Support null filters like [?(@foo == null)] and [?(@foo != null)]

json-path/src/main/java/com/jayway/jsonpath/internal/filter/eval/ExpressionEvaluator.java

@@ -25,9 +25,16 @@ import java.util.Map;
  */
 public class ExpressionEvaluator {
 
+    private static final String NULL_VALUE = "null";
 
     public enum Operator {
-        equal("=="), not_equal("!="), less_or_greater_than("<>"), greater_than(">"), greater_than_or_equal(">="), less_than("<"), less_than_or_equal("<=");
+        equal("=="),
+        not_equal("!="),
+        less_or_greater_than("<>"), //same as not_equal
+        greater_than(">"),
+        greater_than_or_equal(">="),
+        less_than("<"),
+        less_than_or_equal("<=");
 
         private final String representation;
 
@@ -58,6 +65,21 @@ public class ExpressionEvaluator {
             throw new IllegalArgumentException("Unsupported operator " + comparator);
         }
 
+        if(actual == null){
+            if(operator == Operator.equal){
+                return NULL_VALUE.equals(expected);
+            } else if(operator == Operator.not_equal || operator == Operator.less_or_greater_than){
+                return !NULL_VALUE.equals(expected);
+            }
+        } else {
+            if(operator == Operator.not_equal || operator == Operator.less_or_greater_than){
+                if(NULL_VALUE.equals(expected)){
+                    return true;
+                }
+            }
+        }
+
+
         if (actual instanceof Long) {
 
             Long a = (Long) actual;
@@ -173,7 +195,7 @@ public class ExpressionEvaluator {
             BigInteger a = (BigInteger) actual;
             BigInteger e = new BigInteger(expected.trim());
 
-            switch (operator){
+            switch (operator) {
                 case equal:
                     return a.compareTo(e) == 0;
                 case not_equal:
@@ -196,7 +218,7 @@ public class ExpressionEvaluator {
             BigDecimal a = (BigDecimal) actual;
             BigDecimal e = new BigDecimal(expected);
 
-            switch (operator){
+            switch (operator) {
                 case equal:
                     return a.compareTo(e) == 0;
                 case not_equal:

json-path/src/test/java/com/jayway/jsonpath/ExpressionEvalTest.java

@@ -6,7 +6,10 @@ import org.junit.Test;
 
 import java.math.BigDecimal;
 import java.math.BigInteger;
+import java.util.List;
+import java.util.Map;
 
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
@@ -18,6 +21,51 @@ import static org.junit.Assert.assertTrue;
  */
 public class ExpressionEvalTest {
 
+    public static final String DOCUMENT= "{\n" +
+            "    \"characters\": [\n" +
+            "        {\n" +
+            "            \"aliases\": [], \n" +
+            "            \"name\": \"Kleeg Lars\", \n" +
+            "            \"occupation\": \"Moisture farmer\", \n" +
+            "            \"offspring\": []\n" +
+            "        }, \n" +
+            "        {\n" +
+            "            \"aliases\": [], \n" +
+            "            \"name\": \"Shmi Skywalker\", \n" +
+            "            \"occupation\": \"Virgin mother\", \n" +
+            "            \"offspring\": [\n" +
+            "                \"AnakinSkywalker\"\n" +
+            "            ]\n" +
+            "        }, \n" +
+            "        {\n" +
+            "            \"aliases\": [\n" +
+            "                \"Darth Vader\"\n" +
+            "            ], \n" +
+            "            \"name\": \"Annakin Skywalker\", \n" +
+            "            \"occupation\": \"Hand of the Emperor, Lord of the Sith\", \n" +
+            "            \"offspring\": [\n" +
+            "                \"Luke Skywalker\", \n" +
+            "                \"LeiaOrgana\"\n" +
+            "            ]\n" +
+            "        }, \n" +
+            "        {\n" +
+            "            \"aliases\": [\n" +
+            "                \"Nerf herder\"\n" +
+            "            ], \n" +
+            "            \"name\": \"Luke Skywalker\", \n" +
+            "            \"occupation\": \"Farm boy\", \n" +
+            "            \"offspring\": null\n" +
+            "        }, \n" +
+            "        {\n" +
+            "            \"aliases\": [\n" +
+            "                \"Your Highness\"\n" +
+            "            ], \n" +
+            "            \"name\": \"Leia Organa\", \n" +
+            "            \"occupation\": \"Senator\", \n" +
+            "        }\n" +
+            "    ]\n" +
+            "}\n";
+
     @Test
     public void long_eval() throws Exception {
 
@@ -117,8 +165,42 @@ public class ExpressionEvalTest {
         assertFalse(ExpressionEvaluator.eval(false, "==", "true"));
         assertFalse(ExpressionEvaluator.eval(true, "!=", "true"));
         assertFalse(ExpressionEvaluator.eval(true, "<>", "true"));
-        assertFalse(ExpressionEvaluator.eval(false, "!=", "false"));
         assertFalse(ExpressionEvaluator.eval(false, "<>", "false"));
+        assertFalse(ExpressionEvaluator.eval(false, "!=", "false"));
+
+    }
+
+    @Test
+    public void null_eval() throws Exception {
+        assertTrue(ExpressionEvaluator.eval(new Integer(10), "!=", "null"));
+
+        assertTrue(ExpressionEvaluator.eval(null, "==", "null"));
+
+        assertTrue(ExpressionEvaluator.eval(null, "<>", "FOO"));
+        assertTrue(ExpressionEvaluator.eval("FOO", "<>", "null"));
+
+
+        assertTrue(ExpressionEvaluator.eval(null, "!=", "FOO"));
+        assertTrue(ExpressionEvaluator.eval("FOO", "<>", "null"));
+
+        assertTrue(ExpressionEvaluator.eval(null, "!=", "10"));
+    }
+
+
+    @Test
+    public void nulls_filter() {
+
+        List<Map<String, Object>> result = JsonPath.read(DOCUMENT, "$.characters[?(@.offspring == null)]");
+        assertEquals(1, result.size());
+
+        result = JsonPath.read(DOCUMENT, "$.characters[?(@.offspring != null)]");
+        assertEquals(3, result.size());
+
+        result = JsonPath.read(DOCUMENT, "$.characters[?(@.offspring)]");
+        assertEquals(4, result.size());
+
+
+
 
     }