JVM (Java Virtual Machine)

In short: The JVM is the “virtual machine” that executes Java bytecode. It makes Java platform independent: the same bytecode runs on Windows, Linux and macOS because there is a matching JVM for each system.

In more detail: The JVM behaves like a small abstract computer with its own instruction set (bytecode), its own memory model and its own memory management. It sits as a layer between the program and the operating system.

In Depth

Components of the JVM:

• Class loader: Loads .class files on demand, checks them (verifier) and links them.
• Memory areas: Heap (objects), stacks (per thread: calls and local variables), metaspace (class information).
• Execution engine: An interpreter runs bytecode; the JIT compiler (in HotSpot: C1 and C2) translates frequently used methods into fast machine code.
• Garbage collector: Frees memory of objects nobody points to anymore (G1, ZGC, Shenandoah …).
• Security: The bytecode verifier prevents invalid code. (The former Security Manager is deprecated.)

Important startup options:

java -Xms256m -Xmx2g -jar app.jar     # initial and maximum heap size
java -XX:+UseZGC -jar app.jar          # choose a garbage collector

• Other languages: Kotlin, Scala, Groovy and Clojure also produce bytecode and run on the JVM.
• Several JVMs: HotSpot (default in OpenJDK), GraalVM, OpenJ9. The language is standardized, the implementation is not.
• Cost: Startup takes a bit and memory is needed for the JVM itself. In return, long-running programs are very fast after warm-up.
• Troubleshooting: OutOfMemoryError (heap too small or a leak), StackOverflowError (recursion too deep).

See also: Runtime environments: JVM, CLR, BEAM and co., Bytecode, Garbage collector