1446 : Extraction of json block before parse (langchain4j#1516)

Extracts json block before parse and added support for injecting custom
ServiceOutputParser to AiService.

## Issue
Closes langchain4j#1446

## Change
Added method in DefaultServiceOutputParser to find start and end index
of json blocks and substrings result before parse.

## General checklist
<!-- Please double-check the following points and mark them like this:
[X] -->
- [X] There are no breaking changes
- [X] 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/src/main/java/dev/langchain4j/service/DefaultAiServices.java

@@ -240,8 +240,7 @@ public Object invoke(Object proxy, Method method, Object[] args) throws Exceptio
 
                         response = Response.from(response.content(), tokenUsageAccumulator, response.finishReason());
 
-                        Object parsedResponse;
-                        parsedResponse = serviceOutputParser.parse(response, returnType);
+                        Object parsedResponse = serviceOutputParser.parse(response, returnType);
                         if (typeHasRawClass(returnType, Result.class)) {
                             return Result.builder()
                                     .content(parsedResponse)

langchain4j/src/main/java/dev/langchain4j/service/output/ServiceOutputParser.java

@@ -10,6 +10,8 @@
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.*;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 import static dev.langchain4j.exception.IllegalConfigurationException.illegalConfiguration;
 import static dev.langchain4j.internal.ValidationUtils.ensureNotNull;
@@ -18,6 +20,18 @@
 
 public class ServiceOutputParser {
 
+    /**
+     * JSON Pattern:<br />
+     * 
+     * <i>\\{(?:[^{}]|\\{(?:[^{}]|\\{(?:[^{}]|\\{[^{}]*\\})*\\})*\\})*\\}</i>: Matches JSON objects, accounting for nested objects.
+     */
+    private static final String JSON_PATTERN_REGEX = "\\{(?:[^{}]|\\{(?:[^{}]|\\{(?:[^{}]|\\{[^{}]*\\})*\\})*\\})*\\}";
+
+    /**
+     * Pattern.DOTALL: This flag makes the dot . match all characters, including newline characters.
+     */
+    private static final Pattern JSON_BLOCK_PATTERN = Pattern.compile(JSON_PATTERN_REGEX, Pattern.DOTALL);
+
     private final OutputParserFactory outputParserFactory;
 
     public ServiceOutputParser() {
@@ -60,7 +74,8 @@ public Object parse(Response<AiMessage> response, Type returnType) {
             return optionalOutputParser.get().parse(text);
         }
 
-        return Json.fromJson(text, rawReturnClass);
+        String extractedJsonBlock = extractJsonBlock(text);
+        return Json.fromJson(extractedJsonBlock, rawReturnClass);
     }
 
     public String outputFormatInstructions(Type returnType) {
@@ -106,7 +121,7 @@ public String outputFormatInstructions(Type returnType) {
         return "\nYou must answer strictly in the following JSON format: " + jsonStructure((rawClass), new HashSet<>());
     }
 
-    public static String jsonStructure(Class<?> structured, Set<Class<?>> visited) {
+    private static String jsonStructure(Class<?> structured, Set<Class<?>> visited) {
         StringBuilder jsonSchema = new StringBuilder();
 
         jsonSchema.append("{\n");
@@ -195,4 +210,14 @@ private static String simpleTypeName(Type type) {
                 return type.getTypeName();
         }
     }
+
+    private String extractJsonBlock(String text) {
+        Matcher matcher = JSON_BLOCK_PATTERN.matcher(text);
+
+        if (matcher.find()) {
+            return matcher.group();
+        }
+
+        return text;
+    }
 }

langchain4j/src/test/java/dev/langchain4j/service/output/ServiceOutputParserTest.java

@@ -1,10 +1,13 @@
 package dev.langchain4j.service.output;
 
+import com.google.gson.JsonSyntaxException;
 import com.google.gson.reflect.TypeToken;
 import dev.langchain4j.data.message.AiMessage;
 import dev.langchain4j.model.output.Response;
 import dev.langchain4j.model.output.structured.Description;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import org.mockito.stubbing.Answer;
 
 import java.io.Serializable;
@@ -18,6 +21,7 @@
 import java.util.concurrent.atomic.AtomicReference;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.junit.jupiter.api.Assertions.assertInstanceOf;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.doAnswer;
@@ -91,6 +95,79 @@ private void testWhetherProperOutputParserWasCalled(AiMessage aiMessage, Type ra
         assertInstanceOf(expectedOutputParserType, capturedOutputParser);
     }
 
+    /********************************************************************************************
+     * Json output parse tests
+     ********************************************************************************************/
+
+    @ParameterizedTest
+    @ValueSource(strings = {
+        "{\"key\":\"value\"}",
+        "```\n{\"key\":\"value\"}\n```",
+        "```json\n{\"key\":\"value\"}\n```",
+        "Sure, here is your JSON:\n```\n{\"key\":\"value\"}\n```\nLet me know if you need more help."
+    })
+    void makeSureJsonBlockIsExtractedBeforeParse(String json) {
+        // Given
+        AiMessage aiMessage = AiMessage.aiMessage(json);
+        Response<AiMessage> responseStub = Response.from(aiMessage);
+        sut = new ServiceOutputParser();
+
+        // When
+        Object result = sut.parse(responseStub, KeyProperty.class);
+
+        // Then
+        assertInstanceOf(KeyProperty.class, result);
+
+        KeyProperty keyProperty = (KeyProperty) result;
+        assertThat(keyProperty.key).isEqualTo("value");
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {
+        "{\"keyProperty\" : {\"key\" : \"value\"}}",
+        "```\n{\"keyProperty\" :\n {\"key\" : \"value\"}\n}\n```",
+        "```json\n{\"keyProperty\" :\n {\"key\" : \"value\"}\n}\n```",
+        "Sure, here is your JSON:\n```\n{\"keyProperty\" :\n {\"key\" : \"value\"}\n}\n```\nLet me know if you need more help."
+    })
+    void makeSureNestedJsonBlockIsExtractedBeforeParse(String json) {
+        // Given
+        AiMessage aiMessage = AiMessage.aiMessage(json);
+        Response<AiMessage> responseStub = Response.from(aiMessage);
+        sut = new ServiceOutputParser();
+
+        // When
+        Object result = sut.parse(responseStub, KeyPropertyWrapper.class);
+
+        // Then
+        assertInstanceOf(KeyPropertyWrapper.class, result);
+
+        KeyPropertyWrapper keyProperty = (KeyPropertyWrapper) result;
+        assertThat(keyProperty.keyProperty.key).isEqualTo("value");
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {
+        "\"key\":\"value\"}",
+        "{\"key\":\"value\""
+    })
+    void illegalJsonBlockNotExtractedAndFailsParse(String json) {
+        // Given
+        AiMessage aiMessage = AiMessage.aiMessage(json);
+        Response<AiMessage> responseStub = Response.from(aiMessage);
+        sut = new ServiceOutputParser();
+
+        // When / Then
+        assertThatExceptionOfType(JsonSyntaxException.class).isThrownBy(() -> sut.parse(responseStub, KeyProperty.class));
+    }
+
+    static class KeyPropertyWrapper {
+        KeyProperty keyProperty;
+    }
+
+    static class KeyProperty {
+        String key;
+    }
+
     /********************************************************************************************
      * Output format instructions tests
      ********************************************************************************************/