为什么 Jackson ObjectMapper 不能序列化 play.data.validation.ValidationError 类？

Question

我有以下设置：

public class ValidationUtils {

  public static class ValidationResults {
      public List validationErrors;
      public boolean valid;

      public ValidationResults(){
        validationErrors = new ArrayList();
        valid = true;
    }

      @Override
      public String toString() {
        return "ValidationResults [validationErrors=" + validationErrors
                    + ", valid=" + valid + "]";
      }
  }

  // some static methods that create instances of ValidationResults.
}

在 ValidationResults 类的序列化时出现此错误：

com.fasterxml.jackson.databind.JsonMappingException: No serializer found for class play.data.validation.ValidationError and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) ) (through reference chain: utils.ValidationResults["validationErrors"]->java.util.ArrayList[0])

我使用 play.libs.Json 实用程序类，它只是 Jackson 的对象映射器的包装器。 有谁知道问题是什么以及我该如何解决？

ValidationError类的结构如下：

package play.data.validation;

import java.util.*;

import com.google.common.collect.ImmutableList;

/**
 * A form validation error.
 */
public class ValidationError {

    private String key;
    private String message;
    private List arguments;

    /**
     * Constructs a new ValidationError.
     *
     * @param key the error key
     * @param message the error message
     */
    public ValidationError(String key, String message) {
        this(key, message, ImmutableList.of());
    }

    /**
     * Constructs a new ValidationError.
     *
     * @param key the error key
     * @param message the error message
     * @param arguments the error message arguments
     */
    public ValidationError(String key, String message, List arguments) {
        this.key = key;
        this.message = message;
        this.arguments = arguments;
    }

    /**
     * Returns the error key.
     */
    public String key() {
        return key;
    }

    /**
     * Returns the error message.
     */
    public String message() {
        return message;
    }

    /**
     * Returns the error arguments.
     */
    public List arguments() {
        return arguments;
    }

    public String toString() {
        return "ValidationError(" + key + "," + message + "," + arguments + ")";
    }

}

Answer 1

ValidationError 的字段访问器方法既不遵循JavaBean conventions，也没有使用@JsonProperty 注释，因此Jackson 无法识别它们。

以下是您稍作修改的类的工作示例，其中包含更新的方法名称：

public class JacksonGetters {
    public static class ValidationError {

        private String key;
        private String message;
        private List arguments;

        /**
         * Constructs a new ValidationError.
         *
         * @param key the error key
         * @param message the error message
         */
        public ValidationError(String key, String message) {
            this(key, message, null);
        }

        /**
         * Constructs a new ValidationError.
         *
         * @param key the error key
         * @param message the error message
         * @param arguments the error message arguments
         */
        public ValidationError(String key, String message, List arguments) {
            this.key = key;
            this.message = message;
            this.arguments = arguments;
        }

        /**
         * Returns the error key.
         */
        public String getKey() {
            return key;
        }

        /**
         * Returns the error message.
         */
        public String getMessage() {
            return message;
        }

        /**
         * Returns the error arguments.
         */
        public List getArguments() {
            return arguments;
        }

        public String toString() {
            return "ValidationError(" + key + "," + message + "," + arguments + ")";
        }

    }

    public static void main(String[] args) throws JsonProcessingException {
        ValidationError err = new ValidationError("key", "message");
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.writerWithDefaultPrettyPrinter().writeValueAsString(err));
    }

}

输出：

{
  "key" : "key",
  "message" : "message",
  "arguments" : null
}