Java Static Blocks Loading Order
- Static blocks are always ran before the object is created
- When static field and static variable are together, they are in textual order
- Static block of parent is executed firstly because it is loaded firstly
Static field and static variable
This question confused me a lot when I learnt lazy initialization holder class of
Singleton design pattern.
So why here it could be a
Singleton? How is the order of initialization?!
getInstance() is invoked in the first time. It will load
SingletonHolder.instance which leads
SingletonHolder class to be initialized. When
SingletonHolder is initialized, it will initialize its static field. As it’s a static field, so it will only be initialized one time and thread safe.
From this moment, a question comes into my mind? What’s the order to initializer static blocks mixed with inner class and superclass?
Let’s see amore complexe situation:
Static Field Test 1 Construct! Test 2 Construct! Inner Statuc Field Test 3 Construct!
So From the output, we get the following process:
Singleton t = new Singleton(2)-> Initialize Class `Singleton -> Initialize Static Filed -> Initialize Static Variable -> Run its constructor
Singleton.getInstance()-> Initialize Class
Inner-> Initialize Static Filde -> Initialize Static Variable -> Run
Now let’s exchange positions of static field and static variable of inner class:
Static Field Test 1 Construct! Test 2 Construct! Test 3 Construct! Inner Static Field
So here, the order to initialize Static Field and Static Variable exchanges, too!
From section 12.4.2 of the Oracle Java Language Specification:
The procedure for initializing is as follows:
Then, initialize the final class variables and fields of interfaces whose values are compile-time constant expressions (§220.127.116.11, §9.3.1, §13.4.9, §15.28). Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order, as though they were a single block.
In textual order! So this is why we have different out put in the above example!
Parent static block Child static block Parent initialisation block Parent Constructor Child initialisation block Child Constructor
When we are calling constructor of the subclass (child), then this constructor implicitly calls
super() before executing it’s own constructor. So Class
Parent will now be initialized. But static blocks are always ran before the object is created, so we see firstly “Parent static block”, then “Child static block”. What’s more, Initialization block comes into play even before the constructor call.