Google Gson的使用方法,实现Json结构的相互转换

原文链接：https://blog.csdn.net/qxs965266509/article/details/42774691

在Java开发中，有时需要保存一个数据结构成字符串，可能你会考虑用Json，但是当Json字符串转换成Java对象时，转换成的是JsonObject，并不是你想要的Class类型的对象，操作起来就很不是愉悦，下面说的就可以解决了这种问题。

现在，我先自定义一个Class类

	public class Student {
		public int id;
		public String nickName;
		public int age;
		public ArrayList<String> books;
		public HashMap<String, String> booksMap;
	}

案例一，案例二，案例三都是把Java的Class对象使用Gson转换成Json的字符串

案例一：

仅包含基本数据类型的数据结构

Gson gson = new Gson();
	Student student = new Student();
	student.id = 1;
	student.nickName = "乔晓松";
	student.age = 22;
	student.email = "[email protected]";
	Log.e("MainActivity", gson.toJson(student));

输出结果是 ：

{"email":"[email protected]","nickName":"乔晓松","id":1,"age":22}

案例二：

除了基本数据类型还包含了List集合

Gson gson = new Gson();
		Student student = new Student();
		student.id = 1;
		student.nickName = "乔晓松";
		student.age = 22;
		student.email = "[email protected]";
		ArrayList<String> books = new ArrayList<String>();
		books.add("数学");
		books.add("语文");
		books.add("英语");
		books.add("物理");
		books.add("化学");
		books.add("生物");
		student.books = books;
		Log.e("MainActivity", gson.toJson(student));

输出结果是 ：

{"books":["数学","语文","英语","物理","化学","生物"],"email":"[email protected]","nickName":"乔晓松","id":1,"age":22}

案例三：

除了基本数据类型还包含了List和Map集合

Gson gson = new Gson();
		Student student = new Student();
		student.id = 1;
		student.nickName = "乔晓松";
		student.age = 22;
		student.email = "[email protected]";
		ArrayList<String> books = new ArrayList<String>();
		books.add("数学");
		books.add("语文");
		books.add("英语");
		books.add("物理");
		books.add("化学");
		books.add("生物");
		student.books = books;
		HashMap<String, String> booksMap = new HashMap<String, String>();
		booksMap.put("1", "数学");
		booksMap.put("2", "语文");
		booksMap.put("3", "英语");
		booksMap.put("4", "物理");
		booksMap.put("5", "化学");
		booksMap.put("6", "生物");
		student.booksMap = booksMap;
		Log.e("MainActivity", gson.toJson(student));

输出结果是 ：

{"books":["数学","语文","英语","物理","化学","生物"],"booksMap":{"3":"英语","2":"语文","1":"数学","6":"生物","5":"化学","4":"物理"},"email":"[email protected]","nickName":"乔晓松","id":1,"age":22}

案例四：
把案例三输出的字符串使用Gson转换成Student对象

Gson gson = new Gson();
		Student student = new Student();
		student.id = 1;
		student.nickName = "乔晓松";
		student.age = 22;
		student.email = "[email protected]";
		ArrayList<String> books = new ArrayList<String>();
		books.add("数学");
		books.add("语文");
		books.add("英语");
		books.add("物理");
		books.add("化学");
		books.add("生物");
		student.books = books;
		HashMap<String, String> booksMap = new HashMap<String, String>();
		booksMap.put("1", "数学");
		booksMap.put("2", "语文");
		booksMap.put("3", "英语");
		booksMap.put("4", "物理");
		booksMap.put("5", "化学");
		booksMap.put("6", "生物");
		student.booksMap = booksMap;
		String result = gson.toJson(student);
 
		Student studentG = gson.fromJson(result, Student.class);
 
		Log.e("MainActivity", "id:" + studentG.id);
		Log.e("MainActivity", "nickName:" + studentG.nickName);
		Log.e("MainActivity", "age:" + studentG.age);
		Log.e("MainActivity", "email:" + studentG.email);
		Log.e("MainActivity", "books size:" + studentG.books.size());
		Log.e("MainActivity", "booksMap size:" + studentG.booksMap.size());

输出结果是 ：

id:1
nickName:乔晓松
age:22
email:[email protected]
books size:6
booksMap size:6

通过这4个案例我解决你一定就把Gson的基本用法学会了，当然我们的需求可能需要把List或者Map等集合的泛型换成我们自定义个class，这也是可以解决的，请看案例

案例五：

泛型的使用

	public HashMap<String,Book> booksMap;
 
 public class Book{
		public int id;
		public String name;
	}

把booksMap转换成字符串和上面的案例是一样的，但是booksMap的Json字符串换成booksMap的实例对象就有点不同了，因为booksMap有自定义的泛型

HashMap<String, Book> booksMap = gson.fromJson(result, new TypeToken<HashMap<String, Book>>() { }.getType());

栗子：

Gson gson = new Gson();
        HashMap<String, Book> booksMap = new HashMap<>();
        booksMap.put("book1", new Book(1, "Java基础开发"));
        booksMap.put("book2", new Book(2, "Java设计模式"));
        booksMap.put("book3", new Book(3, "Java算法"));
        String dataStr = gson.toJson(booksMap);
        System.out.println("dataStr:" + dataStr);
        HashMap<String, Book> booksMap2 = gson.fromJson(dataStr, new TypeToken<HashMap<String, Book>>() {
        }.getType());
        for (Map.Entry<String, Book> entry : booksMap2.entrySet()) {
            System.out.println("key:"+entry.getKey() + "    value:" + entry.getValue().toString());
        }

输出的结果是：

dataStr:{"book2":{"id":2,"name":"Java设计模式"},"book1":{"id":1,"name":"Java基础开发"},"book3":{"id":3,"name":"Java算法"}}
key:book2    value:Book{id=2, name='Java设计模式'}
key:book1    value:Book{id=1, name='Java基础开发'}
key:book3    value:Book{id=3, name='Java算法'}

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