Question

Are there any practical uses of the Java Virtual Machine’s NOP opcode in today’s JVM? If so, what are some scenarios in which NOPs would be generated in bytecode?

I would even be interested to see an example of Java code that compiles into bytecode with NOPs.

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Update

BCEL’s MethodGen class says,

While generating code it may be necessary to insert NOP operations.

I am guessing other Bytecode generation libraries are in the same boat, as was pointed out in the accepted answer.

Answer 1

Some NOP bytecode use cases are for class file transformations, optimizations and static analysis performed by tools such as Apache BCEL, ASM, FindBugs, PMD, etc. The Apache BCEL manual touches on some uses of NOP for analysis and optimization purposes.

A JVM may use NOP bytecodes for JIT optimizations to ensure code blocks that are at synchronization safepoints are properly aligned to avoid false sharing.

As for some sample code compiled using the JDK javac compiler that contains NOP bytecodes, that’s an interesting challenge. However, I doubt the compiler will generate any class file containing NOP bytecodes since the bytecode instruction stream is only single-byte aligned. I would be curious to see such an example, but I can’t think of any myself.