Enable Spotbugs NP_NONNULL_RETURN_VIOLATION

codestyle/spotbugs-exclude.xml

@@ -60,7 +60,6 @@
     <Bug pattern="NP_LOAD_OF_KNOWN_NULL_VALUE"/>
     <Bug pattern="NP_METHOD_PARAMETER_TIGHTENS_ANNOTATION"/>
     <Bug pattern="NP_NONNULL_FIELD_NOT_INITIALIZED_IN_CONSTRUCTOR"/>
-    <Bug pattern="NP_NONNULL_RETURN_VIOLATION"/>
     <Bug pattern="NP_NULL_ON_SOME_PATH"/>
     <Bug pattern="NP_NULL_ON_SOME_PATH_FROM_RETURN_VALUE"/>
     <Bug pattern="NP_NULL_ON_SOME_PATH_MIGHT_BE_INFEASIBLE"/>

extensions-contrib/time-min-max/src/main/java/org/apache/druid/query/aggregation/TimestampAggregatorFactory.java

@@ -249,6 +249,7 @@ public int hashCode()
     return result;
   }
 
+  @Nullable
   static Long convertLong(TimestampSpec timestampSpec, Object input)
   {
     if (input instanceof Number) {

processing/src/main/java/org/apache/druid/query/groupby/GroupByQueryEngine.java

@@ -269,6 +269,7 @@ public PositionMaintainer(
       this.max = max - increment; // Make sure there is enough room for one more increment
     }
 
+    @Nullable
     public Integer getNext()
     {
       if (nextVal > max) {

processing/src/main/java/org/apache/druid/segment/AbstractSegment.java

@@ -19,9 +19,12 @@
 
 package org.apache.druid.segment;
 
+import javax.annotation.Nullable;
+
 public abstract class AbstractSegment implements Segment
 {
   @Override
+  @Nullable
   public <T> T as(Class<T> clazz)
   {
     if (clazz.equals(QueryableIndex.class)) {

processing/src/main/java/org/apache/druid/segment/ColumnSelectorBitmapIndexSelector.java

@@ -182,6 +182,7 @@ public BitmapFactory getBitmapFactory()
   }
 
   @Override
+  @Nullable
   public BitmapIndex getBitmapIndex(String dimension)
   {
     if (isVirtualColumn(dimension)) {
@@ -204,6 +205,7 @@ public int getCardinality()
         }
 
         @Override
+        @Nullable
         public String getValue(int index)
         {
           return null;

processing/src/main/java/org/apache/druid/segment/DimensionHandler.java

@@ -25,6 +25,8 @@
 import org.apache.druid.segment.selector.settable.SettableColumnValueSelector;
 import org.apache.druid.segment.writeout.SegmentWriteOutMedium;
 
+import javax.annotation.Nullable;
+
 import java.util.Comparator;
 
 /**
@@ -70,6 +72,7 @@ public interface DimensionHandler
    * Get {@link MultiValueHandling} for the column associated with this handler.
    * Only string columns can have {@link MultiValueHandling} currently.
    */
+  @Nullable
   default MultiValueHandling getMultivalueHandling()
   {
     return null;

processing/src/main/java/org/apache/druid/segment/IndexMerger.java

@@ -88,6 +88,7 @@ static List<String> getMergedDimensions(List<IndexableAdapter> indexes)
     }
   }
 
+  @Nullable
   static List<String> getLongestSharedDimOrder(List<IndexableAdapter> indexes)
   {
     int maxSize = 0;

processing/src/main/java/org/apache/druid/segment/MMappedIndex.java

@@ -27,6 +27,8 @@
 import org.apache.druid.segment.data.VSizeColumnarMultiInts;
 import org.joda.time.Interval;
 
+import javax.annotation.Nullable;
+
 import java.util.Map;
 
 /**
@@ -85,15 +87,10 @@ public Interval getDataInterval()
     return dataInterval;
   }
 
+  @Nullable
   public MetricHolder getMetricHolder(String metric)
   {
-    final MetricHolder retVal = metrics.get(metric);
-
-    if (retVal == null) {
-      return null;
-    }
-
-    return retVal;
+    return metrics.get(metric);
   }
 
   public GenericIndexed<String> getDimValueLookup(String dimension)

processing/src/main/java/org/apache/druid/segment/ReferenceCountingSegment.java

@@ -26,6 +26,8 @@
 import org.apache.druid.timeline.partition.ShardSpec;
 import org.joda.time.Interval;
 
