J-LangChain - Agent - 编排一个 ReAct + Function Call 反应链

系列文章索引
J-LangChain 入门

介绍

j‑langchain 是一款基于 Java 的 AIGC 编排框架，致力于集成多种大模型（LLM）调用及 RAG 工具。自 1.0.8 版本起，我们引入了工具函数（Function Call）调用能力，正式实现了 Tools 功能，并将其与 ReAct（Reasoning + Acting） 模式结合，从而构建出功能丰富、交互智能的 Agent 系统。

在本文中，我们将通过一个详实的实例，展示如何利用 Tools 功能编排一个具备 ReAct 反应链的 Agent。不仅能够体验 j‑langchain 的 Function Call 功能，还能深入了解大模型在 ReAct 反应链中如何一步步调用外部工具，实现复杂推理与动态响应。

示例场景

假设我们需要构建一个智能代理，能够回答类似于“上海的天气如何？”的等问题。为此，我们需要：

定义一个提示模板（Prompt Template），指导代理如何处理问题。
配置工具（Tools），例如获取天气和时间的函数。
实现一个带有工具调用的循环逻辑，确保代理能在必要时获取外部信息。
解析并返回最终答案。

以下是实现这一功能的完整代码解析。

代码解析

1. 定义提示模板

提示模板是代理的核心，它告诉模型如何思考和行动。我们使用 PromptTemplate 类定义了一个结构化的模板：

PromptTemplate prompt = PromptTemplate.fromTemplate("""Answer the following questions as best you can. You have access to the following tools:${tools}Use the following format:Question: the input question you must answerThought: Consider whether you already have enough information to answer the question. If so, proceed directly to the final answer.If additional information is needed, take the following steps:- Identify what specific information is missing.- Call the appropriate tool to obtain that information.- Analyze the new information and determine if the question can now be answered.When using tools, follow this structured approach:Action: the action to take, should be one of [${toolNames}]Action Input: the input to the actionObservation: the result of the action- You may use tools **up to 3 times**. If you still lack a complete answer after 3 attempts, summarize the best possible response.- If a tool's result is **irrelevant or does not improve understanding**, do not call the same tool again. Instead, attempt to derive an answer from available information.Thought: Based on the gathered information, determine if you can now provide a final answer. If yes, proceed to:Final Answer: the final answer to the original input question.If not, provide the best possible answer with a note on any remaining uncertainties.Begin!Question: ${input}Thought:""");

${tools} 和 ${toolNames} 是占位符，会在运行时被替换为实际的工具列表和工具名称。
${input} 使用户提出的问题。
模板中明确了代理的思考步骤：先判断是否需要更多信息，若需要则调用工具，最后给出答案。

2. 配置语言模型

我们使用 ChatOllama 作为语言模型，设置 temperature 为 0 以确保输出稳定：

ChatOllama llm = ChatOllama.builder().model("llama3:8b").temperature(0f).build();

当然你可以调试更聪明的模型，但此实例中 llama3:8b 已经可以达到效果。

3. 定义工具

工具是代理获取外部信息的关键。我们定义了两个简单工具：获取天气和获取时间：

Tool getWeather = Tool.builder().name("get_weather").params("location: String").description("Get city weather information and enter the city name").func(location -> String.format("The weather in %s is sunny with a temperature of 25°C", location)).build();Tool getTime = Tool.builder().name("get_time").params("city: String").description("Get city the current time and enter the city name").func(location -> String.format("%s The current time is 12:00 PM", location)).build();List<Tool> tools = List.of(getWeather, getTime);
prompt.withTools(tools);

