Another miserable day by kotlin grammar

The source is a feedback from colleagues , A... Was reported during the test  Kotlin.Lazy  Null pointer exception for ,Lazy Is defined as follows ：

class TestA{
    ...
    val xxxx:Service? by lazy{
         xxxService()   
    }
    ...
}

It looks very ordinary by lazy Why do I report null pointer ？ In the deep lazy When viewing the source code , No suspicious points were found , At that time, the code logic involved concurrent calls , I checked it, too by lazy   The initialization , The default implementation is  SynchronizedLazyImpl, Thread safe operation has been done .

To avoid too much code interference , We will cover by lazy All the places used are copied to one Test Class , And then through  Decompile  Decompile to Java Code to see if it is kotlin The problem of .

Kotlin The code is as follows ：

class TestA {

    init {
        ....
        initView()
    }

    private fun initView() {
        //  call  Service  Method 
        service?.getName()
    }

    private val service: AService? by lazy {
        AService()
    }
}

Decompiled Java Code ：

public final class TestA {
   private final Lazy service$delegate;

   private final void initView() {
      // 1、 obtain  service  example 
      AService var10000 = this.getService();
      if (var10000 != null) {
         var10000.getName();
      }
   }

   private final AService getService() {
      Lazy var1 = this.service$delegate;
      Object var3 = null;
      // 2、 call  Lazy  Of  getValue  Method 
     .return (AService)var1.getValue();
   }

   public TestA() {
      this.initView();
      // 3、 initialization  Lazy  example 
     .this.service$delegate = LazyKt.lazy((Function0)null.INSTANCE);
   }
}

Immediately find the problem through the decompilation of the code ：

1. stay TestA In the construction method of , Execute first  initView Method to get AService Example .

2. but  getService  Methods  Lazy It's not initialized yet , But it directly calls  getValue  Method triggers a null pointer exception .

3. stay initView Do it after you finish Lazy The initialization , It's too late , The exception has already occurred .

How to solve this problem ？ Just put the by lazy Referred to the init In front of the code block , as follows ：

class TestA {
    private val service: AService? by lazy {
        AService()
    }

    init {
        initView()
    }
    ...
}

Decompilation result ：

public final class TestA {
   private final Lazy service$delegate;
   ....
   public TestA() {
      // 1、 initialization  Lazy  example 
     .this.service$delegate = LazyKt.lazy((Function0)null.INSTANCE);
      // 2、 Call again  getService  Method 
     .this.initView();
   }
}

1. Construction is finally initialized first Lazy object .

2. Call again  initView  Method , At this time, the method is   Lazy.getValue  Can be called normally .

Is it a little against common sense ？ Why calling a variable in a method also involves the location of the variable ,Kotlin I'm afraid this advanced grammar candy is even C None of them ( Laugh at , ha-ha ).

that Kotlin Is there really no grammar check on it ？ There is , Let me change the code to show you ：

IDE Will prompt the current service uninitialized ,「 However, this prompt is limited to init Call in code block lazy When it's time to remind , If in init Call an intermediate method in , Then call from the intermediate method lazy, The prompt verification will be invalid 」.

Has been Kotlin Grammar sugar pit miserable day ！！！

The author has a top , Put them together to learn ~

 

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