Classloading and initialization in Java
Understanding of when a class is loaded and initialized in JVM is one of the fundamental concept of Java programming language. Thanks to Java language specification we have everything clearly documented and explained, but many Java programmer still doesn't know when a class is loaded or when a class is initialized in Java. Class loading and initialization seems confusing and complex to many beginners and its true until having some experience in belt its not always easy to get into subtle details of How JVM works in Java. In this Java tutorial we will see when class loading occurs in Java and when and how class and interface are initialized in Java. I will not go into detail of ClasLoader or How ClassLoader works in Java, that is subject of another post I am planning. just to keep this article focused and concise. There are several articles on Java fundamentals in Javarevisited like How HashMap works in Java and How Garbage collection works in Java. If you are interested you can also check those.
When Class is loaded in Java
Class loading is done by ClassLoaders in Java which can be implemented to eagerly load a class as soon as another class references it or lazy load the class until a need of class initialization occurs. If Class is loaded before its actually being used it can sit inside before being initialized. I believe this may vary from JVM to JVM. While its guaranteed by JLS that a class will be loaded when there is a need of static initialization.
When a Class is initialized in Java
After class loading, initialization of class takes place which means initializing all static members of class. A Class is initialized in Java when :
1) an Instance of class is created using either new() keyword or using reflection using class.forName(), which may throw ClassNotFoundException in Java.
2) an static method of Class is invoked.
3) an static field of Class is assigned.
4) an static field of class is used which is not a constant variable.
5) if Class is a top level class and an assert statement lexically nested within class is executed.
Reflection can also cause initialization of class. Some methods of java.lang.reflect package may cause class to be initialized. JLS Strictly says that a class should not be initialized by any reason other than above.
How Class is initialized in Java
Now we know what triggers initialization of a class in Java, which is precisely documented in Java language specification. Its also important to know in which order various fields (static and non static), block (static an non static), various classes (sub class and super class) and various interfaces (sub interface, implementation class and super interface) is initialized in Java. Infact many Core Java interview question and SCJP question based on this concept because it affect final value of any variable if its initialized on multiple places. Here are some of the rules of class initialization in Java:
1) Classes are initialized from top to bottom so field declared on top initialized before field declared in bottom
2) Super Class is initialized before Sub Class or derived class in Java
3) If Class initialization is triggered due to access of static field, only Class which has declared static field is initialized and it doesn't trigger initialization of super class or sub class even if static field is referenced by Type of Sub Class, Sub Interface or by implementation class of interface.
4) interface initialization in Java doesn't cause super interfaces to be initialized.
5) static fields are initialized during static initialization of class while non static fields are initialized when instance of class is created. It means static fields are initialized before non static fields in Java.
6)non static fields are initialized by constructors in Java. sub class constructor implicitly call super class constructor before doing any initialization, which guarantees that non static or instance variables of super class is initialized before sub class.
Examples of class initialization in Java:
Here is an example of when class is initialized in Java. In this example we will see which classes are initialized in Java.
1) Super class is initialized before sub class in Java.
2) Static variables or blocks are initialized before non static blocks or fields.
3) Not used class is not initialized at all because its not been used, none of the cases mentioned on JLS or above which triggers initialization of class is not happened here.
Let's have a look on another example of class initialization in Java:
1. Here class initialization occurs because static field is accessed which is not a compile time constant. had you declare "familyName" compile time constant using final keyword in Java (as shown in commented section) class initialization of super class would not have occurred.
2) Only super class is initialized even though static field is referenced using sub type.
There is another example of class initialization related to interface on JLS which explains clearly that initialization of sub interfaces does not trigger initialization of super interface. I highly recommend reading JLS 14.4 for understating class loading and initialization in more detail.
That's all on When a class is initialized and loaded in Java. We have seen clear guidelines form JLS regarding class initialization. We have also seen the order on which super type and sub type are initialized and order of initialization for both static and non static fields and blocks in Java.
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