Tool.builder() 提供了简洁的方式来定义工具的名称、参数、描述和执行逻辑。
prompt.withTools(tools) 将工具注入提示模板。

4. 辅助节点

为了实现 ReAct 形式交互，我们需要设计一个循环，并实现一些节点处理中间结果，和循环的退出判断，辅助流程：

4.1 处理工具调用的中间结果


TranslateHandler<AIMessage, AIMessage> cut = new TranslateHandler<>(llmResult -> {if (llmResult == null || StringUtils.isEmpty(llmResult.getContent()) || !llmResult.getContent().contains("Observation:")) {return llmResult;}String prefix = llmResult.getContent().substring(0, llmResult.getContent().indexOf("Observation:"));llmResult.setContent(prefix);return llmResult;
});

cut 处理器截取模型输出中工具调用前的部分，确保后续逻辑只处理必要内容。

4.2 解析模型输出

TranslateHandler<Map<String, String>, AIMessage> trans = new TranslateHandler<>(llmResult -> PromptUtil.stringToMap(llmResult.getContent()));

trans 将模型生成的文本解析为键值对（如 Action 和 Action Input）。

4.3 循环控制


int limit = 10;
Function<Integer, Boolean> isFinish = i -> {Map<String, String> map = ContextBus.get().getResult(trans.getNodeId());return i < limit && (map == null || (map.containsKey("Action") && map.containsKey("Action Input")));
};

isFinish 定义了循环退出条件：最多迭代 10 次，或模型不再需要调用工具。

4.4 执行工具调用

// 入参为trans节点键值对结果
TranslateHandler<Object, Map<String, String>> call = new TranslateHandler<>(map -> {// 获取原promt模型，用于追加StringPromptValue promptResult = ContextBus.get().getResult(prompt.getNodeId());// 获取cut节点结果，用于追加AIMessage cutResult = ContextBus.get().getResult(cut.getNodeId());Tool useTool = tools.stream().filter(t -> t.getName().toLowerCase().equals(map.get("Action"))).findAny().orElse(null);if (useTool == null) {promptResult.setText(promptResult.getText().trim() + "again");return promptResult;}String observation = (String) useTool.getFunc().apply(map.get("Action Input"));System.out.println("Observation: " + observation);String prefix = cutResult.getContent();String agentScratchpad = prefix.substring(prefix.indexOf("Thought:") + 8).trim() + "\nObservation:" + observation + "\nThought:";promptResult.setText(promptResult.getText().trim() + agentScratchpad);return promptResult;
});

call 处理器执行工具调用，并将结果（Observation）追加到提示中，供下一次循环使用。

5. 组装完整流程

使用 ChainActor 构建完整的执行链：

FlowInstance chain = chainActor.builder().next(sPrint) // 打印开始标记.next(prompt) // 构建prompt.loop(// 循环是否退出isFinish,// 循环执行llm,chainActor.builder() // 嵌入中间结果执行链.next(cut).next(trans) // 处理每次模型输出.next(Info.c(isCall, call), // 判断需要调用工具Info.c(input -> ContextBus.get().getResult(llm.getNodeId())) // 判断不需要调用工具，直接输出模型结果).build()).next(parser) // 解析结果.next(answer) // 再次处理模型输出结果，截取.next(ePrint) // 打印结束标记.build();

6. 执行并测试

> Entering new AgentExecutor chain...
A straightforward question! Let's see if we can get an answer without using any tools.Thought: We don't have any information about Shanghai's weather yet. But we can try to use the `get_weather` tool to find out!Action: get_weather
Action Input: ShanghaiObservation: The weather in Shanghai is sunny with a temperature of 25°C
Thought: Now that we have the weather information for Shanghai, let's proceed to answer the question.Final Answer: The weather in Shanghai is sunny with a temperature of 25°C.
> Finished chain.
The weather in Shanghai is sunny with a temperature of 25°C.

完整代码实例

https://github.com/flower-trees/j-langchain-example/blob/master/src/main/java/org/salt/jlangchain/demo/rag/tools/ZeroShotReactDescription.java

