With the diffusion of Java in advanced multimedia mobile devices, there is a growing need for speeding up the execution of Java Bytecode beyond the limits of traditional interpreters and just-in-time compilers. In this area, Java coprocessors are viewed as a promising technology, which marries the flexibility of a general purpose microprocessor to run legacy code and lightweight Java methods, with the high performance of a specialized execution engine on speed-critical bytecode. This work proposes and analyzes a microprocessor with FPGA coprocessor architecture with efficient shared-memory communication support. Furthermore, we describe a complete run-time environment that supports dynamic migration of Java methods to the coprocessor, and we quantitatively analyze speedups achievable under a number of system configurations using an accurate complete-system simulator.