Kotlin leaking this in constructor

Issue

I have an abstract class that implements DefaultLifecycleObserver . I’d like to call lifecycle.addObserver(this) from the init block, but it says «Leaking ‘this’ in constructor of non-final class MyAbstractClass».

abstract class MyAbstractClass(protected val activity: AppCompatActivity) : DefaultLifecycleObserver < init < activity.lifecycle.addObserver(this) >. . . > 

I can move this line of code to the init block of each final class that extends this abstract class, but I don’t like the idea, especially because I want to guarantee that each new class that will extend MyAbstractClass in the future will call it as well. Is there a better place to call this without creating a leak?

Solution

I suppose you could post your call so it only happens after the object is fully instantiated:

abstract class MyAbstractClass(protected val activity: AppCompatActivity) : DefaultLifecycleObserver < init < Handler(Looper.getMainLooper()).post < activity.lifecycle.addObserver(this) >> > 

Or it might be less surprising to create an extension function you can tack onto your constructor calls. Then you can explicitly start the observation immediately. You’d have to make activity public, though. By defining it in an extension like this, your subclasses can call this and return themselves so you can chain it to constructor calls.

fun T.alsoBegin(): T < activity.lifecycle.addObserver(this) return this >val foo = SomeImplementation(myActivity).alsoBegin() 

Источник

Kotlin – “Leaking ‘this’ in constructor” warning should apply to final classes as well as open ones

In Kotlin if you have an open class which refers to this in its constructor or init block, you (quite rightly) get a compiler warning:

Leaking ‘this’ in constructor of non-final class

The reason for this is explained here.

My question is: why is this not reported when the class is final? If this is used in the init block before that block has completed, the object is still not in a fully constructed state, so shouldn’t the warning apply there too?

This can even lead to a situation where a val property seems to change at runtime. Take this code as an example:

class Listener < fun onCreated(leaker: Leaker) = println("Listener hears that leaker created with a value of $") > class Leaker(listener: Listener) < val myVal: Int init < listener.onCreated(this) myVal = 1 println("Leaker knows that it's been created with a value of $myVal") >> 

Using these objects as follows:

will result in the following output:

Listener hears that leaker created with a value of 0 Leaker knows that it's been created with a value of 1 

Notice that myVal is initially reported as being 0, then as being 1.

As can be seen, Leaker passes an instance of itself to Listener before Leaker has been fully constructed. Listener can then access the myVal property before it’s been initialized, so it’ll have the default value (0 in this case as it’s an integer). Later on Listener then changes the value of this property (to 1 in this example). This means that the program behaves as if a val has changed.

Should the compiler warn you about this?

Best Solution

tl;dr: https://youtrack.jetbrains.com/issue/KT-22044 is a good fit regarding this issue.

I will cite what Intellij IDEAs inspection called «Leaking ‘this’ in constructor» says about this:

• Accessing a non-final property in constructor
• Calling a non-final function in constructor
• Using this as a function argument in a constructor of a non-final class

abstract class Base < val code = calculate() abstract fun calculate(): Int >class Derived(private val x: Int) : Base() < override fun calculate() = x >fun testIt() < println(Derived(42).code) // Expected: 42, actual: 0 >

I think that there should be warning nonetheless as you were able to access a non-initialized variable. The reason: the compiler already disallows direct access to uninitialized variables, i.e. the following will not compile:

but accessing it indirectly compiles and shows the default value of the variable type:

and there we also have a «leak», basically accessing some before it should 😉

Источник

«Leaking ‘this’ in constructor» warning should apply to final classes as well as open ones?

tl;dr: https://youtrack.jetbrains.com/issue/KT-22044 is a good fit regarding this issue.

