記錄Jackson和Lombok的坑
今天遇到Jackson反序列化json缺少了欄位，後來研究下發現是Jackson的機制和Lombok生成的setter不一致，導致沒有正確呼叫setter。
復現
Java實體類
@Data
public class DemoData{
private Double t;
private Double eDay;
}
Json字串
{
"t":12.23,
"eDay":123.321
}
使用Jackson解析下來，發現只有t有值，而eDay沒有解析到。
原因分析
首先第一反應是Lombok生成的getter和setter也許有問題，於是去掉@Data註解，用IDEA生成getter和setter，再進行反序列化，發現已經可以正常反序列化了。
於是看了下編譯生成的程式碼：
public class DemoData{
private Double t;
private Double eDay;
public Double getT() {
return this.t;
}
public Double getEDay() {
return this.eDay;
}
public void setT(final Double t) {
this.t = t;
}
public void setEDay(final Double eDay) {
this.eDay = eDay;
}
}
去掉lombok的註解，直接用IDEA生成getter和setter，生成之後是這樣的：
public class DemoData{
private Double t;
private Double eDay;
public Double getT() {
return t;
}
public void setT(Double t) {
this.t = t;
}
public Double geteDay() {
return eDay;
}
public void seteDay(Double eDay) {
this.eDay = eDay;
}
}
顯然兩邊的Getter和Setter是不一樣的，那麼Jackson是怎麼尋找屬性和Setter的呢？
Jackson2在初始化序列器時，對pojo型別物件會收集其屬性資訊，屬性包括成員變數及方法，然後屬性名稱和處理過後的方法名稱做為key儲存到一個LinkedHashMap中。
收集過程中會呼叫com.fasterxml.jackson.databind.util.BeanUtil中的legacyManglePropertyName方法用來處理方法名稱，它會將get/set方法字首，即get或set去掉，並將其後面的連續大寫字元轉換成小寫字元返回。
例如: getNEWString會轉變成newstring返回。你的屬性名稱如果有這樣的"nSmallSellCount",lombok自動生成的get方法就會是這樣的"getNSmallSellCount"，處理過後就是這樣的"nsmallSellCount",這與屬性nSmallSellCount並不衝突，可以同時存在於HashMap中。
所以，當Jackson掃描由Lombok生成的POJO時，讀取到setEDay，會把set去掉，拿到EDay，然後轉成eday。由此導致json中的eDay屬性在LinkedHashMap中沒有找到setter方法，反序列化就丟失了欄位。
所以原因已經確定了：當使用Lombok修飾的POJO中存在由aAxxx這樣的（單個小寫字母跟著大寫字母)的屬性時,反序列化會丟失這個欄位。
如何解決
DeLombok
當程式碼中出現這樣的欄位時，由IDEA生成對應的getter和setter，會自動覆蓋lombok生成的方法。
使用Builder來做Jackson的反序列化器
Lombok似乎意識到了這個問題（所以為啥不改下setter的生成呢？？？），編寫了@Jacksonized這個註解來為Jackson反序列提供支援，但是這個註解必須配合@Builder或者@SuperBuilder一起使用才會生效。
我們看下@Jacksonized的官方說明：
/**
* The {@code @Jacksonized} annotation is an add-on annotation for
* {@code @}{@link Builder} and {@code @}{@link SuperBuilder}. It automatically
* configures the generated builder class to be used by Jackson's
* deserialization. It only has an effect if present at a context where there is
* also a {@code @Builder} or a {@code @SuperBuilder}; a warning is emitted
* otherwise.
* <p>
* In particular, the annotation does the following:
* <ul>
* <li>Configure Jackson to use the builder for deserialization using
* {@code @JsonDeserialize(builder=Foobar.FoobarBuilder[Impl].class)}
* on the class (where <em>Foobar</em> is the name of the annotated class).</li>
* <li>Copy Jackson-related configuration annotations (like
* {@code @JsonIgnoreProperties}) from the class to the builder class. This is
* necessary so that Jackson recognizes them when using the builder.</li>
* <li>Insert {@code @JsonPOJOBuilder(withPrefix="")} on the generated builder
* class to override Jackson's default prefix "with". If you configured a
* different prefix in lombok using {@code setterPrefix}, this value is used. If
* you changed the name of the {@code build()} method using using
* {@code buildMethodName}, this is also made known to Jackson.</li>
* <li>For {@code @SuperBuilder}, make the builder implementation class
* package-private.</li>
* </ul>
* This annotation does <em>not</em> change the behavior of the generated builder.
* A {@code @Jacksonized} {@code @SuperBuilder} remains fully compatible to
* regular {@code @SuperBuilder}s.
*/
簡單來說，這個註解會做下面的事：
- 會通過
@JsonDeserialize註解讓Jackson使用Builder來構建物件； - 拷貝Jackson相關的註解到Builder中（比如
@JsonIgnoreProperties）; - 生成的Builder類會新增
@JsonPOJOBuilder註解並寫入prefix；
因此，把上面的Pojo改寫成這樣：
@Data
@Builder
@Jacksonized
public class DemoData {
private Double t;
private Double eDay;
}
會生成下面的POJO:
@JsonDeserialize(
builder = DemoData.DemoDataBuilder.class
)
public class DemoData {
private Double t;
private Double eDay;
DemoData(final Double t, final Double eDay) {
this.t = t;
this.eDay = eDay;
}
public static DemoData.DemoDataBuilder builder() {
return new DemoData.DemoDataBuilder();
}
public Double getT() {
return this.t;
}
public Double getEDay() {
return this.eDay;
}
public void setT(final Double t) {
this.t = t;
}
public void setEDay(final Double eDay) {
this.eDay = eDay;
}
public boolean equals(final Object o) {
if (o == this) {
return true;
} else if (!(o instanceof DemoData)) {
return false;
} else {
DemoData other = (DemoData)o;
if (!other.canEqual(this)) {
return false;
} else {
Object this$t = this.getT();
Object other$t = other.getT();
if (this$t == null) {
if (other$t != null) {
return false;
}
} else if (!this$t.equals(other$t)) {
return false;
}
Object this$eDay = this.getEDay();
Object other$eDay = other.getEDay();
if (this$eDay == null) {
if (other$eDay != null) {
return false;
}
} else if (!this$eDay.equals(other$eDay)) {
return false;
}
return true;
}
}
}
protected boolean canEqual(final Object other) {
return other instanceof DemoData;
}
public int hashCode() {
int PRIME = true;
int result = 1;
Object $t = this.getT();
int result = result * 59 + ($t == null ? 43 : $t.hashCode());
Object $eDay = this.getEDay();
result = result * 59 + ($eDay == null ? 43 : $eDay.hashCode());
return result;
}
public String toString() {
Double var10000 = this.getT();
return "DemoData(t=" + var10000 + ", eDay=" + this.getEDay() + ")";
}
@JsonPOJOBuilder(
withPrefix = "",
buildMethodName = "build"
)
public static class DemoDataBuilder {
private Double t;
private Double eDay;
DemoDataBuilder() {
}
public DemoData.DemoDataBuilder t(final Double t) {
this.t = t;
return this;
}
public DemoData.DemoDataBuilder eDay(final Double eDay) {
this.eDay = eDay;
return this;
}
public DemoData build() {
return new DemoData(this.t, this.eDay);
}
public String toString() {
return "DemoData.DemoDataBuilder(t=" + this.t + ", eDay=" + this.eDay + ")";
}
}
}
此時，Jackson會使用建造者方法來構建物件，寫入屬性，Json也可以正常解析了。