Monthly Archives: November 2015

bash的陷阱(3): return

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bash里的return

当函数hook是一个只有一行return的空函数，并且hook函数被另一个test函数用在结尾的时候，test的返回结果很有趣：

#!/bin/bash

function test {
  ls /notexist >/dev/null 2>&1
  hook
}

function hook() {
    return
}

hook
echo "hook execute result "$?

test
echo "test execute result "$?

如果没有给return语句一个显式的code，它返回的是上一次命令的结果。hook函数这里应该用return 0更严谨一下。

HTTP Header里的If-xxx条件参数

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接上一篇讨论实现静态资源访问的问题，除了可部分获取资源，还有条件参数，服务器端判断当前资源状况满足这些条件才处理请求。

这些条件有If-Match,If-Modified-Since,If-Range,If-Unmodified-Since,If-None-Match等。最常见的If-None-Match这个header，用来在获取资源时比较本地的etag，如果服务器端不一致才获取。

先获取一个资源的etag:

$ curl -I http://localhost:8080/chain.jpg

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Accept-Ranges: bytes
ETag: W/"4932-1448300760000"
Last-Modified: Tue, 24 Nov 2015 01:46:00 CST
Content-Type: image/jpeg
Content-Length: 4932
Date: Mon, 23 Nov 2015 18:12:49 GMT

然后在请求header里增加If-None-Match内容：

$ curl -v -H 'If-None-Match: W/"4932-1448300760000"'  http://localhost:8080/chain.jpg

* Hostname was NOT found in DNS cache
*   Trying ::1...
* Connected to localhost (::1) port 8080 (#0)
> GET /_files_/img/chain.jpg HTTP/1.1
> User-Agent: curl/7.37.1
> Host: localhost:8080
> Accept: */*
> If-None-Match: W/"4932-1448300760000"
>
< HTTP/1.1 304 Not Modified
* Server Apache-Coyote/1.1 is not blacklisted
< Server: Apache-Coyote/1.1
< ETag: W/"4932-1448300760000"
< Date: Mon, 23 Nov 2015 18:13:01 GMT
<
* Connection #0 to host localhost left intact

etag相等的话，条件不符合返回304。

jmap的bug

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这个版本的jdk在执行jmap时居然将旧生代的百分比算成了负数。

$ java -version
java version "1.8.0_45"
Java(TM) SE Runtime Environment (build 1.8.0_45-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)


$ jmap -heap 15903
Attaching to process ID 15903, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.45-b02

using parallel threads in the new generation.
using thread-local object allocation.
Concurrent Mark-Sweep GC

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 3221225472 (3072.0MB)
   NewSize                  = 348913664 (332.75MB)
   MaxNewSize               = 348913664 (332.75MB)
   OldSize                  = 2872311808 (2739.25MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
   capacity = 314048512 (299.5MB)
   used     = 242824544 (231.57553100585938MB)
   free     = 71223968 (67.92446899414062MB)
   77.32071152115505% used
Eden Space:
   capacity = 279183360 (266.25MB)
   used     = 207959392 (198.32553100585938MB)
   free     = 71223968 (67.92446899414062MB)
   74.48846234961854% used
From Space:
   capacity = 34865152 (33.25MB)
   used     = 34865152 (33.25MB)
   free     = 0 (0.0MB)
   100.0% used
To Space:
   capacity = 34865152 (33.25MB)
   used     = 0 (0.0MB)
   free     = 34865152 (33.25MB)
   0.0% used
concurrent mark-sweep generation:
   capacity = 2872311808 (2739.25MB)
   used     = 17503507292142 MB
   free     = 92986414215918728 (8.86787550124347E10MB)
   -3.237336945265481E9% used

HTTP Header里的Range和Content-Range参数

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这个话题是从实现一个http资源的静态访问引发的。http协议从1.1开始支持获取文件的部分内容，这为并行下载以及断点续传提供了技术支持。它通过在Header里两个参数实现的，客户端发请求时对应的是Range，服务器端响应时对应的是Content-Range；通过tomcat看一下这两个参数。

在应用的根目录下放了一张图片”chain.jpg”，图片的大小是4932字节，用curl模拟分段请求，请求时把respons的header给dump到一个文件里：

$ curl -D "resp-header1.txt" -H 'Range: bytes=0-2000' \
    http://localhost:8080/chain.jpg > /tmp/test.jpg 

$ cat resp-header1.txt

HTTP/1.1 206 Partial Content # 返回状态码是206
Server: Apache-Coyote/1.1
Accept-Ranges: bytes
ETag: W/"4932-1447753566000"
Last-Modified: Tue, 17 Nov 2015 09:46:06 GMT
Content-Range: bytes 0-2000/4932
Content-Type: image/jpeg
Content-Length: 2001
Date: Tue, 17 Nov 2015 17:27:45 GMT

这时在mac下用preview程序打开图片看到是部分的，把剩余部分数据也下载下来才行:

$ curl -H 'Range: bytes=2001-4932' \
    http://localhost:8080/chain.jpg >> /tmp/test.jpg

Range参数还支持多个区间，用逗号分隔，下面对另一个内容为”hello world”的文件”a.html”多区间请求，这时response的Content-Type不再是原文件mime类型，而用一种multipart/byteranges类型表示：

$ curl -D 'resp-header' -H 'Range: bytes=0-5,6-10' http://localhost:8080/a.html 
--CATALINA_MIME_BOUNDARY
Content-Type: text/html
Content-Range: bytes 0-5/12

hello
--CATALINA_MIME_BOUNDARY
Content-Type: text/html
Content-Range: bytes 6-10/12

world
--CATALINA_MIME_BOUNDARY--

$ cat resp-header

HTTP/1.1 206 Partial Content
Server: Apache-Coyote/1.1
Accept-Ranges: bytes
ETag: W/"12-1447780011000"
Last-Modified: Tue, 17 Nov 2015 17:06:51 GMT
Content-Type: multipart/byteranges; boundary=CATALINA_MIME_BOUNDARY
Content-Length: 208
Date: Tue, 17 Nov 2015 17:39:30 GMT