如何在REST端点中将结构作为有效JSON返回？

Question

I am fetching some data from an API, which I want to serve in a REST endpoint from my Go application.

the struct is as such:

type Stock struct {
    Stock     string            `json:"message_id,omitempty"`
    StockData  map[string]interface{} `json:"status,omitempty"`
}

//
var StockDataMap Stock

And if printed in the console, it looks as it's supposed to.

My controller is as such:

package lib

import (
    "net/http"
    "log"
    "encoding/json"
)

func returnStocksFromMemory(w http.ResponseWriter, r *http.Request) {
    json.Marshal(StockDataMap)
}


// Expose controller at http://localhost:8081/
func StockMarketDataController() {
    http.HandleFunc("/stocks", returnStocksFromMemory)
    log.Fatal(http.ListenAndServe(":8081", nil))
}

Printing StockDataMap results in a key:hashtable, which is what I want. However, when accessing http://localhost:8081/stocks , it returns nothing.

returnStocksFromMemory surely is the problem. But how do I return the struct to valid JSON in there? Right now, it's empty as said.

Answer 1

It looks like you're coming from Ruby or another language where a value in the tail position is the return value and exceptions are used to handle errors. In Go you need to be explicit with both error handling and returns.

This is a good article for developing a web app in Go;

https://golang.org/doc/articles/wiki/

In terms of your returnStocksFromMemory I would make the following changes ensuring you update HandleFunc with the new name appropriately:

func stocksHandler(w http.ResponseWriter, r *http.Request) {
    enc := json.NewEncoder(w)
    err := enc.Encode(StockDataMap)
    if err != nil {
      http.Error(w, err.Error(), http.StatusInternalServerError)
      return
    }
}

Note as your code is outlined it will only return {} . You need to populate the StockDataMap with values. The change in function name is two-fold;

1. Idiomatic Go uses concise names.
2. Handler is generally used as a postfix for http handlers.

I would encourage you to read Effective Go article to help you map your current model of development to Go;

https://golang.org/doc/effective_go.html

On naming things you can review this slide deck;

https://talks.golang.org/2014/names.slide#1