Gson用法补遗

  本文小结基本用法和一些值得关注的gson用法
1 基本用法
   假设有对象pojo，如下：
 

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 + "]";
  }

    然后是转换为JSON以及对JSON的反向转换：

final Gson gson = new Gson();

ModelObject modelObject = new ModelObject("myname", 12, true, 2.3);
System.out.println("toJson ---");
System.out.println("Original Java object : " + modelObject);

String json = gson.toJson(modelObject);
System.out.println("Converted JSON string is : " + json);
 
System.out.println("fromJson----");

System.out.println("Original JSON string is : " + json);

ModelObject modelObject1 = gson.fromJson(json, ModelObject.class);
System.out.println("Converted Java object : " + modelObject1);

  输出：
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） 如果是对于泛型的
 
  则要使用：
  Type collectionType =new TypeToken<{ generic-object-with-type-information }>(){}.getType();
  
比如：
  

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

 
 
  使用方法：
 

Gson gson = new Gson();
 
System.out.println("A generic object demo");

GenericModel<Integer> model = new GenericModel<>(12);
 

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

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");

List<String> listOfString = new ArrayList<>();
listOfString.add("ajduke");
listOfString.add("ajduchess");
 

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);

3 如果不希望有某些属性，则使用transient屏蔽，如：

    transient int val;

4 介绍一下如何输出漂亮的json格式，因为默认的输出是compact格式，就是比如：
  {"name":"ajduke","languagesKnown":["Java","Scala","Ruby"]}

  可以使用如下的方式输出漂亮格式的：
 

GsonBuilder gsonBuilder = new GsonBuilder();
Gson prettyGson = gsonBuilder.setPrettyPrinting().create();
json = prettyGson.toJson(new Developer());

   就是setPrettyPrinting选项了，输出的格式为：
   ******* Pretty formatting *********
{
  "name": "ajduke",
  "languagesKnown": [
    "Java",
    "Scala",
    "Ruby"
  ]
}
 
5 把null值也转换，默认是不转换null值的，可以选择也转换，比如
  

class Developer {
 
  private String name;
  private String classz;
  List<String> languagesKnown;
  
  public Developer() {
    name = "ajduke";
 
    languagesKnown = new ArrayList<>();
    languagesKnown.add("Java");
    languagesKnown.add("Scala");
    languagesKnown.add("Ruby");
  }
}

   则：

Gson gson = new Gson();
 
System.out.println("Default behaviour ");
GsonBuilder gsonBuilder = new GsonBuilder();
 
Gson prettyGson = gsonBuilder.setPrettyPrinting().create();
String json = prettyGson.toJson(new Developer());
System.out.println(json);
 
System.out.println("Including the nulls ");
Gson includeNullsGson = gsonBuilder.serializeNulls().create();
String json2 = includeNullsGson.toJson(new Developer());
System.out.println(json2);

  输出：
Default behaviour
{
  "name": "ajduke",
  "languagesKnown": [
    "Java",
    "Scala",
    "Ruby"
  ]
}
Including the nulls
{
  "name": "ajduke",
  "classz": null,
  "languagesKnown": [
    "Java",
    "Scala",
    "Ruby"
  ]
}

6 输出的json使用另外一个名字，默认转换出来的json中和对象的字段是一样的，当然也
可以设置成不同，使用注解
   @SerializedName("firstName")
  private String name;
则指定了别名

7 可以输出到流
  

 Gson gson = new Gson();
  System.out.println("Writing JSON to file ....");

  try (FileWriter writer = new FileWriter("d:/output.json")) { 
    gson.toJson(new Developer(), writer); // writing to file
  }
  

8 使用@Since注解去维护版本，比如你有一个REST的API,并且有多个版本的JSON，如果下一个版本JSON中增加了字段，但又不希望所有的版本都在使用这些字段的话，就可以使用，比如：
 

public class Example33 {
  public static void main(String[] args) {
    Gson gson = new GsonBuilder().setVersion(2.0).create();
    String json = gson.toJson(new ExampleClass());
    System.out.println("Output for version 2.0...");
    System.out.println(json);
    
    gson= new GsonBuilder().setVersion(1.0).create();
    json = gson.toJson(new ExampleClass());
    System.out.println("\nOutput for version 1.0...");
    System.out.println(json);
    
    gson= new Gson();
    json = gson.toJson(new ExampleClass());
    System.out.println("\nOutput for No version set...");
    System.out.println(json);
  }
}
 
class ExampleClass{
  String field=  "field";
  // this is in version 1.0
  @Since(1.0) String newField1 = "field 1";
  // following will be included in the version 1.1
  @Since(2.0) String newField2 = "field 2";
}

输出为：
Output for version 2.0...
{"field":"field","newField1":"field 1","newField2":"field 2"}

Output for version 1.0...
{"field":"field","newField1":"field 1"}

Output for No version set...
{"field":"field","newField1":"field 1","newField2":"field 2"}

9 HTML的转换
   默认是GSON把HTML 转义的，但也可以设置不转义，比如：

public class Example34 {
  public static void main(String[] args) {
    String str ="<myval>";
    Gson gson = new Gson();
    System.out.println("Normal behaviour...");
    System.out.println(gson.toJson(str));
    
    System.out.println("\nDisabled html escaping...");
    gson = new GsonBuilder().disableHtmlEscaping().create();
    System.out.println(gson.toJson(str));
  }
}

输出：
  Normal behaviour...
"\u003cmyval\u003e"

Disabled html escaping...
"<myval>"


10 指定某些修饰符，比如private等不输出为json，比如：
  GsonBuilder excludeFieldsWithModifiers = gsonBuilder
    .excludeFieldsWithModifiers(Modifier.PRIVATE);

Gson create = excludeFieldsWithModifiers.create();
String json2 = create.toJson(new Developer());

   当然也可以将要输出字段用@Expose去注解输出：

11甚至可以指定将原来对象的字段，输出成什么样的JSON，比如全部转换为小写，并用空格或者下划线分隔
  比如：
 

public static void main(String[] args) {
    
    Gson gson = new Gson();
    String json = gson.toJson(new JsonClass());
    System.out.println("Default behaviour....");
    System.out.println(json);
    
    gson = new GsonBuilder().setFieldNamingPolicy(
    FieldNamingPolicy.LOWER_CASE_WITH_DASHES).create();
    json = gson.toJson(new JsonClass());
    System.out.println("\nFields with lower case with dashes...");
    System.out.println(json);
    
    gson = new GsonBuilder().setFieldNamingPolicy(
    FieldNamingPolicy.UPPER_CAMEL_CASE_WITH_SPACES).create();
    json = gson.toJson(new JsonClass());
    System.out.println("\nFields with upper case with spaces...");
    System.out.println(json);
    
  }
}
 
class JsonClass {
  String myField = "value1";
  String myAnotherField = "value2";
}

  输出：
Default behaviour....
{"myField":"value1","myAnotherField":"value2"}

Fields with lower case and dashes...
{"my-field":"value1","my-another-field":"value2"}

Fields with lower case and dashes...
{"My Field":"value1","My Another Field":"value2"}