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;
}
}