Migrate Ollama from gson to jackson (langchain4j#1697)

## Issue
Closes langchain4j#1688 

## Change
Migrate Ollama from gson to jackson

## General checklist
<!-- Please double-check the following points and mark them like this:
[X] -->
- [x] There are no breaking changes
- [ ] I have added unit and integration tests for my change
- [x] I have manually run all the unit and integration tests in the
module I have added/changed, and they are all green
- [x] I have manually run all the unit and integration tests in the
[core](https://github.com/langchain4j/langchain4j/tree/main/langchain4j-core)
and
[main](https://github.com/langchain4j/langchain4j/tree/main/langchain4j)
modules, and they are all green
<!-- Before adding documentation and example(s) (below), please wait
until the PR is reviewed and approved. -->
- [ ] I have added/updated the
[documentation](https://github.com/langchain4j/langchain4j/tree/main/docs/docs)
- [ ] I have added an example in the [examples
repo](https://github.com/langchain4j/langchain4j-examples) (only for
"big" features)
- [ ] I have added/updated [Spring Boot
starter(s)](https://github.com/langchain4j/langchain4j-spring) (if
applicable)

langchain4j-ollama/src/main/java/dev/langchain4j/model/ollama/OllamaClient.java

@@ -1,6 +1,5 @@
 package dev.langchain4j.model.ollama;
 
-import com.fasterxml.jackson.databind.ObjectMapper;
 import dev.langchain4j.data.message.AiMessage;
 import dev.langchain4j.internal.Utils;
 import dev.langchain4j.model.StreamingResponseHandler;
@@ -27,15 +26,13 @@
 import java.util.Map;
 import java.util.Optional;
 
-import static com.fasterxml.jackson.databind.SerializationFeature.INDENT_OUTPUT;
+import static dev.langchain4j.model.ollama.OllamaJsonUtils.getObjectMapper;
+import static dev.langchain4j.model.ollama.OllamaJsonUtils.toObject;
 import static java.lang.Boolean.TRUE;
 
 @Slf4j
 class OllamaClient {
 
-    private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper()
-            .enable(INDENT_OUTPUT);
-
     private final OllamaApi ollamaApi;
     private final boolean logStreamingResponses;
 
@@ -66,7 +63,7 @@ public OllamaClient(String baseUrl,
         Retrofit retrofit = new Retrofit.Builder()
                 .baseUrl(Utils.ensureTrailingForwardSlash(baseUrl))
                 .client(okHttpClient)
-                .addConverterFactory(JacksonConverterFactory.create(OBJECT_MAPPER))
+                .addConverterFactory(JacksonConverterFactory.create(getObjectMapper()))
                 .build();
 
         ollamaApi = retrofit.create(OllamaApi.class);
@@ -118,7 +115,7 @@ public void onResponse(Call<ResponseBody> call, retrofit2.Response<ResponseBody>
                             log.debug("Streaming partial response: {}", partialResponse);
                         }
 
-                        CompletionResponse completionResponse = OBJECT_MAPPER.readValue(partialResponse, CompletionResponse.class);
+                        CompletionResponse completionResponse = toObject(partialResponse, CompletionResponse.class);
                         contentBuilder.append(completionResponse.getResponse());
                         handler.onNext(completionResponse.getResponse());
 
@@ -161,7 +158,7 @@ public void onResponse(Call<ResponseBody> call, retrofit2.Response<ResponseBody>
                                 log.debug("Streaming partial response: {}", partialResponse);
                             }
 
-                            ChatResponse chatResponse = OBJECT_MAPPER.readValue(partialResponse, ChatResponse.class);
+                            ChatResponse chatResponse = toObject(partialResponse, ChatResponse.class);
                             String content = chatResponse.getMessage().getContent();
                             contentBuilder.append(content);
                             handler.onNext(content);

langchain4j-ollama/src/main/java/dev/langchain4j/model/ollama/OllamaJsonUtils.java

