要将 Java 对象或 POJO（普通旧 Java 对象）转换为 JSON，我们可以使用JSONObject将对象作为参数的构造函数之一。在下面的示例中，我们将StudentPOJO 转换为 JSON 字符串。Student类必须提供 getter 方法，JSONObject通过调用这些方法创建 JSON 字符串。

在此代码段中，我们执行以下操作：

使用 setter 方法创建Student对象并设置其属性。创建JSONObject调用object并将Student对象用作其构造函数的参数。JSONObject使用 getter 方法生成 JSON 字符串。调用object.toString()方法获取 JSON 字符串。

 
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.google.gson.Gson;
import org.json.JSONObject;
 
import java.util.Arrays;
 
public class PojoToJSON {
 
public static void main(String[] args) throws JsonProcessingException {
Student student = new Student();
student.setId(1L);
student.setName("Alice");
student.setAge(20);
student.setCourses(Arrays.asList("Engineering", "Finance", "Chemistry"));
 
JSONObject object = new JSONObject(student);
String json = object.toString();
System.out.println(json);
System.out.println(new Gson().toJson(student));
// Creating Object of ObjectMapper define in Jackson API
ObjectMapper Obj = new ObjectMapper();
 
// Converting the Java object into a JSON string
String jsonStr = Obj.writeValueAsString(student);
// Displaying Java object into a JSON string
System.out.println(jsonStr);
 
}
}

运行此代码会产生以下结果：

{"courses":["Engineering","Finance","Chemistry"],"name":"Alice","id":1,"age":20}{"id":1,"name":"Alice","age":20,"courses":["Engineering","Finance","Chemistry"]}{"id":1,"name":"Alice","age":20,"courses":["Engineering","Finance","Chemistry"]}

上面代码中使用的Student类：

 
import java.util.List;
 
public class Student {
 
private Long id;
private String name;
private int age;
private List<String> courses;
 
public Student(Long id, String name, int age, List<String> courses) {
this.id = id;
this.name = name;
this.age = age;
this.courses = courses;
}
 
Student() {
 
}
 
public Long getId() {
return id;
}
 
public void setId(Long id) {
this.id = id;
}
 
public String getName() {
return name;
}
 
public void setName(String name) {
this.name = name;
}
 
public int getAge() {
return age;
}
 
public void setAge(int age) {
this.age = age;
}
 
public List<String> getCourses() {
return courses;
}
 
public void setCourses(List<String> courses) {
this.courses = courses;
}
 
}

Maven 依赖项

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.example.javaobjectjson</groupId>
<artifactId>JavaObjectJSON</artifactId>
<version>1.0-SNAPSHOT</version>
<dependencies>
 
<!-- https://mvnrepository.com/artifact/org.json/json -->
<dependency>
<groupId>org.json</groupId>
<artifactId>json</artifactId>
<version>20211205</version>
</dependency>
<!-- https://mvnrepository.com/artifact/com.google.code.gson/gson -->
<dependency>
<groupId>com.google.code.gson</groupId>
<artifactId>gson</artifactId>
<version>2.9.0</version>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.12.1</version>
</dependency>  
</dependencies>
</project>