Java Virtual Machine - Class Loader

The JVM manages the process of loading, linking and initializing classes and interfaces in a dynamic manner. During the loading process, the JVM finds the binary representation of a class and creates it.

During the linking process, the loaded classes are combined into the run-time state of the JVM so that they can be executed during the initialization phase. The JVM basically uses the symbol table stored in the run-time constant pool for the linking process. Initialization consists of actually executing the linked classes.

Types of Loaders

The BootStrap class loader is on the top of the class loader hierarchy. It loads the standard JDK classes in the JRE’s lib directory.

The Extension class loader is in the middle of the class loader hierarchy and is the immediate child of the bootstrap class loader and loads the classes in the JRE’s lib\ext directory.

The Application class loader is at the bottom of the class loader hierarchy and is the immediate child of the application class loader. It loads the jars and classes specified by the CLASSPATH ENV variable.

Linking

The linking process consists of the following three steps −

Verification − This is done by the Bytecode verifier to ensure that the generated .class files (the Bytecode) are valid. If not, an error is thrown and the linking process comes to a halt.

Preparation − Memory is allocated to all static variables of a class and they are initialized with the default values.

Resolution − All symbolic memory references are replaced with the original references. To accomplish this, the symbol table in the run-time constant memory of the method area of the class is used.

Initialization

This is the final phase of the class-loading process. Static variables are assigned original values and static blocks are executed.