Deep Thoughts On JVM (Java Virtual Machine)

Java Virtual Machine (JVM)

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.

JVM is specifically responsible for converting bytecode to machine-specific code and is necessary in both JDK and JRE. It is also platform-dependent and performs many functions, including memory management and security.

JVM Architecture

It is mainly responsible for three activities. 

• Loading
• Linking
• Initialization

Loading

The Class loader reads the “.class” file, generate the corresponding binary data and save it in the method area. For each “.class” file, JVM stores the following information in the method area.

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. 

• 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 the class hierarchy.

There are three class loaders : 

• 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 the “JAVA_HOME/jre/lib” directory.

• 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.

• System/Application class loader: It is a 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.

JVM 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 calls. 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 executes the “.class” (bytecode). It reads the byte-code line by line, uses data and information present in various memory area and executes 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 an interpreter. 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.

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.