Jackson是Java平台非常流行的JSON解析框架，而且扩展性很强，本文通过添加一个Module来加强JSON字段的解析来实现单个字段的加密与解密功能。

基本想法是使用Jackson来处理自定义的注解，在需要加密/解密的字段上添加上相应的注解，先定义个自定义的注解：

@JacksonAnnotation
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER})
public @interface Encrypt {}

然后在需要在POJO中对字段添加@Encrypt注解：

public  final class User {

    @Encrypt
    private String name;
    private String guid;

    @Encrypt
    private User first;

    @Encrypt
    private int age;
    
    //... getters/setters
}

完成上面的工作后，然后再来扩展Jackson来处理@Encrypt注解进行字段加密，先定义个Jackson的Module：

public class EncryptionModule extends Module {

    public final static String ARTIFACT_ID = "jackson-hb-encryption";
    public final static Version VERSION = new Version(1, 0, 0, null);

    private EncryptionModule(){

    }

    ...

    @Override
    public void setupModule(SetupContext setupContext) {
        setupContext.addBeanSerializerModifier(new EncryptedSerializerModifier());
        setupContext.addBeanDeserializerModifier(new EncryptedDeserializerModifier());
    }


    /**
     * 创建一个{@link ObjectMapper}对象，支持{@link Encrypt}注解。
     * @return
     */
    public static ObjectMapper createMapper(){
        ObjectMapper objectMapper  = new ObjectMapper();
        objectMapper.setSerializationInclusion(JsonSerialize.Inclusion.NON_NULL);
        objectMapper.configure(DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        objectMapper.configure( SerializationConfig.Feature.FAIL_ON_EMPTY_BEANS, false);
        objectMapper.registerModule(new EncryptionModule());
        return objectMapper;
    }
}

创建module只需要继承org.codehaus.jackson.map.Module类，EncryptionModule在setupModule方法中添加了一个序列化修改器和反序列化修改器，EncryptedSerializerModifier重载changeProperties方法，遍历属性如果属性有@Encrypt注解那么需要将属性的JsonSerializer设置为EncryptedJsonSerializer。

public class EncryptedSerializerModifier extends BeanSerializerModifier {


    @Override
    public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BasicBeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {

        /*
            遍历beanProperties处理Encrypt.class注解
         */
        List<BeanPropertyWriter> newWriter = new ArrayList<>();
        for(BeanPropertyWriter writer : beanProperties){
            if(null == writer.getAnnotation(Encrypt.class)){
                newWriter.add(writer);
            }else{
                JsonSerializer<Object> serializer = new EncryptedJsonSerializer(writer.getSerializer());
                newWriter.add(writer.withSerializer(serializer));
            }
        }

        return newWriter;
    }
}

EncryptedJsonSerializer是自定义的序列化工具，它实现属性字段的加密功能：

public class EncryptedJsonSerializer extends JsonSerializer<Object> {

    /**
     * 默认序列化工具对象
     */
    private final JsonSerializer<Object> serializer;

    public EncryptedJsonSerializer(JsonSerializer<Object> serializer) {
        this.serializer = serializer;
    }

    @Override
    public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {
        StringWriter stringWriter = new StringWriter();
        ObjectCodec objectCodec = jsonGenerator.getCodec();
        JsonGenerator nestedGenerator = null;

        //空对象或空字符串不处理。
        if(o == null || Strings.isNullOrEmpty(String.valueOf(o))){
            if (serializer == null) {
                serializerProvider.defaultSerializeValue(o, jsonGenerator);
            }else{
                serializer.serialize(o, jsonGenerator, serializerProvider);
            }

            return;
        }

        /*
            生成一个新的JsonGenerator，用于将o写入。
         */
        if(objectCodec instanceof ObjectMapper){
            nestedGenerator = ((ObjectMapper) objectCodec).getJsonFactory().createJsonGenerator(stringWriter);
        }

        if (nestedGenerator == null) {
            throw new NullPointerException("nestedGenerator == null");
        }

        /*
            将数据写入到新生成的JsonGenerator中
         */
        if (serializer == null) {
            serializerProvider.defaultSerializeValue(o, nestedGenerator);
        }else{
            serializer.serialize(o, nestedGenerator, serializerProvider);
        }

        nestedGenerator.close();
        /*
            JsonGenerator会生成一个带双引号的字符串， 将数据加密后写入。
         */
        String value = stringWriter.getBuffer().toString();
        try {
            //空字符串不加密
            jsonGenerator.writeString(AESTools.encrypt(value));
        } catch (Exception e) {
            throw new IllegalStateException(e);
        }
    }
}

EncryptedJsonSerializer的处理是先判断是否为空对象或空字符串，如果是那就将它们直接收入到jsonGenerator，否则的话创建一个新的nestedGenerator并将数据写入进去，然后再拿出来进行AES加密，最后写入到原始的jsonGenerator中，这样就完成了属性的加密以久序列化工作。

参考资源