
xlm	application/vnd.ms-excel
xls	application/vnd.ms-excel
xlt	application/vnd.ms-excel
xlw	application/vnd.ms-excel
xof	x-world/x-vrml
xpm	image/x-xpixmap
xwd	image/x-xwindowdump
z	application/x-compress
zip	application/zip