AI‑Powered JUnit Rule for Automatic Error Reporting to GPT

This article is a step‑by‑step AI development tutorial tailored for Java engineers. It explains how to create a simple JUnit Rule that, after a test method fails, automatically sends the error information to GPT and receives code modification advice.

Greeting the AI and generating JSON‑based class files

First, the tutorial demonstrates how to ask the AI to generate Java classes in JSON format, making it easier to use the data without manually writing JSON strings.

package com.jd.jr.parrot.llm; import lombok.Data; /** * created on 2023/7/25 20:07 * @author Dragonchu */ @Data public class Choice { private String finish_reason; private int index; private Message message; }

package com.jd.jr.parrot.llm; import lombok.Data; /** * created on 2023/7/25 20:07 * @author Dragonchu */ @Data public class Message { private String role; private String content; }

package com.jd.jr.parrot.llm; import lombok.Data; /** * created on 2023/7/25 20:06 * @author Dragonchu */ @Data public class Usage { private int completion_tokens; private int prompt_tokens; private int total_tokens; }

package com.jd.jr.parrot.llm; import java.util.List; import lombok.Data; /** * created on 2023/7/25 20:17 * @author Dragonchu */ @Data public class OpenAiRequest { private String erp; private List messages; private String model; }

package com.jd.jr.parrot.llm; import java.util.List; import lombok.Data; /** * created on 2023/7/25 20:04 * @author Dragonchu */ @Data public class OpenAiResponse { private long created; private Usage usage; private String model; private String id; private List choices; private String object; }

package com.jd.jr.parrot.llm; import com.google.gson.Gson; import org.apache.http.client.methods.CloseableHttpResponse; import org.apache.http.client.methods.HttpPost; import org.apache.http.entity.StringEntity; import org.apache.http.impl.client.CloseableHttpClient; import org.apache.http.impl.client.HttpClients; import org.apache.http.util.EntityUtils; import java.io.IOException; import java.nio.charset.Charset; import java.util.Arrays; import java.util.List; import lombok.extern.slf4j.Slf4j; /** * created on 2023/7/25 20:07 * @author Dragonchu */ @Slf4j public class LLMUtil { private static String url = "Your url"; private static String apiKey = "Your key"; public static String sendMsg(String msg){ OpenAiRequest request = new OpenAiRequest(); Message message = new Message(); message.setRole("user"); message.setContent(msg); request.setMessages(Arrays.asList(message)); request.setModel("gpt-3.5-turbo"); Gson gson = new Gson(); String result = postGPT(url, gson.toJson(request)); OpenAiResponse openAiResponse = gson.fromJson(result, OpenAiResponse.class); return openAiResponse.getChoices().get(0).getMessage().getContent(); } public static String postGPT(String url, String jsonStr) { CloseableHttpClient httpClient = HttpClients.createDefault(); HttpPost httpPost = new HttpPost(url); httpPost.setHeader("Content-type", "application/json;charset=utf-8"); httpPost.setHeader("Authorization", apiKey); StringEntity entity = new StringEntity(jsonStr, Charset.forName("UTF-8")); entity.setContentEncoding("UTF-8"); entity.setContentType("application/json"); httpPost.setEntity(entity); String result = null; try { CloseableHttpResponse httpResponse = httpClient.execute(httpPost); result = EntityUtils.toString(httpResponse.getEntity(), "UTF-8"); if (httpResponse.getStatusLine().getStatusCode() != 200) { log.error("post->return error，result: {}", result); } } catch (IOException e) { log.error("postStream->error", e); e.printStackTrace(); } return result; } public static void main(String[] args) { log.info(sendMsg("Who are you?")); } }

