eugenp · maibin · Jul 24, 2025 · Jul 21, 2025 · Jul 22, 2025 · Jul 22, 2025
diff --git a/...modules/jackson-core/src/main/java/com/baeldung/jackson/staticobjectmapper/JsonUtils.java b/...modules/jackson-core/src/main/java/com/baeldung/jackson/staticobjectmapper/JsonUtils.java
@@ -0,0 +1,7 @@
+package com.baeldung.jackson.staticobjectmapper;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+public class JsonUtils {
+    public static final ObjectMapper MAPPER = new ObjectMapper();
+}
diff --git a/...rc/test/java/com/baeldung/jackson/staticobjectmapper/ConflictingRequirementsUnitTest.java b/...rc/test/java/com/baeldung/jackson/staticobjectmapper/ConflictingRequirementsUnitTest.java
@@ -0,0 +1,47 @@
+package com.baeldung.jackson.staticobjectmapper;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import com.fasterxml.jackson.databind.PropertyNamingStrategy;
+import org.junit.jupiter.api.Test;
+
+import java.text.SimpleDateFormat;
+import java.time.LocalDate;
+import java.time.ZoneId;
+import java.time.ZoneOffset;
+import java.util.Collections;
+import java.util.Date;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+
+class ConflictingRequirementsUnitTest {
+
+    private static final ObjectMapper GLOBAL_MAPPER = new ObjectMapper();
+
+    @Test
+    void whenSwitchingDateFormatGlobally_thenEndpointsCollide() throws Exception {
+        SimpleDateFormat iso = new SimpleDateFormat("yyyy-MM-dd");
+        GLOBAL_MAPPER.setDateFormat(iso);
+
+        Map<String, Date> payload = Collections.singletonMap(
+                "dob",
+                Date.from(LocalDate.of(1990, 10, 5)
+                        .atTime(12, 0)
+                        .toInstant(ZoneOffset.UTC)));
+
+        String forA = GLOBAL_MAPPER.writeValueAsString(payload);
+        assertEquals("{\"dob\":\"1990-10-05\"}", forA);
+
+        SimpleDateFormat european = new SimpleDateFormat("dd/MM/yyyy");
+        GLOBAL_MAPPER.setDateFormat(european);
+
+        String forB = GLOBAL_MAPPER.writeValueAsString(payload);
+        assertEquals("{\"dob\":\"05/10/1990\"}", forB);
+
+        String nowBrokenForA = GLOBAL_MAPPER.writeValueAsString(payload);
+        assertNotEquals(forA, nowBrokenForA);
+    }
+
+}
diff --git a/...on-core/src/test/java/com/baeldung/jackson/staticobjectmapper/HiddenCouplingUnitTest.java b/...on-core/src/test/java/com/baeldung/jackson/staticobjectmapper/HiddenCouplingUnitTest.java
@@ -0,0 +1,38 @@
+package com.baeldung.jackson.staticobjectmapper;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
+import org.junit.jupiter.api.Order;
+import org.junit.jupiter.api.Test;
+
+import java.text.SimpleDateFormat;
+import java.time.LocalDate;
+import java.time.ZoneOffset;
+import java.util.Collections;
+import java.util.Date;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+
+class HiddenCouplingUnitTest {
+
+    private static final ObjectMapper GLOBAL_MAPPER = new ObjectMapper();
+
+    @Test
+    @Order(1)
+    void givenCustomDateFormat_whenConfiguredFirst_thenPasses() throws Exception {
+        GLOBAL_MAPPER.setDateFormat(new SimpleDateFormat("dd-MM-yyyy"));
+        Map<String, Date> payload = Collections.singletonMap("date", Date.from(LocalDate.of(1998, 2, 9).atTime(12, 0).toInstant(ZoneOffset.UTC)));
+        String json = GLOBAL_MAPPER.writeValueAsString(payload);
+        assertEquals("{\"date\":\"09-02-1998\"}", json);
+    }
+
+    @Test 
+    @Order(2)
+    void givenDefaultDateFormat_whenRunAfterMutation_thenFails() throws Exception {
+        Map<String, Date> payload = Collections.singletonMap("date", Date.from(LocalDate.of(1998, 2, 9).atTime(12, 0).toInstant(ZoneOffset.UTC)));
+        String json = GLOBAL_MAPPER.writeValueAsString(payload);
+        assertNotEquals("{\"date\":887025600000}", json);
+    }
+}
diff --git a/...c/test/java/com/baeldung/jackson/staticobjectmapper/ObjectMapperThreadSafetyUnitTest.java b/...c/test/java/com/baeldung/jackson/staticobjectmapper/ObjectMapperThreadSafetyUnitTest.java
@@ -0,0 +1,79 @@
+package com.baeldung.jackson.staticobjectmapper;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.SerializationFeature;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.text.SimpleDateFormat;
+import java.time.LocalDate;
+import java.time.ZoneOffset;
+import java.util.Date;
+import java.util.Map;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
+import static java.util.Collections.singletonMap;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+
+class ObjectMapperThreadSafetyUnitTest {
+
+    private ObjectMapper GLOBAL_MAPPER = new ObjectMapper();
+
+    /**
+     * two real threads, created once and reused for every repetition
+     */
+    private static final ExecutorService POOL = Executors.newFixedThreadPool(2);
+
+    @Test
+    void whenRegisteringDateFormatGlobally_thenAffectsAllConsumers() throws Exception {
+        Map<String, Date> payload = singletonMap("today", Date.from(LocalDate.of(1998, 2, 9).atTime(12, 0).toInstant(ZoneOffset.UTC)));
+
+        String before = GLOBAL_MAPPER.writeValueAsString(payload);
+        assertEquals("{\"today\":887025600000}", before);
+
+        GLOBAL_MAPPER.setDateFormat(new SimpleDateFormat("yyyy-MM-dd"));
+
+        String after = GLOBAL_MAPPER.writeValueAsString(payload);
+        assertEquals("{\"today\":\"1998-02-09\"}", after);
+    }
+
+    @Test
+    void whenSimpleDateFormatChanges_thenConflictHappens() throws Exception {
+        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
+        GLOBAL_MAPPER.setDateFormat(format);
+
+        Callable<String> task = () -> GLOBAL_MAPPER.writeValueAsString(singletonMap("key", Date.from(LocalDate.of(1998, 2, 9).atTime(12, 0).toInstant(ZoneOffset.UTC))));
+        Callable<Void> mutator = () -> {
+            format.applyPattern("dd-MM-yyyy");
+            return null;
+        };
+
+        Future<String> taskResult1 = POOL.submit(task);
+        assertEquals("{\"key\":\"1998-02-09\"}", taskResult1.get());
+        POOL.submit(mutator).get();
+        Future<String> taskResult2 = POOL.submit(task);
+        assertEquals("{\"key\":\"09-02-1998\"}", taskResult2.get());
+    }
+
+    @Test
+    void whenUsingCopyScopedMapper_thenNoInterference() throws Exception {
+        ObjectMapper localCopy = GLOBAL_MAPPER.copy().enable(SerializationFeature.INDENT_OUTPUT);
+        assertEquals("{\n  \"key\" : \"value\"\n}", localCopy.writeValueAsString(singletonMap("key", "value")));
+        assertEquals("{\"key\":\"value\"}", GLOBAL_MAPPER.writeValueAsString(singletonMap("key", "value")));
+    }
+
+    @BeforeEach
+    void setup() {
+        GLOBAL_MAPPER = new ObjectMapper();
+    }
+
+    @AfterAll
+    static void shutdownPool() {
+        POOL.shutdownNow();
+    }
+}