Using the Google Gemini Large Language Model APIs in Java

  1 package com.markwatson.gemini;
  2 
  3 import org.json.JSONArray;
  4 import org.json.JSONObject;
  5 
  6 import java.io.IOException;
  7 import java.net.URI;
  8 import java.net.http.HttpClient;
  9 import java.net.http.HttpRequest;
 10 import java.net.http.HttpResponse;
 11 import java.nio.file.Files;
 12 import java.nio.file.Path;
 13 
 14 public class GeminiCompletions {
 15 
 16     private static final String DEFAULT_MODEL = "gemini-2.5-flash";
 17     public static String model = DEFAULT_MODEL;
 18 
 19     public static void main(String[] args) throws Exception {
 20         String prompt = "How much is 11 + 22?";
 21         String completion = getCompletion(prompt);
 22         System.out.println("completion: " + completion);
 23     }
 24 
 25     public static String getCompletion(String prompt) throws Exception {
 26         var jsonBody = buildRequestBody(prompt);
 27         return executeRequest(jsonBody);
 28     }
 29 
 30     public static String getCompletionWithSearch(String prompt) throws Exception {
 31         var textPart = new JSONObject().put("text", prompt);
 32         var parts = new JSONArray().put(textPart);
 33         var content = new JSONObject().put("parts", parts);
 34         var contents = new JSONArray().put(content);
 35 
 36         var googleSearchTool = new JSONObject().put("google_search", new JSONObject());
 37         var tools = new JSONArray().put(googleSearchTool);
 38 
 39         var body = new JSONObject()
 40                 .put("contents", contents)
 41                 .put("tools", tools);
 42 
 43         return executeRequest(body.toString());
 44     }
 45 
 46     private static String buildRequestBody(String prompt) {
 47         var textPart = new JSONObject().put("text", prompt);
 48         var parts = new JSONArray().put(textPart);
 49         var content = new JSONObject().put("parts", parts);
 50         var contents = new JSONArray().put(content);
 51         return new JSONObject().put("contents", contents).toString();
 52     }
 53 
 54     private static String executeRequest(String jsonBody) throws Exception {
 55         String apiKey = System.getenv("GOOGLE_API_KEY");
 56         if (apiKey == null || apiKey.isEmpty()) {
 57             throw new IOException("GOOGLE_API_KEY environment variable not set.");
 58         }
 59 
 60         var uri = URI.create(
 61                 "https://generativelanguage.googleapis.com/v1beta/models/" + model + ":generateContent?key=" + apiKey);
 62 
 63         var request = HttpRequest.newBuilder()
 64                 .uri(uri)
 65                 .header("Content-Type", "application/json")
 66                 .POST(HttpRequest.BodyPublishers.ofString(jsonBody))
 67                 .build();
 68 
 69         HttpResponse<String> response;
 70         try (var client = HttpClient.newHttpClient()) {
 71             response = client.send(request, HttpResponse.BodyHandlers.ofString());
 72         }
 73 
 74         if (response.statusCode() != 200) {
 75             throw new IOException("Gemini API request failed (HTTP %d): %s"
 76                     .formatted(response.statusCode(), response.body()));
 77         }
 78 
 79         var jsonObject = new JSONObject(response.body());
 80         return jsonObject.getJSONArray("candidates").getJSONObject(0).getJSONObject("content").getJSONArray("parts")
 81                 .getJSONObject(0).getString("text");
 82     }
 83 
 84     /***
 85      * Utilities for using the Gemini API
 86      */
 87 
 88     // read the contents of a file path into a Java string
 89     public static String readFileToString(String filePath) throws IOException {
 90         return Files.readString(Path.of(filePath)).replace("\"", "\\\"");
 91     }
 92 
 93     public static String replaceSubstring(String originalString, String substringToReplace, String replacementString) {
 94         return originalString.replace(substringToReplace, replacementString);
 95     }
 96 
 97     public static String promptVar(String prompt0, String varName, String varValue) {
 98         String prompt = replaceSubstring(prompt0, varName, varValue);
 99         return replaceSubstring(prompt, varName, varValue);
100     }
101 }

