java - 为什么主构造函数主体在字段初始化之前执行？

Question

所以我正在重写 Android 应用程序的一些遗留代码。

该更改的一部分包括引入 View 模型。其中的一部分包括将曾经是对象的UserManager类更改为AndroidViewModel。

class UserManager(application: Application) : AndroidViewModel(application) {

  private val userData: MutableMap<User, MutableMap<String, Any>> = object : HashMap<User, MutableMap<String, Any>>() {
      override fun get(key: User): MutableMap<String, Any>? {
          val former = super.get(key)
          val current = former ?: mutableMapOf()
          if (current !== former) this.put(key, current)
          return current
      }
  }

  init {
    restoreActiveUsers()
  }
  
  override fun onCleared() {
      persistActiveUsersData()
  }

  private fun restoreActiveUsers() {
    val decodedUsers: List<User> = ... load users from persistent storage ...
    
    decodedUsers.forEach { userData[it] } //create an entry in [userData] with the user as key, if none exists

    ...
  }
}

init block 是新的，因为在我进行转换之前，它曾经在对象实例上被调用，这是我困惑的根源。

因为尝试像这样运行应用程序会给我带来异常 decodedUsers.forEach { userData[it] } 因为

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bla.bla.bla.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.bla.bla.bla..user.service.UserManager
          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
          at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
          at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
          ...
       Caused by: java.lang.RuntimeException: Cannot create an instance of class com.,bla.blab.bla.user.service.UserManager
          at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:275)
          at androidx.lifecycle.SavedStateViewModelFactory.create(SavedStateViewModelFactory.java:106)
          ...
          at com.,bla.blab.bla.app.ui.MainActivity.getUserManager(Unknown Source:7)
          at com.,bla.blab.bla.app.ui.MainActivity.onCreate(MainActivity.kt:71)
          at android.app.Activity.performCreate(Activity.java:7802)
          ...
      Caused by: java.lang.reflect.InvocationTargetException
          at java.lang.reflect.Constructor.newInstance0(Native Method)
          at java.lang.reflect.Constructor.newInstance(Constructor.java:343)
          at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:267)
          at androidx.lifecycle.SavedStateViewModelFactory.create(SavedStateViewModelFactory.java:106)
          ...
      Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.Map.get(java.lang.Object)' on a null object reference
          at com.bla.bla.bla.user.service.UserManager.restoreActiveUsers(UserManager.kt:178)
          at com.bla.bla.bla.user.service.UserManager.<init>(UserManager.kt:60)

我用调试器检查过，userData确实是null。

但这没有意义。

因为我没有其他想法，并且不顾 AndroidStudio 的抗议，我还是改用了辅助构造函数。

constructor(application: Application) : super(application) {
    restoreActiveUsers()
}

这样就成功了。

不过，我很难理解为什么。

根据jvm specs :

Whenever a new class instance is created, memory space is allocated for it with room for all the instance variables declared in the class type and all the instance variables declared in each superclass of the class type, including all the instance variables that may be hidden (§8.3).

If there is not sufficient space available to allocate memory for the object, then creation of the class instance completes abruptly with an OutOfMemoryError. Otherwise, all the instance variables in the new object, including those declared in superclasses, are initialized to their default values (§4.12.5).

Just before a reference to the newly created object is returned as the result, the indicated constructor is processed to initialize the new object using the following procedure:

1. Assign the arguments for the constructor to newly created parameter variables for this constructor invocation.

2. If this constructor begins with an explicit constructor invocation (§8.8.7.1) of another constructor in the same class (using this), then evaluate the arguments and process that constructor invocation recursively using these same five steps. If that constructor invocation completes abruptly, then this procedure completes abruptly for the same reason; otherwise, continue with step 5.

3. This constructor does not begin with an explicit constructor invocation of another constructor in the same class (using this). If this constructor is for a class other than Object, then this constructor will begin with an explicit or implicit invocation of a superclass constructor (using super). Evaluate the arguments and process that superclass constructor invocation recursively using these same five steps. If that constructor invocation completes abruptly, then this procedure completes abruptly for the same reason. Otherwise, continue with step 4.

4. Execute the instance initializers and instance variable initializers for this class, assigning the values of instance variable initializers to the corresponding instance variables, in the left-to-right order in which they appear textually in the source code for the class. If execution of any of these initializers results in an exception, then no further initializers are processed and this procedure completes abruptly with that same exception. Otherwise, continue with step 5.

5. Execute the rest of the body of this constructor. If that execution completes abruptly, then this procedure completes abruptly for the same reason. Otherwise, this procedure completes normally.

如果我没看错的话，实例变量应该总是在构造函数体执行之前初始化。

这意味着 init{...} 在构造函数之前执行。

但这也没有意义，因为根据 these docs ,

The Java compiler copies initializer blocks into every constructor.

这会让它们在实例变量初始化之后执行，不是吗？

那么......这里发生了什么？

为什么userData出现在上面的类null中，而它不应该是这样？

Answer 1

TL;DR

在 Kotlin 端找不到任何错误，可能是 Android 行为。

Kotlin 的初始化顺序

1. 主构造函数
2. 辅助构造函数
3. 属性和初始化 block - 取决于它们的(自上而下)顺序

During an instance initialization, the initializer blocks are executed in the same order as they appear in the class body, interleaved with the property initializers

Check out the code example in kotlindoc

Note that code in initializer blocks effectively becomes part of the primary constructor. Delegation to the primary constructor happens as the first statement of a secondary constructor, so the code in all initializer blocks and property initializers is executed before the secondary constructor body. Even if the class has no primary constructor, the delegation still happens implicitly, and the initializer blocks are still executed

结论

您的代码应该执行
第一UserManager(application: Application) ,
然后AndroidViewModel(application) ,
然后private val userData: MutableMap<User, MutableMap<String, Any>> = ... ,
然后init { restoreActiveUsers() }

我尝试在 EDI 中编写此示例(扩展普通类而不是 AndroidViewModel )，但我无法重现该异常:

private open class Boo (private val input: Int)

private class Foo : Boo(1) {

    private val logger = LoggerFactory.getLogger(this::class.java)

    val userData: MutableMap<String, MutableMap<String, Any>> = object : HashMap<String, MutableMap<String, Any>>() {
        override fun get(key: String): MutableMap<String, Any>? {
            val former = super.get(key)
            val current = former ?: mutableMapOf()
            if (current !== former) this.put(key, current)
            return current
        }
    }

    init {
        restoreActiveUsers()
    }

     private fun restoreActiveUsers() {
        (1..3).forEach { _ -> logger.info {  "${userData["notInside"]}"  } }
    }
}

输出:
{}
{}
{}

-

您的问题表明行为非常奇怪，因为字段 userData是一个不可为空 val并且不会产生编译错误，如果将 init block 写在该字段上方，则会产生编译错误!因此，当纯粹涉及 kotlin 时，必须先初始化该字段。
我没有 Android 开发经验，也不知道初始化部分是如何工作的，但我强烈建议在那里查找问题。