使用杰克逊对象映射器将 JSON 数组转换为 POJO

Question

这可能是一个重复的问题。请参考link。
我能够将一个 json 对象映射到 POJO。但是如何使用相同的 jackson 框架将 json 对象数组转换为 pojo。

private void jsonToPojo(){
   ObjectMapper mapper=new ObjectMapper();

    try {
        User1 user1=mapper.readValue(readFromFile(), User1.class);
        User1[] user2=mapper.readValue(readFromFile(), User1[].class);
        System.out.println(user1);
        Toast.makeText(getApplicationContext(), "inside try", 0).show();
    } catch (JsonParseException e) {
        // TODO Auto-generated catch block
        Log.i("Exception", "jsonparseexception");
        e.printStackTrace();
    } catch (JsonMappingException e) {
        // TODO Auto-generated catch block
        Log.i("Exception", "jsonmapping exception");
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        Log.i("Exception", "ioexception");
        e.printStackTrace();
    }
}

这是类用户对象。

public class User {
private int age = 23;
private String name = "amal";
private List<String> messages = new ArrayList<String>() {
    {
        add("hi");
        add("how");
        add("are u.");
    }
};

//getter and setter methods

@Override
public String toString() {
    return "User [age=" + age + ", name=" + name + ", " +
            "messages=" + messages + "]";
}

这是我尝试做的：（readFromFile() 从文件中获取 json）

User1[] user2=mapper.readValue(readFromFile(), User1[].class);

jsonToPojo() 仅适用于一个对象。但是，如果我尝试上面的代码行，它不会采用以下 json：

[ 
 {
   "age":"23",
   "messages":["hi","how","are u."],
   "name":"amal"
 },

 {
   "age":"98",
   "messages":["Reply","my","question"],
   "name":"You"
 }
]

Answer 1

试试这样的：

public class Test {

/**
 * @param args
 */
public static void main(String[] args) {
    User a = new User(23, "amal");
    ArrayList<String> message = new ArrayList<String>();
    message.add("m1");
    message.add("m2");
    a.setMessages(message);

    User b = new User(58, "pete");
    User[] ab = new User[] {a, b};

    ObjectMapper mapper = new ObjectMapper();

    try {
        String s1 = getJson1(a);
        System.out.println(s1);
        User user1 = mapper.readValue(s1, User.class);
        System.out.println(user1);

        System.out.println("----------------");

        String s2 = getJson2(ab);
        System.out.println(s2);
        User[] user2 = mapper.readValue(s2, User[].class);
        for (User u : user2)
            System.out.println(u);

    } catch (Exception e) {
        e.printStackTrace();
    }

}

    private static String getJson1(User user) throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        return mapper.writeValueAsString(user);
    }

    private static String getJson2(User[] ab) throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        return mapper.writeValueAsString(ab);
    }

}

public class User {
    private int               age;
    private String            name;
    private ArrayList<String> messages;

    public User() {
        super();
    }

    public User(int age, String name) {
        this();
        this.age = age;
        this.name = name;
    }

    // ****************
    // Getter & Setter ....
    // ****************
}

你会得到这个输出：

{"age":23,"name":"amal","messages":["m1","m2"]}
User [age=23, name=amal, messages=[m1, m2]]
----------------
[{"age":23,"name":"amal","messages":["m1","m2"]},{"age":58,"name":"pete","messages":null}]
User [age=23, name=amal, messages=[m1, m2]]
User [age=58, name=pete, messages=null]