gson常用用法小结

gson是来自google的十分不错的json转换器,转换起来十分方便,今天小结下其中的一些用法:

1) 最基本的对象和json相互转换
POJO:
 

public class ModelObject {
  String name;
  int val;
  boolean status;
  double f;
  
  public ModelObject(String name, int val, 
  boolean status, double f) {
    super();
    this.name = name;
    this.val = val;
    this.status = status;
    this.f = f;
  }
  
  @Override
  public String toString() {
    return "ModelObject [name=" + name + ",
    val=" + val + ", status="
    + status + ", f=" + f + "]";
  }
 

  相互转换的代码:

final Gson gson = new Gson();
// original object instantiation
ModelObject modelObject = new ModelObject("myname", 12, true, 2.3);
System.out.println("toJson ---");
System.out.println("Original Java object : " + modelObject);
// converting an object to json object
String json = gson.toJson(modelObject);
System.out.println("Converted JSON string is : " + json);
 
System.out.println("fromJson----");
// getting object from json representation
System.out.println("Original JSON string is : " + json);
// converting json to object
ModelObject modelObject1 = gson.fromJson(json, ModelObject.class);
System.out.println("Converted Java object : " + modelObject1);

输出如下:

toJson ---
Original Java object : ModelObject [name=myname, val=12, status=true, f=2.3]
Converted JSON string is : {"name":"myname","val":12,"status":true,"f":2.3}
fromJson----
Original JSON string is : {"name":"myname","val":12,"status":true,"f":2.3}
Converted Java object : ModelObject [name=myname, val=12, status=true, f=2.3]



2) 针对泛型
   先看一个泛型的例子定义

 

public class GenericModel<T> {
  T value;
  
  public GenericModel(T value) {
    super();
    this.value = value;
  }
  
  @Override
  public String toString() {
    return "Model2 [value=" + value + "]";
  }
}

   然后转换方法如下:
Gson gson = new Gson();

System.out.println("A generic object demo");
// a generified object
GenericModel<Integer> model = new GenericModel<>(12);

// converting to json representation
String json = gson.toJson(model);
System.out.println("json representation :" + json);

// converting back to object
Type collectionType = new TypeToken<GenericModel<Integer>>() {
}.getType();
GenericModel<Integer> modelObj =
                  gson.fromJson(json, collectionType);
System.out.println("converted object representation: " + modelObj);

System.out.println("\nA object from collection framework\n");
// for collection framework objects
List<String> listOfString = new ArrayList<>();
listOfString.add("ajduke");
listOfString.add("ajduchess");

// conversion to json
String jsonStr = gson.toJson(listOfString);
System.out.println("json representation :" + jsonStr);

Type collectionType2 = new TypeToken<List<String>>() {
}.getType();
List<String> listObj = gson.fromJson(jsonStr, collectionType2);
System.out.println("converted object representation: " + listObj);

输出:
A generic object demo
json representation :{"value":12}
converted object representation: Model2 [value=12]

A object from collection framework

json representation :["ajduke","ajduchess"]
converted object representation: [ajduke, ajduchess]

也就是说,GSON把json转换为JAVA泛型对象的时候,要先定义好
Type collectionType = new TypeToken<GenericModel<Integer>>() {
}.getType()
告诉到底用什么类型,然后再用fromjson方法

3) 使用transient指定不需要转换为json的属性

 

public class ModelObject {
  String name;
  transient int val;
  boolean status;
  double f;
  
  public ModelObject(String name, int val, 
   boolean status, double f) {
    super();
    this.name = name;
    this.val = val;
    this.status = status;
    this.f = f;
  }
  
  @Override
  public String toString() {
    return "ModelObject [name=" + name + ",
    val=" + val + ", status="
    + status + ", f=" + f + "]";
  }
}

  则转换程序:

Gson gson = new Gson();
// original object
ModelObject modelObject = new ModelObject("namesake", 50, true, 4.3);
System.out.print("Original Java object : ");
System.out.println(modelObject);
 
// converting to an json representation
String json = gson.toJson(modelObject);
System.out.print("Converted JSON string is : ");
System.out.println(json);
 
// getting back the object from json representation
ModelObject modelObject3 = gson.fromJson(json, ModelObject.class);
System.out.print("Converted Java object : ");
System.out.println(modelObject3);

  则输出的信息中,就不会转换val了.

4) 也可以使用注解,指定哪些是要暴露转换的属性,如:
 

	@Expose
	private Integer businessId;

但这个时候要用
Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation()
.create();
BusinessSystem bus = (BusinessSystem) (gson.fromJson(data,
BusinessSystem.class));

这样类似的方式转换;