记录Jackson和Lombok的坑

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：

1. 会通过@JsonDeserialize注解让Jackson使用Builder来构建对象；
2. 拷贝Jackson相关的注解到Builder中（比如@JsonIgnoreProperties）;
3. 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。