Issue apache#2362 Evaluate JsonPath functions in the JSON Input transform

docs/hop-user-manual/modules/ROOT/pages/pipeline/transforms/jsoninput.adoc

@@ -180,6 +180,14 @@ You can use Regex in the Path expression.
 
 *Example Path to filter on those that start with the letter 'a':* $.data[?(@=~/a.*/i)]
 
+*JsonPath functions*
+
+The aggregation functions provided by JayWay can be used at the end of a Path expression, e.g. `length()` (or its alias `size()`), `sum()`, `avg()`, `min()`, `max()`, `stddev()`, `keys()`, `first()`, `last()` and `index()`.
+
+*Example Path to retrieve the number of elements of an array:* $.someArray.length()
+
+A function always produces a single value, and therefore a single output row. Combining a function field with a field whose Path matches several elements is not supported: the transform reports a structure error because the two Paths do not yield the same number of rows. Read such a value in a dedicated JSON Input transform when the other fields return multiple rows.
+
 
 === Additional output fields tab
 

plugins/transforms/json/src/main/java/org/apache/hop/pipeline/transforms/jsoninput/reader/FastJsonReader.java

@@ -29,6 +29,7 @@
 import java.io.InputStream;
 import java.util.AbstractList;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.EnumSet;
 import java.util.List;
 import java.util.RandomAccess;
@@ -299,7 +300,7 @@ private List<List<?>> evalCombinedResult() throws JsonInputException {
     int i = 0;
     for (JsonPath path : paths) {
       Object raw = getReadContext().read(path);
-      List<Object> result = normalizeJsonPathResult(raw);
+      List<Object> result = raw == null ? readFunctionPath(path) : normalizeJsonPathResult(raw);
       if (result.size() != lastSize && lastSize > 0 && !result.isEmpty()) {
         throw new JsonInputException(
             BaseMessages.getString(
@@ -322,6 +323,27 @@ private List<List<?>> evalCombinedResult() throws JsonInputException {
     return results;
   }
 
+  /**
+   * JsonPath never evaluates a function path (length(), sum(), keys(), ...) while
+   * ALWAYS_RETURN_LIST is set: combined with SUPPRESS_EXCEPTIONS it silently yields null. Such a
+   * path is re-evaluated here without that option, and the scalar it produces is exposed as a
+   * single row.
+   */
+  private List<Object> readFunctionPath(JsonPath path) throws JsonInputException {
+    ReadContext context = getReadContext();
+    EnumSet<Option> options = EnumSet.noneOf(Option.class);
+    options.addAll(context.configuration().getOptions());
+    options.remove(Option.ALWAYS_RETURN_LIST);
+    Configuration functionConfiguration =
+        context.configuration().setOptions(options.toArray(new Option[0]));
+    Object document = context.json();
+    Object value = path.read(document, functionConfiguration);
+    if (value instanceof List<?> || value instanceof ArrayNode) {
+      return normalizeJsonPathResult(value);
+    }
+    return Collections.singletonList(value);
+  }
+
   public static boolean isAllNull(Iterable<?> list) {
     for (Object obj : list) {
       if (obj != null) {

plugins/transforms/json/src/test/java/org/apache/hop/pipeline/transforms/jsoninput/reader/FastJsonReaderTest.java

@@ -20,15 +20,20 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.Mockito.mock;
 
+import com.fasterxml.jackson.databind.ObjectMapper;
 import com.jayway.jsonpath.Option;
+import java.io.ByteArrayInputStream;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.List;
+import org.apache.hop.core.IRowSet;
 import org.apache.hop.core.exception.HopException;
 import org.apache.hop.core.logging.ILogChannel;
 import org.apache.hop.pipeline.transforms.jsoninput.JsonInputField;
@@ -110,4 +115,57 @@ void testFastJsonReaderGetMaxRowSize() {
     mainList.add(l3);
     assertEquals(3, FastJsonReader.getMaxRowSize(Collections.singletonList(mainList)));
   }
+
+  private static final String ARRAY_JSON = "{\"someArray\":[{\"x\":1},{\"x\":2}]}";
+
+  private static IRowSet readString(String json, String path) throws HopException {
+    JsonInputField field = new JsonInputField("value");
+    field.setPath(path);
+    FastJsonReader reader =
+        new FastJsonReader(new JsonInputField[] {field}, mock(ILogChannel.class));
+    return reader.parseStringValue(new ByteArrayInputStream(json.getBytes(StandardCharsets.UTF_8)));
+  }
+
+  @Test
+  void testLengthFunctionPathReturnsNumberOfArrayElements() throws Exception {
+    IRowSet rowSet = readString(ARRAY_JSON, "$.someArray.length()");
+    Object[] row = rowSet.getRow();
+    assertNotNull(row);
+    assertEquals(2, ((Number) row[0]).intValue());
+  }
+
+  @Test
+  void testSumFunctionPathIsEvaluated() throws Exception {
+    IRowSet rowSet = readString("{\"values\":[1,2,4]}", "$.values.sum()");
+    Object[] row = rowSet.getRow();
+    assertNotNull(row);
+    assertEquals(7, ((Number) row[0]).intValue());
+  }
+
+  @Test
+  void testFunctionPathReturnsSingleRow() throws Exception {
+    IRowSet rowSet = readString(ARRAY_JSON, "$.someArray.length()");
+    assertNotNull(rowSet.getRow());
+    assertNull(rowSet.getRow());
+  }
+
+  @Test
+  void testLengthFunctionPathOnJsonNodeInput() throws Exception {
+    JsonInputField field = new JsonInputField("value");
+    field.setPath("$.someArray.length()");
+    FastJsonReader reader =
+        new FastJsonReader(new JsonInputField[] {field}, mock(ILogChannel.class));
+    IRowSet rowSet = reader.parseJsonNodeValue(new ObjectMapper().readTree(ARRAY_JSON));
+    Object[] row = rowSet.getRow();
+    assertNotNull(row);
+    assertEquals(2, ((Number) row[0]).intValue());
+  }
+
+  @Test
+  void testRegularPathStillReturnsOneRowPerMatch() throws Exception {
+    IRowSet rowSet = readString(ARRAY_JSON, "$.someArray[*].x");
+    assertNotNull(rowSet.getRow());
+    assertNotNull(rowSet.getRow());
+    assertNull(rowSet.getRow());
+  }
 }