Is this a bug in Kotlin or I missing something?

Question

I don't have to much experience in multi-threading. So not sure if I got right the following Java code decompiled from Kotlin.

Here is the Kotlin code:

companion object {
    @Volatile private var INSTANCE: SomeDatabase? = null

    fun getInstance(context: Context): SomeDatabase =
            INSTANCE ?: synchronized(this) {
                INSTANCE ?: buildDatabase(context).also { INSTANCE = it }
            }
}

Here is the decompiled code in Java:

     SomeDatabase var10000 = ((SomeDatabase.Companion)this).getINSTANCE();
     if (var10000 == null) {
        synchronized(this){}

        SomeDatabase var4;
        try {
           var10000 = SomeDatabase.Companion.getINSTANCE();
           if (var10000 == null) {

               ...

        var10000 = var4;
     }

     return var10000;

Doesn't this mean that code is actually not synchronized because of empty block in synchronized(this){}?

Answer 1

If you look at the bytecode itself without decompiling it, you'll see that the synchronization happens as it should - this here is a version stripped of lots of load and store operations, line numbers, etc., but what matters is where the MONITORENTER and MONITOREXIT instructions are:

public final getInstance(Landroid/content/Context;)Lcom/example/SomeDatabase;
    LDC "context"
    INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull (Ljava/lang/Object;Ljava/lang/String;)V
    INVOKESTATIC com/example/Foo.access$getINSTANCE$cp ()Lcom/example/SomeDatabase;
    MONITORENTER
    INVOKESTATIC com/example/Foo.access$getINSTANCE$cp ()Lcom/example/SomeDatabase;
    INVOKESTATIC com/example/FooKt.buildDatabase (Landroid/content/Context;)Lcom/example/SomeDatabase;
    INVOKESTATIC com/example/Foo.access$setINSTANCE$cp (Lcom/example/SomeDatabase;)V
    MONITOREXIT
    MONITOREXIT
    ARETURN

The issue you're seeing is not a bug in the compiler, but rather an issue with the decompiler, which isn't particularly rare. It's quite the challenge to decompile arbitrary generated bytecode back to Java.