Multiparty session typing (MPST) is a method to automatically prove safety and liveness of protocol implementations relative to specifications. We present BGJ: a new tool to apply the MPST method in combination with Java. The checks performed using our tool are purely static (all errors are reported early at compile-time) and resource-efficient (near-zero cost abstractions at run-time), thereby addressing two issues of existing tools. BGJ is built using VerCors, but our approach is general.