I will cite what Intellij IDEAs inspection called «Leaking ‘this’ in constructor» says about this:

• Accessing a non-final property in constructor
• Calling a non-final function in constructor
• Using this as a function argument in a constructor of a non-final class

abstract class Base < val code = calculate() abstract fun calculate(): Int >class Derived(private val x: Int) : Base() < override fun calculate() = x >fun testIt() < println(Derived(42).code) // Expected: 42, actual: 0 >

I think that there should be warning nonetheless as you were able to access a non-initialized variable. The reason: the compiler already disallows direct access to uninitialized variables, i.e. the following will not compile:

but accessing it indirectly compiles and shows the default value of the variable type:

and there we also have a «leak», basically accessing some before it should 😉

Related videos on Youtube

Yoni Gibbs

Comments

In Kotlin if you have an open class which refers to this in its constructor or init block, you (quite rightly) get a compiler warning:

The reason for this is explained here. My question is: why is this not reported when the class is final? If this is used in the init block before that block has completed, the object is still not in a fully constructed state, so shouldn’t the warning apply there too? This can even lead to a situation where a val property seems to change at runtime. Take this code as an example:

class Listener < fun onCreated(leaker: Leaker) = println("Listener hears that leaker created with a value of $") > class Leaker(listener: Listener) < val myVal: Int init < listener.onCreated(this) myVal = 1 println("Leaker knows that it's been created with a value of $myVal") >> 

Listener hears that leaker created with a value of 0 Leaker knows that it's been created with a value of 1 

Notice that myVal is initially reported as being 0, then as being 1. As can be seen, Leaker passes an instance of itself to Listener before Leaker has been fully constructed. Listener can then access the myVal property before it’s been initialized, so it’ll have the default value (0 in this case as it’s an integer). Later on Listener then changes the value of this property (to 1 in this example). This means that the program behaves as if a val has changed. Should the compiler warn you about this?

Источник

[Solved]-«Leaking ‘this’ in constructor» warning should apply to final classes as well as open ones?-kotlin

tl;dr: https://youtrack.jetbrains.com/issue/KT-22044 is a good fit regarding this issue.

I will cite what Intellij IDEAs inspection called «Leaking ‘this’ in constructor» says about this:

• Accessing a non-final property in constructor
• Calling a non-final function in constructor
• Using this as a function argument in a constructor of a non-final class

abstract class Base < val code = calculate() abstract fun calculate(): Int >class Derived(private val x: Int) : Base() < override fun calculate() = x >fun testIt() < println(Derived(42).code) // Expected: 42, actual: 0 >

I think that there should be warning nonetheless as you were able to access a non-initialized variable. The reason: the compiler already disallows direct access to uninitialized variables, i.e. the following will not compile:

but accessing it indirectly compiles and shows the default value of the variable type:

and there we also have a «leak», basically accessing some before it should 😉

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Источник

Предупреждение «Утечка этого» в конструкторе должно относиться как к конечным классам, так и к открытым?

В Kotlin, если у вас есть класс open , который ссылается на this в своем конструкторе или блоке init , вы (совершенно справедливо) получите предупреждение компилятора:

Leaking ‘this’ in constructor of non-final class

Причина этого объясняется здесь.

Мой вопрос: почему этот нет сообщается, когда класс является окончательным? Если this используется в блоке init до того, как этот блок завершен, объект все еще не находится в полностью сконструированном состоянии, поэтому разве предупреждение не должно применяться и там?

Это может даже привести к тому, что свойство val изменится во время выполнения. В качестве примера возьмем этот код:

class Listener < fun onCreated(leaker: Leaker) = println("Listener hears that leaker created with a value of $") > class Leaker(listener: Listener) < val myVal: Int init < listener.onCreated(this) myVal = 1 println("Leaker knows that it's been created with a value of $myVal") >> 

Использование этих объектов следующим образом:

приведет к следующему выводу:

Listener hears that leaker created with a value of 0 Leaker knows that it's been created with a value of 1 

Обратите внимание, что myVal сначала отображается как 0, а затем как 1.

Как можно видеть, Leaker передает свой экземпляр Listener до того, как Leaker будет полностью сконструирован. Затем Listener может получить доступ к свойству myVal до его инициализации, поэтому оно будет иметь значение по умолчанию (в данном случае 0, поскольку это целое число). Позже Listener изменяет значение этого свойства (в этом примере на 1). Это означает, что программа ведет себя так, как если бы val был изменен.

Должен ли компилятор вас об этом предупредить?

Источник