[BUG]FastJSON2 Issue Report: StringIndexOutOfBoundsException When Serializing Objects with Large BigDecimal

FastJSON2 Issue Report: StringIndexOutOfBoundsException When Serializing Objects with Large BigDecimal Maps

Summary

When using FastJSON2 (version 2.0.57) to serialize an object containing large Map and other BigDecimal variables with many entries (150+), a StringIndexOutOfBoundsException occurs in JSONWriterUTF8.writeDecimal() method. This happens even with JSONWriter.Feature.LargeObject applied both through annotations and
explicitly in the serialization method.

Environment

- FastJSON2 version: 2.0.57
- JDK/JRE: Java 21 Kotlin
- OS: Linux
- Application type: Kotlin financial data processing application

Issue Description

The application processes financial market data and serializes state objects to JSON for downstream systems. One particular object containing a map with approximately 150-200 entries of market index weights (represented as BigDecimal values) and other financial datas consistently fails with a StringIndexOutOfBoundsException
when serialized using FastJSON2.

The error indicates a buffer overflow in the JSONWriterUTF8.writeDecimal() method, specifically hitting the default 8KB buffer limit (8192 bytes). This happens despite both:
1. Adding JSONWriter.Feature.LargeObject to the class annotation
2. Explicitly adding JSONWriter.Feature.LargeObject when calling JSON.toJSONBytes()

Exception Details

java.lang.StringIndexOutOfBoundsException: Range [8187, 8187 + 6) out of bounds for length 8192
at java.base/jdk.internal.util.Preconditions$1.apply(Preconditions.java:55)
at java.base/jdk.internal.util.Preconditions$1.apply(Preconditions.java:52)
at java.base/jdk.internal.util.Preconditions$4.apply(Preconditions.java:213)
at java.base/jdk.internal.util.Preconditions$4.apply(Preconditions.java:210)
at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:98)
at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckFromIndexSize(Preconditions.java:118)
at java.base/jdk.internal.util.Preconditions.checkFromIndexSize(Preconditions.java:397)
at java.base/java.lang.String.checkBoundsOffCount(String.java:4853)
at java.base/java.lang.String.getBytes(String.java:1765)
at com.alibaba.fastjson2.JSONWriterUTF8.writeDecimal(JSONWriterUTF8.java:2382)
at com.alibaba.fastjson2.writer.FieldWriterBigDecimalField.write(FieldWriterBigDecimalField.java:32)
at com.alibaba.fastjson2.writer.ObjectWriterAdapter.write(ObjectWriterAdapter.java:312)
at com.alibaba.fastjson2.JSON.toJSONBytes(JSON.java:3298)
at com.company.processor.EntityProcessor.createOutput(EntityProcessor.kt:211)

ExampleData:
[anonymized_data_sample.txt](https://github.com/user-attachments/files/19856134/anonymized_data_sample.txt)

Start with JSONWriterUTF8.writeDecimal() at JSONWriterUTF8.java:2382 and follow the call from FieldWriterBigDecimalField.java. Reproduce the failure through EntityProcessor.kt:211 using the anonymized sample and the reported LargeObject options. Done means serializing the large BigDecimal map completes without StringIndexOutOfBoundsException.

Written by the indexing model from the issue text.