一個(gè)Getter引發(fā)的血案
本文轉(zhuǎn)載自微信公眾號(hào)「你呀不牛」，作者不牛。轉(zhuǎn)載本文請(qǐng)聯(lián)系你呀不牛公眾號(hào)。
1需求
最近做一了個(gè)需求，調(diào)用其他服務(wù)的REST接口，感覺(jué)很簡(jiǎn)單，于是迅速就搞起來(lái)了
構(gòu)造Request類(lèi)
- public class User {
- private String name;
- private Integer age;
- public User(String name, Integer age) {
- this.name = name;
- this.age = age;
- }
- }
啪，我上來(lái)就一new
- service.sendRequest(new User("niu", 18));
打完，收工，又是努力工作(摸魚(yú))的一天。
2定位
但是，某天晚上8點(diǎn)，測(cè)試人員突然給我打電話，說(shuō)調(diào)用失敗，同時(shí)本身又缺少打印，沒(méi)有辦法具體哪出問(wèn)題了。
我是不會(huì)認(rèn)為這么簡(jiǎn)單的代碼自己會(huì)出錯(cuò)的，不可能!!
經(jīng)過(guò)網(wǎng)絡(luò)抓包后發(fā)現(xiàn)，收到的參數(shù)都是null，但是我這邊明明調(diào)用構(gòu)造器傳入?yún)?shù)了
難道出現(xiàn)靈異事件了?
經(jīng)過(guò)分析，整體數(shù)據(jù)流為：
能出現(xiàn)問(wèn)題的地方只能是序列化JSON地方，于是本地測(cè)試驗(yàn)證了這一結(jié)論：
- public static void main(String[] args) throws IOException {
- ObjectMapper objectMapper = new ObjectMapper();
- String request = objectMapper.writeValueAsString(new User("niu", 18));
- System.out.println(request);
- }
雖然是出問(wèn)題了，但是序列化并沒(méi)有轉(zhuǎn)為屬性為null的對(duì)象，而是直接拋出異常
- Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class online.jvm.bean.User and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)
- at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77)
通過(guò)查詢(xún)異常資料，解決掉這種異常需要在增加Jackson的序列化配置FAIL_ON_EMPTY_BEANS，F(xiàn)AIL_ON_EMPTY_BEANS這個(gè)配置表示如果某個(gè)bean序列化為空時(shí)不會(huì)異常失敗
- public static void main(String[] args) throws IOException {
- ObjectMapper objectMapper = new ObjectMapper();
- objectMapper.configure(FAIL_ON_EMPTY_BEANS, false);
- String request = objectMapper.writeValueAsString(new User("niu", 18));
- System.out.println(request);
- }
這種就不會(huì)報(bào)錯(cuò)，而是返回序列化成空串，也就導(dǎo)致接受方為屬性都為null
通過(guò)看自研RPC框架看到是有該FAIL_ON_EMPTY_BEANS的配置
3解決
再來(lái)分析一下原因，Jackson序列化時(shí)需要調(diào)用bean的getter方法
1、寫(xiě)上getter后再看下結(jié)果：
- public class User {
- private String name;
- private Integer age;
- public User(String name, Integer age) {
- this.name = name;
- this.age = age;
- }
- public String getName() {
- return name;
- }
- public Integer getAge() {
- return age;
- }
- public static void main(String[] args) throws IOException {
- ObjectMapper objectMapper = new ObjectMapper();
- String request = objectMapper.writeValueAsString(new User("niu", 18));
- System.out.println(request);
- // 輸出正常 ： {"name":"niu","age":18}
- }
- }
2、或者把屬性訪問(wèn)權(quán)限改為public
- public class User {
- public String name;
- public Integer age;
- public User(String name, Integer age) {
- this.name = name;
- this.age = age;
- }
- public static void main(String[] args) throws IOException {
- ObjectMapper objectMapper = new ObjectMapper();
- String request = objectMapper.writeValueAsString(new User("niu", 18));
- System.out.println(request);
- // 輸出正常 ： {"name":"niu","age":18}
- }
- }
但是如果要求不能暴露bean的屬性即使是getter也不行呢?
