The Java runtime contains ~8MLOC; of them, about 1.5M are C++ (and Assembly generated by a C++ DSL), and more and more of the runtime is being written in Java (

The Java runtime contains ~8MLOC; of them, about 1.5M are C++ (and Assembly generated by a C++ DSL), and more and more of the runtime is being written in Java (e.g. the virtual thread scheduler is written in Java). The two main pieces that aren't are the bytecode interpreter, the JITs, and the GCs (although the Graal JIT is written in Java). Interestingly, the more latency-critical stuff is written in Java (the JITs and GCs work mostly in the background nowadays, and the interpreter is used only at startup and on deoptimisation slow-paths). Some of the reason is historical, and some has to do with warmup. Project Leyden and Graal's Native Image will help compile more Java AOT, allowing even more of the runtime to gradually be written in Java. It will take some time as it's not a top priority: it won't immediately deliver user-facing functionality, and most of the work on the JDK is already done in Java code anyway.