http://matt.aimonetti.net/posts/2013/07/01/golang-multipart-file-upload-example/
The Go language is one of my favorite programming languages. However,sometimes doing simple things can seem a bit harder than it should. However,most of the time,the problem is just to find out how to do things the easy way. While Go’s documention isn’t bad,the real key to finding out how to do things is often to look at thesource codeand thetest suite.
I’m not yet super familiar with all the std lib packages,so when I wanted to test my Go web services,I wrote a few lines of code to create a multipart file upload function that was building the body from scratch. Once I was done messing with the varIoUs headers,boundary protocol etc.. I started testing some edge cases,I found some bugs in my code. Looking at Go’s packages,I realized that all the tools were already available for me to use. I was just lacking a good example. Walking through the test suite I finally figured out how to write a simple multipart file upload example with some extra query params.
Hopefully this example will be helpful to some of you.
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package main import ( "bytes" "fmt" "io" "log" "mime/multipart" "net/http" "os" "path/filepath" ) // Creates a new file upload http request with optional extra params func newfileUploadRequest(uri string, params map[]paramNamepath ) (*http.Requesterror{ fileerr := osOpenpath) if != nil { return nilerr } defer Close() body := &bytesBuffer{} writer multipartNewWriterbody) partwriterCreateFormFilefilepathBase)) } _= ioCopy) for keyval := range params _ WriteFieldval) () } NewRequest("POST"uri) } main() Getwd() path += "/test.pdf" extraParams { "title": "My Document" "author""Matt Aimonetti""description""A document with all the Go programming language secrets" request"https://google.com/upload"extraParams"file""/tmp/doc.pdf"logFatalerrclient ClientrespclientDo} else ReadFromBodyfmtPrintlnStatusCodeHeader} } |
Example’s source code on GitHub
All the work is done in thenewfileUploadRequestfunction and really,themime/multipartpackage hides all the complexity of creating a multipart request.
The key is to set a newmultipart.Writer:
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The writer will do all the work and will write directly to our body (which itself is a buffer of bytes).
We then create a part for the file form entry with the name of the file param and the name of the file (that we extracted using thepath/filepathpackage). We need to add the content of the file to the file part,we use theio.Copy()to do so. In the first version of this article,I had usedio/IoUtilReadallto read the content of the file (see codehere). However a few readers rightfully mentioned that I should instead copy content from the file to the part instead of temporarily loading the content of the file in memory.Hereis an even more optimized version using goroutine to stream the data,andhereis the full example using a pipe.
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Themultipart.Writertakes care of setting the boundary and formating the form data for us,nice isn’t it?!
Then for any extra params passed as a map of string keys to string value,we use another function of themultipart.Writertype:
Once again,the writer takes care of creating the right headers,and to add the passed value. |
