无法使用GSON解析JSON文件

Question

I have a pretty simple JSON file that I need to get the field names and values out of. I'm using Java 1.7 with gson in eclipse. Here is my JSON file:

{
   "header": [
      {
         "document_number": "document_number",     
         "report": "report",
         "version": "version"
      }
   ],

   "summary": [
      {
         "row_type": "row_type",
         "crs_id": "crs_id",
         "report_begin": "report_begin",
         "report_end": "report_end",
         "gross_tax": "gross_tax",
         "compensating_tax": "compensating_tax",
         "withholding_tax": "withholding_tax",
         "total_due_tax": "total_due_tax",
         "penalty": "penalty",
         "interest": "interest",
         "total_due_due": "total_due_due"
      }
   ],

   "detail": [

      {

         "row_type": "row_type",
         "county_name": "county_name",
         "rate_type": "rate_type",
         "location_code": "location_code",
         "gross_receipts": "gross_receipts",
         "deductions": "deductions",
         "taxable_gross_receipts": "taxable_gross_receipts",
         "tax_rate": "tax_rate","gross_tax": "gross_tax"

      }

   ]

}

I need to navigate the file withour knowing the different field names. In my mind I want to load the JSON file, then return the first field value (in this case header). And then return the field names and values under that root. Sorry if my terminology is off, I'm new to JSON. I've found different gson documentation online and its all horrible. I couldn't find anything for pulling values without knowing the field names. Thanks

Answer 1

Here is the code. For more info read inline comments or ask me.

Create some POJO classes that is replica of your JSON string.

class MyJSON {
    ArrayList<Header> header;
    ArrayList<Summary> summary;
    ArrayList<Detail> detail;
}

class Header {
    String document_number;
    String report;
    String version;
}
// create classes for Summary and Detail as well in the similar way of Header
// the variable name must be same as it is in JSON string along with case.

...

// read JSON from the file
StringBuilder builder = new StringBuilder();
BufferedReader reader = new BufferedReader(new FileReader(new File("resources/json1.txt")));
String line = null;
while ((line = reader.readLine()) != null) {
    builder.append(line);
}
reader.close();

// create a new Gson object
Gson gson = new Gson();
// convert JSON string to POJO object
MyJSON object = gson.fromJson(builder.toString(), MyJSON.class);

// printing only header
for (Header header : object.header) {
    System.out.println(header.document_number + ", " + header.report + ", "
            + header.version);
}

output (printing only header)

document_number, report, version

Answer 2

This is my ready to run solution.

package stackoverflow.questions;

import java.util.*;

import com.google.gson.*;

public class Q23704415 {

    public static void main(String[] args) {
       String json = "{                                                                      "+
               "   \"header\": [                                                       "+
               "      {                                                                "+
               "         \"document_number\": \"document_number\",                     "+
               "         \"report\": \"report\",                                       "+
               "         \"version\": \"version\"                                      "+
               "      }                                                                "+
               "   ],                                                                  "+
               "                                                                       "+
               "   \"summary\": [                                                      "+
               "      {                                                                "+
               "         \"row_type\": \"row_type\",                                   "+
               "         \"crs_id\": \"crs_id\",                                       "+
               "         \"report_begin\": \"report_begin\",                           "+
               "         \"report_end\": \"report_end\",                               "+
               "         \"gross_tax\": \"gross_tax\",                                 "+
               "         \"compensating_tax\": \"compensating_tax\",                   "+
               "         \"withholding_tax\": \"withholding_tax\",                     "+
               "         \"total_due_tax\": \"total_due_tax\",                         "+
               "         \"penalty\": \"penalty\",                                     "+
               "         \"interest\": \"interest\",                                   "+
               "         \"total_due_due\": \"total_due_due\"                          "+
               "      }                                                                "+
               "   ],                                                                  "+
               "                                                                       "+
               "   \"detail\": [                                                       "+
               "                                                                       "+
               "      {                                                                "+
               "                                                                       "+
               "         \"row_type\": \"row_type\",                                   "+
               "         \"county_name\": \"county_name\",                             "+
               "         \"rate_type\": \"rate_type\",                                 "+
               "         \"location_code\": \"location_code\",                         "+
               "         \"gross_receipts\": \"gross_receipts\",                       "+
               "         \"deductions\": \"deductions\",                               "+
               "         \"taxable_gross_receipts\": \"taxable_gross_receipts\",       "+
               "         \"tax_rate\": \"tax_rate\",\"gross_tax\": \"gross_tax\"       "+
               "                                                                       "+
               "      }                                                                "+
               "                                                                       "+
               "   ]                                                                   "+
               "                                                                       "+
               "}                                                                      ";


         JsonElement root = new JsonParser().parse(json);
         JsonObject jo = root.getAsJsonObject();

         JsonObject header = jo.get("header").getAsJsonArray().get(0).getAsJsonObject();

         Set<Map.Entry<String, JsonElement>> set = header.entrySet();
         for(Map.Entry<String, JsonElement> e : set){
             System.out.println(e.getKey() + "->" + e.getValue());
         }



    }

}

Some notes on what you wrote.

1. Your JSON is a bit weird, header for example, is an array that contains one JSON object only and the same is for the rest of the JSON. If you are in control of JSON you can simplify it
2. You said that you want the "first" field, but root element of you JSON is an object ie a map ie an associative array, so "first" makes no sense here so you can access by name and not by position
3. You said that you want field under header but it's an array so you have to unpack it first and get first position

I did not use Gson class since it usually works better with a class hierarchy as the one Braj provided you with. With JsonParser you are more flexible assuming that the "structure" does not change too much. That is, you have an object that contains arrays with only one element that is a map.

So this line

JsonObject header = jo.get("header").getAsJsonArray().get(0).getAsJsonObject();

essentially traverse your JSON to get to the right info. I was forgetting: this is the result of my code:

document_number->"document_number"
report->"report"
version->"version"