78-string-builder-shared

Example 78 — Shared StringBuilder

Detector: StringBuilderDetector
Select it with: includes = DetectorType.STRING_BUILDER

The Problem

RequestLogger keeps a single StringBuilder log as an instance field. Its append(String msg) method calls log.append(msg).append("\n") with no synchronization. StringBuilder is explicitly documented as not thread-safe (use StringBuffer or explicit locking if shared).

Under concurrent access:

• The internal char[] buffer can be grown by one thread while another is copying characters, producing ArrayIndexOutOfBoundsException.
• Two threads interleaving their appends produce garbled, interleaved output where log entries from different threads are merged into one line.

How to Reproduce

Remove the @Disabled annotation from test_concurrent_detectsSharedBuilder and run the test. StringBuilderDetector records every append from each thread and flags StringBuilder instances accessed concurrently from more than one thread.

The Fix

Use StringBuffer instead of StringBuilder, wrap the appends in a synchronized block, or accumulate per-thread strings and merge them at the end with a StringJoiner.