3、注解 @JsonProperty
這是就需要使用Jackson提供的注解 @JsonProperty
- public class User {
- @JsonProperty("userName")
- private String name;
- @JsonProperty
- private Integer age;
- public User(String name, Integer age) {
- this.name = name;
- this.age = age;
- }
- public static void main(String[] args) throws IOException {
- ObjectMapper objectMapper = new ObjectMapper();
- String request = objectMapper.writeValueAsString(new User("niu", 18));
- System.out.println(request);
- // {"userName":"niu","age":18}
- }
- }
來(lái)看下注解@JsonProperty的源碼注釋
- Marker annotation that can be used to define a non-static method as a "setter" or "getter" for a logical property (depending on its signature), or non-static object field to be used (serialized, deserialized) as a logical property.
大體意思是注解如果用在屬性上相當(dāng)于為該屬性定義getter和setter。
那如果既有g(shù)etter又有@JsonProperty注解，以哪個(gè)為準(zhǔn)呢?
- public class User {
- @JsonProperty("userName")
- private String name;
- @JsonProperty
- private Integer age;
- public User(String name, Integer age) {
- this.name = name;
- this.age = age;
- }
- public String getName() {
- return name;
- }
- public static void main(String[] args) throws IOException {
- ObjectMapper objectMapper = new ObjectMapper();
- String request = objectMapper.writeValueAsString(new User("niu", 18));
- System.out.println(request);
- // {"age":18,"userName":"niu"}
- }
- }
如果getter一個(gè)沒(méi)有的屬性，效果如何呢?
- public class User {
- @JsonProperty("userName")
- private String name;
- @JsonProperty
- private Integer age;
- public User(String name, Integer age) {
- this.name = name;
- this.age = age;
- }
- public String getName2() {
- return name;
- }
- public static void main(String[] args) throws IOException {
- ObjectMapper objectMapper = new ObjectMapper();
- String request = objectMapper.writeValueAsString(new User("niu", 18));
- System.out.println(request);
- // {"age":18,"name2":"niu","userName":"niu"}
- }
- }
這說(shuō)明如果有@JsonProperty注解，先以注解為準(zhǔn)
然后利用反射找到對(duì)象類(lèi)的所有g(shù)et方法，接下來(lái)去get，然后小寫(xiě)化，作為json的每個(gè)key值，而get方法的返回值作為value。接下來(lái)再反射field，添加到j(luò)son中。
4、特殊情況
還有一種比較特殊的情況, getter方法由lombok生成，且屬性的次首字母是大寫(xiě)：
- @Getter
- public class User {
- @JsonProperty
- private String nAme;
- @JsonProperty
- private Integer age;
- public User(String name, Integer age) {
- this.nAme = name;
- this.age = age;
- }
- public static void main(String[] args) throws IOException {
- ObjectMapper objectMapper = new ObjectMapper();
- String request = objectMapper.writeValueAsString(new User("niu", 18));
- System.out.println(request);
- // {"nAme":"niu","age":18,"name":"niu"}
- }
- }
這是因?yàn)閘ombok生成的getter會(huì)把屬性的第一個(gè)字母變成大寫(xiě)，
序列化時(shí)會(huì)把get后與小寫(xiě)字母中間的大寫(xiě)變成小寫(xiě)，也就是會(huì)把NA變成小寫(xiě)
所以序列化結(jié)果會(huì)有name(getter獲取)和nAme(注解獲取)兩個(gè)屬性
- public String getNAme() {
- return this.nAme;
- }
如果我們自己用idea快捷鍵生成getter，
此時(shí)之后序列化nAme
- public String getnAme() {
- return nAme;
- }
4小結(jié)
許多bug都是在自以為沒(méi)有問(wèn)題的地方產(chǎn)生，看似簡(jiǎn)單，更需要小心，同時(shí)也需要多注意序列化原理，整體感覺(jué)序列化還是用Gson更省心，完全不用關(guān)心Getter和Setter方法，會(huì)完全按照屬性名來(lái)序列化。
本文的涉及的bug過(guò)程和解決方式希望對(duì)你也有所幫助，再見(jiàn)。