@Component
public class ZeroShotReactDescription {@AutowiredChainActor chainActor;public void run() {PromptTemplate prompt = PromptTemplate.fromTemplate("""Answer the following questions as best you can. You have access to the following tools:${tools}Use the following format:Question: the input question you must answerThought: Consider whether you already have enough information to answer the question. If so, proceed directly to the final answer.If additional information is needed, take the following steps:- Identify what specific information is missing.- Call the appropriate tool to obtain that information.- Analyze the new information and determine if the question can now be answered.When using tools, follow this structured approach:Action: the action to take, should be one of [${toolNames}]Action Input: the input to the actionObservation: the result of the action- You may use tools **up to 3 times**. If you still lack a complete answer after 3 attempts, summarize the best possible response.- If a tool's result is **irrelevant or does not improve understanding**, do not call the same tool again. Instead, attempt to derive an answer from available information.Thought: Based on the gathered information, determine if you can now provide a final answer. If yes, proceed to:Final Answer: the final answer to the original input question.If not, provide the best possible answer with a note on any remaining uncertainties.Begin!Question: ${input}Thought:""");ChatOllama llm = ChatOllama.builder().model("llama3:8b").temperature(0f).build();Tool getWeather = Tool.builder().name("get_weather").params("location: String").description("Get city weather information and enter the city name").func(location -> String.format("The weather in %s is sunny with a temperature of 25°C", location)).build();Tool getTime = Tool.builder().name("get_time").params("city: String").description("Get city the current time and enter the city name").func(location -> String.format("%s The current time is 12:00 PM", location)).build();List<Tool> tools = List.of(getWeather, getTime);prompt.withTools(tools);TranslateHandler<AIMessage, AIMessage> cut = new TranslateHandler<>(llmResult -> {if (llmResult == null || StringUtils.isEmpty(llmResult.getContent()) || !llmResult.getContent().contains("Observation:")) {if (llmResult != null) {System.out.println(llmResult.getContent());}return llmResult;}String prefix = llmResult.getContent().substring(0, llmResult.getContent().indexOf("Observation:"));System.out.println(prefix);llmResult.setContent(prefix);return llmResult;});TranslateHandler<Map<String, String>, AIMessage> trans = new TranslateHandler<>(llmResult -> PromptUtil.stringToMap(llmResult.getContent()));int limit = 10;Function<Integer, Boolean> isFinish = i -> {Map<String, String> map = ContextBus.get().getResult(trans.getNodeId());return i < limit && (map == null || (map.containsKey("Action") && map.containsKey("Action Input")));};Function<Object, Boolean> isCall = map -> ((Map<String, String>) map).containsKey("Action") && ((Map<String, String>) map).containsKey("Action Input");TranslateHandler<Object, Map<String, String>> call = new TranslateHandler<>(map -> {StringPromptValue promptResult = ContextBus.get().getResult(prompt.getNodeId());AIMessage cutResult = ContextBus.get().getResult(cut.getNodeId());Tool useTool = tools.stream().filter(t -> t.getName().toLowerCase().equals(map.get("Action"))).findAny().orElse(null);if (useTool == null) {promptResult.setText(promptResult.getText().trim() + "again");return promptResult;}String observation = (String) useTool.getFunc().apply(map.get("Action Input"));System.out.println("Observation: " + observation);String prefix = cutResult.getContent();String agentScratchpad = prefix.substring(prefix.indexOf("Thought:") + 8).trim() + "\nObservation:" + observation + "\nThought:";promptResult.setText(promptResult.getText().trim() + agentScratchpad);return promptResult;});StrOutputParser parser = new StrOutputParser();TranslateHandler<Object, Object> answer = new TranslateHandler<>(input -> {ChatGeneration generation = (ChatGeneration) input;String content = generation.getText();if (content.contains("Final Answer:")) {int start = content.indexOf("Final Answer:") + 13;int end = content.indexOf("\n", start);if (end > 0) {generation.setText(content.substring(start, end).trim());} else {generation.setText(content.substring(start).trim());}}return generation;});ConsumerHandler<?> sPrint = new ConsumerHandler<>(input -> System.out.println("> Entering new AgentExecutor chain..."));ConsumerHandler<?> ePrint = new ConsumerHandler<>(input -> System.out.println("> Finished chain."));FlowInstance chain = chainActor.builder().next(sPrint) // print start.next(prompt).loop(// Loop Exit ConditionsisFinish,// Loop Flowllm,chainActor.builder().next(cut).next(trans) // convert content generated by mll.next(Info.c(isCall, call), // need call functionInfo.c(input -> ContextBus.get().getResult(llm.getNodeId())) // else no need, return mll result).build()).next(parser).next(answer) // deal result.next(ePrint) // print end.build();ChatGeneration result = chainActor.invoke(chain, Map.of("input", "What's the weather like in Shanghai?"));System.out.println(result);}
}