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Mvel Compiler

drools-model/drools-mvel-compiler/README.md

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Mvel Compiler

This is a compiler that translates from MVEL source code to Java. It's used mostly in consequences that use dialect "MVEL"

It's also used to preprocess Java consequences that use modify and with blocks (see ModifyCompilerTest). This assumes that every Java consequence that contains a modify could be parsed by the mvel.jj grammar in drools-mvel-parser module.

Transformation rules are described in the MvelCompilerTest

The idea of the algorithm is split into three phases

  1. Preprocess modify and with statements and transform that to java-like (see PreprocessPhase)
  2. Call the Statement Visitor that will ensure that every child statement is processed accordingly
  3. For each ExpressionStatement type the expression and transform accessors to getter in RHSPhase
  4. Use the expression type to set the type of the undeclared variables in an assignment expression and transform field accessor to setters in LHSPhase
  5. Reprocess the RHS in ReProcessRHSPHase to further coerce BigDecimals

The first phase consists of a type inference algorithm that uses reflection to type check the fields and methods and transform to valid accessor. The .class files have to be present in the classloader before running the mvel-compiler. If not, this algorithm will fail.

For example this MVEL expression

java

person.name;

when name is a private field gets transformed into

java

person.getName();

If .name is a public field, the expression is left as it is.