Most frequently asked Java Virtual Machine (JVM) Interview Questions

1. What is JVM ?
2. What is the use of JVM?
3. What memory areas can be found in the JVM?
4. How does a JIT Compiler work?
5. What is Metaspace?
6. What are some common arguments used with JVM?
7. What is the distinction between the JVM, JRE, and JDK?
8. How can the Java programming language remain platform-independent?
9. What are Java Class Loaders and what are their main characteristics?

What is JVM ?

Java Virtual Machine (JVM) is an abstract computing machine that enables a computer to run a Java program. It provides the runtime environment in which Java bytecode can be executed. In addition to enabling the execution of Java programs, the JVM also provides an extensive framework of features to support the development of secure, reliable, and high-performance applications.
At the most basic level, the JVM consists of three components: a virtual processor, memory, and an execution engine. The virtual processor executes the Java bytecode instructions that make up the program, while the memory holds the data associated with the program. The execution engine coordinates the operations of the virtual processor, allowing the program to access data stored in the memory.
The following is a code snippet that illustrates how the JVM works. The code creates a new instance of a class called "MyClass", which contains an integer variable called "myInt":

public class MyClass {
    int myInt;

    public MyClass() {
    public MyClass() {
        myInt = 0;
    }
} 

MyClass myObj = new MyClass(); // Create a new object from MyClass. 
int value = myObj.myInt; // Access the value of myInt from the object.

What is the use of JVM?

The Java Virtual Machine (JVM) is an essential component of any Java application. It provides a virtual environment for the execution of compiled Java code and is responsible for managing memory and other system resources. Without the JVM, it would be impossible to run Java applications on a variety of platforms.
Some of the primary functions performed by the JVM include the compilation of Java bytecode into native machine code, memory management, garbage collection, and security. Through the compilation of Java bytecode into native code, the JVM allows Java applications to run on different operating systems without requiring platform-specific adaptations. Memory management is handled by the JVM, which creates and maintains the memory areas used by the application. Garbage collection recycles unused memory, eliminating the need to manually manage memory allocation. Finally, the JVM includes built-in security features to prevent malicious code from running on the system.
The JVM is also used to provide additional features such as thread support and native method invocation. Native methods are used to call code written in languages other than Java, and thread support allows multiple threads of execution to be handled concurrently, improving performance. All of these features enable the JVM to provide a secure and reliable environment for executing Java applications.

What memory areas can be found in the JVM?

The Java Virtual Machine (JVM) has three main areas of memory: the heap, the stack, and the metaspace.
The heap is where objects are stored. When an object is created, it is stored in the heap. The size of the heap is determined by the JVM, and it can grow and shrink as necessary. This area of memory also includes garbage collection - a process that reclaims memory from objects that are no longer being used.
The stack is an area of memory used to store information about the current method being executed. When a method is called, the parameters and local variables of that method are stored in the stack. The size of the stack is limited, and when it is exceeded, an error will be thrown.
The metaspace is an area of memory that stores JVM metadata, such as class and method information. This area of memory does not have a fixed size, and will grow or shrink as necessary.
It is important to keep these memory areas in mind when developing applications, as memory usage can affect performance and stability. Here is a code snippet to illustrate how the JVM's memory areas are used:

// Create an object
MyObject obj = new MyObject();

// Store the object in the heap
obj.onHeap = true;

// Store information about the method being run in the stack
int x = 10;

// Store information about the class being used in the metaspace
Class c = MyObject.class;

How does a JIT Compiler work?

A Just-in-Time (JIT) compiler is a type of compiler that takes program source code and translates it into machine language code in real time during the program's execution. This process drastically improves the performance of the program as well as reduces the size of the compiled code. A JIT compiler consists of two parts. The first is a parser that reads the program source code and understands the structure and syntax of the language. The second part is an optimizer, which rewrites the parsed code so that it can run faster. The compiler then generates native machine code which can be directly executed by the processor. Here is an example of how a JIT compiler works:

// Code written in C#
int x = 10;
int y = 20;
int z = x * y;

// Code after being compiled with a JIT compiler
mov eax, 10 ; move value 10 into register eax
mov ebx, 20 ; move value 20 into register ebx
mul ebx ; multiply values from registers eax and ebx and store result in eax
mov [z], eax ; move result from register eax to memory address z

As you can see from this example, a JIT compiler compiles the source code into machine language instructions, making the code much faster and more efficient. In addition, by optimizing the code for the specific architecture, it is possible to achieve even greater performance benefits.

What is Metaspace?

Metaspace is a software platform that enables developers to build, deploy, and manage AI models in a streamlined enterprise-grade environment. It provides features such as a modular, extensible API and API library, custom data preprocessing tools, a custom scoring engine, peer-reviewed algorithms and models, and a sophisticated visualization system. The platform also offers advanced decision analytics, monitoring, and optimization of AI models, helping developers measure and iterate on their models. In addition, it features an integrated security system, to ensure the safety and privacy of your data, and an intelligent repository of algorithms and models that can be re-used, modified, and shared. An example code snippet of Metaspace might look something like this:

// Create a new deployment for your model
deployment = metaspace.createDeployment(model);

// Initialize a scoring engine
engine = metaspace.initializeScoringEngine(deployment);

// Generate scores from the engine
scores = engine.generateScores();

// Visualize the scores
metaspace.visualizeScores(scores);

What are some common arguments used with JVM?

