Architectural advancement in hardware implementation of Java increases the performance. Java processors reduce the overhead of execution time and memory accesses of traditional implementation of JVM in embedded systems. To improve the performance of Java processors and decrease the execution time, we decided to customize a processor is called JOP. We design a Reconfigurable Functional Unit (RFU) which is integrated to JOP's core for executing the Custom Instructions (CIs) that are generated offline. In this article the efficiency of an extensible Java processor is analyzed with the number of clock cycles in several signal processing application benchmarks. Using this architecture, performance is improved by 31.8% in average compared to basic JOP.