Describe how Lombok works under the hood (annotation processing)?

Lombok hooks into the Java compilation process as an annotation processor, a standard extension point the javac compiler supports for running code during compilation that can inspect and modify the program being compiled. Unlike most annotation processors, which are restricted to only generating new, separate source files, Lombok uses (officially unsupported, internal) compiler APIs to directly modify the in-memory abstract syntax tree (AST) of the class being compiled.

flowchart LR
    A[Source .java file with annotations] --> B[javac parses to AST]
    B --> C[Lombok annotation processor runs, modifies AST in place]
    C --> D[javac continues compiling the modified AST]
    D --> E[Generated .class bytecode includes Lombok's additions]

This is why Lombok can add whole new methods to an existing class (something standard annotation processors can't do) — it directly edits the parsed representation of your class before the compiler finishes turning it into bytecode, rather than generating a separate file alongside it.

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