fix issue #819 and add test cases

json-path/src/main/java/com/jayway/jsonpath/internal/function/text/Length.java

@@ -49,6 +49,9 @@ public class Length implements PathFunction {
             // for length - to the wildcard such that we request all of its children so we can get back an array and
             // take its length.
             Parameter lengthOfParameter = parameters.get(0);
+            if(lengthOfParameter.getPath().isDefinite()){
+                return ctx.configuration().jsonProvider().length(lengthOfParameter.getValue());
+            }
             if (!lengthOfParameter.getPath().isFunctionPath()) {
                 Path path = lengthOfParameter.getPath();
                 if (path instanceof CompiledPath) {

json-path/src/test/java/com/jayway/jsonpath/internal/function/LengthFunctionTest.java

@@ -0,0 +1,42 @@
+package com.jayway.jsonpath.internal.function;
+
+import com.jayway.jsonpath.JsonPath;
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Test for length function. Length class: {@link com.jayway.jsonpath.internal.function.text.Length}
+ */
+public class LengthFunctionTest {
+
+    private static final String testString = "{\n" +
+            "\t\"store\": {\n" +
+            "\t\t\"book\": [[\"book0-1\",\"book0-2\"],\"book1\", \"book2\", [\"book3\"]]" +
+            "\t},\n" +
+            "    \"store1\": {\n" +
+            "\t\t\"book\": [\"book4\", [\"book5-1\",\"book5-2\"], \"book6\",[\"book7\"]]\n" +
+            "\t}\n" +
+            "}";
+
+    /**
+     * Test case for definite path
+     */
+    @Test
+    public void testDefinite(){
+        assertThat((int)JsonPath.read(testString,"$.length($.store.book)")).isEqualTo(4);
+        assertThat((int)JsonPath.read(testString,"$.length($.store.book[0])")).isEqualTo(2);
+        assertThat((int)JsonPath.read(testString,"$.length($.store1.book)")).isEqualTo(4);
+    }
+    /**
+     * Test case for indefinite path
+     */
+    @Test
+    public void testIndefinite(){
+        assertThat((int)JsonPath.read(testString,"$.length($..book)")).isEqualTo(8);
+        assertThat((int)JsonPath.read(testString,"$.length($.store..book)")).isEqualTo(4);
+        assertThat((int)JsonPath.read(testString,"$.length($.store1..book)")).isEqualTo(4);
+        assertThat((int)JsonPath.read(testString,"$.length($.store1..book[1])")).isEqualTo(2);
+    }
+
+}