JAVA Virtual Machine

In this topic, we will explain the java virtual machine.

JVM(Java Virtual Machine) acts as a run-time engine to run Java applications. JVM is the one that actually calls the main method present in a java code. JVM is a part of JRE(Java Runtime Environment).

Java applications are called WORA (Write Once Run Anywhere). This means a programmer can develop Java code on one system and can expect it to run on any other Java-enabled system without any adjustment. This is all possible because of JVM.

When we compile a .java file, .class files(contains byte-code) with the same class names present in the .java file are generated by the Java compiler. This .class file goes into various steps when we run it. These steps together describe the whole JVM.

JAVA Virtual Machine

ClassLoader Subsystem It is mainly responsible for three activities:

  • Loading
  • Linking
  • Initialization

Loading: The Classloader reads the .class file, generates the corresponding binary data, and saves it in the method area. For each .class file, JVM stores the following information in the method area.

  • The fully qualified name of the loaded class and its immediate parent class.
  • Whether .class file is related to Class or Interface or Enum.
  • Modifier, Variables and Method information, etc.

After loading the .class file, JVM creates an object of type Class to represent this file in the heap memory. Please note that this object is of type Class predefined in java.lang package. This Class object can be used by the programmer for getting class level information like the name of the class, parent name, methods, and variable information, etc. To get this object reference we can use the getClass() method of Object class.

Linking: Performs verification, preparation, and (optionally) resolution.

  • Verification: It ensures the correctness of the .class file i.e. it checks whether this file is properly formatted and generated by a valid compiler or not. If verification fails, we get run-time exception java. lang.VerifyError.
  • Preparation: JVM allocates memory for class variables and initializing the memory to default values.
  • Resolution: It is the process of replacing symbolic references from the type with direct references. It is done by searching into the method area to locate the referenced entity.

Initialization: In this phase, all static variables are assigned with their values defined in the code and static block(if any). This is executed from top to bottom in a class and from parent to child in a class hierarchy. In general, there are three class loaders:

  1. Bootstrap class loader: Every JVM implementation must have a bootstrap class loader, capable of loading trusted classes. It loads core java API classes present in JAVA_HOME/JRE/lib directory. This path is popularly known as the bootstrap path. It is implemented in native languages like C, C++.
  2. Extension class loader: It is a child of the bootstrap class loader. It loads the classes present in the extensions directories JAVA_HOME/JRE/lib/ext(Extension path) or any other directory specified by the java. ext.dirs system property. It is implemented in java by the sun.misc.Launcher$ExtClassLoader class.
  3. System/Application class loader: It is the child of the extension class loader. It is responsible to load classes from the application classpath. It internally uses Environment Variable which mapped to java.class.path. It is also implemented in Java by the sun.misc.Launcher$AppClassLoader class.

Note: Java Virtual Machine follows the Delegation-Hierarchy principle to load classes. System class loader delegate load request to extension class loader and extension class loader delegate request to the bootstrap class loader. If a class found in the boot-strap path, the class is loaded otherwise request again transfers to the extension class loader and then to the system class loader. At last, if the system class loader fails to load the class, then we get run-time exception java.lang.ClassNotFoundException.



JAVA Virtual Machine Memory:

Method area: In the method area, all class level information like class name, immediate parent class name, methods and variables information, etc. are stored, including static variables. There is only one method area per JVM, and it is a shared resource.

Heap area: Information of all objects is stored in the heap area. There is also one Heap Area per JVM. It is also a shared resource.

Stack area: For every thread, JVM creates one run-time stack which is stored here. Every block of this stack is called activation record/stack frame which stores methods call. All local variables of that method are stored in their corresponding frame. After a thread terminates, its run-time stack will be destroyed by JVM. It is not a shared resource.

PC Registers: Store address of current execution instruction of a thread. Obviously, each thread has separate PC Registers.

Native method stacks: For every thread, a separate native stack is created. It stores native method information.



Execution Engine
Execution engine execute the .class (bytecode). It reads the byte-code line by line, use data and information present in various memory area and execute instructions. It can be classified into three parts:

  • Interpreter: It interprets the bytecode line by line and then executes. The disadvantage here is that when one method is called multiple times, every time interpretation is required.
  • Just-In-Time Compiler(JIT): It is used to increase the efficiency of interpreters. It compiles the entire bytecode and changes it to native code so whenever the interpreter sees repeated method calls, JIT provides direct native code for that part so re-interpretation is not required, thus efficiency is improved.
  • Garbage Collector: It destroys un-referenced objects. For more on Garbage Collector, refer to Garbage Collector.

Java Native Interface (JNI):

It is an interface that interacts with the Native Method
Libraries and provides the native libraries(C, C++) required for the execution. It enables JVM to call C/C++ libraries and to be called by C/C++ libraries which may be specific to hardware.

Native Method Libraries: It is a collection of the Native Libraries(C, C++) which are required by the Execution Engine.