Handling JSON Post Request in Go

Question

So I have the following  which seems incredibly hacky  and I ve been thinking to myself that Go has better designed libraries than this  but I can t find an example of Go handling a POST request of JSON data  They are all form POSTs    Here is an example request  curl -X POST -d     test      that     http   localhost 8082 test  And here is the code  with the logs embedded   package main  import        encoding json       log       net http     type test struct struct       Test string    func test rw http ResponseWriter  req  http Request        req ParseForm       log Println req Form        LOG  map   test    that           var t test struct     for key       range req Form           log Println key            LOG    test    that           err    json Unmarshal   byte key    amp t          if err    nil               log Println err Error                        log Println t Test        LOG  that    func main         http HandleFunc   test   test      log Fatal http ListenAndServe   8082   nil       There s got to be a better way  right  I m just stumped in finding what the best practice could be     Go is also known as Golang to the search engines  and mentioned here so others can find it

User · Accepted Answer

Please use json Decoder instead of json Unmarshal    func test rw http ResponseWriter  req  http Request        decoder    json NewDecoder req Body      var t test struct     err    decoder Decode  amp t      if err    nil           panic err            log Println t Test

User · Answer

I was driving myself crazy with this exact problem   My JSON Marshaller and Unmarshaller were not populating my Go struct   Then I found the solution at https   eager io blog go-and-json       As with all structs in Go  it   s important to remember that only   fields with a capital first letter are visible to external programs   like the JSON Marshaller     After that  my Marshaller and Unmarshaller worked perfectly

User · Answer

I found the following example from the docs really helpful  source here    package main  import        encoding json       fmt       io       log       strings     func main         const jsonStream               Name    Ed    Text    Knock knock              Name    Sam    Text    Who s there              Name    Ed    Text    Go fmt              Name    Sam    Text    Go fmt who              Name    Ed    Text    Go fmt yourself              type Message struct           Name  Text string           dec    json NewDecoder strings NewReader jsonStream       for           var m Message         if err    dec Decode  amp m   err    io EOF               break           else if err    nil               log Fatal err                    fmt Printf   s   s n   m Name  m Text            The key here being that the OP was looking to decode  type test struct struct       Test string        in which case we would drop the const jsonStream  and replace the Message struct with the test struct   func test rw http ResponseWriter  req  http Request        dec    json NewDecoder req Body      for           var t test struct         if err    dec Decode  amp t   err    io EOF               break           else if err    nil               log Fatal err                    log Printf   s n   t Test            Update  I would also add that this post provides some great data about responding with JSON as well  The author explains struct tags  which I was not aware of   Since JSON does not normally look like   Test    test    SomeKey    SomeVal    but rather   test    test    somekey    some value    you can restructure your struct like this   type test struct struct       Test string  json  test       SomeKey string  json  some-key          and now your handler will parse JSON using  some-key  as opposed to  SomeKey   which you will be using internally

User · Answer

You need to read from req Body   The ParseForm method is reading from the req Body and then parsing it in standard HTTP encoded format   What you want is to read the body and parse it in JSON format   Here s your code updated   package main  import        encoding json       log       net http       io ioutil     type test struct struct       Test string    func test rw http ResponseWriter  req  http Request        body  err    ioutil ReadAll req Body      if err    nil           panic err            log Println string body       var t test struct     err   json Unmarshal body   amp t      if err    nil           panic err            log Println t Test     func main         http HandleFunc   test   test      log Fatal http ListenAndServe   8082   nil

User · Answer

There are two reasons why json Decoder should be preferred over json Unmarshal - that are not addressed in the most popular answer from 2013    February 2018  go 1 10 introduced a new method json Decoder DisallowUnknownFields    which addresses the concern of detecting unwanted JSON-input req Body is already an io Reader  Reading its entire contents and then performing json Unmarshal wastes resources if the stream was  say a 10MB block of invalid JSON  Parsing the request body  with json Decoder  as it streams in would trigger an early parse error if invalid JSON was encountered  Processing I O streams in realtime is the preferred go-way       Addressing some of the user comments about detecting bad user input   To enforce mandatory fields  and other sanitation checks  try   d    json NewDecoder req Body  d DisallowUnknownFields      catch unwanted fields     anonymous struct type  handy for one-time use t    struct       Test  string  json  test      pointer so we can test for field absence      err    d Decode  amp t  if err    nil          bad JSON or unrecognized json field     http Error rw  err Error    http StatusBadRequest      return    if t Test    nil       http Error rw   missing field  test  from JSON object   http StatusBadRequest      return       optional extra check if d More         http Error rw   extraneous data after JSON object   http StatusBadRequest      return       got the input we expected  no more  no less log Println  t Test    Playground  Typical output     curl -X POST -d      http   localhost 8082 strict test  expected json field  test     curl -X POST -d     Test     maybe      Unwanted     1     http   localhost 8082 strict test  json  unknown field  Unwanted     curl -X POST -d     Test     oops   g4rB4g3      amp    http   localhost 8082 strict test  extraneous data after JSON    curl -X POST -d     Test     Works     http   localhost 8082 strict test   log  2019 03 07 16 03 13 Works

User · Answer

I like to define custom structs locally  So     my handler func func addImage w http ResponseWriter  r  http Request            define custom type     type Input struct           Url        string   json  quot url quot           Name       string   json  quot name quot           Priority   int8     json  quot priority quot                 define a var      var input Input         decode input or return error     err    json NewDecoder r Body  Decode  amp input      if err    nil           w WriteHeader 400          fmt Fprintf w   quot Decode error  please check your JSON formating  quot           return               print user inputs     fmt Fprintf w   quot Inputed name   s quot   input Name

User · Answer

type test struct       Test string  json  test      func test w http ResponseWriter  req  http Request        var t test struct      body       ioutil ReadAll req Body      json Unmarshal body   amp t       fmt Println t

[json] Handling JSON Post Request in Go

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