Why MyBatis Treats Integer 0 as an Empty String in <if> Conditions

Problem

A MyBatis mapper uses a dynamic <if> clause to filter rows by a state column:

<if test="req.state != null and req.state != ''">
    AND md.state = #{req.state}
</if>

The front‑end sends state as an Integer (0 or 1). When req.state is 0, the condition is evaluated as false and the state filter is omitted, causing incorrect query results.

Root Cause

MyBatis evaluates the test attribute with OGNL. In OGNL, numeric values are converted to BigDecimal and considered true only if they are non‑zero. The expression state != '' forces OGNL to compare an Integer with an empty string, which results in the integer being treated as false when its value is 0. Consequently the whole expression state != null and state != '' evaluates to false for 0.

MyBatis Internals

The parsing flow is:

private void initNodeHandlerMap() {
    nodeHandlerMap.put("if", new IfHandler());
    // other handlers omitted
}

IfHandler

creates an IfSqlNode that stores the test string and delegates evaluation to ExpressionEvaluator:

public class IfHandler implements NodeHandler {
    public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
        MixedSqlNode mixedSqlNode = parseDynamicTags(nodeToHandle);
        String test = nodeToHandle.getStringAttribute("test");
        IfSqlNode ifSqlNode = new IfSqlNode(mixedSqlNode, test);
        targetContents.add(ifSqlNode);
    }
}

The IfSqlNode applies the expression:

public boolean apply(DynamicContext context) {
    if (evaluator.evaluateBoolean(test, context.getBindings())) {
        contents.apply(context);
        return true;
    }
    return false;
}

Key part – ExpressionEvaluator.evaluateBoolean:

public boolean evaluateBoolean(String expression, Object parameterObject) {
    Object value = OgnlCache.getValue(expression, parameterObject);
    if (value instanceof Boolean) {
        return (Boolean) value;
    }
    if (value instanceof Number) {
        return new BigDecimal(String.valueOf(value)).compareTo(BigDecimal.ZERO) != 0;
    }
    return value != null;
}

When the expression is state != null and state != '' and state = 0, OGNL evaluates the second part as false, making the whole test false.

Reproduction

Add OGNL dependency and run a simple test:

<dependency>
    <groupId>ognl</groupId>
    <artifactId>ognl</artifactId>
    <version>2.7.3</version>
</dependency>

public static void main(String[] args) {
    Map<String, Object> map = new HashMap<>();
    map.put("state", 0);
    Object result = OgnlCache.getValue("state != null and state != ''", map);
    System.out.println(result); // prints false
}

The program prints false, confirming the OGNL behaviour.

Solution

Do not compare numeric parameters with an empty string. Typical fixes are:

Remove the empty‑string check:

<if test="req.state != null">
    AND md.state = #{req.state}
</if>

Explicitly handle zero:

<if test="req.state != null and req.state != 0">
    AND md.state = #{req.state}
</if>

Either approach ensures that a legitimate 0 value is not filtered out.