Optimize resource lookup in DevTools restart

[hoonyworld]

Optimize resource lookup in DevTools restart

This pull request significantly improves the performance of resource lookups within the DevTools restart mechanism, particularly benefiting projects with a large number of files and source directories.

Motivation and Problem

The existing ClassLoaderFilesResourcePatternResolver and its underlying ClassLoaderFiles implementation suffer from performance degradation in large-scale projects. Key methods such as getFile(), size(), isDeleted(), and getAdditionalResources() rely on nested loops that iterate through all source directories and their respective files. This results in a time complexity of O(n*m) for many critical operations, where 'n' is the number of source directories and 'm' is the number of files. As a project grows, these repeated iterations can introduce noticeable delays during application restarts.

Solution

This PR addresses the performance bottleneck by refactoring ClassLoaderFiles to use a pre-computed, flattened map for direct lookups, and updating ClassLoaderFilesResourcePatternResolver to leverage this new, efficient structure.

1. ClassLoaderFiles Refactoring:

• Introduced a Flattened Map:
  A new Map<String, ClassLoaderFile> filesByName field has been added. This map is populated once at construction time and serves as a unified, central cache for all files across all source directories.
• O(1) Complexity for Core Methods:
  • getFile(name) now performs a direct get() on the filesByName map, reducing its complexity from O(n) to O(1).
  • size() now directly returns filesByName.size(), reducing its complexity from O(n) to O(1).
• Data Consistency:
  The addFile() and removeAll() methods have been updated to ensure that both the original sourceDirectories map and the new filesByName map are kept in sync, guaranteeing data integrity.
• New Efficient Iterator:
  A new getFileEntries() method provides a way to iterate over all files in a single loop, which is used by the resolver.

2. ClassLoaderFilesResourcePatternResolver Optimization:

• Optimized isDeleted():
  This method has been refactored to eliminate the nested loop structure (from O(n*m) to O(m)). It now iterates over the single, unified collection provided by classLoaderFiles.getFileEntries(), significantly reducing the complexity of the operation while preserving the original exception handling behavior.
• Optimized getAdditionalResources():
  Similarly, this method now iterates over the single, unified collection provided by classLoaderFiles.getFileEntries(), eliminating the outer loop and improving readability and performance.

Benefits

Significant Performance Improvement:
Reduces the time complexity of critical file lookup operations. Direct lookups like getFile are improved from O(n) to O(1), while iterative searches like isDeleted are improved from O(n*m) to O(m), leading to faster application restarts in DevTools.

Enhanced Code Quality:
Eliminates nested loops, which simplifies the code, improves readability, and makes the logic easier to maintain.

Minimal Memory Overhead:
The optimization introduces a map of references, not data duplicates. The trade-off of a negligible increase in memory usage for a substantial performance gain is highly favorable.

This change maintains full backward compatibility with the existing API while providing a much-needed performance boost for developers working on large applications.