依赖

<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId>
    <version>4.8.1</version>
</dependency>

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.6</version>
</dependency>

<dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.13</version>
    <scope>test</scope>
</dependency>

代码示例

package com.demo;

import com.google.gson.Gson;
import okhttp3.*;
import org.junit.Test;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class Demo {
    public static final MediaType JSON = MediaType.parse("application/json; charset=utf-8");

    /**
     * 发送GET请求
     */
    @Test
    public void getRequest() throws IOException {
        OkHttpClient client = new OkHttpClient();

        Request request = new Request.Builder()
                .url("http:///get")
                .build();

        Response response = client.newCall(request).execute();

        if (response.isSuccessful()) {
            System.out.println(response.body().string());
        }

    }

    /**
     * POST请求
     */
    @Test
    public void postRequest() throws IOException {
        OkHttpClient client = new OkHttpClient();

        // 提交FormData
        FormBody.Builder form = new FormBody.Builder();
        form.add("name", "Tom");
        form.add("age", "23");

        Request request = new Request.Builder()
                .url("http:///post")
                .post(form.build())
                .build();

        Response response = client.newCall(request).execute();
        System.out.println(response.body().string());

    }

    /**
     * 提交Json
     */
    @Test
    public void postJsonRequest() throws IOException {
        OkHttpClient client = new OkHttpClient();

        Map<String, Object> map = new HashMap<>();
        map.put("name", "Tom");
        map.put("age", 23);

        Gson gson = new Gson();
        String data = gson.toJson(map);

        RequestBody requestBody = RequestBody.create(data, JSON);

        Request request = new Request.Builder()
                .url("http:///post")
                .post(requestBody)
                .build();

        Response response = client.newCall(request).execute();
        System.out.println(response.body().string());

    }
}