There are a few common arguments that are used with Java Virtual Machines (JVM). These include memory options, class path options, JVM version options, garbage collection options and debugging options.

• Memory Options: -Xms sets the initial heap size, -Xmx sets the maximum heap size.
• Class Path Options: -cp or -classpath is used to include external dependency or library files.
• JVM Version Options: -target sets the Java version.
• Garbage Collection Options: -verbose:gc prints out information about each garbage collection event.
• Debugging Options: -Xdebug enables remote debugging and -Xrunjdwp specifies the transport address for debugging.

Below is an example of a code snippet which includes some of these arguments: CODE:

java -Xms4G -Xmx4G -cp lib/*.jar -target 1.8 -verbose:gc -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005 ClassNameToRun
 

The flags being used in the above code snippet are -Xms4G, -Xmx4G, -cp, -target, -verbose:gc, -Xdebug, and -Xrunjdwp. These arguments set the initial heap size (-Xms4G), maximum heap size (-Xmx4G), class path which includes all libraries (-cp), target Java version (-target 1.8), activating verbose garbage collection (-verbose:gc), enabling remote debugging (-Xdebug) and setting the transport address for debugging (-Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005).

What is the distinction between the JVM, JRE, and JDK?

The difference between the Java Virtual Machine (JVM), Java Runtime Environment (JRE), and Java Development Kit (JDK) can be summarized as follows:
The JVM is a virtual machine that provides a platform-independent environment to execute Java bytecode. It is an abstract computing machine, which runs a program on a real machine or another virtual machine. JVM is the abstract layer which is provided by the class libraries and runtime support code. The JVM allows the same Java code to be executed on different platforms with different architectures.
The JRE is a software package which contains the necessary files and libraries required to execute Java applications or applets. It is a complete implementation of the JVM, where you can find all the libraries, plugins, and other necessary components to run a Java application. In addition, the JRE also includes the Java Plug-in, which is required for running applets in a browser.
The JDK is a software development toolkit for Java. It is a package of Java compiler, interpreter, and other tools that are used for developing software applications written in Java. This package includes various tools for debugging, compiling and executing Java programs. In addition, it also contains the Java Runtime Environment (JRE) so that the developer can develop, debug and compile Java applications.
In short, the JVM provides the platform to execute Java programs, the JRE provides the necessary files and libraries to execute Java programs, and the JDK provides the necessary tools to develop Java applications.

How can the Java programming language remain platform-independent?

Java is platform-independent because of the use of the Java Virtual Machine (JVM). The Java code that the programmer writes is compiled into bytecode, a low-level machine-readable version of the program. The bytecode is then interpreted by the JVM, which makes it possible to run the same bytecode on any computer, regardless of operating system. This means that once compiled, the same program can be used across multiple platforms without any additional effort from the programmer.
To demonstrate this, consider the following Java code snippet:

public class PlatformIndependent{
	public static void main(String[] args){
		System.out.println("This program is platform independent!");
	}
}

This code will produce the message "This program is platform independent!" on any computer running Java, regardless of the operating system or hardware architecture. The only requirement is that a JVM must be present on the system in order to interpret the bytecode.
By utilizing the JVM, Java has become a popular language for developing cross-platform applications, such as web browsers and mobile applications. As long as the JVM is available on the system, the same Java code can be run on any computer, regardless of the operating system or hardware architecture.

Can you elaborate on how the JVM runs Java programs?

Absolutely! The Java Virtual Machine (JVM) is responsible for running Java programs. Essentially, when a program written in the Java language is compiled, the compiler creates a file containing Java Bytecode - the instructions that will be interpreted and executed by the JVM.
The JVM performs several processes in order to run a Java program, such as loading code, verifying code, and executing it. When a Java program is loaded into memory, the JVM verifies all the components of the program - such as classes and methods - to ensure they are all valid. The JVM then allocates memory space for the program and starts executing the bytecode instructions, thus running the program and producing the desired output.
Here is an example code snippet illustrating the structure of a Java program:

public class Example {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

This program, when run in a JVM, will print out "Hello, World!" on the console. As you can see, the main method is where the program execution begins. The JVM looks for the main method in the class and calls it, following which the code within the method is executed.

What are Java Class Loaders and what are their main characteristics?

Java Class Loaders can be thought of as an entity responsible for loading Java classes into the JVM. They are responsible for finding a class definition and loading it into the JVM in order to be used in the Java application. The main characteristics of a Class Loader are as follows:

• 1. Visibility: This refers to what classes the Class Loader can actually see and use. All child Class Loaders have visibility of their parent Class Loaders but not vice-versa.
• 2. Delegation: This is the concept that when trying to find and load a class definition, the Class Loader first checks its own list of classes before attempting to ask its parent to do the same.
• 3. Security: This is the idea that when loading a class, the Class Loader applies certain security constraints to ensure that only classes that are expected are being loaded.
• 4. Garbage Collection: This refers to the idea that when a class is no longer in use, the Class Loader can automatically unload it from memory.

In order to demonstrate the concept of a Class Loader, consider the following code snippet:

ClassLoader classLoader = new ClassLoader() {
   public Class<?> loadClass(String name) throws ClassNotFoundException {
       return findClass(name);
   }
};
Class myObject = classLoader.loadClass("com.example.MyObject");