jhy/jsoup · error · ValidationException

Object must not be null

Error message

Object must not be null

What it means

Validate.notNull(obj) is jsoup's basic null assertion helper. It throws a ValidationException with the message "Object must not be null" whenever any jsoup API that guards its inputs receives a null object reference.

Solutions

1. Check for null before calling the jsoup API, or use Optional to model the absence
2. Use Validate.expectNotNull()/notNullParam() in your own code to fail fast with context at the call site
3. Trace which argument was null by wrapping the call and logging inputs on failure

Example fix

// before
doc.select("div.main").first().html(); // NPE path -> ValidationException if null guarded
// after
Element main = doc.select("div.main").first();
if (main != null) System.out.println(main.html());

if (element == null) {
    // handle absence explicitly instead of passing null into the API
    return Optional.empty();
}

Optional<Element> safe = Optional.ofNullable(doc.selectFirst("div.main"));

try {
    apiCall(maybeNull);
} catch (ValidationException e) {
    if (e.getMessage().equals("Object must not be null")) {
        log.error("Required object was null at call site", e);
    } else { throw e; }
}

When it happens

Trigger: Passing null to any jsoup method that internally calls Validate.notNull — e.g. a null Element, null tag name, null attribute key, or null selector string at an API boundary guarded by this helper.

Common situations: Chaining off a query that returned null (element.select(...).first() can be null); map lookups or optional unwrapping that yielded null before being handed to jsoup; version changes where an API that previously tolerated null became strict.

Related errors

• The parameter ' ' must not be null.
• msg
• The ' ' parameter must not be empty.
• Pattern syntax error:
• Pattern complexity error

AI-assisted analysis of jhy/jsoup@9851ac5d9c (2026-09-08). Data as JSON: /api/errors/400ee9b80e314a3d. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/org/jsoup/helper/Validate.java:19

package org.jsoup.helper;

import org.jspecify.annotations.Nullable;

/**
 * Validators to check that method arguments meet expectations. 
 */
public final class Validate {
    
    private Validate() {}

    /**
     * Validates that the object is not null
     * @param obj object to test
     * @throws ValidationException if the object is null
     */
    public static void notNull(@Nullable Object obj) {
        if (obj == null)
            throw new ValidationException("Object must not be null");
    }

    /**
     Validates that the parameter is not null

     * @param obj the parameter to test
     * @param param the name of the parameter, for presentation in the validation exception.
     * @throws ValidationException if the object is null
     */
    public static void notNullParam(@Nullable final Object obj, final String param) {
        if (obj == null)
            throw new ValidationException(String.format("The parameter '%s' must not be null.", param));
    }

    /**
     * Validates that the object is not null
     * @param obj object to test
     * @param msg message to include in the Exception if validation fails

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