@@ -0,0 +1,46 @@
+package dev.langchain4j.model.ollama;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+import static com.fasterxml.jackson.databind.SerializationFeature.INDENT_OUTPUT;
+
+class OllamaJsonUtils {
+
+    private OllamaJsonUtils() throws InstantiationException {
+        throw new InstantiationException("Can't instantiate this utility class.");
+    }
+
+    private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper()
+            .enable(INDENT_OUTPUT);
+
+    static String toJson(Object object) {
+        try {
+            return OBJECT_MAPPER.writeValueAsString(object);
+        } catch (JsonProcessingException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    static <T> T toObject(String jsonStr, Class<T> clazz) {
+        try {
+            return OBJECT_MAPPER.readValue(jsonStr, clazz);
+        } catch (JsonProcessingException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    static <T> T toObject(String jsonStr, TypeReference<T> typeReference) {
+        try {
+            return OBJECT_MAPPER.readValue(jsonStr, typeReference);
+        } catch (JsonProcessingException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    static ObjectMapper getObjectMapper() {
+        return OBJECT_MAPPER;
+    }
+
+}

langchain4j-ollama/src/main/java/dev/langchain4j/model/ollama/OllamaMessagesUtils.java

@@ -3,7 +3,6 @@
 import dev.langchain4j.agent.tool.ToolExecutionRequest;
 import dev.langchain4j.agent.tool.ToolSpecification;
 import dev.langchain4j.data.message.*;
-import dev.langchain4j.internal.Json;
 
 import java.util.List;
 import java.util.Map;
@@ -12,6 +11,7 @@
 
 import static dev.langchain4j.data.message.ContentType.IMAGE;
 import static dev.langchain4j.data.message.ContentType.TEXT;
+import static dev.langchain4j.model.ollama.OllamaJsonUtils.toJson;
 
 class OllamaMessagesUtils {
 
@@ -48,7 +48,7 @@ static List<ToolExecutionRequest> toToolExecutionRequest(List<ToolCall> toolCall
         return toolCalls.stream().map(toolCall ->
                         ToolExecutionRequest.builder()
                                 .name(toolCall.getFunction().getName())
-                                .arguments(Json.toJson(toolCall.getFunction().getArguments()))
+                                .arguments(toJson(toolCall.getFunction().getArguments()))
                                 .build())
                 .collect(Collectors.toList());
     }

langchain4j-ollama/src/test/java/dev/langchain4j/model/ollama/OllamaApiIT.java

@@ -1,29 +1,25 @@
 package dev.langchain4j.model.ollama;
 
-import com.google.gson.Gson;
-import com.google.gson.GsonBuilder;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
 import okhttp3.mockwebserver.Dispatcher;
 import okhttp3.mockwebserver.MockResponse;
 import okhttp3.mockwebserver.MockWebServer;
 import okhttp3.mockwebserver.RecordedRequest;
-import org.jetbrains.annotations.NotNull;
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 
 import java.io.IOException;
 import java.time.Duration;
 
-import static com.google.gson.FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES;
 import static org.assertj.core.api.Assertions.assertThat;
 
 public class OllamaApiIT {
 
     private static MockWebServer mockWebServer;
 
-    private static final Gson GSON = new GsonBuilder()
-            .setFieldNamingPolicy(LOWER_CASE_WITH_UNDERSCORES)
-            .create();
+    private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
 
     @BeforeAll
     public static void init() throws IOException {
@@ -40,9 +36,16 @@ public MockResponse dispatch(RecordedRequest recordedRequest) {
                         .message(message)
                         .build();
 
+                String jsonBody;
+                try {
+                    jsonBody = OBJECT_MAPPER.writeValueAsString(chatResponse);
+                } catch (JsonProcessingException e) {
+                    throw new RuntimeException(e);
+                }
+
                 return new MockResponse()
                         .setResponseCode(200)
-                        .setBody(GSON.toJson(chatResponse));
+                        .setBody(jsonBody);
             }
         };
         mockWebServer.setDispatcher(dispatcher);