杰克逊-com.fasterxml.jackson.databind.exc.MismatchedInputException

Question

我正在尝试从引用自己的Comment对象的网络中获取JSON，并通过读取JSON获得相同的Web。

这是Comment类：

@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
public class Comment {

private int                 id;

private String              text;

private Collection<Comment> parents;

private Collection<Comment> replies;

private static int          next_id = 0;


public Comment(final int id, final String text, final Collection<Comment> parents, final Collection<Comment> replies) {
    super();
    this.id = id;
    this.text = text;
    this.parents = parents;
    this.replies = replies;
}

public Comment(final String text) {
    Comment.next_id++;
    this.id = Comment.next_id;
    this.text = text;
    this.parents = new HashSet<Comment>();
    this.replies = new HashSet<Comment>();
}

public int getId() {
    return this.id;
}

public void setId(final int id) {
    this.id = id;
}

public String getText() {
    return this.text;
}

public void setText(final String text) {
    this.text = text;
}

public Collection<Comment> getParents() {
    return this.parents;
}

public void setParents(final HashSet<Comment> parents) {
    this.parents = parents;
}

public void addParent(final Comment c) {
    this.parents.add(c);
}

public Collection<Comment> getReplies() {
    return this.replies;
}

public void setReplies(final HashSet<Comment> replies) {
    this.replies = replies;
}

public void addReply(final Comment c) {
    this.replies.add(c);
}

public static int getNext_id() {
    return Comment.next_id;
}

public static void setNext_id(final int next_id) {
    Comment.next_id = next_id;
}

@Override
public String toString() {
    return "Comment [id=" + this.id + ", text=" + this.text + ", replies=" + this.replies + "]";
}

我在这里创建一些评论并将其链接：

public class CommentsJSONGenerator {

public static void main(final String[] args) throws JsonGenerationException, JsonMappingException, IOException {

    final Comment c1 = new Comment("This is a sample comment");
    final Comment c2 = new Comment("This is an answer for the first comment");
    final Comment c3 = new Comment("This is second answer for the first comment");
    final Comment c4 = new Comment("This is an answer for the third comment");

    c1.addReply(c2);
    c1.addReply(c3);
    c2.addParent(c1);
    c3.addParent(c1);
    c3.addReply(c4);
    c4.addParent(c3);

    final ObjectMapper m = new ObjectMapper();
    final ObjectWriter w = m.writer(new DefaultPrettyPrinter());
    w.writeValue(new File("src/main/resources/comments.json"), c1);

}

这是我得到的JSON：

{
"id" : 1,
"text" : "This is a sample comment",
"parents" : [ ],
"replies" : [ {
  "id" : 2,
  "text" : "This is an answer for the first comment",
  "parents" : [ 1 ],
  "replies" : [ ]
}, {
  "id" : 3,
  "text" : "This is second answer for the first comment",
  "parents" : [ 1 ],
  "replies" : [ {
    "id" : 4,
    "text" : "This is an answer for the third comment",
    "parents" : [ 3 ],
    "replies" : [ ]
  } ]
} ]
}

之后，我尝试做相反的事情，从JSON获取相同的对象。

public class JSONReader {


public static void main(final String[] args) throws JsonParseException, JsonMappingException, IOException {
    final ObjectMapper mapper = new ObjectMapper();
    final Comment c = mapper.readValue(new File("src/main/resources/comments.json"), Comment.class);
}

}

但是它抛出了这个异常：

Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `domain.Comment` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)