Why a Single Log Line Crashed My Java Service: Fastjson Getter Pitfalls Explained

1. Problem Discovery

During an iteration, a newly added log statement caused the main workflow to throw an exception. The log code was:

log.info("batch save receipt context:{}", JSON.toJSONString(context));

The error log screenshot showed a Fastjson JSONException caused by a null field during serialization.

2. Solution

Investigation revealed a newly added getDutyLevelNumber() method that processed a field without null‑checking. Fastjson automatically calls getter methods during serialization, so the null field caused an NPE. Renaming the method to fetchDutyLevelNumber() stopped Fastjson from invoking it and resolved the issue.

To avoid similar problems, you can:

Rename custom getters to avoid the get prefix.

Use SerializerFeature.IgnoreNonFieldGetter when calling toJSONString.

Annotate getters with @JSONField(serialize = false) to exclude them.

Centralize JSON conversion in a utility class that handles exceptions.

3. Serialization Process

Fastjson’s serialization starts with JSON.toJSONString:

public static String toJSONString(Object object, int defaultFeatures, SerializerFeature... features) {
    SerializeWriter out = new SerializeWriter((Writer) null, defaultFeatures, features);
    try {
        JSONSerializer serializer = new JSONSerializer(out);
        serializer.write(object);
        String outString = out.toString();
        int len = outString.length();
        if (len > 0 && outString.charAt(len - 1) == '.' && object instanceof Number && !out.isEnabled(SerializerFeature.WriteClassName)) {
            return outString.substring(0, len - 1);
        }
        return outString;
    } finally {
        out.close();
    }
}

The JSONSerializer.write method obtains an ObjectSerializer for the object's class and delegates the actual conversion:

public final void write(Object object) {
    if (object == null) {
        out.writeNull();
        return;
    }
    Class<?> clazz = object.getClass();
    ObjectSerializer writer = getObjectWriter(clazz);
    try {
        writer.write(this, object, null, null, 0);
    } catch (IOException e) {
        throw new JSONException(e.getMessage(), e);
    }
}

The serializer is retrieved via SerializeConfig.getObjectWriter, which may create a JavaBeanSerializer for custom objects. JavaBeanSerializer uses TypeUtils.buildBeanInfo to collect field information, and TypeUtils.computeGetters to turn getter methods into FieldInfo entries. During serialization, each FieldInfo invokes its associated getter, which is why getDutyLevelNumber() was executed and caused the NPE.

4. Practical Summary

When using Fastjson:

Avoid custom getters that can be invoked unintentionally; rename them or exclude them with annotations.

Prefer field‑based serialization if getters are not needed.

Be aware that Fastjson follows the JavaBeans convention, calling any method that starts with get or is.

Consider the small performance cost of method calls versus the safety and flexibility they provide.