在Java中使用Jackson時，何時使用@JsonValue註解？

@JsonValue 註解在方法層級很有用。我們可以使用此註解從 java 物件產生 JSON 字串。如果我們想要列印序列化對象，則重寫 toString() 方法。但使用@JsonValue註解，我們可以定義java物件序列化的方式。

語法

<strong>@Target(value={ANNOTATION_TYPE,METHOD,FIELD})
@Retention(value=RUNTIME)
public @interface JsonValue</strong>

範例

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonValue;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.IOException;
public class JsonValueAnnotationTest {
   public static void main(String args[]) throws Exception {
      <strong>ObjectMapper </strong>mapper = new <strong>ObjectMapper()</strong>;
      String jsonString = mapper.<strong>writeValueAsString</strong>(new Student());
      System.out.println(jsonString);
   }
}
<strong>// Student class</strong>
class Student {
   <strong>@JsonProperty</strong>
   private int studentId = 115;
  <strong> @JsonProperty</strong>
   private String studentName = "Sai Adithya";
   <strong>@JsonValue</strong>
   public String <strong>toJson</strong>() {
      return this.studentName + "," + this.studentId + "," + this.toString();
   }
   <strong>@Override</strong>
   public String toString() {
      return "Student[" +
            "studentId = " + studentId +
      ", studentName = " + studentName +
      &#39;]&#39;;
   }
}

輸出

<strong>"Sai Adithya,115,Student[studentId = 115, studentName = Sai Adithya]"</strong>

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