Kotlin Json 序列化2 - 添加注解支持

全部代码：

1. https://github.com/zhaoyubetter/kotlinDemo/tree/master/src/com/better/json

1. 添加排除注解

在一些data class中，有些property 不需要被序列化，我们可通过为期添加注解；然后在获取KProperty时，排除掉即可；

a. 添加注解类

@Target(AnnotationTarget.PROPERTY)
@Retention(AnnotationRetention.RUNTIME)
annotation class JsonExclude

b. 在serialize方法添加过滤如下：

private inline fun StringBuilder.serializeObj(o: Any) {
    // ==== 1. 获取类的所有`KProperty`
    o.javaClass.kotlin.memberProperties.filter { 
         it.findAnnotation<JsonExclude>() == null }.
        ....
}

c. 测试代码

data class UserBean(@JsonExclude val username: String, val age: Int)
val user = UserBean("better", 30)
println(serialize(user))    // {"age":30}

2. 添加属性名替换注解

a. 新增注解类：

/**
 * 替换名字
 */
@Target(AnnotationTarget.PROPERTY)
@Retention(AnnotationRetention.RUNTIME)
annotation class JsonName(val name: String)

b. 修改 serializeProperty方法如下：

private inline fun StringBuilder.serializeProperty(property: KProperty1<Any, 
      *>, receiver: Any) {
    // check Annotation JsonName
    val jsonName = property.findAnnotation<JsonName>()?.name ?: property.name
    serializeString(jsonName)
    ...
}

c. 测试一下

// change json key
data class UserBean(@JsonExclude val username: String, @JsonName("myAge") val age: Int)

3. 添加自定义解析器

比如：对于时间戳，序列化时，我们将其转换成 yyyy-MM-dd;

a. 添加转换器与转换注解

interface ValueSerializer<in T> {
    fun toJson(value: T?): Any?    // 将值转换成某个具体类型
}

// ValueSerializer 协变，用来 表示 ValueSerializer 的子类
annotation class CustomSerializer(val kClazz: KClass<out ValueSerializer<*>>)

b. 修改 serializeProperty 方法，判断是否有 CustomSerializer 注解

private inline fun StringBuilder.serializeProperty(property: KProperty1<Any, *>, receiver: Any) {
    
    ...
    // 获取属性值
    var value = property.invoke(receiver)
    // has custom serialize 判断是否有 CustomSerializer 注解
    val customSerializeClass = property.findAnnotation<CustomSerializer>()?.kClazz
    if(value != null && customSerializeClass != null) {
        // 创建KClass实例
        val valueSerializer = customSerializeClass.objectInstance ?: customSerializeClass.createInstance() 
        // * 需要强制转一下
        value =(valueSerializer as ValueSerializer<Any?>).toJson(value)
    }
    serializePropertyValue(value)
}

再利用扩展函数优化一下：

private inline fun StringBuilder.serializeProperty(property: KProperty1<Any, *>, receiver: Any) {
    ...
    val value = property.invoke(receiver)
    val jsonValue = property.getCustomSerializer(property)?.toJson(value) ?: value
    serializePropertyValue(jsonValue)
}

// 新增扩展函数
private inline fun KProperty<*>.getCustomSerializer(property: 
  KProperty1<Any, *>): ValueSerializer<Any?>? {
    // has custom serialize
    val customSerializeClass = property.findAnnotation<CustomSerializer>()?.kClazz
    if (customSerializeClass != null) {
        // 创建KClass 示例
        val valueSerializer = customSerializeClass.objectInstance ?: 
            customSerializeClass.createInstance()
        // * 需要强制转一下
        return (valueSerializer as ValueSerializer<Any?>?)
    }
    return null
}

c. 测试代码

data class MyTime(@CustomSerializer(DateSerializer::class) val birthday: Long?, val name: String)

// 具体类
object DateSerializer : ValueSerializer<Long> {
    private val dmt = SimpleDateFormat("yyyy-MM-dd")
    override fun toJson(value: Long?): Any? = if (value == null) null else dmt.format(Date())
}

// 测试
val test = MyTime(Calendar.getInstance().timeInMillis, "better")
println(serialize(test))