// 这是Kotlin代码，Java也是一样原理，都是用@JsonView实现单向透明。// 原理：使用@JsonView和withView(JvXxx) 让jackson从某一方向扫描时，直接忽略某些字段。// 所以一对多/多对多的循环引用问题都能解决，从任意方向开始引用的都行。 由于这种玩法似乎挤占了JsonView的原有用途，所以是有代价的，本篇讨论了代价及如何避坑。// 高级篇：模拟真实案例中JsonView的复杂关系，估计没几个人会遇到这么复杂的情况。

import com.fasterxml.jackson.annotation.JsonView

import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper

// 各种各样的 JsonView，跟数据库的View是同样用途private interface JvClientMini //给客户端的简略信息private interface JvClient : JvClientMini //给客户端的完整信息private interface JvPersist //用于数据存储

// 为了从各个方向都能随心所欲地切断循环引用，// 下边又定义了一批以JvUser和JvRole为前缀的JsonView。// 这些定义本来有可能不必要的，但这就是代价。// 不过代价倒也不算太高，毕竟输出一个个符合特定要求的Json也可能要费不少代码。private interface JvUserClientMini : JvClientMini

private interface JvUserClient : JvUserClientMini, JvClient

private interface JvUserPersist : JvPersist

private interface JvRoleClient : JvClient

private interface JvRolePersist : JvPersist

private class User(

var id: Long = 0,

var name: String = "",

@JsonView(value = [JvUserPersist::class])

var password: String = "123456",

@JsonView(value = [JvUserClient::class, JvUserPersist::class])

var roles: List = listOf()

)

private class Role(

var id: Long = 0,

var role: String = "",

@JsonView(value = [JvPersist::class])

var auth: String = "all",

@JsonView(value = [JvRoleClient::class])

var users: List = listOf()

) {

@JsonView(value = [JvClient::class])

fun usersCount() = users.size

}

private var writer = jacksonObjectMapper().writer().withDefaultPrettyPrinter()

private fun printJsonWithView(obj: Any, jv: Class) {

println("JsonView: ${jv.simpleName}");

writer.withView(jv).writeValueAsString(obj).let { println(it) }

}

fun main() {

//construct many to many reference val list = listOf(1, 2)

val users = list.map { User(it.toLong(), "name-$it") }

val roles = list.map { Role(it.toLong(), "role-$it") }

users.forEach { it.roles = roles }

roles.forEach { it.users = users }

//use @JsonView & withView(FromXXX) to resolve recursion reference println("解决json一对多&多对多循环引用问题，从任意方向开始引用的都没问题")

println("user -> roles -> users -> roles ... OK")

printJsonWithView(users[0], JvUserClientMini::class.java)

printJsonWithView(users[0], JvUserClient::class.java)

printJsonWithView(users[0], JvUserPersist::class.java)

println("role -> users -> roles -> users ... OK")

printJsonWithView(roles[0], JvRoleClient::class.java)

printJsonWithView(roles[0], JvRolePersist::class.java)

// Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: // Infinite recursion (StackOverflowError) (through reference chain: // println(writer.writeValueAsString(users[0])) // println(writer.writeValueAsString(roles[0]))

// 解决jackson循环引用问题，网上其他常见的套路： // 一对多可行，单向引用可行，多向引用比较麻烦 // @JsonBackReference // @JsonManagedReference // @JsonIdentityReference // @JsonIdentityInfo}

/* 输出结果："""解决json一对多&多对多循环引用问题，从任意方向开始引用的都没问题user -> roles -> users -> roles ... OKJsonView: JvUserClientMini{"id" : 1,"name" : "name-1"}JsonView: JvUserClient{"id" : 1,"name" : "name-1","roles" : [ {"id" : 1,"role" : "role-1","usersCount" : 2}, {"id" : 2,"role" : "role-2","usersCount" : 2} ]}JsonView: JvUserPersist{"id" : 1,"name" : "name-1","password" : "123456","roles" : [ {"id" : 1,"role" : "role-1","auth" : "all"}, {"id" : 2,"role" : "role-2","auth" : "all"} ]}role -> users -> roles -> users ... OKJsonView: JvRoleClient{"id" : 1,"role" : "role-1","users" : [ {"id" : 1,"name" : "name-1"}, {"id" : 2,"name" : "name-2"} ],"usersCount" : 2}JsonView: JvRolePersist{"id" : 1,"role" : "role-1","auth" : "all"}"""*/