JacksonSampleSimplePojoMapper

1. 1. page was renamed from SimplePojoMapper <<TableOfContents(4)>>

A Simple POJO Mapper

If you just need to map POJOs (Plain Old Java Objects) back and forth from JSON -- maybe because you want to pass data between Java and some other programming language, or maybe because you want to persist POJOs in a JSON format -- just below is a simple PojoMapper class that does this.

Example Usage

   // Create a POJO
   MyPojo pojo = new MyPojo();

   // Populate the POJO

   // Convert the POJO to a JSON string
   String pojoAsString = PojoMapper.toJson(pojo, true);

   // Convert the JSON string back into a POJO
   pojo = (MyPojo) PojoMapper.fromJson(pojoAsString, MyPojo.class);

   // Convert the POJO to a JSON string that gets stored in a file
   FileWriter fw = new FileWriter("pojo.txt");
   PojoMapper.toJson(pojo3, fw, true);

   // Convert the JSON string in the file back into a POJO
   FileReader fr = new FileReader("pojo.txt");
   pojo = (MyPojo) PojoMapper.fromJson(fr, MyPojo.class);

The PojoMapper class

import org.codehaus.jackson.JsonFactory;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.JsonParseException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;

public class PojoMapper {

    private static ObjectMapper m = new ObjectMapper();
    private static JsonFactory jf = new JsonFactory();

    public static <T> Object fromJson(String jsonAsString, Class<T> pojoClass)
    throws JsonMappingException, JsonParseException, IOException {
        return m.readValue(jsonAsString, pojoClass);
    }

    public static <T> Object fromJson(FileReader fr, Class<T> pojoClass)
    throws JsonParseException, IOException
    {
        return m.readValue(fr, pojoClass);
    }

    public static String toJson(Object pojo, boolean prettyPrint)
    throws JsonMappingException, JsonGenerationException, IOException {
        StringWriter sw = new StringWriter();
        JsonGenerator jg = jf.createJsonGenerator(sw);
        if (prettyPrint) {
            jg.useDefaultPrettyPrinter();
        }
        m.writeValue(jg, pojo);
        return sw.toString();
    }

    public static void toJson(Object pojo, FileWriter fw, boolean prettyPrint)
    throws JsonMappingException, JsonGenerationException, IOException {
        JsonGenerator jg = jf.createJsonGenerator(fw);
        if (prettyPrint) {
            jg.useDefaultPrettyPrinter();
        }
        m.writeValue(jg, pojo);
    }
}

The MyPojo class

import java.io.FileReader;
import java.io.FileWriter;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

public class MyPojo {
    private boolean on;
    private String name;
    private Map<String,Date> map;

    public void setOn(boolean on) {
        this.on = on;
    }

    public boolean isOn() {
        return this.on;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getName() {
        return this.name;
    }

    public void setMap(Map<String,Date> map) {
        this.map = map;
    }

    public Map<String,Date> getMap() {
        return this.map;
    }

    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("on=" + on);
        sb.append(" name=" + name);
        sb.append(" map={");
        for (Map.Entry<String,Date> entry : this.map.entrySet()) {
            sb.append("" + entry.getKey() + "=" + entry.getValue());
        }
        sb.append("}");
        return sb.toString();
    }

    public static void main(String[] args) {

        // Create a POJO
        MyPojo pojo = new MyPojo();
        // Populate it with some data
        pojo.setName("Lucky");
        pojo.setOn(true);
        Map<String,Date> map = new HashMap<String,Date>();
        map.put("now", new Date());
        pojo.setMap(map);

        // Map it to JSON and then back again
        try {
            String pojoAsString = PojoMapper.toJson(pojo, true);
            System.out.println("POJO as string:\n" + pojoAsString + "\n");
            MyPojo pojo2 = (MyPojo) PojoMapper.fromJson(pojoAsString, MyPojo.class);
            System.out.println("POJO toString():\n" + pojo2 + "\n");
        } catch (Exception e) {
            e.printStackTrace();
        }

        // Create another POJO
        MyPojo pojo3 = new MyPojo();
        pojo3.setName("Baldwin");
        pojo3.setOn(false);
        Map<String,Date> map2 = new HashMap<String,Date>();
        map2.put("now", new Date());
        pojo3.setMap(map2);

        // Map it to JSON, store it on disk and then read it back
        try {
            FileWriter fw = new FileWriter("pojo.txt");
            PojoMapper.toJson(pojo3, fw, true);

            FileReader fr = new FileReader("pojo.txt");

            MyPojo pojo4 = (MyPojo) PojoMapper.fromJson(fr, MyPojo.class);
            System.out.println("POJO read from file:\n" + pojo4);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

main() method output

POJO as string:
{
  "name" : "Lucky",
  "on" : true,
  "map" : {
    "now" : 1270349496303
  }
}

POJO toString():
on=true name=Lucky map={now=Sat Apr 03 19:51:36 PDT 2010}

POJO read from file:
on=false name=Baldwin map={now=Sat Apr 03 19:51:36 PDT 2010}

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