杰克逊数据绑定反序列化：跳过依赖于值的元素

Question

我正在使用 jackson 库来反序列化 json 数据。 如果属性不符合条件，有没有办法跳过某些元素？

举例

Java 类：

@JsonIgnoreProperties(ignoreUnknown = true)
public class Group
{
   private String name;
   private int id;
   private List<User> userList;
}

@JsonIgnoreProperties(ignoreUnknown = true)
public class User
{
   private String firstName;
   private String lastName;
   private boolean deleted;
}

Json 文件：

["test.Group", {
    "name" : "testgroup1",
    "id" : 3
    "userList" : [ "java.util.ArrayList", [ 
       ["test.User", {
        "firstName" : "John",
        "lastName"  :  "Doe",
        "deleted"  :  false } ],
       ["test.User", {
        "firstName" : "John",
        "lastName"  :  "Doe",
        "deleted"  :  true } ],
       ["test.User", {
        "firstName" : "John",
        "lastName"  :  "Doe",
        "deleted"  :  false } ] ] ]
}]

通常我是这样反序列化的：

ObjectMapper mapper = new ObjectMapper();
mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL);
test.Group g1 = mapper.readValue(jsonString,test.Group.class);

现在，是否可以跳过属性“deleted”的值为 true 的每个用户元素？ 我有办法通过数据绑定来做到这一点，还是必须使用另一种方法，如树或流式传输？

编辑

我正在为安卓而不是桌面开发。 这个问题的原因是，可能有数千个 User 元素，我想尽量减少内存使用。

Answer 1

不跳过，而是在使用 Java 8 阅读后删除（假设您的 User 有一个用于 deleted 的 getter）：

g1.getUserList().removeIf(User::isDeleted);

Answer 2

您可以通过解析树节点然后过滤节点来注入中介。一个例子：

public static void main(String[] args) 
{

    Group g = new Group();
    g.setId(1);
    g.setName("Test");

    User u1 = new User();
    u1.setDeleted(false);
    u1.setFirstName("John");
    u1.setLastName("Jones");

    User u2 = new User();
    u2.setDeleted(true);
    u2.setFirstName("Jane");
    u2.setLastName("Jones");

    g.addUser(u1);
    g.addUser(u2);

    try
    {
        ObjectMapper mapper = new ObjectMapper();
        String jsonVal = mapper.writeValueAsString(g);
        JsonNode node = mapper.readTree(jsonVal);

        for (Iterator<Entry<String, JsonNode>> it = node.fields(); it.hasNext(); ) 
        {
            Map.Entry<String, JsonNode> field = it.next();
            String key = field.getKey();

            if ("userList".equals(key))
            {                   
                JsonNode users = field.getValue();

                if (users.isArray())
                {
                    for (Iterator<JsonNode> x = users.iterator(); x.hasNext();)
                    {
                        JsonNode entry = x.next();
                        if (entry.get("deleted").asBoolean())
                        {
                            System.out.println("Remove " + entry.get("firstName").asText() + " " + entry.get("lastName").asText());
                            x.remove();
                        }
                        else
                        {
                            System.out.println("Don't remove " + entry.get("firstName").asText() + " " + entry.get("lastName").asText());
                        }
                    }
                }
            }
        }

        Group grp = mapper.treeToValue(node, Group.class);
        System.out.println("Final group: " + grp);
    }
    catch (Exception e)
    {
        System.out.println("Something went wrong...");
        e.printStackTrace();
    }
}

输出结果：

Don't remove John Jones
Remove Jane Jones
Final group: Group [name=Test, id=1, userList=[User [firstName=John, lastName=Jones, deleted=false]]]

这是在 Group 对象上使用自定义反序列化器的第二种方法。这只是我阅读的内容，因此可能会增加效率：

public class Answer28536024 {

@JsonDeserialize(using = GroupDeserializer.class)
public static class Group
{
   private String name;
   private int id;
   private List<User> userList;

   public Group()
   {
       userList = new ArrayList<User>();
   }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

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

    public void addUser(User u)
    {
        userList.add(u);
    }

    public List<User> getUserList() {
        return userList;
    }

    public void setUserList(List<User> userList) {
        this.userList = userList;
    }

    @Override
    public String toString() {
        return "Group [name=" + name + ", id=" + id + ", userList=" + userList
                + "]";
    }      
}

public static class GroupDeserializer extends JsonDeserializer<Group>
{

    @Override
    public Group deserialize(JsonParser jp, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        JsonNode node = jp.getCodec().readTree(jp);

        Group group = new Group();
        group.setName(node.get("name").asText());
        group.setId(node.get("id").asInt());


        JsonNode users = node.get("userList");
        if (users.isArray())
        {
            for (JsonNode userNode : users)
            {
                if (!userNode.get("deleted").asBoolean())
                {
                    User user = new User();
                    user.setFirstName(userNode.get("firstName").asText());
                    user.setLastName(userNode.get("lastName").asText());
                    user.setDeleted(false);

                    group.addUser(user);
                }
            }
        }

        return group;
    }       
}


public static class User
{
   private String firstName;
   private String lastName;
   private boolean deleted;

   public User()
   {

   }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public boolean isDeleted() {
        return deleted;
    }

    public void setDeleted(boolean deleted) {
        this.deleted = deleted;
    }

    @Override
    public String toString() {
        return "User [firstName=" + firstName + ", lastName=" + lastName
                + ", deleted=" + deleted + "]";
    }
}

public static void main(String[] args) 
{       
    Group g = new Group();
    g.setId(1);
    g.setName("Test");

    User u1 = new User();
    u1.setDeleted(false);
    u1.setFirstName("John");
    u1.setLastName("Jones");

    User u2 = new User();
    u2.setDeleted(true);
    u2.setFirstName("Jane");
    u2.setLastName("Jones");

    g.addUser(u1);
    g.addUser(u2);

    try
    {
        ObjectMapper mapper = new ObjectMapper();

        String jsonVal = mapper.writeValueAsString(g);
        System.out.println(jsonVal);

        Group grp = mapper.readValue(jsonVal, Group.class);
        System.out.println("Final group: " + grp);
    }
    catch (Exception e)
    {
        System.out.println("Something went wrong...");
        e.printStackTrace();
    }
}

}

这个的输出：

{"name":"Test","id":1,"userList":[{"firstName":"John","lastName":"Jones","deleted":false},{"firstName":"Jane","lastName":"Jones","deleted":true}]}
Final group: Group [name=Test, id=1, userList=[User [firstName=John, lastName=Jones, deleted=false]]]