<p>This thesis presents an in-depth investigation into the opportunities of optimizing the performance (throughput) of fuzzing on Java Virtual Machine (JVM)-based languages. The study identifies five main areas for potential optimization, each of which contributes to the performance bottlenecks in the existing state-of-the-art Java fuzzer, Jazzer.</p> <p><br></p> <p>Firstly, the use of coverage probes is recognized as costly due to the native method call, including call frame generation and destruction, while it only performs a simple byte increment. Secondly, the probes may become exhausted, which subsequently cease to generate signals for new interesting inputs, while the associated costs persist. Thirdly, the scanning of the coverage map is expensive, particularly for targets with a large loaded bytecode. Given that test inputs can only execute a portion of these, the probes for most bytecodes are scanned repeatedly without generating any signals, indicating a need for a more structured coverage map design to skip the code probes effectively. Lastly, exception handling in JVM is costly as it automatically fills in the stack trace whenever an exception object is created, even when most targets don't utilize this information. </p> <p><br></p> <p>The study then designs and implements optimization techniques for these opportunities. We believe we provide the optimal solution for the first opportunity, while better optimizations could be proposed for the second, third, and fourth. The collective improvement brought about by these implementations is on average 138% and up to 441% in throughput. This work, thus, offers valuable insights into enhancing the efficiency of fuzz testing in JVM languages and paves the way for further research in optimizing other areas of JVM-based-language fuzzing performance.</p>