JSON正确反序列化ArrayList

Question

我以前从未使用过JSON，我想序列化ArrayList

我的writer类如下所示：

public class Writer {

public void write(){
    ArrayList<Person> personList = new ArrayList<>();

    Person p1 = new Person("James", "Bond", LocalDate.of(1997,9,22));
    Person p2 = new Person("Santa", "Claus", LocalDate.of(1918,11,6));
    Person p3 = new Person("Peter", "Griffin", LocalDate.of(1978,3,24));
    Person p4 = new Person("Lois", "Griffin", LocalDate.of(1982,7,14));

    personList.add(p1);
    personList.add(p2);
    personList.add(p3);
    personList.add(p4);

    ObjectMapper mapper = new ObjectMapper();
    ObjectWriter writer = mapper.writer(new DefaultPrettyPrinter());

    try {
        writer.writeValue(new File(System.getProperty("user.dir")+"/File/Person.json"), personList);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

我的读者课是这样的：

public class Reader {

public void read(){
    ObjectMapper mapper = new ObjectMapper();

    try {
        ArrayList<Person> liste = mapper.readValue(new FileInputStream("File/Personen.json"), ArrayList.class);
        System.out.println("Read: " + liste.get(0));
    } catch (IOException e) {
        e.printStackTrace();
    }
}

列表中的第一个对象如下：Gelesen:{firstname=James，lastname=Bond，生日={year=1997，month=九月，monthValue=9，dayOfMonth=22，年表={id=ISO，calendarType=iso8601}，era=CE，dayOfYear=265，dayOfWeek=MONDAY，leapYear=false}

如何将此JSON字符串转换回类"人"的java对象？我的序列化/反序列化有什么问题吗？

编辑：我想检查从JSON文件反序列化的列表中我的人是否与原始人相同，因此我编写了系统。出来println（list.get）（0）。getFirstname（）然后我得到了一个java。ClassCastException:java。base/java。util。LinkedHashMap无法强制转换为Person




Answer 1

你能试试这个代码吗？我想你需要把你的数组列表转换成json格式。并以json格式检索它。

public void write(){
    ArrayList<Person> personList = new ArrayList<>();
    JSONObject obj = new JSONObject();//JSON object.
    Person p1 = new Person("James", "Bond", LocalDate.of(1997,9,22));
    Person p2 = new Person("Santa", "Claus", LocalDate.of(1918,11,6));
    Person p3 = new Person("Peter", "Griffin", LocalDate.of(1978,3,24));
    Person p4 = new Person("Lois", "Griffin", LocalDate.of(1982,7,14));

    personList.add(p1);
    personList.add(p2);
    personList.add(p3);
    personList.add(p4);
    obj.put("list", personList);
    ObjectMapper mapper = new ObjectMapper();
    ObjectWriter writer = mapper.writer(new DefaultPrettyPrinter());
    try {
        writer.writeValue(new File(System.getProperty("user.dir")+"/File/Person.json"), obj);
    } catch (IOException e) {
        e.printStackTrace();
    } 
}

public void read(){
    ObjectMapper mapper = new ObjectMapper();
    try {
        JSONObject obj = mapper.readValue(new File("G:\\Personen.json"));
        // Convert JSON string from file to Object
        Person person = mapper.readValue(new File("G:\\Personen.json"), Person.class);
        System.out.println("Read: " + obj);

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

Answer 2

匿名用户

第一个问题：您正在反序列化到没有任何泛型类型的ArrayList，因此您将收到LinkedHashMap的ArrayList。Jackson不知道您想要的是人员列表，所以它使用LinkedHashMap作为适用于任何json的类型。

要反序列化到您应使用的人员列表，请执行以下操作：

ArrayList<Person> list = mapper.readValue(new FileInputStream("./person.json"),
                    mapper.getTypeFactory().constructCollectionType(ArrayList.class, Person.class));

第二个问题：你正在使用Java8次（LocalDate类），但杰克逊不知道。为了能够正确处理它，您需要从jackson-datatype-jsr310依赖项中添加JavaTimeMoules并注册它。

因此，生成的代码将是这样的：

public void write() {
    ArrayList<Person> personList = new ArrayList<>();

    Person p1 = new Person("James", "Bond", LocalDate.of(1997, 9, 22));
    Person p2 = new Person("Santa", "Claus", LocalDate.of(1918, 11, 6));
    Person p3 = new Person("Peter", "Griffin", LocalDate.of(1978, 3, 24));
    Person p4 = new Person("Lois", "Griffin", LocalDate.of(1982, 7, 14));

    personList.add(p1);
    personList.add(p2);
    personList.add(p3);
    personList.add(p4);

    ObjectMapper mapper = new ObjectMapper().registerModule(new JavaTimeModule());
    ObjectWriter writer = mapper.writer(new DefaultPrettyPrinter());

    try {
        writer.writeValue(new File("./person.json"), personList);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

public void read() {
    ObjectMapper mapper = new ObjectMapper().registerModule(new JavaTimeModule());

    try {
        ArrayList<Person> list = mapper.readValue(new FileInputStream("./person.json"),
                mapper.getTypeFactory().constructCollectionType(ArrayList.class, Person.class));
        System.out.println("Read: " + list.get(0).getFirstname());
    } catch (IOException e) {
        e.printStackTrace();
    }

}