协程中一个lateinit变量初始化成功，另一个失败的原因排查

在Android项目的MainActivity中，通过lifecycleScope发起两个Retrofit异步请求，使用async与await从API获取数据时，todoAdapter可成功初始化并赋值，但dropdownAdapter未初始化，抛出kotlin.UninitializedPropertyAccessException异常。

问题代码 #

package com.example.retrofittodo

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import android.widget.ArrayAdapter
import androidx.core.view.isVisible
import androidx.lifecycle.lifecycleScope
import androidx.recyclerview.widget.LinearLayoutManager
import com.example.retrofittodo.databinding.ActivityMainBinding
import kotlinx.coroutines.async
import retrofit2.HttpException
import retrofit2.Response
import java.io.IOException

const val TAG = "MainActivity"

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding
    private lateinit var todoAdapter: TodoAdapter
    private lateinit var dropdownAdapter: ArrayAdapter<User>

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        // Get data
        lifecycleScope.launchWhenCreated {
            binding.progressBar.isVisible = true

            val todosDeferred = async { RetrofitInstance.api.getTodos(null) }
            val usersDeferred = async { RetrofitInstance.api.getUsers() }

            val todoResponse = try {
                todosDeferred.await()
            } catch (e: IOException) {
                // Most likely no internet connection or maybe output stream closed
                Log.e(TAG, "IOException, you might not have internet connection")
                binding.progressBar.isVisible = false
                return@launchWhenCreated
            } catch (e: HttpException) {
                // If response code does not start with digit 2 then something is unusual
                Log.e(TAG, "HttpException, unexpected response")
                binding.progressBar.isVisible = false
                return@launchWhenCreated
            }

            val userResponse = try {
                usersDeferred.await()
            } catch (e: IOException) {
                // Most likely no internet connection or maybe output stream closed
                Log.e(TAG, "IOException, you might not have internet connection")
                binding.progressBar.isVisible = false
                return@launchWhenCreated
            } catch (e: HttpException) {
                // If response code does not start with digit 2 then something is unusual
                Log.e(TAG, "HttpException, unexpected response")
                binding.progressBar.isVisible = false
                return@launchWhenCreated
            }

            if (checkResponse(todoResponse)) {
                todoAdapter.todos = todoResponse.body()!! //succeeds
            }
            else {
                Log.e(TAG, "Todo response not successful")
            }

            if (checkResponse(userResponse)) {
                dropdownAdapter = ArrayAdapter(this@MainActivity, R.layout.item_user, userResponse.body()!!) // fails
            }
            else {
                Log.e(TAG, "User response not successful")
            }

            binding.progressBar.isVisible = false
        }

        setupRecyclerView()
        binding.dropdownUser.setAdapter(dropdownAdapter)
    }

    private fun <T : Any> checkResponse(response: Response<List<T>>) =
        response.isSuccessful && response.body() != null

    private fun setupRecyclerView() = binding.rvTodos.apply {
        todoAdapter = TodoAdapter()
        adapter = todoAdapter
        layoutManager = LinearLayoutManager(this@MainActivity)
    }
}

报错日志 #

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.retrofittodo, PID: 8917
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.retrofittodo/com.example.retrofittodo.MainActivity}: kotlin.UninitializedPropertyAccessException: lateinit property dropdownAdapter has not been initialized
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3635)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3792)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2210)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loopOnce(Looper.java:201)
        at android.os.Looper.loop(Looper.java:288)
        at android.app.ActivityThread.main(ActivityThread.java:7839)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
     Caused by: kotlin.UninitializedPropertyAccessException: lateinit property dropdownAdapter has not been initialized
        at com.example.retrofittodo.MainActivity.onCreate(MainActivity.kt:119)
        at android.app.Activity.performCreate(Activity.java:8051)
        at android.app.Activity.performCreate(Activity.java:8031)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1329)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3608)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3792) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2210) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loopOnce(Looper.java:201) 
        at android.os.Looper.loop(Looper.java:288) 
        at android.app.ActivityThread.main(ActivityThread.java:7839) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003) 
W/System: A resource failed to call close. 

原因分析 #

核心问题是代码执行顺序和初始化时机的差异：

• todoAdapter的初始化是同步执行的：在onCreate中调用setupRecyclerView()时，该方法内部直接实例化了todoAdapter = TodoAdapter()，所以当协程后续给todoAdapter.todos赋值时，todoAdapter已经完成初始化，不会触发异常。
• dropdownAdapter的初始化是异步延迟的：dropdownAdapter的实例化被放在了协程内部的网络请求回调里，而binding.dropdownUser.setAdapter(dropdownAdapter)是在协程启动后立刻同步执行的。由于协程是异步挂起执行的，网络请求需要时间，所以setAdapter执行时，dropdownAdapter还没有被赋值，此时访问lateinit修饰的变量就会抛出UninitializedPropertyAccessException异常。

修复方案 #

方案一：将setAdapter移到协程内部初始化之后 #

把设置适配器的代码放在dropdownAdapter实例化之后，确保初始化完成再使用：

if (checkResponse(userResponse)) {
    dropdownAdapter = ArrayAdapter(this@MainActivity, R.layout.item_user, userResponse.body()!!)
    // 移到这里执行
    binding.dropdownUser.setAdapter(dropdownAdapter)
}

方案二：提前初始化dropdownAdapter #

给dropdownAdapter一个默认的空适配器初始化，避免lateinit变量未赋值就被访问，后续再更新数据：

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = ActivityMainBinding.inflate(layoutInflater)
    setContentView(binding.root)

    setupRecyclerView()
    // 提前初始化空适配器
    dropdownAdapter = ArrayAdapter(this, R.layout.item_user, emptyList())
    binding.dropdownUser.setAdapter(dropdownAdapter)

    // Get data
    lifecycleScope.launchWhenCreated {
        // ...其他代码
        if (checkResponse(userResponse)) {
            // 更新适配器数据
            dropdownAdapter.clear()
            dropdownAdapter.addAll(userResponse.body()!!)
        }
    }
}

内容的提问来源于stack exchange，提问作者Siddharth Manthan