 1     @Test
 2     @DisplayName("Simple completion returns a non-empty response")
 3     void testCompletion() throws Exception {
 4         String r = GeminiCompletions
 5                 .getCompletion("Translate the following English text to French: 'Hello, how are you?'");
 6         System.out.println("completion: " + r);
 7         assertNotNull(r, "Completion should not be null");
 8         assertFalse(r.isEmpty(), "Completion should not be empty");
 9     }
10 
11     @Test
12     @DisplayName("Two-shot template prompt returns a non-empty response")
13     void testTwoShotTemplate() throws Exception {
14         String input_text = "Mark Johnson enjoys living in Berkeley California at 102 Dunston Street and use mjess@foobar.com for contacting him.";
15         String prompt0 = GeminiCompletions.readFileToString("../prompts/two-shot-2-var.txt");
16         System.out.println("prompt0: " + prompt0);
17         String prompt = GeminiCompletions.promptVar(prompt0, "{input_text}", input_text);
18         System.out.println("prompt: " + prompt);
19         String r = GeminiCompletions.getCompletion(prompt);
20         System.out.println("two shot extraction completion: " + r);
21         assertNotNull(r, "Two-shot completion should not be null");
22         assertFalse(r.isEmpty(), "Two-shot completion should not be empty");
23     }
24 
25     @Test
26     @DisplayName("Summarization prompt returns a non-empty response")
27     void testSummarization() throws Exception {
28         String input_text = "Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter is one of the brightest objects visible to the naked eye in the night sky, and has been known to ancient civilizations since before recorded history. It is named after the Roman god Jupiter.[19] When viewed from Earth, Jupiter can be bright enough for its reflected light to cast visible shadows,[ and is on average the third-brightest natural object in the night sky after the Moon and Venus.";
29         String prompt0 = GeminiCompletions.readFileToString("../prompts/summarization_prompt.txt");
30         System.out.println("prompt0: " + prompt0);
31         String prompt = GeminiCompletions.promptVar(prompt0, "{input_text}", input_text);
32         System.out.println("prompt: " + prompt);
33         String r = GeminiCompletions.getCompletion(prompt);
34         System.out.println("summarization completion: " + r);
35         assertNotNull(r, "Summarization result should not be null");
36         assertFalse(r.isEmpty(), "Summarization result should not be empty");
37     }
38 
39     @Test
40     @DisplayName("Completion with Google Search grounding returns a non-empty response")
41     void testCompletionWithSearch() throws Exception {
42         String prompt = "What is the current stock price of Google?";
43         String r = GeminiCompletions.getCompletionWithSearch(prompt);
44         System.out.println("Search completion: " + r);
45         assertNotNull(r, "Search completion should not be null");
46         assertFalse(r.isEmpty(), "Search completion should not be empty");
47     }

 1 $ make
 2 mvn test -q # run test in quiet mode
 3 completion: Here are a few ways to say this in French, depending on the level of formality:
 4 
 5 *   **Formal/Standard:** *Bonjour, comment allez-vous ?*
 6 *   **Informal/Casual:** *Salut, ça va ?*
 7 *   **Neutral:** *Bonjour, comment ça va ?*
 8 
 9 Search completion: As of the market close on December 18, 2025, the stock prices for **Alphabet Inc. (Google)** are as follows:
10 
11 *   **Alphabet Inc. Class A (GOOGL):** ~$302.01
12 *   **Alphabet Inc. Class C (GOOG):** ~$303.29
13 
14 
15 prompt0: Given the two examples below, extract the names, addresses, and email addresses of individuals mentioned later as Process Text. Format the extracted information in JSON, with keys for \"name\", \"address\", and \"email\". If any information is missing, use \"null\" for that field. Be very concise in your output by providing only the output JSON.
16 
17 Example 1:
18 Text: \"John Doe lives at 1234 Maple Street, Springfield. His email is johndoe@example.com.\"
19 Output: 
20 {
21   \"name\": \"John Doe\",
22   \"address\": \"1234 Maple Street, Springfield\",
23   \"email\": \"johndoe@example.com\"
24 }
25 
26 Example 2:
27 Text: \"Jane Smith has recently moved to 5678 Oak Avenue, Anytown. She hasn't updated her email yet.\"
28 Output: 
29 {
30   \"name\": \"Jane Smith\",
31   \"address\": \"5678 Oak Avenue, Anytown\",
32   \"email\": null
33 }
34 
35 Process Text: \"{input_text}\"
36 Output:
37 
38 prompt: Given the two examples below, extract the names, addresses, and email addresses of individuals mentioned later as Process Text. Format the extracted information in JSON, with keys for \"name\", \"address\", and \"email\". If any information is missing, use \"null\" for that field. Be very concise in your output by providing only the output JSON.
39 
40 Example 1:
41 Text: \"John Doe lives at 1234 Maple Street, Springfield. His email is johndoe@example.com.\"
42 Output: 
43 {
44   \"name\": \"John Doe\",
45   \"address\": \"1234 Maple Street, Springfield\",
46   \"email\": \"johndoe@example.com\"
47 }
48 
49 Example 2:
50 Text: \"Jane Smith has recently moved to 5678 Oak Avenue, Anytown. She hasn't updated her email yet.\"
51 Output: 
52 {
53   \"name\": \"Jane Smith\",
54   \"address\": \"5678 Oak Avenue, Anytown\",
55   \"email\": null
56 }
57 
58 Process Text: \"Mark Johnson enjoys living in Berkeley California at 102 Dunston Street and use mjess@foobar.com for contacting him.\"
59 Output:
60 
61 two shot extraction completion: {
62   "name": "Mark Johnson",
63   "address": "102 Dunston Street, Berkeley California",
64   "email": "mjess@foobar.com"
65 }
66 prompt0: Summarize the following text: \"{input_text}\"
67 Output:
68 
69 prompt: Summarize the following text: \"Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter is one of the brightest objects visible to the naked eye in the night sky, and has been known to ancient civilizations since before recorded history. It is named after the Roman god Jupiter.[19] When viewed from Earth, Jupiter can be bright enough for its reflected light to cast visible shadows,[ and is on average the third-brightest natural object in the night sky after the Moon and Venus.\"
70 Output:
71 
72 summarization completion: Jupiter is the fifth planet from the Sun and the largest in the Solar System. A gas giant named after the Roman god, it is more massive than all other planets combined and stands as the third-brightest natural object in the night sky.