[DAT-21391] - introduce standard Claude structure and documents for LB Core

.claude/CLAUDE.md

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+# Liquibase - Claude Code Instructions
+
+## Tech Stack
+
+- **Language**: Java 21
+- **Build**: Maven 3.8.4+ (multi-module project)
+- **Testing**: JUnit Jupiter 5.13.3, Spock 2.4-M6-groovy-4.0, Mockito 5.18.0
+- **Logging**: Custom logging framework (liquibase.logging)
+- **Serialization**: SnakeYAML 2.4 for YAML support
+- **Code Quality**: JaCoCo for coverage, SonarCloud for analysis
+
+## Project Structure
+
+```
+liquibase/
++-- pom.xml (parent POM)
++-- liquibase-core/ (core engine)
++-- liquibase-maven-plugin/ (Maven integration)
++-- liquibase-integration-tests/ (integration tests)
++-- liquibase-debian/ (Linux deb packaging)
++-- liquibase-rpm/ (Linux rpm packaging)
+```
+
+## Build Commands
+
+```bash
+# Full build
+mvn clean install
+
+# Skip tests
+mvn clean install -DskipTests
+
+# Run tests only
+mvn test
+
+# Build specific module
+mvn clean install -pl liquibase-core
+
+# Build with dependencies
+mvn clean install -pl liquibase-maven-plugin -am
+
+# Run integration tests
+mvn verify -pl liquibase-integration-tests
+
+# Run with Oracle profile
+mvn verify -Poracle
+```
+
+## Code Patterns
+
+### Adding a New Change Type
+
+1. Create class in `liquibase-core/src/main/java/liquibase/change/core/`
+2. Extend `AbstractChange`
+3. Annotate with `@DatabaseChange`
+4. Implement `generateStatements()` and `createInverses()`
+5. Register in `META-INF/services/liquibase.change.Change`
+
+### Adding Database Support
+
+1. Create class in `liquibase-core/src/main/java/liquibase/database/core/`
+2. Extend `AbstractJdbcDatabase`
+3. Override database-specific methods
+4. Register in `META-INF/services/liquibase.database.Database`
+
+### Adding SQL Generator
+
+1. Create class in `liquibase-core/src/main/java/liquibase/sqlgenerator/core/`
+2. Extend `AbstractSqlGenerator<StatementType>`
+3. Set priority for database-specific generators
+4. Register in `META-INF/services/liquibase.sqlgenerator.SqlGenerator`
+
+## Anti-Patterns
+
+**AVOID:**
+- Direct SQL string construction without escaping (use `database.escapeObjectName()`)
+- Hardcoding database-specific syntax in Change classes (use SqlGenerator)
+- Breaking backwards compatibility in changelog parsing
+- Modifying DATABASECHANGELOG table structure without migration path
+- Ignoring transaction boundaries in multi-statement changes
+
+**DO NOT:**
+- Add new dependencies without updating parent POM dependencyManagement
+- Skip precondition validation in change execution
+- Bypass LockService for database modifications
+- Create new changelog formats without parser registration
+
+## Quality Gates
+
+### Before Commit
+
+1. Run `mvn verify` to ensure all tests pass
+2. Check for new compiler warnings
+3. Ensure changelog formats remain backwards compatible
+4. Verify precondition checks for new changes
+
+### Code Review Focus
+
+- Database abstraction maintained
+- SQL injection prevention
+- Rollback support implemented
+- Thread safety considerations
+- Backwards compatibility
+
+## Testing Strategy
+
+### Unit Tests
+
+- Location: `src/test/java/` in each module
+- Framework: JUnit Jupiter
+- Mock external dependencies with Mockito
+- Test SQL generation with MockDatabase
+
+### Integration Tests
+
+- Location: `liquibase-integration-tests/`
+- Framework: Spock + JUnit
+- Tests run against H2, HSQLDB, Derby (embedded)
+- Use `@Disabled` for database-specific tests requiring external setup
+
+### Running Specific Tests
+
+```bash
+# Single test class
+mvn test -Dtest=CreateTableChangeTest
+
+# Single test method
+mvn test -Dtest=CreateTableChangeTest#testGenerateStatements
+
+# Pattern matching
+mvn test -Dtest=*OracleTest
+```
+
+## File Organization
+
+### Source Files
+
+```
+src/main/java/liquibase/
++-- change/           # Change type definitions
++-- changelog/        # Changelog parsing and processing
++-- database/         # Database abstractions
++-- diff/             # Database comparison
++-- executor/         # SQL execution
++-- parser/           # Changelog file parsers
++-- precondition/     # Precondition checks
++-- snapshot/         # Database state capture
++-- sql/              # SQL statements
++-- sqlgenerator/     # SQL generation
+```
+
+### Resource Files
+
+```
+src/main/resources/
++-- META-INF/
+    +-- services/     # ServiceLoader registrations
+    +-- MANIFEST.MF   # OSGi bundle manifest
++-- liquibase.build.properties
+```
+
+## Naming Conventions
+
+### Classes
+
+- Changes: `{Action}{Object}Change` (e.g., `CreateTableChange`, `AddColumnChange`)
+- Databases: `{DatabaseName}Database` (e.g., `OracleDatabase`, `PostgresDatabase`)
+- Generators: `{Statement}Generator{Database}` (e.g., `CreateTableGeneratorOracle`)
+- Statements: `{Action}{Object}Statement` (e.g., `CreateTableStatement`)
+
+### Packages
+
+- Database-specific: `liquibase.*.core` (e.g., `liquibase.database.core`)
+- Integration: `liquibase.integration.*` (e.g., `liquibase.integration.ant`)
+
+### Test Classes
+
+- Unit tests: `{ClassName}Test`
+- Spock specs: `{ClassName}Spec`
+- Integration tests: `{Feature}IntegrationTest`
+
+## IDE Setup
+
+### IntelliJ IDEA
+
+- Import as Maven project
+- Enable Groovy plugin for Spock tests
+- Set Java SDK to 21
+- Configure Groovy SDK from Maven dependency
+
+### Useful Run Configurations
+
+- `mvn clean install -DskipTests` - Fast build
+- `mvn test -pl liquibase-core` - Core tests only
+- `mvn verify` - Full verification including integration tests