My previous blog explains how a class is loaded. Once Java class is compiled, it’s binary is loaded into the JVM. Next two phase are Linking and Initialization.
Linking
Linking involves three steps.
Verification
Preparation
Resolution.
Verification:
One of the advantage of java architecture is that once a java source file is compiled and converted to bytecode it can be executed in any JVM. This also introduce security risk as the source is untrustworthy. So verification is like a security guard which check the byte code before it is allowed to be executed by JVM. Verifier is a part of VM and programmer does not have any control over it.
Preparation:
Memory allocation and initialization of class variable to default value is done at this steps
Two important point is class variable are assigned initial value later and the value for Boolean is not false but integer zero(0).
Two important point is class variable are assigned initial value later and the value for Boolean is not false but integer zero(0).
Resolution:
Resolution involves resolving symbolic reference of classes, interfaces, fields and methods with direct reference.
"symbolic references" are strings that can be used to retrieve the actual object
Class and Interface resolution
Field Resolution
Exception :
NoSuchFiledError : Field that cannot be resolved
IllegalAccessError : Field is not accessible
Method Resolution:
Exception
NoSuchMethodError: Method Resolution Fails.
AbstractMethodError: Method resolution succeed but method is abstract and class is not abstract
IllegalAccessError: Field is not accessible
Initialization
Once linking is completed the last phase Initialization is executed. In summary this phase assign initial value to class variable and static initializers.
JVM compiles the java class and collects the class variable and static initializer into a no name methods call “”. Programmer does not have any control on this special methods and only VM execute this method to assign initial value to class variable and static initializer
JVM compiles the java class and collects the class variable and static initializer into a no name methods call “”. Programmer does not have any control on this special methods and only VM execute this method to assign initial value to class variable and static initializer
This special methods is called Class Initialization method for class and Interface initialization method for interface.
Note: If a java class does not have class variable or static initializer then the compiled class will not have no name special method “”.
Class Initialization
Initialize the class’s direct superclass if not initialized and recursively keep on going up unless all the superclass in the hierarchy is initialize or reached at the top of hierarchy that is object class
Initialize the class Initialization method.
Interface Initialization
1. Initialize the interface Initialization method.
Additional resource can be found here
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