


How can I efficiently process large JSON arrays without loading them entirely into memory in Go?
Dec 25, 2024 am 09:00 AMDecoding Large Stream JSON
When dealing with massive JSON arrays stored in files, it's crucial to avoid loading the entire array into memory, as this can lead to out-of-memory errors. Instead, consider streaming the JSON data element by element.
One approach to achieve this is demonstrated in the encoding/json documentation:
package main import ( "encoding/json" "fmt" "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)) // read open bracket t, err := dec.Token() if err != nil { log.Fatal(err) } fmt.Printf("%T: %v\n", t, t) // while the array contains values for dec.More() { var m Message // decode an array value (Message) err := dec.Decode(&m) if err != nil { log.Fatal(err) } fmt.Printf("%v: %v\n", m.Name, m.Text) } // read closing bracket t, err = dec.Token() if err != nil { log.Fatal(err) } fmt.Printf("%T: %v\n", t, t) }
In this sample code:
- A decoder object (dec) is created to read JSON data from a string.
- The decoder reads tokens one by one, starting with the opening bracket of the array.
- A for loop iterates over array elements, parsing each into a Message struct.
- The names and text of each message are printed.
- The loop continues until no more elements are available in the array, indicated by the dec.More() function.
- Finally, the decoder reads and prints the closing bracket of the array.
This approach allows you to stream a large JSON array without loading the entire data structure into memory, optimizing resource usage and enabling efficient processing.
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