Add jackson-core dependency and refactor JSON handling

The jackson-core dependency was added in `pom.xml` to enhance JSON handling in the application. The `DeviceMeasurement` class was refactored to handle null fields when generating JSON output. Helper methods were introduced for managing `externalDataReference`, `dateCreated`, and `location` fields. This change improves output cleanliness and standardizes data for users and systems.

pom.xml

@@ -73,6 +73,11 @@
             <artifactId>slf4j-nop</artifactId>
             <version>2.0.6</version>
         </dependency>
+        <dependency>
+            <groupId>com.fasterxml.jackson.core</groupId>
+            <artifactId>jackson-core</artifactId>
+            <version>2.16.1</version>
+        </dependency>
         <dependency>
             <groupId>org.junit.jupiter</groupId>
             <artifactId>junit-jupiter-engine</artifactId>

src/main/java/de/app/fivegla/fiware/model/DeviceMeasurement.java

@@ -98,27 +98,46 @@ public String asJson() {
                 "  \"name\":\"" + name + "\"," +
                 "  \"type\":\"" + type + "\"," +
                 "  \"value\":" + value + "," +
-                "  \"externalDataLink\":" + (StringUtils.isNotBlank(externalDataReference) ? externalDataReference : " ") + "\"," +
+                "  \"externalDataLink\":" + externalDataReferenceAsJson() + "," +
                 "  \"dateCreated\":" + dateCreatedAsJson() + "," +
                 "  \"location\":" + locationAsJson() +
                 "}";
     }
 
+    private String externalDataReferenceAsJson() {
+        if (StringUtils.isBlank(externalDataReference)) {
+            return "{}";
+        } else {
+            return "{" +
+                    "  \"type\":\"" + FiwareType.TEXT.getKey() + "\"," +
+                    "  \"value\":\"" + externalDataReference + "\"" +
+                    "}";
+        }
+    }
+
     private String dateCreatedAsJson() {
-        return "{" +
-                "  \"type\":\"" + FiwareType.DATE_TIME.getKey() + "\"," +
-                "  \"value\":\"" + formatter.format(dateCreated) + "\"" +
-                "}";
+        if (null == dateCreated) {
+            return "{}";
+        } else {
+            return "{" +
+                    "  \"type\":\"" + FiwareType.DATE_TIME.getKey() + "\"," +
+                    "  \"value\":\"" + formatter.format(dateCreated) + "\"" +
+                    "}";
+        }
     }
 
     private String locationAsJson() {
-        return "{" +
-                "  \"type\":\"" + FiwareType.GEO_POINT.getKey() + "\"," +
-                "  \"value\": {" +
-                "    \"type\":\"Point\"," +
-                "    \"coordinates\": [" + longitude + "," + latitude + "]" +
-                "  }" +
-                "}";
+        if (latitude == 0.0 && longitude == 0.0) {
+            return "{}";
+        } else {
+            return "{" +
+                    "  \"type\":\"" + FiwareType.GEO_POINT.getKey() + "\"," +
+                    "  \"value\": {" +
+                    "    \"type\":\"Point\"," +
+                    "    \"coordinates\": [" + longitude + "," + latitude + "]" +
+                    "  }" +
+                    "}";
+        }
     }
 
     @Override

src/test/java/de/app/fivegla/fiware/api/DeviceMeasurementValidJsonTest.java

@@ -0,0 +1,43 @@
+package de.app.fivegla.fiware.api;
+
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import de.app.fivegla.fiware.model.builder.DeviceMeasurementBuilder;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.time.Instant;
+import java.util.UUID;
+
+class DevxiceMeasurementValidJsonTest {
+
+    @Test
+    void givenDeviceMeasurement_WhenCreatingJson_ThenTheResultShouldBeValidJson() throws Exception {
+        var deviceMeasurementAsJson = new DeviceMeasurementBuilder()
+                .withId(UUID.randomUUID().toString())
+                .withType(FiwareType.TEXT.name())
+                .withMeasurement("value", FiwareType.TEXT, 2.0, Instant.now(), 4.0, 8.0)
+                .build()
+                .asJson();
+        var objectMapper = new ObjectMapper();
+        var jsonNode = objectMapper.readTree(deviceMeasurementAsJson);
+        Assertions.assertEquals(FiwareType.TEXT.getKey(), jsonNode.get("type").asText());
+        Assertions.assertEquals(2.0, jsonNode.get("value").asDouble());
+    }
+
+    @Test
+    void givenDeviceMeasurementWithExternalDataReference_WhenCreatingJson_ThenTheResultShouldBeValidJson() throws Exception {
+        var deviceMeasurementAsJson = new DeviceMeasurementBuilder()
+                .withId(UUID.randomUUID().toString())
+                .withType(FiwareType.TEXT.name())
+                .withMeasurement("value", FiwareType.TEXT, 2.0, Instant.now(), 4.0, 8.0)
+                .withExternalDataReference("http://example.com")
+                .build()
+                .asJson();
+        var objectMapper = new ObjectMapper();
+        var jsonNode = objectMapper.readTree(deviceMeasurementAsJson);
+        Assertions.assertEquals(FiwareType.TEXT.getKey(), jsonNode.get("type").asText());
+        Assertions.assertEquals(2.0, jsonNode.get("value").asDouble());
+    }
+
+}