Abstract Syntax Trees
The "semantic" behind "semantic code search and transformation."
OpenRewrite's Abstract Syntax Tree (AST) has a unique set of characteristics that make it suitable for both single-repository and mass transformation operations:
    Type-attributed. Each AST element is imbued with type information. For a field reference, for example, the source code may just refer to myField. The OpenRewrite AST element for myField would also contain information about what the type of myField is, even if it isn't defined in the same source file or even the same project.
    Format-preserving. Whitespace before and after AST elements is preserved in the tree so the tree can be printed out to reconstitute the original source code without clobbering formatting. Additionally, refactoring operations that insert code are sensitive to the local style of the code around them, and match the local style.
    Acyclic and serializable. Most AST's containing type information are potentially cyclic. Cycles usually come from generic type signatures like class A<T extends A<T>>. This kind of pattern is generally found in things like abstract builder types in Java. OpenRewrite cuts these cycles off and adds serialization annotations to its types so the AST can be serialized/deserialized with libraries like Jackson.
All of these properties are necessary for scalable organization-wide search and transformation.
Format-preservation is necessary in any organization that doesn't have absolutely consistent formatting across its whole codebase.
Type attribution is necessary for the accurate matching of patterns. For example, if we are looking for SLF4J log statements and we see a statement like the following, without type attribution how do we know if logger is an SLF4J or a Logback logger?
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logger.info("Hi");
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The production of type-attributed ASTs for a whole organization is arbitrarily computationally complex, since it requires dependency resolution, parsing of the source code, and type attribution (basically Java compilation up to the point of bytecode generation). Since OpenRewrite ASTs are serializable, we can store them off centrally as a byproduct of compilation and then operate on them en masse later.
Once we have a serialized AST for a particular source file, and since it also contains type information, it can be refactored/searched completely independently of other source files in the same source package or repository. This makes mass search and refactoring an approximately linearly scalable operation.
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