在这篇快速文章中，我们将使用GSON来序列化和反序列化集合。在这个例子中，我们将Integer和Employee对象的集合序列化为JSON表示，并使用TypeToken将整数的集合反序列化为任意的Java对象。

GSON的Maven依赖性

要在Maven2/3中使用Gson，你可以通过添加以下依赖关系来使用Maven Central中的Gson版本。

<dependencies>
    <!--  Gson: Java to Json conversion -->
    <dependency>
      <groupId>com.google.code.gson</groupId>
      <artifactId>gson</artifactId>
      <version>2.8.5</version>
      <scope>compile</scope>
    </dependency>
</dependencies>

###将雇员集合序列化

我们先来创建一个Employee POJO类。

class Employee {
    private String firstName;
    private String lastName;
    public Employee(String firstName, String lastName) {
        super();
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

现在，让我们写一段代码来序列化一个雇员对象的集合。

Gson gson = new Gson();
// Serialization of collection of employees
Collection<Employee> employees = Arrays.asList(new Employee("firstName1", "lastName1"),
  new Employee("firstName2", "lastName2"),
  new Employee("firstName3", "lastName3"),
  new Employee("firstName4", "lastName4"),
  new Employee("firstName5", "lastName5"));
String empJson = gson.toJson(employees); 
System.out.println(empJson);

解除序列化雇员的集合

// De-serialization of employee json to Collection of employee Java objects
String employeeJson = "[\r\n" + 
  "  {\r\n" + 
  "    \"firstName\": \"firstName1\",\r\n" + 
  "    \"lastName\": \"lastName1\"\r\n" + 
  "  },\r\n" + 
  "  {\r\n" + 
  "    \"firstName\": \"firstName2\",\r\n" + 
  "    \"lastName\": \"lastName2\"\r\n" + 
  "  },\r\n" + 
  "  {\r\n" + 
  "    \"firstName\": \"firstName3\",\r\n" + 
  "    \"lastName\": \"lastName3\"\r\n" + 
  "  },\r\n" + 
  "  {\r\n" + 
  "    \"firstName\": \"firstName4\",\r\n" + 
  "    \"lastName\": \"lastName4\"\r\n" + 
  "  },\r\n" + 
  "  {\r\n" + 
  "    \"firstName\": \"firstName5\",\r\n" + 
  "    \"lastName\": \"lastName5\"\r\n" + 
  "  }\r\n" + 
  "]";
Type type = new TypeToken<Collection<Employee>>() {}.getType();
Collection<Employee> collectionOfEmp = gson.fromJson(employeeJson, type);
System.out.println(collectionOfEmp);

让我们再看一个例子，序列化和反序列化整数的集合。

对整数集合进行序列化和反序列化

Gson gson = new GsonBuilder().setPrettyPrinting().create();
Collection<Integer> ints = Arrays.asList(1, 2, 3, 4, 5);
// Serialization of integer
String json = gson.toJson(ints); 
System.out.println(json);
// Deserialization of integer
Type collectionType = new TypeToken<Collection<Integer>>() {}.getType();
Collection<Integer> ints2 = gson.fromJson(json, collectionType);
System.out.println(ints2);

完整的程序供参考

package net.javaguides.gson;
import java.lang.reflect.Type;
import java.util.Arrays;
import java.util.Collection;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.reflect.TypeToken;
public class GSONCollectionsExample {
    public static void main(String[] args) {
        Gson gson = new GsonBuilder().setPrettyPrinting().create();
        Collection < Integer > ints = Arrays.asList(1, 2, 3, 4, 5);
        // Serialization of integer
        String json = gson.toJson(ints);
        System.out.println(json);
        // Serialization of collection of employees
        Collection < Employee > employees = Arrays.asList(new Employee("firstName1", "lastName1"),
            new Employee("firstName2", "lastName2"),
            new Employee("firstName3", "lastName3"),
            new Employee("firstName4", "lastName4"),
            new Employee("firstName5", "lastName5"));
        String empJson = gson.toJson(employees);
        System.out.println(empJson);
        // Deserialization of integer
        Type collectionType = new TypeToken < Collection < Integer >> () {}.getType();
        Collection < Integer > ints2 = gson.fromJson(json, collectionType);
        System.out.println(ints2);
        // De-serialization of employee json to Collection of employee Java objects
        String employeeJson = "[\r\n" +
            "  {\r\n" +
            "    \"firstName\": \"firstName1\",\r\n" +
            "    \"lastName\": \"lastName1\"\r\n" +
            "  },\r\n" +
            "  {\r\n" +
            "    \"firstName\": \"firstName2\",\r\n" +
            "    \"lastName\": \"lastName2\"\r\n" +
            "  },\r\n" +
            "  {\r\n" +
            "    \"firstName\": \"firstName3\",\r\n" +
            "    \"lastName\": \"lastName3\"\r\n" +
            "  },\r\n" +
            "  {\r\n" +
            "    \"firstName\": \"firstName4\",\r\n" +
            "    \"lastName\": \"lastName4\"\r\n" +
            "  },\r\n" +
            "  {\r\n" +
            "    \"firstName\": \"firstName5\",\r\n" +
            "    \"lastName\": \"lastName5\"\r\n" +
            "  }\r\n" +
            "]";
        Type type = new TypeToken < Collection < Employee >> () {}.getType();
        Collection < Employee > collectionOfEmp = gson.fromJson(employeeJson, type);
        System.out.println(collectionOfEmp);
    }
}
class Employee {
    private String firstName;
    private String lastName;
    public Employee(String firstName, String lastName) {
        super();
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

输出。

[
  1,
  2,
  3,
  4,
  5
]
[
  {
    "firstName": "firstName1",
    "lastName": "lastName1"
  },
  {
    "firstName": "firstName2",
    "lastName": "lastName2"
  },
  {
    "firstName": "firstName3",
    "lastName": "lastName3"
  },
  {
    "firstName": "firstName4",
    "lastName": "lastName4"
  },
  {
    "firstName": "firstName5",
    "lastName": "lastName5"
  }
]
[1, 2, 3, 4, 5]
[net.javaguides.gson.Employee@1698c449, net.javaguides.gson.Employee@5ef04b5, net.javaguides.gson.Employee@5f4da5c3, net.javaguides.gson.Employee@443b7951, net.javaguides.gson.Employee@14514713]

集合的限制

Gson可以序列化一个任意对象的集合，但不能从中反序列化，因为用户没有办法指出结果对象的类型。相反，在反序列化时，集合必须是一个特定的、通用的类型。这是有道理的，在遵循良好的Java编码实践时，很少会出现问题。