Convert flat JSON structure to object and add another property

Question

I have a simple object saved in json I would like to convert back to object and add a new property at the same time:

The json string is this:

{"Database":0,"SalesForce":0,"Watson":0,"AIC":0,"Luise":0}

I want to add:

"Total":0

So the object I get back should be:

class myObj{
    private int Database;
    private int SalesForce;
    private int Watson;
    private int AIC;
    private int Luise;
    private int Total;
}

I don't have an actual class, I would like to do this dynamically. IE, as a dynamic object.

I don't think a dynamic class is possible in this case, you would still need to refer to a class in your code to use these attributes. Have you considered using a Map? The Jackson Project also has a class to represent an arbitrary JSON object (JsonNode) — ecarlos, May 10 '18 at 21:28
Can your JSON contain nested objects, or are all the values simple? — evnica, May 10 '18 at 21:57

score 1 · Accepted Answer · answered May 10 '18 at 21:34

1

Just append it to your string like so

yourJsonString = yourJsonString.substring(0, yourJsonString.length() - 1) + ",\"Total\": 0}"

answered May 10 '18 at 21:34

Trash Can

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hehe, that's a smart and easy way to do it – Bill Software Engineer May 10 '18 at 21:56

score 0 · Answer 2 · answered May 10 '18 at 22:49

Since the question was how to convert a JSON structure to an object adding another property and not how to modify an original JSON String, I would like to post an answer that actually does what the title of the question says: JSON to Java object conversion.

The code below will work only if there are no nested objects and is not efficient if the JSON structure is large (because of the String concatenation).

Also, please note that creating classes dynamically might not be such a good idea (e.g. as discussed here). However, if you need it, a possible solution is given below. In this solution, a JSON String is converted to a Map and a new key-value pair is added. Then the source code of the target class is written and compiled, the class is loaded and a test method is called for demo purposes.

import java.io.File;
import java.io.FileWriter;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.Collections;
import java.util.Map;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.StandardLocation;
import javax.tools.ToolProvider;


public class JsonToJava

{
    private static Map<String, Object> map;
    private static  String json = "{\"Database\":0,\"SalesForce\":0,\"Watson\":0,\"AIC\":0,\"Luise\":0}";
    private static String fileName = "Test";

    public static void main(String[] args) throws Exception {

        // create an empty source file
        File sourceFile = File.createTempFile(fileName, ".java");
        sourceFile.deleteOnExit();

        java.lang.reflect.Type mapType = new TypeToken<Map<String, Object>>(){}.getType();
        Gson gson = new Gson();
        map = gson.fromJson(json, mapType );

        addProperty( "Total", "0" );

        String fields = "";

        for (Map.Entry<String, Object> entry : map.entrySet())
        {
            fields += "\tprivate " ;
            try{
                Double.parseDouble( entry.getValue().toString() );
                fields += "double ";
            }
            catch ( NumberFormatException e ){
                try{
                    Integer.parseInt( entry.getValue().toString() );
                    fields += "int ";
                }
                catch ( NumberFormatException ex ){
                    fields += "String " + entry.getValue();
                }
            }
            fields += entry.getKey() + ";\n";
        }

        // generate the source code
        String classname = sourceFile.getName().split("\\.")[0];
        String sourceCode = "public class " + classname + "{\n\n" +
                 fields +
                "\n\tpublic void hello(){\n\t\t System.out.print(\"Hello world\");\n\t}\n}";

        // write the source code into the source file
        System.out.println(sourceCode);
        FileWriter writer = new FileWriter(sourceFile);
        writer.write(sourceCode);
        writer.close();

        // compile the source file
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        StandardJavaFileManager fileManager = compiler.getStandardFileManager(null, null, null);
        File parentDirectory = sourceFile.getParentFile();
        fileManager.setLocation(StandardLocation.CLASS_OUTPUT, Collections.singletonList( parentDirectory ) );
        Iterable<? extends JavaFileObject> compilationUnits =
                fileManager.getJavaFileObjectsFromFiles( Collections.singletonList( sourceFile ) );
        compiler.getTask(null, fileManager, null, null, null, compilationUnits).call();
        fileManager.close();

        // load the class
        URLClassLoader classLoader = URLClassLoader.newInstance(new URL[] { parentDirectory.toURI().toURL() });
        Class<?> helloClass = classLoader.loadClass(classname);

        // call a method of the newly compiled and loaded class
        Method helloMethod = helloClass.getDeclaredMethod("hello");
        helloMethod.invoke(helloClass.newInstance());
    }

    private static void addProperty(String key, String value)
    {
        map.put( key, value );
    }
}

Convert flat JSON structure to object and add another property

2 Answers2