FastJson: What It Is, Performance Comparison, Popularity Issues, and Why to Abandon It

1. What is FastJson

FastJson is an open‑source JSON parsing library from Alibaba that can parse JSON strings, serialize Java beans to JSON, and deserialize JSON back to Java beans.

It is a commonly used library among Java developers, and its main selling point is speed.

2. Is it really fast?

Without research there is no right to speak; a simple benchmark comparing FastJson with the widely used Jackson and Google’s Gson was performed on JDK 8, AMD 3700X, 3200 MHz RAM, testing both simple and complex objects for 10 million conversions.

The results show FastJson is indeed the fastest, about 20 % faster than Jackson, while Gson is the slowest by a large margin. However, the test does not cover memory usage or large documents, and speed alone should not be the sole criterion for library selection.

In modern applications even the slowest Gson meets requirements; other optimizations such as I/O tuning and parallel processing are more effective than choosing a marginally faster library.

3. FastJson is not that popular

FastJson’s Maven usage is far lower than Jackson and Gson, and it lacks the extensive ecosystem of the Jackson family.

Some Chinese forums claim FastJson is a low‑quality domestic library, though the author’s own projects have used it without bugs.

There are over 1300 unresolved issues on its GitHub repository, some of which are demonstrated in the following screenshots.

Test code (shown as an image) was used, and after updating to the latest version the same problems persisted, revealing serious logical errors in the library’s handling of certain date‑time formats.

Incorrect formats such as “1970‑01‑01 00:00:00.000000000.000000000” are parsed successfully, while correct formats like “1970‑01‑01 00:00:00” fail.

Combined with community feedback, the author concludes FastJson is not as excellent as claimed.

4. Abandon FastJson

In the author’s projects, Jackson is used as the JSON parser to keep dependencies minimal, while some colleagues still use Gson or FastJson without strict guidelines.

Jackson’s popularity has intrinsic reasons, and future projects will favor Jackson and gradually phase out FastJson.

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