使用java注解实现toJson方法

如果我有一个对象user，它有几个属性，我想把该对象序列化成一个json字符串，怎么做？我怎么把这种类型的问题实现成一个函数？

注解类似于在被注解的对象上，添加一些简单的属性。在运行时解析这些属性，以实现不同的功能。

在以下代码中，我通过两种方式：1、在属性上添加注解；或者2、在类上添加注解。来实现这个功能。

为简化问题，我仅处理字段类型为String和ArrayList<String>的属性。

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.HashMap;

class JsonFormaterTest{
    public static void main(String[] args){
        ArrayList<String> fruits = new ArrayList<String>();
        fruits.add("apple");
        fruits.add("orange");
        User user = new User("abc@163.com", "abc", fruits);
        JsonFormater jf = new JsonFormater(user);
        System.out.println(jf.toJson(1));

        Person person = new Person("abc@163.com", "abc", fruits);
        jf = new JsonFormater(person);
        System.out.println(jf.toJson(2));
    }
}

class JsonFormater{
    private Object object;
    public JsonFormater(Object obj){
        this.object = obj;
    }
    public ArrayList<ArrayList<String>> toJsonArr(){
        Field[] fields = this.object.getClass().getDeclaredFields();
        ArrayList<ArrayList<String>> json = new ArrayList<ArrayList<String>>();
        for(Field field : fields){
            if(field.isAnnotationPresent(JsonFieldFormat.class)){
                JsonFieldFormat jff = (JsonFieldFormat) field.getAnnotation(JsonFieldFormat.class);
                ArrayList<String> aFieldArr = new ArrayList<String>();
                String fieldName;
                if(jff.name().equals("")){
                    fieldName = field.getName();
                }
                else {
                    fieldName = jff.name();
                }
                aFieldArr.add(fieldName);
                try {
                    if (jff.type().equals("String")) {
                        Field aField = this.object.getClass().getDeclaredField(field.getName());
                        aField.setAccessible(true);
                        String aFieldValue = (String) aField.get(this.object);
                        aFieldArr.add("\""+aFieldValue+"\"");
//                        Method aFieldMethod = this.object.getClass().getDeclaredMethod(fieldName, null);
//                        aFieldMethod.setAccessible(true);
//                        String aFieldValue = (String) aFieldMethod.invoke(this.object, null);
                    } else if (jff.type().equals("ArrayList")) {
                        Field aField = this.object.getClass().getDeclaredField(field.getName());
                        aField.setAccessible(true);
                        ArrayList<String> aFieldValue = (ArrayList<String>) aField.get(this.object);
                        String fieldValues = "[";
                        for (int i = 0; i < aFieldValue.size(); i++) {
                            String str = aFieldValue.get(i);
                            fieldValues = fieldValues + "\"" + str + "\"";
                            if(i!=aFieldValue.size()-1){
                                fieldValues = fieldValues + ",";
                            }
                        }
                        fieldValues = fieldValues + "]";
                        aFieldArr.add(fieldValues);
                    }
                    json.add(aFieldArr);
                }
                catch (Exception e){
                    //
                }
            }
            else {
//                System.out.println("unused field");
//                System.out.println(field.getName());
            }
        }
        return json;
    }
    public ArrayList<ArrayList<String>> toJsonArr2(){
        Field[] fields = this.object.getClass().getDeclaredFields();
        ArrayList<ArrayList<String>> json = new ArrayList<ArrayList<String>>();
        if(this.object.getClass().isAnnotationPresent(JsonClassFormat.class)){
            JsonClassFormat jcf = (JsonClassFormat) this.object.getClass().getAnnotation(JsonClassFormat.class);
            String[] nameTypes = jcf.nameTypes();
            int len = nameTypes.length;
            int fieldLen = len/3;
            HashMap<String, String> types = new HashMap<String, String>();
            HashMap<String, String> aliasNames = new HashMap<String, String>();
            for(int i=0;i<fieldLen;i++){
                types.put(nameTypes[i*3], nameTypes[i*3+1]);
                aliasNames.put(nameTypes[i*3], nameTypes[i*3+2]);
            }
            for(Field field : fields){
                if(types.get(field.getName()) != null){
                    ArrayList<String> aFieldArr = new ArrayList<String>();
                    String fieldName;
                    if(aliasNames.get(field.getName()).equals("")){
                        fieldName = field.getName();
                    }
                    else {
                        fieldName = aliasNames.get(field.getName());
                    }
                    aFieldArr.add(fieldName);
                    try{
                        Field aField = this.object.getClass().getDeclaredField(field.getName());
                        aField.setAccessible(true);
                        if(types.get(field.getName()).equals("String")){
                            String aFieldValue = (String) aField.get(this.object);
                            aFieldArr.add("\""+aFieldValue+"\"");
                        }
                        else if(types.get(field.getName()).equals("ArrayList")){
                            ArrayList<String> aFieldValue = (ArrayList<String>) aField.get(this.object);
                            String fieldValues = "[";
                            for(int i=0;i<aFieldValue.size();i++){
                                String str = aFieldValue.get(i);
                                fieldValues = fieldValues + "\"" + str + "\"";
                                if(i!=aFieldValue.size()-1){
                                    fieldValues = fieldValues + ",";
                                }
                            }
                            fieldValues = fieldValues + "]";
                            aFieldArr.add(fieldValues);
                        }
                        json.add(aFieldArr);
                    }
                    catch (Exception e){
                        //
                    }
                }
                else{
//                    System.out.println("unused field");
//                    System.out.println(field.getName());
                }
            }
        }
        return json;
    }
    public String toJson(int x){
        ArrayList<ArrayList<String>> json;
        if(x==1){
            json = toJsonArr();
        }
        else{
            json = toJsonArr2();
        }
        ArrayList<String> allFields = new ArrayList<String>();
        for(int i=0;i<json.size();i++){
            ArrayList<String> str = json.get(i);
            allFields.add("\""+str.get(0)+"\":"+str.get(1)+"");
        }
        String jsonStr = "{";
        for(int i=0;i<allFields.size();i++){
            jsonStr = jsonStr + allFields.get(i);
            if(i!=allFields.size()-1){
                jsonStr = jsonStr + ",";
            }
        }
        jsonStr = jsonStr + "}";
        return jsonStr;
    }
}

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface JsonFieldFormat {
    String type() default "";
    String name() default "";
}

class User{
    @JsonFieldFormat(type="String")
    private String email;
    @JsonFieldFormat(type="String", name="alias")
    private String name;
    @JsonFieldFormat(type="ArrayList")
    private ArrayList<String> favFruits;

    private String otherField;

    public User(String email, String name, ArrayList<String> favFruits){
        this.email = email;
        this.name = name;
        this.favFruits = favFruits;
    }
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface JsonClassFormat {
    String[] nameTypes();
}

@JsonClassFormat(nameTypes={"email", "String", "", "name", "String", "alias", "favFruits", "ArrayList", ""})
class Person{
    private String email;
    private String name;
    private ArrayList<String> favFruits;

    private String otherField;

    public Person(String email, String name, ArrayList<String> favFruits){
        this.email = email;
        this.name = name;
        this.favFruits = favFruits;
    }
}