app/code/Vendor/ModuleApi/
├── etc
│   ├── module.xml
│   └── webapi.xml
├── Model
│   └── ItemRepository.php
└── registration.php

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

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Vendor_ModuleApi" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Webapi"/>
        </sequence>
    </module>
</config>

<?xml version="1.0"?>
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Webapi:etc/webapi.xsd">
    <route url="/V1/module/items" method="GET">
        <service class="Vendor\ModuleApi\Model\ItemRepository" method="getItems"/>
        <resources>
            <resource ref="anonymous"/>
        </resources>
    </route>
</routes>

<?php
namespace Vendor\ModuleApi\Model;

use Magento\Framework\Webapi\Rest\Request;
use Magento\Framework\Webapi\Rest\Response;

class ItemRepository
{
    protected $request;
    protected $response;
    
    public function __construct(
        Request $request,
        Response $response
    ) {
        $this->request = $request;
        $this->response = $response;
    }
    
    public function getItems()
    {
        try {
            // 获取项目数据的逻辑
            $items = [
                ['id' => 1, 'name' => 'Item 1'],
                ['id' => 2, 'name' => 'Item 2']
            ];
            
            // 设置响应头为JSON
            $this->response->setHeader('Content-Type', 'application/json');
            
            return json_encode($items);
        } catch (\Exception $e) {
            $this->response->setStatusCode(500);
            return json_encode(['error' => $e->getMessage()]);
        }
    }
}