json to java object(using mapping)?

EDIT
Use data model from my GSON example. Now you can do that in this way:

String response = "{\"response\":{\"numFound\":5,\"start\":0,\"docs\":[{\"id\":1},{\"id\":2},{\"id\":3},{\"id\":4},{\"id\":5}]}}";
ObjectMapper mapper = new ObjectMapper();
ObjectReader objectReader = mapper.reader(Body.class);
Body body = (Body) objectReader.readValue(response);
System.out.println(body.getResponse().getDocs());

GSON EXAMPLE
You should use GSON to JSON parsing. Create data model which help you working with "response" string. See my example:

import java.util.Arrays;
import java.util.List;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class SourceCodeProgram {

    public static void main(String[] args) throws Exception {
        String response = "{\"response\":{\"numFound\":5,\"start\":0,\"docs\":[{\"id\":1},{\"id\":2},{\"id\":3},{\"id\":4},{\"id\":5}]}}";
        Gson gson = new GsonBuilder().serializeNulls().create();

        System.out.println("To JSON");
        System.out.println(gson.toJson(createTestBody()));
        System.out.println("From JSON");
        System.out.println(gson.fromJson(response, Body.class));
    }

    private static Body createTestBody() {
        Response response = new Response();
        response.setNumFound(5);
        response.setStart(0);
        response.setDocs(Arrays.asList(new Doc(1), new Doc(2), new Doc(3),
                new Doc(4), new Doc(5)));

        Body body = new Body();
        body.setResponse(response);
        return body;
    }
}

class Body {

    private Response response;

    public Response getResponse() {
        return response;
    }

    public void setResponse(Response response) {
        this.response = response;
    }

    @Override
    public String toString() {
        return "Body [response=" + response + "]";
    }
}

class Response {

    private int numFound;
    private int start;
    private List<Doc> docs;

    public int getNumFound() {
        return numFound;
    }

    public void setNumFound(int numFound) {
        this.numFound = numFound;
    }

    public int getStart() {
        return start;
    }

    public void setStart(int start) {
        this.start = start;
    }

    public List<Doc> getDocs() {
        return docs;
    }

    public void setDocs(List<Doc> docs) {
        this.docs = docs;
    }

    @Override
    public String toString() {
        return "Response [numFound=" + numFound + ", start=" + start
                + ", docs=" + docs + "]";
    }
}

class Doc {

    private int id;

    public Doc() {
    }

    public Doc(int id) {
        this.id = id;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @Override
    public String toString() {
        return Integer.toString(id);
    }
}

Output:

To JSON
{"response":{"numFound":5,"start":0,"docs":[{"id":1},{"id":2},{"id":3},{"id":4},{"id":5}]}}
From JSON
Body [response=Response [numFound=5, start=0, docs=[1, 2, 3, 4, 5]]]

And "docs" you can retrieve in this way:

Body body = gson.fromJson(response, Body.class);
System.out.println(body.getResponse().getDocs());