apache/flink · error · RuntimeException

Unable to instantiate the hadoop input format

Error message

Unable to instantiate the hadoop input format

What it means

Thrown by HadoopInputFormatBase.readFields (open/configure phase) when Class.forName(hadoopInputFormatClassName).newInstance() fails. The Hadoop InputFormat class name is read from serialized job configuration; if the class cannot be found, is abstract, lacks a no-arg constructor, or throws in its constructor, this RuntimeException wraps the underlying cause.

Source

Thrown at flink-connectors/flink-hadoop-compatibility/src/main/java/org/apache/flink/api/java/hadoop/mapred/HadoopInputFormatBase.java:299

        super.read(in);

        String hadoopInputFormatClassName = in.readUTF();
        String keyClassName = in.readUTF();
        String valueClassName = in.readUTF();
        if (jobConf == null) {
            jobConf = new JobConf();
        }
        jobConf.readFields(in);
        try {
            this.mapredInputFormat =
                    (org.apache.hadoop.mapred.InputFormat<K, V>)
                            Class.forName(
                                            hadoopInputFormatClassName,
                                            true,
                                            Thread.currentThread().getContextClassLoader())
                                    .newInstance();
        } catch (Exception e) {
            throw new RuntimeException("Unable to instantiate the hadoop input format", e);
        }
        try {
            this.keyClass =
                    (Class<K>)
                            Class.forName(
                                    keyClassName,
                                    true,
                                    Thread.currentThread().getContextClassLoader());
        } catch (Exception e) {
            throw new RuntimeException("Unable to find key class.", e);
        }
        try {
            this.valueClass =
                    (Class<V>)
                            Class.forName(
                                    valueClassName,
                                    true,
                                    Thread.currentThread().getContextClassLoader());

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Add the JAR containing the Hadoop InputFormat class to the Flink job classpath (via -C / lib folder / fat JAR).
2. Verify the class name passed to HadoopInputFormat is the fully qualified name of a concrete, instantiable class.
3. Inspect the wrapped exception (getCause()) for the root ClassNotFoundException, InstantiationException, or IllegalAccessException.
4. Ensure the Hadoop version on the classpath matches the version the InputFormat was compiled against.

// Validate the InputFormat class is loadable before submitting the job
String className = hadoopInputFormatClass.getName();
try {
    Class<?> clazz = Class.forName(className);
    if (clazz.isInterface() || java.lang.reflect.Modifier.isAbstract(clazz.getModifiers())) {
        throw new IllegalArgumentException("InputFormat class must be concrete: " + className);
    }
    clazz.getDeclaredConstructor().newInstance(); // test instantiation
} catch (Exception e) {
    throw new IllegalArgumentException("Cannot instantiate InputFormat: " + className, e);
}

When it happens

Trigger: Deserializing a HadoopInputFormat whose hadoopInputFormatClassName is not on the task classpath, or whose class cannot be instantiated (abstract, interface, private constructor, or constructor exception).

Common situations: Missing Hadoop InputFormat JAR in the Flink job classpath; typo in the class name configured via HadoopInputFormat; version mismatch where the InputFormat class was renamed or removed between Hadoop versions; the InputFormat constructor throws due to missing native libraries.

Related errors

• Unable to find key class.
• Unable to find value class.
• Unable to instantiate the hadoop output format
• The program's entry point class '{className}' could not be l
• Could not load class for serialization config

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/34f3e4f860943918. Report an issue: GitHub.