google的@SerializedName和@Expose注解

注解了@SerializedName的字段会被序列化到JSON中，输出的JSON格式中的名字即为注解时给定的名字。public class SomeClassWithFields { @SerializedName("name") private final String someField; private final String someOtherField; public SomeClassWithFields(String a, String b) { this.someField = a; this.someOtherField = b; } } SomeClassWithFields objectToSerialize = new SomeClassWithFields("a", "b"); Gson gson = new Gson(); String jsonRepresentation = gson.toJson(objectToSerialize); System.out.println(jsonRepresentation); ===== OUTPUT ===== {"name":"a","someOtherField":"b"}

上面是google API的例子。这个注解可以用在需要以JSON格式输出pojo类信息的情况。

同样@Expose注解的意思是：An annotation that indicates this member should be exposed for JSON serialization or deserialization。这时google API上的一些话：

This annotation has no effect unless you build com.google.gson.Gson with a com.google.gson.GsonBuilder and invoke com.google.gson.GsonBuilder.excludeFieldsWithoutExposeAnnotation() method.

Here is an example of how this annotation is meant to be used:

 public class User {
   @Expose private String firstName;
   @Expose private String lastName;
   @Expose private String emailAddress;
   private String password;
 }
 

If you created Gson with new Gson(), the toJson() and fromJson() methods will use the password field along-with firstName, lastName, and emailAddress for serialization and deserialization. However, if you created Gson with Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create() then the toJson() and fromJson() methods of Gson will exclude the password field. This is because the password field is not marked with the @Expose annotation.

Note that another way to achieve the same effect would have been to just mark the password field as transient, and Gson would have excluded it even with default settings. The @Expose annotation is useful in a style of programming where you want to explicitly specify all fields that should get considered for serialization or deserialization.