Boosting Template Rendering 10×: Replace String.replace with StringBuilder

Background

Alipay Card Pack stores users' membership cards and coupons. Both the card cell and card details are rendered by combining a static template with dynamic data, which is then presented to the end user.

Problem Discovery

The original template variable replacement logic iterates over the template string, using indexOf and substring to replace variables delimited by $. This approach is executed on every request and was suspected to have performance issues.

Performance Optimization V1

Removed the repeated indexOf and substring calls. The new method first extracts all variables from the template using a double‑pointer scan, stores them in a collection, and then replaces each variable in a single pass.

Performance Optimization V2

Introduced a local cache (using Google Guava) to store the mapping between a template ID and its variable set, eliminating the need to extract variables on every request and reducing pressure on the backend database.

Performance Optimization V3

Replaced String.replace with StringBuilder to avoid template recompilation and object creation for each replacement.

Performance Optimization V4

Combined the cache‑free approach of V3 with StringBuilder, achieving the highest speed gain (over 10×) while keeping the code relatively simple.

Performance Optimization V5

Removed the cache dependency, kept the variable extraction, and used StringBuilder to replace String.replace, resulting in a readable implementation with excellent performance.

Performance Comparison

Benchmarks show that V4 > V3 > V5 > V2 > V1 > original version. The dominant bottleneck is the String.replace call; replacing it with StringBuilder yields more than tenfold speed improvement, while caching provides additional but diminishing gains.

Conclusion

The key takeaways are that String.replace incurs template compilation and creates many temporary strings, consuming CPU and memory, while StringBuilder avoids these costs. Caching template‑variable mappings can further reduce load, but the most effective optimization is replacing String.replace with StringBuilder.