java - gson.toJson(jObj) stackoverflowerror .TypeAdapters$25.write(TypeAdapters.java :704)

Question

我正在尝试使用 gson 将 json 对象转换为字符串。下面是代码

List<Student> studs = //from db
        int count = studs.size();
        Integer tot_pages= count/limit;
        if(page>tot_pages){
            page=tot_pages;
        }
        Integer start=limit*page-limit;
        Gson gson = new Gson();
        JsonObject jObj= new JsonObject();
        jObj.addProperty("page", page);
        jObj.addProperty("total",tot_pages);
        jObj.addProperty("records",count);
        JsonArray cell = new JsonArray();
        JsonArray rows = new JsonArray();
        JsonObject row= new JsonObject();
        for(Student stud: studs){
            row= new JsonObject();
            row.addProperty("id", stud.getId());
            cell=new JsonArray();
            JsonPrimitive name = new JsonPrimitive(stud.getName());
            JsonPrimitive rollno = new JsonPrimitive(stud.getRollno());
            JsonPrimitive yr = new JsonPrimitive(stud.getYr());
            cell.add(name);
            cell.add(rollno);
            cell.add(yr);
            row.add("cell", row);
            rows.add(row);
            System.out.println(stud.getId());
        }
        System.out.println("done");

        jObj.add("rows", rows);
        System.out.println("done1");
        return gson.toJson(jObj);

并出现以下错误

Caused by: java.lang.StackOverflowError at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:416) at java.lang.StringBuffer.append(StringBuffer.java:237) at java.io.StringWriter.write(StringWriter.java:101) at com.google.gson.stream.JsonWriter.string(JsonWriter.java:534) at com.google.gson.stream.JsonWriter.writeDeferredName(JsonWriter.java:402) at com.google.gson.stream.JsonWriter.value(JsonWriter.java:495) at com.google.gson.internal.bind.TypeAdapters$25.write(TypeAdapters.java:686) at com.google.gson.internal.bind.TypeAdapters$25.write(TypeAdapters.java:704) at com.google.gson.internal.bind.TypeAdapters$25.write(TypeAdapters.java:704) at com.google.gson.internal.bind.TypeAdapters$25.write(TypeAdapters.java:704) at com.google.gson.internal.bind.TypeAdapters$25.write(TypeAdapters.java:704) at com.google.gson.internal.bind.TypeAdapters$25.write(TypeAdapters.java:704) at com.google.gson.internal.bind.TypeAdapters$25.write(TypeAdapters.java:704) <-----this line is repeated many times.

请有人帮助我。

Answer 1

更改:

row.add("cell", row);

致:

row.add("cell", cell);

顺便说一句，Gson 对象非常强大，并且应该能够序列化您的学生列表，而无需您现在使用的所有手动代码。最坏的情况是，您应该考虑创建一个 TypeAdapter为您的学生类(class)，将其注册到 Gson，并序列化为列表:

假设的学生类(class):

public class Student implements Serializable {
    private static final long serialVersionUID = 1L;

    private String ssn;
    private String schoolId;
    private String firstName;
    private String middleName;
    private String lastName;
    private Calendar enrollDate;

    // Constructors, getters/setters

    @Override
    public String toString() {
        return String.format("Student[%s,%s (%s)]", getLastName(),
                getFirstName(), getSchoolId());
    }
}

对应的适配器:

public class StudentAdapter extends TypeAdapter<Student> {

    public StudentAdapter() {
        super();
    }

    @Override
    public Student read(final JsonReader reader) throws IOException {
        if (reader.peek() == JsonToken.NULL) {
            reader.nextNull();
            return null;
        }

        final Student student = new Student();

        reader.beginObject();
        while (reader.hasNext()) {
            final String name = reader.nextName();
            if ("firstName".equals(name)) {
                student.setFirstName(reader.nextString());
            } else if ("lastName".equals(name)) {
                student.setLastName(reader.nextString());
            } else if ("schoolId".equals(name)) {
                student.setSchoolId(reader.nextString());
            }
        }
        reader.endObject();

        return student;
    }

    @Override
    public void write(final JsonWriter writer, final Student student)
            throws IOException {
        if (student == null) {
            writer.nullValue();
            return;
        }

        writer.beginObject();
        writer.name("firstName");
        writer.value(student.getFirstName());
        writer.name("lastName");
        writer.value(student.getLastName());
        writer.name("schoolId");
        writer.value(student.getSchoolId());
        writer.endObject();
    }
}

测试代码:

final List<Student> origStudentList = Arrays.asList(new Student(
        "B12J9", "Samantha", "Cole"), new Student("A09K2", "Adebisi",
        "Onihan"), new Student(null, "Miguel", "Rodriguez"));

final Gson gson = new GsonBuilder().registerTypeAdapter(Student.class,
        new StudentAdapter()).create();

final String json = gson.toJson(origStudentList);
System.out.println(json);

final List<Student> unmarshalledStudentList = gson.fromJson(json,
        new TypeToken<List<Student>>() {
        }.getType());
System.out.println(unmarshalledStudentList);

在上面的示例中，我演示了如何创建一个适配器，该适配器有选择地序列化以下属性的 schoolId、firstName 和 lastName 属性:假设的 Student 对象。您可以将此作为起点，并且我链接的 Javadoc 中还有其他类型适配器的示例。