如何在Jackson序列化时，当字段a为false时忽略字段b的序列化？

要实现当MyClass的a字段为false时忽略b字段的序列化，你之前用的@JsonInclude(Include.CUSTOM)只支持基于字段自身值的过滤，无法依赖其他字段，所以需要用以下两种方案：

方案一：使用@JsonIgnoreCondition（Jackson 2.12+ 推荐） #

这个注解支持自定义忽略条件，能访问当前序列化对象的其他字段值。

1. 自定义忽略条件类 #

import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.annotation.JsonIgnoreCondition;
import com.fasterxml.jackson.databind.introspect.Annotated;
import com.fasterxml.jackson.databind.introspect.AnnotatedField;

public class IgnoreBWhenAFalseCondition implements JsonIgnoreCondition {
    @Override
    public boolean shouldIgnore(Annotated annotated, SerializerProvider provider) throws JsonMappingException {
        if (annotated instanceof AnnotatedField) {
            // 获取当前序列化的对象实例
            Object instance = ((AnnotatedField) annotated).getOwner().getAnnotated().getInstance();
            if (instance instanceof MyClass) {
                MyClass myObj = (MyClass) instance;
                // 当a为false时，返回true表示忽略b字段
                return !myObj.isA();
            }
        }
        return false;
    }
}

2. 修改MyClass，给b字段加注解 #

public class MyClass {
    private boolean a;
    @JsonIgnore(condition = IgnoreBWhenAFalseCondition.class)
    private int b;

    // 必须提供getter方法，Jackson需要获取字段值和对象实例
    public boolean isA() {
        return a;
    }

    public void setA(boolean a) {
        this.a = a;
    }

    public int getB() {
        return b;
    }

    public void setB(int b) {
        this.b = b;
    }
}

测试验证 #

import com.fasterxml.jackson.databind.ObjectMapper;

public class SerializationTest {
    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();

        MyClass objWithTrueA = new MyClass();
        objWithTrueA.setA(true);
        objWithTrueA.setB(10);
        System.out.println(mapper.writeValueAsString(objWithTrueA)); 
        // 输出：{"a":true,"b":10}

        MyClass objWithFalseA = new MyClass();
        objWithFalseA.setA(false);
        objWithFalseA.setB(20);
        System.out.println(mapper.writeValueAsString(objWithFalseA)); 
        // 输出：{"a":false}
    }
}

方案二：使用BeanSerializerModifier（兼容低版本Jackson） #

如果你的Jackson版本低于2.12，无法使用@JsonIgnoreCondition，可以通过修改序列化器逻辑实现：

1. 自定义序列化器修改器 #

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;
import java.io.IOException;
import java.util.List;

public class MyClassSerializerModifier extends BeanSerializerModifier {
    @Override
    public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {
        for (BeanPropertyWriter writer : beanProperties) {
            if ("b".equals(writer.getName())) {
                // 为b字段替换自定义序列化逻辑
                writer.assignSerializer(new JsonSerializer<Integer>() {
                    @Override
                    public void serialize(Integer value, JsonGenerator gen, com.fasterxml.jackson.databind.SerializerProvider serializers) throws IOException {
                        // 获取当前正在序列化的对象实例
                        MyClass currentObj = (MyClass) gen.getCurrentValue();
                        if (currentObj.isA()) {
                            // 只有a为true时才序列化b
                            gen.writeNumber(value);
                        }
                    }
                });
            }
        }
        return beanProperties;
    }
}

2. 注册修改器到ObjectMapper #

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.module.SimpleModule;

public class SerializationTest {
    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        // 注册自定义修改器
        mapper.registerModule(new SimpleModule() {
            @Override
            public void setupModule(SetupContext context) {
                super.setupModule(context);
                context.addBeanSerializerModifier(new MyClassSerializerModifier());
            }
        });

        // 测试代码同方案一，结果一致
    }
}

内容的提问来源于stack exchange，提问作者MiaoZi