Add langchain4j-interop: Spring AI 2.0 and LangChain4j 1.22 side by side

Same order assistant in both frameworks against a local fake OpenAI
server: starter bean collision, wire differences, tool failure and loop
bounds, observability meters, MCP-first tools, and a compile check of
the 2025 article's snippets and Maven coordinates.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01G8ikz8xdWuTP5yun8DZ1hk

langchain4j-interop/src/broken/OldCallAroundAdvisor.java

@@ -0,0 +1,18 @@
+import org.springframework.ai.chat.client.advisor.api.AdvisedRequest;
+import org.springframework.ai.chat.client.advisor.api.AdvisedResponse;
+import org.springframework.ai.chat.client.advisor.api.CallAroundAdvisor;
+import org.springframework.ai.chat.client.advisor.api.CallAroundAdvisorChain;
+
+// From the original TokenBudgetAdvisor.
+class OldCallAroundAdvisor implements CallAroundAdvisor {
+    @Override
+    public AdvisedResponse aroundCall(AdvisedRequest request, CallAroundAdvisorChain chain) {
+        return chain.nextAroundCall(request);
+    }
+
+    @Override
+    public String getName() { return "x"; }
+
+    @Override
+    public int getOrder() { return 0; }
+}

langchain4j-interop/src/broken/OldChatService.java

@@ -0,0 +1,32 @@
+import java.util.List;
+
+import org.springframework.ai.chat.client.ChatClient;
+import reactor.core.publisher.Flux;
+
+// The original ChatService, unchanged: this one is expected to still compile.
+class OldChatService {
+    private final ChatClient chatClient;
+
+    OldChatService(ChatClient.Builder builder) {
+        this.chatClient = builder
+            .defaultSystem("You are a senior Java architect. Be precise and concise.")
+            .build();
+    }
+
+    public String ask(String userMessage) {
+        return chatClient.prompt().user(userMessage).call().content();
+    }
+
+    public Flux<String> stream(String userMessage) {
+        return chatClient.prompt().user(userMessage).stream().content();
+    }
+
+    public ArchitectureReview reviewCode(String codeSnippet) {
+        return chatClient.prompt()
+            .user(u -> u.text("Review this Java code for production readiness: {code}").param("code", codeSnippet))
+            .call()
+            .entity(ArchitectureReview.class);
+    }
+
+    public record ArchitectureReview(String verdict, List<String> issues, List<String> suggestions, int score) { }
+}

langchain4j-interop/src/broken/OldDocumentGetContent.java

@@ -0,0 +1,8 @@
+import org.springframework.ai.document.Document;
+
+// From the original RAGService: Document::getContent
+class OldDocumentGetContent {
+    String text(Document d) {
+        return d.getContent();
+    }
+}

langchain4j-interop/src/broken/OldLangChain4jBuilder.java

@@ -0,0 +1,11 @@
+import dev.langchain4j.model.openai.OpenAiChatModel;
+import dev.langchain4j.service.AiServices;
+
+// From the original AgentConfig.
+class OldLangChain4jBuilder {
+    interface Agent { String chat(String m); }
+
+    Agent agent(OpenAiChatModel model) {
+        return AiServices.builder(Agent.class).chatLanguageModel(model).build();
+    }
+}

langchain4j-interop/src/broken/OldMessageGetContent.java

@@ -0,0 +1,8 @@
+import org.springframework.ai.chat.messages.Message;
+
+// From the original TokenBudgetAdvisor.estimateTokens: m.getContent()
+class OldMessageGetContent {
+    int length(Message m) {
+        return m.getContent().length();
+    }
+}

langchain4j-interop/src/broken/OldMeterRegistryCustomizer.java

@@ -0,0 +1,6 @@
+import org.springframework.boot.actuate.autoconfigure.metrics.MeterRegistryCustomizer;
+
+// From the original ObservabilityConfig.
+class OldMeterRegistryCustomizer {
+    MeterRegistryCustomizer<?> unused;
+}

langchain4j-interop/src/broken/OldPdfConfig.java

@@ -0,0 +1,8 @@
+import org.springframework.ai.reader.pdf.config.PdfDocumentReaderConfig;
+
+// From the original DocumentIngestionService.
+class OldPdfConfig {
+    Object config() {
+        return PdfDocumentReaderConfig.builder().withPageTopMargin(0).build();
+    }
+}

langchain4j-interop/src/broken/OldSearchRequest.java

@@ -0,0 +1,8 @@
+import org.springframework.ai.vectorstore.SearchRequest;
+
+// From the original RAGService and SemanticCacheService.
+class OldSearchRequest {
+    Object build(String question, int topK) {
+        return SearchRequest.query(question).withTopK(topK).withSimilarityThreshold(0.75);
+    }
+}

langchain4j-interop/src/broken/OldTokenTextSplitter.java

@@ -0,0 +1,6 @@
+import org.springframework.ai.transformer.splitter.TokenTextSplitter;
+
+// From the original DocumentIngestionService: this one is expected to still compile.
+class OldTokenTextSplitter {
+    TokenTextSplitter splitter = new TokenTextSplitter(512, 50, 5, 10000, true);
+}