Lead Java Developer/Solution Architect (6-month contract) in London

gravity9 is recruiting Java engineers and architects for a portfolio of engagements across UK central government. These are large-scale, data-intensive modernisation programmes: replacing legacy systems that carry statutory weight, decomposing monoliths that serve millions of users, and building secure data platforms where access control and auditability are first-class architectural concerns rather than late additions.

We are hiring across two overlapping profiles — Senior Java Developer and Application / Migration Architect — and expect most people to move between programmes as engagements start and complete. Several of these are hands-on architect roles: you will own design decisions and also write production code.

The following are representative of current and upcoming engagements. Individual roles will draw on some of this, not all of it — we are more interested in engineers who can work confidently at the intersection of Java and data than in a checklist match against any one programme.

• Secure data catalogue and access-control platform: A greenfield metadata catalogue acting as a federated index over data held across independent domains, so that authorised parties can locate and retrieve data the catalogue itself never holds.
• Real-time decision-support data architecture: Replacing point-to-point integration across a multi-vendor estate with a microservices architecture and a common data layer, so that operational data flows in near real time instead of over days.
• Legacy-to-document-model migration at national scale: Migrating hundreds of millions of records and tens of terabytes of XML out of a legacy relational platform into a sharded document store, in support of a COBOL-to-Java microservices rewrite.
• Monolith decomposition and data-ownership migration: Breaking a distributed monolith of well over a hundred Java services sharing a handful of database clusters into domain-owned services.

What you will be doing: Emphasis varies by role and programme, but MongoDB is the common thread. Every one of these engagements involves modelling for it, migrating onto it, or building on top of it, and genuine depth here is what most differentiates candidates.

• Architecture and design: Produce high- and low-level designs, security and data architectures, and architecture decision records.
• Hands-on build: Build production Java services and RESTful APIs (Spring Boot or similar), designed for integration by multiple consuming systems.
• Data modernisation and migration: Model relational, XML or other legacy schemas onto MongoDB document models.
• Security, access control and assurance: Design and implement attribute- or role-based access control models.
• Client-facing delivery and enablement: Work as an embedded member of a small delivery team, frequently alongside client engineers and other suppliers.

Job requirements:

• Essential: Strong hands-on Java; MongoDB; data-intensive systems; API design; testing; infrastructure and deployment; communication; UK resident.
• For architect-level roles: Proven design authority on a delivery of comparable size and sensitivity.
• Desirable: UK public sector experience; Kafka; legacy modernisation patterns; domain-driven design; attribute-based access control; search; event-driven architecture.

Working context: Engagement is contract-based, typically three to twelve months per programme. SC clearance is the standard requirement for these roles. All work on these programmes is delivered from the UK by UK-onshore staff. The working model is predominantly remote with periodic onsite attendance.