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javap — the class file disassembler

# The javap disassembler — reading bytecode and the constant pool to verify what the compiler actually generated.

Conceptsaved 2026-08-09 #java#jdk#tooling#bytecode

Overview

javap prints the contents of compiled class files. Plain javap Foo shows the public API; -c disassembles method bodies to bytecode; -v adds the constant pool, flags, attributes and stack-map frames. It is the ground truth for "what did javac actually emit" questions — desugaring, synthetic members, and invokedynamic call sites are all visible here and nowhere in the source.

Key points

  • Flags: -c (bytecode), -v (verbose: constant pool + attributes), -p (include private members), -s (internal signatures/descriptors), -l (line/local tables).
  • Reading output: descriptors like (Ljava/lang/String;I)V; the constant pool is a numbered table that instructions reference (#7); Code: blocks show operand-stack bytecode (aload_0, invokevirtual…).
  • String concatenation: since JEP 280, + compiles to a single invokedynamic on StringConcatFactory — not StringBuilder chains; visible immediately with -c.
  • Records: -v shows the Record attribute, generated accessors, and equals/hashCode/toString bootstrapped via ObjectMethods invokedynamic.
  • Switch desugaring: string switches become hashCode + equals cascades over lookupswitch; pattern-matching switches bootstrap through SwitchBootstraps.typeSwitch invokedynamic.
  • Other uses: confirming a class file's major version (-v header) when chasing UnsupportedClassVersionError; inspecting synthetic bridge methods from generics; works on anything on the classpath, including JDK classes (javap -c java.lang.String).

Examples

javap -p -v -cp build/classes com.example.Point   # a record, in full detail
javap -c HelloConcat | grep -A2 invokedynamic     # see JEP 280 concat

Related

  • The JDK, the JRE & their tools — parent catalog of the toolchain.
  • javac — javap verifies its output; the pair closes the loop.
  • jdeps — the other static class-file analyser, at dependency granularity.
  • Deep dive Java — bytecode is the input to the JIT story told there.