将对象序列化为Json时发生NullPointerException

Question

I have an object of type User (as defined below) which upon serialization to Json throws this particular error:

com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: com.providenceuniversal.gim.AuthenticationRequest["user"]->com.providenceuniversal.gim.User["lastSeenToLocal"])

User definition (with relevant properties and methods):

public class User implements ServerMessage {

    //.....

    private final @JsonInclude(Include.NON_NULL) Instant lastSeen;

    @JsonCreator
    User(@JsonProperty("username") String username) {
            if (!isValidUsername(username))
                  throw new IllegalArgumentException("Invalid constructor argument values");
            this.username = username.trim();
            this.firstName = null;
            this.lastName = null;
            this.email = null;
            this.status = null;
            this.joinDate = null;
            this.lastSeen = null;
            this.dateOfBirth = null;
    }

    @JsonCreator
    User(
         @JsonProperty("username") String username,
         @JsonProperty("firstName") String firstName,
         @JsonProperty("lastName") String lastName,
         @JsonProperty("email") String email,
         @JsonProperty("status") String status,
         @JsonProperty("joinDate") Instant joinDate,
         @JsonProperty("lastSeen") Instant lastSeen,
         @JsonProperty("dateOfBirth") LocalDate dateOfBirth) {

            if (username == null || username.trim().length() == 0)
                  throw new IllegalArgumentException("Invalid constructor argument values");
            this.username = username.trim();
            this.firstName = firstName;
            this.lastName = lastName;
            this.email = email;
            this.status = status;
            this.joinDate = joinDate;
            this.lastSeen = lastSeen;
            this.dateOfBirth = dateOfBirth;
    }

    //.....

    public Instant getLastSeen() {
            return lastSeen;
    }

    public LocalDateTime getLastSeenToLocal() {
            return getLastSeen().atZone(ZoneId.systemDefault()).toLocalDateTime();
    }

From the exception it's obvious that the problem is being caused by the getLastSeenToLocal() method (which tries to manipulate a null object in lastSeen ), whose relevance to the serialization process I can't see. Does Jackson call all getters by default, whether or not the fields they return are listed as JsonProperty s, or there is something I'm missing (clearly)?

Answer 1

Jackson by default uses getters() of class. So you can

• Use @JsonAutoDetect to use fields
• Add @JsonIgnore to getLastSeenToLocal() method
• improve getLastSeenToLocal() to check related fields is not null (or any other conditions)