27. Gson

Gson 的使用

对象和 json 字符串的相互转换

package com.itguigu.demo;

import org.junit.jupiter.api.Test;

import com.google.gson.Gson;
import com.itguigu.bean.Student;

public class Demo {
	@Test
	public void testJsonStrObject() {
		Gson gson = new  Gson();
		// 将 student 转换为 json 格式字符串
		Student student = new Student("zhangsan", 20);
		String json = gson.toJson(student);
		System.out.println(json); // {"name":"zhangsan","age":20}
		
		// 将 json 字符串转换为对象
		Student student2 = gson.fromJson(json, Student.class);
		System.out.println(student2); // Student [name=zhangsan, age=20]
	}
}

带泛型的 List。因为有了泛型，所以我们在使用 fromJson 的时候就不能使用上面的那种方式了，而是要使用 TypeToke，将类型传递进去，然后通过 getType 获取类型。

@Test
public void testJsonStrList() {
	ArrayList<Student> students = new ArrayList<>();
	students.add(new Student("lisi", 24));
	students.add(new Student("wangwu", 88));
	students.add(new Student("zhaoliu", 99));
	
	Gson gson = new  Gson();
	String json = gson.toJson(students);
	System.out.println(json);
	// [{"name":"lisi","age":24},{"name":"wangwu","age":88},{"name":"zhaoliu","age":99}]
	
	// 下面这种使用方式有问题，只能转换外层的 ArrayList，里面不是 Student
	// ArrayList<Student> arrayList = gson.fromJson(json, ArrayList.class);
	// System.out.println(arrayList.get(0));
	
	// 使用 typeToken 的方式。直接传入我们需要的集合的泛型 ArrayList<Student>，然后获取类型即可
	ArrayList<Student> students2 = gson.fromJson(json, new TypeToken<ArrayList<Student>>() {}.getType());
	System.out.println(students2.get(0));
	System.out.println(students2.get(0).getName());
}

带泛型的 Map

@Test
public void testJsonStrMap() {
	HashMap<String, Student> map = new HashMap<>();
	map.put("zhangsan", new Student("zhangsan", 99));
	map.put("lisi", new Student("lisi", 44));
	
	Gson gson = new Gson();
	String json = gson.toJson(map);
	System.out.println(json);
	// {"lisi":{"name":"lisi","age":44},"zhangsan":{"name":"zhangsan","age":99}}
	
	HashMap<String, Student> map2 = gson.fromJson(json, new TypeToken<HashMap<String, Student>>() {}.getType());
	System.out.println(map2);
	System.out.println(map2.get("lisi"));
	System.out.println(map2.get("lisi").getAge());
}