如何將java對象轉換成json數據

package cn.hopetesting.com.test;

import cn.hopetesting.com.domain.User;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.Test;

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;


/**
 * @author newcityman
 * @date 2019/9/17 - 23:12
 步驟一、導入jackson的三個包
 二、建立一個user對象，並賦值
 三、建立jackson的核心對象ObjectMapper
 四、調用ObjectMapper的writevalue等相關方法
 */
public class TestJson {
 @Test
 public void test1() throws IOException {
 //一、建立對象
 User user = new User("zmy",20,"male");
 //二、建立jackson的核心對象 ObjectMapper
 ObjectMapper mapper = new ObjectMapper();
 //三、轉換
 /*try {
 String json = mapper.writeValueAsString(user);
 System.out.println(json);
 } catch (JsonProcessingException e) {
 e.printStackTrace();
 }*/
 //將數據寫到d://a.txt文件中
// mapper.writeValue(new File("d://a.txt"),user);
 //將數據關聯到writer中
 mapper.writeValue(new FileWriter("d://b.txt"),user);

 }

 @Test
 public void test2() throws IOException {
 //一、建立對象
 User user = new User("zmy",20,"male");
 user.setBirthday(new Date());
 //二、建立jackson的核心對象 ObjectMapper
 ObjectMapper mapper = new ObjectMapper();
 String json = mapper.writeValueAsString(user);
 System.out.println(json);

 }

 @Test
 public void test3() throws IOException {
 //一、建立user和list對象
 User user = new User("zmy",20,"male",new Date());
 User user1 = new User("light",10,"male",new Date());
 User user2 = new User("cityboy",5,"male",new Date());

 List<User> ps = new ArrayList<User>();
 ps.add(user);
 ps.add(user1);
 ps.add(user2);
 //二、建立jackson的核心對象 ObjectMapper
 ObjectMapper mapper = new ObjectMapper();
 String json = mapper.writeValueAsString(ps);
 System.out.println(json);

 }

 @Test
 public void test4() throws IOException {
 Map<String,Object> map = new HashMap<String, Object>();
 map.put("name","zmy");
 map.put("age",15);
 map.put("gender","男");

 ObjectMapper mapper = new ObjectMapper();
 String json = mapper.writeValueAsString(map);
 System.out.println(json);
 }

 //演示 json字符串轉換爲java對象
 @Test
 public void test5() throws IOException {
 //初始化json字符串
 String json="{\"gender\":\"男\",\"name\":\"zmy\",\"age\":15}";
 //建立ObjectMapper對象
 ObjectMapper mapper = new ObjectMapper();
 //轉換json字符串爲java對象
 User user = mapper.readValue(json, User.class);
 System.out.println(user);
 }}