apache/hadoop · error · NullPointerException

Key may not be null

Error message

Key may not be null

What it means

CountingBloomFilter.delete(Key) removes a key by decrementing the counters it previously incremented; it first requires a non-null key and throws NullPointerException('Key may not be null') as an explicit guard. This mirrors add() and membershipTest() in the same class, failing fast with a readable message instead of an opaque NPE from the hashing layer.

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/bloom/CountingBloomFilter.java:137

      long bucketValue = (buckets[wordNum] & bucketMask) >>> bucketShift;
      
      // only increment if the count in the bucket is less than BUCKET_MAX_VALUE
      if(bucketValue < BUCKET_MAX_VALUE) {
        // increment by 1
        buckets[wordNum] = (buckets[wordNum] & ~bucketMask) | ((bucketValue + 1) << bucketShift);
      }
    }
  }

  /**
   * Removes a specified key from <i>this</i> counting Bloom filter.
   * <p>
   * <b>Invariant</b>: nothing happens if the specified key does not belong to <i>this</i> counter Bloom filter.
   * @param key The key to remove.
   */
  public void delete(Key key) {
    if(key == null) {
      throw new NullPointerException("Key may not be null");
    }
    if(!membershipTest(key)) {
      throw new IllegalArgumentException("Key is not a member");
    }

    int[] h = hash.hash(key);
    hash.clear();

    for(int i = 0; i < nbHash; i++) {
      // find the bucket
      int wordNum = h[i] >> 4;          // div 16
      int bucketShift = (h[i] & 0x0f) << 2;  // (mod 16) * 4
      
      long bucketMask = 15L << bucketShift;
      long bucketValue = (buckets[wordNum] & bucketMask) >>> bucketShift;
      
      // only decrement if the count in the bucket is between 0 and BUCKET_MAX_VALUE
      if(bucketValue >= 1 && bucketValue < BUCKET_MAX_VALUE) {

View on GitHub (pinned to 2add963021)

Solutions

1. Null-check before calling delete() and skip null entries.
2. Filter nulls out of the deletion candidate list before the loop.
3. Fix the upstream nullable source of Keys.

Example fix

// before
for (Key k : expiredKeys) {
  cbf.delete(k); // list contained a null
}
// throws NullPointerException: Key may not be null

// after
for (Key k : expiredKeys) {
  if (k != null) {
    cbf.delete(k);
  }
}

for (Key k : expiredKeys) {
  if (k != null) {
    cbf.delete(k);
  }
}

When it happens

Trigger: cbf.delete(null); deleting keys from a nullable lookup result; maintenance jobs sweeping collections that contain null entries.

Common situations: Expiry/removal passes over caches where some entries were never populated; the null key arriving from deserialization or a cleared map slot.

Related errors

• key can not be null
• key cannot be null
• Key is not a member
• filters cannot be and-ed
• Value length unknown.

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/d2a82f4376dcdeca. Report an issue: GitHub.