<?php
$servidor="localhost";
$nombre_bd="test2";
$username="root";
$password="";
$conexion = mysql_connect($servidor, $username, $password) or die ("No se ha podido conectar al servidor de Base de datos");

mysql_select_db($nombre_bd, $conexion) or die ("xD");
$sql = "SELECT * FROM usuario";
$result = mysql_query($sql, $conexion);
$array = array();
$i = 0;
while($row = mysql_fetch_array($result)) {
  $array[$i] = $row;
  $i ++;
}
$json = json_encode($array, JSON_FORCE_OBJECT);
echo $json;
?>

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package prueba.comsumir.webservice;

import com.google.gson.Gson;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;

/**
 *
 * @author Pablo
 */
public class PruebaComsumirWebservice {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Usuario usuario = new Usuario(1, "User1", "123");
        System.out.println(usuario.toString());
        URL url;
        try {
            // Creando un objeto URL
            url = new URL("http://localhost:8080/Sitio%201/code.php");

            // Realizando la petición GET
            URLConnection con = url.openConnection();

            // Leyendo el resultado
            BufferedReader in = new BufferedReader(new InputStreamReader(
                    con.getInputStream()));

            String linea;
            String json = "";
            while ((linea = in.readLine()) != null) {
                System.out.println(linea);
                json += linea;
            }
            //{"id":1,"usuario":"usuario1","contrasenia":"asd"}
            Usuario usuario1 = new Gson().fromJson(json, Usuario.class);
            System.out.println(usuario.getUsuario());

        } catch (IOException e) {
            System.out.println(e.getMessage());
        }
    }


/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package prueba.comsumir.webservice;

/**
 *
 * @author Pablo
 */
public class Usuario {
    private int id;
    private String usuario;
    private String contrasenia;

    public Usuario(int id, String usuario, String contrasenia) {
        this.id = id;
        this.usuario = usuario;
        this.contrasenia = contrasenia;
    }

    public int getId() {
        return id;
    }

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

    public String getUsuario() {
        return usuario;
    }

    public void setUsuario(String usuario) {
        this.usuario = usuario;
    }

    public String getContrasenia() {
        return contrasenia;
    }

    public void setContrasenia(String contrasenia) {
        this.contrasenia = contrasenia;
    }

    @Override
    public String toString() {
        return String.format("id:%s, usuario:%s, contrasenia:%s", id, usuario, contrasenia);
    }
}

{"0":{"0":"1","id":"1","1":"user1","usuario":"user1","2":"asd","contrasenia":"asd"},"1":{"0":"2","id":"2","1":"user2","usuario":"user2","2":"dsa","contrasenia":"dsa"}}
Exception in thread "main" com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 169 path $
    at com.google.gson.Gson.assertFullConsumption(Gson.java:855)
    at com.google.gson.Gson.fromJson(Gson.java:845)
    at com.google.gson.Gson.fromJson(Gson.java:793)
    at com.google.gson.Gson.fromJson(Gson.java:765)
    at prueba.comsumir.webservice.PruebaComsumirWebservice.main(PruebaComsumirWebservice.java:48)
Caused by: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 169 path $
    at com.google.gson.stream.JsonReader.syntaxError(JsonReader.java:1567)
    at com.google.gson.stream.JsonReader.checkLenient(JsonReader.java:1416)
    at com.google.gson.stream.JsonReader.doPeek(JsonReader.java:546)
    at com.google.gson.stream.JsonReader.peek(JsonReader.java:429)
    at com.google.gson.Gson.assertFullConsumption(Gson.java:851)
    ... 4 more
C:UsersPabloAppDataLocalNetBeansCache8.2executor-snippetsrun.xml:53: Java returned: 1
BUILD FAILED (total time: 1 second)

$con=mysqli_connect($servidor,$username,$password,$nombre_bd);

if(!$con) die('error conectando');

$sql = "SELECT * FROM usuario";
$result = mysqli_query($con, $sql);

$rtn = array();

if($result){
    $row=mysqli_fetch_assoc($result);
    while($row !== null){
      $rtn[] = $row;
      $row=mysqli_fetch_assoc($result);
    }
    mysqli_close($con);
}

header('Content-Type: application/json');
return json_encode($rtn);

public class Usuarios{
    private List<Usuario> usuarios;
    // getter & setter
}

//{"id":1, "usuario":"usuario1", "contrasenia":"asd"}
Usuarios usuarios = new Gson().fromJson(json, Usuarios.class);