Implementing Refactorings in IntelliJ IDEA

Implementing Refactorings in IntelliJ IDEA Dmit ry Jemerov Development Lead JetBrains, Inc. ht tp://www.jetbrains.com/

Agenda
Demo of new refactoring features
Refactoring building blocks
Structural search and replace
Problems and future directions

Building Blocks
Not a set of Lego bricks
A workflow framework into which you can plug your code
And a set of utility functions and classes to implement the essence of refactoring
Every refactoring is somewhat different

Lexing and Parsing
Lexer converts text into stream of lexems
Recursive-descent parser builds PSI (Program Structure Interface) tree
Contains all tokens including whitespace
Error markers at places of syntax errors
Supports high-level semantic operations
Supports write operations (add, replace, delete)

References
Usages of an element declared elsewhere
Can cross languages (e.g. XML to Java)
Key operations:
Resolve (get declaration by usage)
Callbacks on move/rename of target

General Refactoring Workflow
Show UI for entering options
Find usages of target element
Check for conflicts (accessibility etc.)
Show list of affected usages, allow user to exclude some
Modify target element and each usage as appropriate

Find Usages
Finds all references to PSI element
Based on inverted index of all identifiers
All words that look like a reference are checked if they indeed resolve to target

Code Modification
Stub of new element created from text
Targets of references in fragment to be moved are remembered
Stub contents is replaced with fragment to be moved
Modifiers etc. are updated via PSI
References are updated so that they point to the same targets
Updated fragment is reformatted

Formatter
Defines a set of constraints on layout of pairs of elements
Minimum spacing / line breaks
Indentation
Alignment
Whitespace not violating constraints is preserved

Agenda

Structural Search
QBE interface for navigating and modifying Java and XML/HTML code
Code fragment with some variables
Constraints for variables
Replacement pattern specifying how occurrences should be changed (optional)
Allows to perform custom refactoring operations

Structural Search Template

Structural Search Problems
Very hard to use by non-expert users
Patterns are written in a non-obvious way
Unintuitive UI
Doesn’t cover all operations
No façade around internal API for writing scripts

Agenda

Context Dependence in Java
Java language is not uniform
Many contexts in which only certain code is allowed
Constructors, super constructor calls
Enum constants
Anonymous classes
Initializer blocks
Lots of special-case code needed to handle weird context changes

Refactoring Quality
Users typically don’t review the results of applying an automated refactoring
Expect their tool to be 100% correct
Solutions which provide 90% correctness with simple implementation aren’t acceptable
No way to escape hand-coding corner cases

DSL for Code Transformations
If the complexity can’t be avoided, maybe it can be compressed?
Desire to replace verbose Java code with a domain-specific language
Internal DSL
Groovy/Ruby
JetBrains MPS

Patterns
Internal DSL for matching AST fragments

Language Independent Refactorings
Currently only the basic refactorings (Rename and Move) are mostly language-agnostic
All other refactorings are reimplemented from scratch for every new language
Desire to extract language-independent essence to common part

Summary
Refactorings in IntelliJ IDEA involve a lot of complex code with limited reuse
Complexity caused by large number of concepts in Java language
Necessity of 100% correctness means that complexity is essential
Main hope for improvement is building powerful code transformation DSLs

Q&A Dmitry Jemerov [email_address]

Implementing Refactorings in IntelliJ IDEA Dmit ry Jemerov Development Lead JetBrains, Inc. ht tp://www.jetbrains.com/

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Implementing Refactorings in IntelliJ IDEA

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