Gson - 自定义类型适配器

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Gson 使用其内置适配器执行对象的序列化/反序列化。它还支持自定义适配器。让我们讨论如何创建自定义适配器以及如何使用它。

创建自定义适配器

通过扩展TypeAdapter类并向其传递目标对象的类型来创建自定义适配器。重写读取和写入方法以分别执行自定义反序列化和序列化。

class StudentAdapter extends TypeAdapter<Student> { 
   @Override 
   public Student read(JsonReader reader) throws IOException { 
      ... 
   } 
   
   @Override 
   public void write(JsonWriter writer, Student student) throws IOException { 
   } 
}

注册自定义适配器

使用 GsonBuilder 注册自定义适配器并使用GsonBuilder创建 Gson 实例。

GsonBuilder builder = new GsonBuilder(); 
builder.registerTypeAdapter(Student.class, new StudentAdapter()); 
Gson gson = builder.create(); 

使用适配器

Gson 现在将使用自定义适配器将 Json 文本转换为对象，反之亦然。

String jsonString = "{\"name\":\"Mahesh\", \"rollNo\":1}"; 
Student student = gson.fromJson(jsonString, Student.class); 
System.out.println(student);  
jsonString = gson.toJson(student); 
System.out.println(jsonString);

例子

让我们看看自定义类型适配器的实际应用示例。在 C:\>GSON_WORKSPACE 中创建名为GsonTester的 Java 类文件。

文件 - GsonTester.java

import java.io.IOException;  

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder; 
import com.google.gson.TypeAdapter; 
import com.google.gson.stream.JsonReader; 
import com.google.gson.stream.JsonToken; 
import com.google.gson.stream.JsonWriter;  

public class GsonTester { 
   public static void main(String args[]) { 
   
      GsonBuilder builder = new GsonBuilder(); 
      builder.registerTypeAdapter(Student.class, new StudentAdapter()); 
      builder.setPrettyPrinting(); 
      Gson gson = builder.create();  
      
      String jsonString = "{\"name\":\"Mahesh\", \"rollNo\":1}";  
      Student student = gson.fromJson(jsonString, Student.class); 
      System.out.println(student);  
      
      jsonString = gson.toJson(student); 
      System.out.println(jsonString);  
   }      
}  

class StudentAdapter extends TypeAdapter<Student> { 
   @Override 
   public Student read(JsonReader reader) throws IOException { 
      Student student = new Student(); 
      reader.beginObject(); 
      String fieldname = null; 
      
      while (reader.hasNext()) { 
         JsonToken token = reader.peek();            
         
         if (token.equals(JsonToken.NAME)) {     
            //get the current token 
            fieldname = reader.nextName(); 
         } 
         
         if ("name".equals(fieldname)) {       
            //move to next token 
            token = reader.peek(); 
            student.setName(reader.nextString()); 
         } 
         
         if("rollNo".equals(fieldname)) { 
            //move to next token 
            token = reader.peek(); 
            student.setRollNo(reader.nextInt()); 
         }               
      } 
      reader.endObject(); 
      return student; 
   }  
   
   @Override 
   public void write(JsonWriter writer, Student student) throws IOException { 
      writer.beginObject(); 
      writer.name("name"); 
      writer.value(student.getName()); 
      writer.name("rollNo"); 
      writer.value(student.getRollNo()); 
      writer.endObject(); 
   } 
}  

class Student { 
   private int rollNo; 
   private String name;  
   
   public int getRollNo() { 
      return rollNo; 
   } 
   
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   public String getName() { 
      return name; 
   }  
   
   public void setName(String name) { 
      this.name = name; 
   }   
   
   public String toString() { 
      return "Student[ name = "+name+", roll no: "+rollNo+ "]"; 
   } 
} 

验证结果

使用javac编译器编译类，如下所示 -

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行GsonTester查看结果 -

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Student[ name = Mahesh, roll no: 1] 
{ 
   "name": "Mahesh", 
   "rollNo": 1 
}