+import javax.annotation.Nullable;
+
 import java.io.Closeable;
 import java.util.concurrent.Phaser;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -111,6 +113,7 @@ private ReferenceCountingSegment(
     this.atomicUpdateGroupSize = atomicUpdateGroupSize;
   }
 
+  @Nullable
   public Segment getBaseSegment()
   {
     return !isClosed() ? baseSegment : null;
@@ -127,24 +130,28 @@ public boolean isClosed()
   }
 
   @Override
+  @Nullable
   public SegmentId getId()
   {
     return !isClosed() ? baseSegment.getId() : null;
   }
 
   @Override
+  @Nullable
   public Interval getDataInterval()
   {
     return !isClosed() ? baseSegment.getDataInterval() : null;
   }
 
   @Override
+  @Nullable
   public QueryableIndex asQueryableIndex()
   {
     return !isClosed() ? baseSegment.asQueryableIndex() : null;
   }
 
   @Override
+  @Nullable
   public StorageAdapter asStorageAdapter()
   {
     return !isClosed() ? baseSegment.asStorageAdapter() : null;

processing/src/main/java/org/apache/druid/segment/Segment.java

@@ -57,5 +57,6 @@ public interface Segment extends Closeable
    * @param <T> desired interface
    * @return instance of clazz, or null if the interface is not supported by this segment
    */
+  @Nullable
   <T> T as(Class<T> clazz);
 }

processing/src/main/java/org/apache/druid/segment/StringDimensionIndexer.java

@@ -101,6 +101,7 @@ public int getId(@Nullable String value)
       }
     }
 
+    @Nullable
     public String getValue(int id)
     {
       lock.readLock().lock();

processing/src/main/java/org/apache/druid/segment/VirtualColumns.java

@@ -173,6 +173,7 @@ public DimensionSelector makeDimensionSelector(DimensionSpec dimensionSpec, Colu
     }
   }
 
+  @Nullable
   public BitmapIndex getBitmapIndex(String columnName, ColumnSelector columnSelector)
   {
     final VirtualColumn virtualColumn = getVirtualColumn(columnName);
@@ -273,7 +274,7 @@ public byte[] getCacheKey()
     return new CacheKeyBuilder((byte) 0).appendCacheablesIgnoringOrder(virtualColumns).build();
   }
 
-  private void detectCycles(VirtualColumn virtualColumn, Set<String> columnNames)
+  private void detectCycles(VirtualColumn virtualColumn, @Nullable Set<String> columnNames)
   {
     // Copy columnNames to avoid modifying it
     final Set<String> nextSet = columnNames == null

processing/src/main/java/org/apache/druid/segment/incremental/IncrementalIndex.java

@@ -726,6 +726,7 @@ IncrementalIndexRowResult toIncrementalIndexRow(InputRow row)
     return new IncrementalIndexRowResult(incrementalIndexRow, parseExceptionMessages);
   }
 
+  @Nullable
   public static ParseException getCombinedParseException(
       InputRow row,
       @Nullable List<String> dimParseExceptionMessages,

processing/src/main/java/org/apache/druid/segment/incremental/SpatialDimensionRowTransformer.java

@@ -33,6 +33,8 @@
 import org.apache.druid.java.util.common.ISE;
 import org.joda.time.DateTime;
 
+import javax.annotation.Nullable;
+
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
@@ -208,6 +210,7 @@ private boolean isJoinedSpatialDimValValid(String dimVal)
     return true;
   }
 
+  @Nullable
   private static Float tryParseFloat(String val)
   {
     try {
@@ -225,6 +228,7 @@ private static Float tryParseFloat(String val)
    *
    * @return decoded coordinate, or null if it could not be decoded
    */
+  @Nullable
   public static float[] decode(final String encodedCoordinate)
   {
     if (encodedCoordinate == null) {

processing/src/main/java/org/apache/druid/segment/serde/ComplexMetricSerde.java

@@ -70,6 +70,7 @@ public abstract class ComplexMetricSerde
    *
    * @return A function that can compute the size of the complex object or null if you cannot/do not want to compute it
    */
+  @Nullable
   public Function<Object, Long> inputSizeFn()
   {
     return null;