Magento – How to get a response from REST API in JSON format in Magento 2

Below are the custom api module with key pair value
To get JSON response, in client set Response headers to "Content-Type: application/json; charset=utf-8"

and if you need key pair value that is in response should have key and value as like /rest/V1/categories we need to create data interface

To test in Chrome download plugin called rest client app call the url

below module url will be http://yourdomein.com/magento2/rest/V1/getinfo

app\code\Sugarcode\Customapi\registration.php:

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Sugarcode_Customapi',
    __DIR__
);

app\code\Sugarcode\Customapi\etc\module.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Module/etc/module.xsd">
    <module name="Sugarcode_Customapi" setup_version="2.0.0"/>
</config>

app\code\Sugarcode\Customapi\etc\di.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">

    <preference for="Sugarcode\Customapi\Api\TestInterface"
                type="Sugarcode\Customapi\Model\Test" />

    <preference for="Sugarcode\Customapi\Api\Data\TestdataInterface" type="Sugarcode\Customapi\Model\Testmodel" />
</config>

app\code\Sugarcode\Customapi\etc\webapi.xml:

<?xml version="1.0"?>
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../app/code/Magento/Webapi/etc/webapi.xsd">

    <!-- Example: curl http://127.0.0.1/index.php/rest/V1/calculator/add/1/2 -->
    <route url="/V1/getinfo" method="GET">
        <service class="Sugarcode\Customapi\Api\TestInterface" method="getinfo" />
        <resources>
            <resource ref="anonymous" />
        </resources>
    </route>

</routes>

app\code\Sugarcode\Customapi\Api\TestInterface.php:

<?php

namespace Sugarcode\Customapi\Api;

use Sugarcode\Customapi\Api\Data\TestdataInterface;

interface TestInterface
{
    /**
     * Retrieve list of info
     *
     * @throws \Magento\Framework\Exception\NoSuchEntityException If ID is not found
     * @return \Sugarcode\Customapi\Api\Data\TestdataInterface containing Tree objects
     */
    public function getinfo();

}

don't remove comments its most important

app\code\Sugarcode\Customapi\Api\Data\TestdataInterface.php (set and get data):

<?php

namespace Sugarcode\Customapi\Api\Data;

/**
 * @api
 */
interface TestdataInterface
{

    /**
     * Get name
     *
     * @return string
     */
    public function getName();

      /**
     * Set name
     *
     * @param string $name
     * @return $this
     */
    public function setName($id);

}

app\code\Sugarcode\Customapi\Model\Test.php:

<?php

namespace Sugarcode\Customapi\Model;

use Sugarcode\Customapi\Api\TestInterface;

/**
 * Defines the implementaiton class of the calculator service contract.
 */
class Test implements TestInterface
{
    /**
     * Return the sum of the two numbers.
     *
     * @api
     * @param int $num1 Left hand operand.
     * @param int $num2 Right hand operand.
     * @return int The sum of the two values.
     */
    protected $dataFactory;
    public function __construct(\Sugarcode\Customapi\Api\Data\TestdataInterfaceFactory $dataFactory)
    {
        $this->dataFactory = $dataFactory;
    }



    public function getinfo() {
        $page_object = $this->dataFactory->create();
        $page_object->setName('Hello');
        return $page_object;        

    }

}

app\code\Sugarcode\Customapi\Model\Testmodel.php:

<?php

namespace Sugarcode\Customapi\Model;

class Testmodel extends \Magento\Framework\Model\AbstractModel implements
    \Sugarcode\Customapi\Api\Data\TestdataInterface 
{
    const KEY_NAME = 'name';


     public function __construct(
        \Magento\Framework\Model\Context $context,
        \Magento\Framework\Registry $registry,
        \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
        \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
        array $data = []
    ) {
        parent::__construct($context, $registry, $resource, $resourceCollection, $data);
    }


    public function getName()
    {
        return $this->_getData(self::KEY_NAME);
    }


    /**
     * Set name
     *
     * @param string $name
     * @return $this
     */
    public function setName($name)
    {
        return $this->setData(self::KEY_NAME, $name);
    }


}