EC-CUBE4 エンティティ拡張で既存メソッドを書き換える ref: https://qiita.com/izayoi256/items/cdd8074f7264b5d90f90

Customer.php

// 前略
        public function getRoles()
        {return $this->getTraitRoles();}
//後略

CustomerTrait.php

<?php
// 前略
    public function getTraitRoles() // 関数名を変更
// 後略

EntityProxyService.php

<?php
namespace Customize\Service;

use Eccube\Service\EntityProxyService as BaseService;
use PhpCsFixer\Tokenizer\Token;
use PhpCsFixer\Tokenizer\Tokens;
use Symfony\Component\Console\Output\OutputInterface;

class EntityProxyService extends BaseService
{
    /**
     * @var BaseService
     */
    private $baseService;

    public function __construct(EntityManagerInterface $entityManager, BaseService $baseService)
    {
        parent::__construct($entityManager);
        $this->baseService = $baseService;
    }

    public function generate($includesDirs, $excludeDirs, $outputDir, OutputInterface $output = null)
    {
        // 生成されたプロキシファイル一覧
        $generatedFiles = $this->baseService->generate($includesDirs, $excludeDirs, $outputDir, $output);

        foreach ($generatedFiles as $generatedFile) {

            // 末尾が/Customer.phpなら
            if (preg_match('#/Customer.php$#', $generatedFile)) {
                $entityTokens = Tokens::fromCode(file_get_contents($generatedFile));
                $functionIndex = 0;
                // T_FUNCTIONを探す
                while(($functionIndex = $entityTokens->getNextTokenOfKind($functionIndex, [[T_FUNCTION]])) > 0) {

                    $nextIndex = $entityTokens->getNextMeaningfulToken($functionIndex);
                    $nextToken = $entityTokens[$nextIndex];

                    // T_FUNCTIONの次の空白以外のトークンがT_STRINGで、getRolesなら
                    if ($nextToken->isGivenKind(T_STRING) && $nextToken->getContent() === 'getRoles') {

                        // 次の'{'から対応する'}'まで
                        $startIndex = $entityTokens->getNextTokenOfKind($nextIndex, ['{']);
                        $endIndex = $entityTokens->findBlockEnd(Tokens::BLOCK_TYPE_CURLY_BRACE, $startIndex);

                        // を'{return $this->getTraitRoles();}'で置き換える
                        $newTokens = Tokens::fromCode('{return $this->getTraitRoles();}');
                        $entityTokens->overrideRange($startIndex, $endIndex, $newTokens);
                        $code = $entityTokens->generateCode();
                        file_put_contents($generatedFile, $code);
                        break;
                    }
                }
            }
        }

        return $generatedFiles;
    }
}

file1.sh

$ php bin/console eccube:generate:proxies
gen -> /var/www/html/app/proxy/entity/Customer.php

file10.sh

$ bin/console cache:clear --no-warmup
$ rm app/proxy/entity/Customer.php
$ php bin/console eccube:generate:proxies

file3.php

var_export((new \Eccube\Entity\Customer())->getRoles());
// array (
//   0 => 'ROLE_CUSTOMER',
// )

file7.sh

$ php bin/console eccube:generate:proxies
gen -> /var/www/html/app/proxy/entity/Customer.php

file9.php

var_export((new \Eccube\Entity\Customer())->getRoles());
// array (
//   0 => 'ROLE_CUSTOMER',
//   1 => 'ROLE_HOGE',
// )

services.yaml

# ファイル末尾
    Customize\Service\EntityProxyService:
        decorates: Eccube\Service\EntityProxyService
        arguments:
        - '@doctrine.orm.default_entity_manager'
        - '@Customize\Service\EntityProxyService.inner'