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JSON has 3 basic types: booleans, numbers, strings, combined using arrays and objects to build complex structures. Go's terminology calls marshal the process of generating a JSON string from a data structure, and unmarshal the act of parsing JSON to a data structure.
To unmarshal a JSON array into a slice, Unmarshal resets the slice length to zero and then appends each element to the slice. As a special case, to unmarshal an empty JSON array into a slice, Unmarshal replaces the slice with a new empty slice.
To encode JSON data we use the Marshal function. Only data structures that can be represented as valid JSON will be encoded: JSON objects only support strings as keys; to encode a Go map type it must be of the form map[string]T (where T is any Go type supported by the json package).
It really depends on what your input is. If you look at the implementation of the Decode method of json.Decoder, it buffers the entire JSON value in memory before unmarshalling it into a Go value. So in most cases it won't be any more memory efficient (although this could easily change in a future version of the language).
So a better rule of thumb is this:
json.Decoder if your data is coming from an io.Reader stream, or you need to decode multiple values from a stream of data.json.Unmarshal if you already have the JSON data in memory.For the case of reading from an HTTP request, I'd pick json.Decoder since you're obviously reading from a stream.
I found this paragraph in the Go web programming book. But there is no explanation given
So when do we use Decoder versus Unmarshal?
That depends on the input. If your data is coming from an io.Reader stream, like the Body of an http.Request, use Decoder. If you have the data in a string or somewhere in memory, use Unmarshal.
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