Microbenchmarking consists of evaluating, in isolation, the performance of small code segments that play a critical role in large applications. The accuracy of a microbenchmark depends on two critical tasks: wrap the code segment into a payload that faithfully recreates the execution conditions that occur in the large application; build a scaffold that runs the payload a large number of times to get a statistical estimate of the execution time. While recent frameworks such as the Java Microbenchmark Harness (JMH) take care of the scaffold challenge, developers have very limited support to build a correct payload. In this work, we focus on the automatic generation of pay-loads, starting from a code segment selected in a large application. In particular, we aim at preventing two of the most common mistakes made in microbenchmarks: dead code elimination and constant folding. Since a microbench-mark is such a small program, if not designed carefully, it will be over-optimized by the JIT and result in distorted time measures. Our technique hence automatically extracts the segment into a compilable payload and generates additional code to prevent the risks of over-optimization. The whole approach is embedded in a tool called AutoJMH, which generates payloads for JMH scaffolds. We validate the capabilities AutoJMH, showing that the tool is able to process a large percentage of segments in real programs. We also show that AutoJMH can match the quality of payloads handwritten by performance experts and outperform those written by professional Java developers without experience un microbenchmarking.