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For this, we reuse the annotation notation, and those annotations should implement org. The compiler will discover them and apply the transformation on these code elements. Groovy AST transformations must be performed in one of the nine defined compilation phases (org. Global transformations may be applied in any phase, but local transformations may only be applied in the semantic analysis phase or later.

Instruction Selection: instruction set is chosen, for example Java 6 or Java 7 bytecode levelGenerally speaking, there is more type information available later in the phases. If your transformation is concerned with reading the AST, then a later phase where information is more plentiful might be a good choice.

If your transformation is concerned with writing AST, then an earlier phase where the tree is more sparse might be more convenient. Local AST transformations are relative to the context they are applied to. In most cases, the context is defined by an annotation that will define the scope of the transform.

For example, annotating a field would mean that the transformation applies to the field, while annotating the class would mean that the transformation applies to the whole class. It requires two things:an implementation of org. ASTTransformation that adds the logging expressions to the methodAn ASTTransformation is a callback that gives you access to the org.

SourceUnit, through which you can get a reference to the org. The AST (Abstract Syntax Tree) is a tree structure consisting mostly of org. Expression (expressions) or org. An easy way to learn about the AST is to explore it in a debugger. The local transformation annotation is the simple part. WithLoggingASTTransformation is the fully qualified class name of the ASTTransformation we are going to write. This line wires the annotation to the transformation. With this in place, the Groovy compiler is going to invoke gep.

Any breakpoint set within LoggingASTTransformation will now be hit within the IDE when running the sample script. The ASTTransformation class is a little more complex.

GroovyASTTransformation to tell at which compilation phase the transform needs to run. This exercise is left to the reader. Note the creation of the new println statements in the createPrintlnAst(String) method. Creating AST for code is not always simple.

A common error by beginners is to have the AST transformation code in the same source tree as a class that uses the transformation.

Being in the same source tree in general means that they are compiled at the same time. In conclusion, AST transformations need to be precompiled before you can use them. In general, it is as easy as having them in a separate source tree. Global transformations Global AST transformation are similar to local one with a major difference: they do not need an annotation, meaning that they are applied globally, that is to say on each class being compiled.

It is therefore very important to limit their use to last resort, because it can have a significant impact on the compiler performance. This class provides several utility methods that make AST transformations easier to write. Almost all AST transformations included in Groovy extend this class. It is a common use case to be able to transform an expression into another. Groovy provides a class which makes it very easy to do this: org.

In particular it requires knowledge about the AST classes. Since those classes are internal, there are chances that the API will change in the future, meaning that your transformations could break.