[Feature][SDK][Transform] Support parsing SQL NULL literal via NullValueParser #12213

Description

This feature adds first-class support for the SQL NULL literal in Transform SDK expressions.

Previously, a SELECT expression that used NULL directly (e.g. ifnull(null, 3), coalesce(null, col)) failed during parser construction:

java.lang.ClassCastException: net.sf.jsqlparser.expression.NullValue cannot be cast to net.sf.jsqlparser.schema.Column
	at org.apache.inlong.sdk.transform.process.parser.ParserTools.getTransformParser(ParserTools.java:72)
	at org.apache.inlong.sdk.transform.process.operator.OperatorTools.buildParser(OperatorTools.java:119)
	at org.apache.inlong.sdk.transform.process.function.condition.IfNullFunction.<init>(IfNullFunction.java:57)

With this feature, the NULL literal is resolved to a constant Java null, enabling ifnull(null, x) / coalesce(null, x) / concat_ws(..., null, ...) and similar expressions to work as expected.

Use case

Component

inlong-sdk / transform-sdk

• org.apache.inlong.sdk.transform.process.parser.NullValueParser (new)
• Test: org.apache.inlong.sdk.transform.process.function.condition.TestIfNullFunction

Description

This feature adds first-class support for the SQL NULL literal in Transform SDK expressions.

Previously, a SELECT expression that used NULL directly (e.g. ifnull(null, 3), coalesce(null, col)) failed during parser construction:

java.lang.ClassCastException: net.sf.jsqlparser.expression.NullValue cannot be cast to net.sf.jsqlparser.schema.Column
	at org.apache.inlong.sdk.transform.process.parser.ParserTools.getTransformParser(ParserTools.java:72)
	at org.apache.inlong.sdk.transform.process.operator.OperatorTools.buildParser(OperatorTools.java:119)
	at org.apache.inlong.sdk.transform.process.function.condition.IfNullFunction.<init>(IfNullFunction.java:57)

With this feature, the NULL literal is resolved to a constant Java null, enabling ifnull(null, x) / coalesce(null, x) / concat_ws(..., null, ...) and similar expressions to work as expected.

Background

ParserTools.getTransformParser(Expression expr) resolves parsers through a reflective scan that maps Expression sub-types to their parser classes (parserMap). Expression types that are not registered currently fall back to a Column cast:

Class<?> clazz = parserMap.get(expr.getClass());
if (clazz == null) {
    return new ColumnParser((Column) expr);   // NULL literal hits here and crashes
}

The SQL NULL literal is parsed by JSqlParser as net.sf.jsqlparser.expression.NullValue, which is not a Column. Because no parser was registered for it, any NULL literal hit the fallback branch and raised a ClassCastException.

Proposed Change

Introduce a NullValueParser following the existing StringParser pattern:

@TransformParser(values = NullValue.class)
public class NullValueParser implements ValueParser {

    public NullValueParser(NullValue expr) {
        // The NULL literal carries no value; nothing to store.
    }

    @Override
    public Object parse(SourceData sourceData, int rowIndex, Context context) {
        return null;
    }
}

@TransformParser(values = NullValue.class) registers NullValue.class in parserMap, so getTransformParser resolves it through the reflective constructor branch, and NULL literals evaluate to constant null.

Resulting Behavior

• ifnull(null, numeric2) evaluates its first argument to null, so IfNullFunction falls back to the second argument as expected.
• Other functions that receive a NULL literal as an argument (e.g. coalesce, num_nulls, concat_ws) no longer fail during parser construction.

Tests

Added a true-NULL-literal case to TestIfNullFunction:

// case6: ifnull(null, numeric2) -- true NULL literal
transformSql = "select ifnull(null,numeric2) from source";
data = "5|3|3|5";
Assert.assertEquals("result=3", output.get(0));

Verified: Tests run: 1, Failures: 0, Errors: 0, Skipped: 0.

Backward Compatibility

• Fully backward compatible.
• No public API or method-signature change; only a new parser class is added.
• Existing column / function / literal parsers are unaffected.

Files Changed

• inlong-sdk/transform-sdk/src/main/java/org/apache/inlong/sdk/transform/process/parser/NullValueParser.java (new)
• inlong-sdk/transform-sdk/src/test/java/org/apache/inlong/sdk/transform/process/function/condition/TestIfNullFunction.java

Are you willing to submit PR?

• Yes, I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct