记录Jackson和Lombok的坑
今天遇到Jackson反序列化json缺少了字段，后来研究下发现是Jackson的机制和Lombok生成的setter不一致，导致没有正确调用setter。
复现
Java实体类
@Data
public class DemoData{
private Double t;
private Double eDay;
}
Json字符串
{
"t":12.23,
"eDay":123.321
}
使用Jackson解析下来，发现只有t有值，而eDay没有解析到。
原因分析
首先第一反应是Lombok生成的getter和setter也许有问题，于是去掉@Data注解，用IDEA生成getter和setter，再进行反序列化，发现已经可以正常反序列化了。
于是看了下编译生成的代码：
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的注解，直接用IDEA生成getter和setter，生成之后是这样的：
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;
}
}
显然两边的Getter和Setter是不一样的，那么Jackson是怎么寻找属性和Setter的呢？
Jackson2在初始化序列器时，对pojo类型对象会收集其属性信息，属性包括成员变量及方法，然后属性名称和处理过后的方法名称做为key保存到一个LinkedHashMap中。
收集过程中会调用com.fasterxml.jackson.databind.util.BeanUtil中的legacyManglePropertyName方法用来处理方法名称，它会将get/set方法前缀，即get或set去掉，并将其后面的连续大写字符转换成小写字符返回。
例如: getNEWString会转变成newstring返回。你的属性名称如果有这样的"nSmallSellCount",lombok自动生成的get方法就会是这样的"getNSmallSellCount"，处理过后就是这样的"nsmallSellCount",这与属性nSmallSellCount并不冲突，可以同时存在于HashMap中。
所以，当Jackson扫描由Lombok生成的POJO时，读取到setEDay，会把set去掉，拿到EDay，然后转成eday。由此导致json中的eDay属性在LinkedHashMap中没有找到setter方法，反序列化就丢失了字段。
所以原因已经确定了：当使用Lombok修饰的POJO中存在由aAxxx这样的（单个小写字母跟着大写字母)的属性时,反序列化会丢失这个字段。
如何解决
DeLombok
当代码中出现这样的字段时，由IDEA生成对应的getter和setter，会自动覆盖lombok生成的方法。
使用Builder来做Jackson的反序列化器
Lombok似乎意识到了这个问题（所以为啥不改下setter的生成呢？？？），编写了@Jacksonized这个注解来为Jackson反序列提供支持，但是这个注解必须配合@Builder或者@SuperBuilder一起使用才会生效。
我们看下@Jacksonized的官方说明：
/**
* 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.
*/
简单来说，这个注解会做下面的事：
- 会通过
@JsonDeserialize注解让Jackson使用Builder来构建对象； - 拷贝Jackson相关的注解到Builder中（比如
@JsonIgnoreProperties）; - 生成的Builder类会添加
@JsonPOJOBuilder注解并写入prefix；
因此，把上面的Pojo改写成这样：
@Data
@Builder
@Jacksonized
public class DemoData {
private Double t;
private Double eDay;
}
会生成下面的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 + ")";
}
}
}
此时，Jackson会使用建造者方法来构建对象，写入属性，Json也可以正常解析了。