Exception utility for extracting file information from stack traces

package com.jd.jr.parrot; import java.io.BufferedReader; import java.io.File; import java.io.FileReader; import java.io.IOException; import java.nio.file.Path; import java.nio.file.Paths; import java.util.regex.Matcher; import java.util.regex.Pattern; /** * created on 2023/7/28 15:08 * @author Dragonchu */ public class ExceptionUtil { public static String getFileName(String stackTraceLine) { Pattern pattern = Pattern.compile("\\((.*\\.java)"); Matcher matcher = pattern.matcher(stackTraceLine); if (matcher.find()) { return matcher.group(1); } return null; } public static Path getPath(String stackTraceLine) { int firstParenthesisIndex = stackTraceLine.indexOf("("); int firstDotIndex = stackTraceLine.lastIndexOf(".", firstParenthesisIndex); String packagePath = stackTraceLine.substring(0, firstDotIndex).replace(".", File.separator); String rootPath = new File("").getAbsolutePath(); return Paths.get(rootPath, "src", "main", "java", packagePath + ".java"); } public static String getCodeFile(String stackTraceLine) { Path filePath = getPath(stackTraceLine); if (filePath.toFile().exists()) { try (BufferedReader reader = new BufferedReader(new FileReader(filePath.toFile()))) { StringBuilder content = new StringBuilder(); String line; while ((line = reader.readLine()) != null) { content.append(line).append("\n"); } return content.toString(); } catch (IOException e) { e.printStackTrace(); } } return null; } public static void main(String[] args) { String stackTraceLine = "com.jd.jr.parrot.ExceptionExample.main(ExceptionExample.java:24)"; System.out.println("fileName = " + getFileName(stackTraceLine)); System.out.println("path = " + getPath(stackTraceLine).toString()); System.out.println("codeFile = " + getCodeFile(stackTraceLine)); } }

Prompt definition for GPT

private static final String prompt = "\"You are a good Java engineer, please analyze the following error and provide possible solutions." + "The error message is : %s, The error stack is : %s. The code file is : %s";

OverWatch TestWatcher implementation

package com.jd.jr.parrot.junit; import com.jd.jr.parrot.ExceptionUtil; import com.jd.jr.parrot.llm.LLMUtil; import java.util.Arrays; import lombok.extern.slf4j.Slf4j; import org.junit.rules.TestWatcher; import org.junit.runner.Description; /** * created on 2023/7/28 14:07 * @author Dragonchu */ @Slf4j public class OverWatch extends TestWatcher { private static final String prompt = "\"You are a good Java engineer, please analyze the following error and provide possible solutions." + "The error message is : %s, The error stack is : %s. The code file is : %s"; @Override protected void failed(Throwable e, Description description) { super.failed(e, description); String codeFile = ExceptionUtil.getCodeFile(e.getStackTrace()[0].toString()); log.info(LLMUtil.sendMsg(String.format(prompt, e.getMessage(), Arrays.toString(e.getStackTrace()), codeFile))); } }

AI‑generated example of error‑prone code

The tutorial includes a conversation with GPT where the model returns a Java snippet that uses the external library jsoup to fetch a web page, then deliberately accesses a non‑existent element, causing a NullPointerException that can be hard to trace.

import org.jsoup.Jsoup; import org.jsoup.nodes.Document; import org.jsoup.nodes.Element; import java.io.IOException; public class ExceptionExample { public static void main(String[] args) { try { Document doc = Jsoup.connect("https://www.example.com").get(); Element title = doc.select("title").first(); System.out.println("Title: " + title.text()); // Attempt to get a non‑existent element Element nonExistentElement = doc.select("#nonexistent-element").first(); // This will throw NullPointerException nonExistentElement.text(); } catch (IOException e) { e.printStackTrace(); } } }

JUnit test that triggers the AI analysis

package com.jd.jr.parrot; import com.jd.jr.parrot.junit.OverWatch; import org.junit.FixMethodOrder; import org.junit.Rule; import org.junit.Test; import org.junit.runners.MethodSorters; import java.io.IOException; @FixMethodOrder(MethodSorters.NAME_ASCENDING) public class OverWatchTest { @Rule public OverWatch overWatch = new OverWatch(); @Test public void jsoupTest() throws IOException { ExceptionExample.main(new String[]{}); } }

When the test runs and fails, the OverWatch rule captures the exception, extracts the source file via ExceptionUtil , formats the prompt, and sends it to GPT using LLMUtil . The AI’s response (shown in the article) provides a possible fix for the NullPointerException .