Senior Java Developer | Aon Netherlands | Rotterdam | 38.75 hours | Self-Employed / Contract

About Aon

Aon is a leading international provider of risk management, insurance, pension advisory, and health solutions. With thousands of colleagues worldwide, Aon helps organizations make better decisions through data, expertise, and innovative technology.

We are looking for an experienced Senior Java Developer to temporarily join the Web Development Team in Rotterdam to provide additional capacity. You will work on business-critical applications and digital services within a professional and innovative IT environment.

The assignment

As a Senior Java Developer, you will be part of a multidisciplinary development team of approximately 10 to 15 professionals. You will work closely with business stakeholders, product owners, analysts, and other developers on the further development, modernization, and integration of business-critical applications.

This assignment arose from a combination of temporary capacity expansion and backfilling. That’s why we’re looking for an independent developer who can start quickly and requires minimal supervision.
 

Your responsibilities will include:

• Developing new features within existing applications;
• Designing and implementing REST APIs and system integrations;
• Translating functional requirements into technical solutions;
• Developing automated tests and ensuring software quality;
• Improving and modernizing existing applications;
• Work closely with sales teams, business stakeholders, and development teams.
   

What are you bringing?
 

Must-haves

• t least5 years of experience as a Java Developer;
• Experience with Java and Spring Boot;
• Proven experience with REST API development;
• Experience with system integrations and data exchange between applications;
• You are self-reliant and can quickly become productive;
• Good communication skills;
• Proficiency in both Dutch and English.
   

Pre

• Experience with Spring Security;
• Experience with Spring JPA;
• Experience with Spring Web;
• Experience with OKTA;
• Experience with Azure Cloud Services, such as App Services, Key Vault, and Blob Storage;
• Knowledge of Secure Software Development;
• Experience with API integrations, data mapping, and validation processes.

Practical information

• Start date: preferably August 14, 2026, no later than September 2026;
• Contract duration: through November 30, 2026, with an option to extend;
• Hours: 40 hours per week;
• Location: Rotterdam. Offices in Amsterdam and Eindhoven are also available, but you must be present in Rotterdam at least two days a week;
• Hybrid work: During the onboarding period, you are expected to be in the office approximately four days a week. After that, hybrid work is possible, with an average of two days working from home;
• Maximum hourly rate: €105, excluding VAT;
• Contract Type: Both self-employment and secondment are possible.
   

Job Application Process

1. Initial online consultation;
2. Second on-site interview in Rotterdam.