JDK, JRE and JVM Questions and answers

JDK (Java Development Kit)

1. What is JDK?
   • JDK, or Java Development Kit, is a software development environment used for developing Java applications and applets. It includes the JRE, an interpreter/loader (Java), a compiler (javac), an archiver (jar), a documentation generator (javadoc), and other tools needed in Java development.
2. What tools are included in the JDK?
   • The JDK includes tools such as the Java compiler (javac), Java Virtual Machine (JVM), Java Runtime Environment (JRE), and other utility tools like javadoc and jar.
3. How does JDK differ from JRE?
   • JDK is a development toolkit that includes everything in the JRE plus compilers and tools for developing Java applications, whereas JRE is a runtime environment for executing Java applications.
4. Can you run Java applications using only JDK?
   • Yes, since JDK includes JRE, you can run Java applications with it. JDK is intended for Java development, including running Java applications.
5. Is it necessary to install JDK on a server hosting Java web applications?
   • It depends. If the server is only used to run Java applications, installing the JRE is sufficient. However, if you need to compile Java code or use JDK tools, then JDK must be installed.

JRE (Java Runtime Environment)

6. What is JRE?
   • JRE stands for Java Runtime Environment. It is a part of the Java Development Kit (JDK) but can be downloaded separately. JRE is a package of libraries, Java Virtual Machine (JVM), and other components necessary to run Java applications.
7. What components are included in the JRE?
   • The JRE includes the Java Virtual Machine (JVM), core classes, and supporting libraries required to run Java applications that are not specific to the Java development environment.
8. Can you develop Java applications using only JRE?
   • No, JRE is intended for running Java applications, not for developing them. It does not include development tools such as the compiler or debugger.
9. How does JRE relate to JVM?
   • JRE includes JVM, which is the engine that runs Java applications. JRE provides the libraries and resources necessary for JVM to execute Java applications.
10. Is it possible to run a Java application without JRE?
    • No, running a Java application requires JRE, as it contains the JVM and standard libraries necessary for execution.

JVM (Java Virtual Machine)

11. What is JVM?
    • JVM, or Java Virtual Machine, is an abstract machine that enables a computer to run Java programs as well as programs written in other languages compiled to Java bytecode.
12. How does JVM work?
    • JVM works by loading class files (compiled Java bytecode), verifying the code, executing the code, and providing runtime environments like memory management and security settings.
13. What are the main components of JVM?
    • The main components include the Class Loader, Runtime Data Areas, Execution Engine, and Native Method Interface.
14. Can JVM run programs written in languages other than Java?
    • Yes, JVM can run programs written in languages other than Java, which are compiled to Java bytecode, such as Scala or Kotlin.
15. Is JVM platform-independent?
    • The bytecode JVM executes is platform-independent, but the JVM itself is platform-specific, as it needs to convert bytecode into machine code that runs on a specific hardware platform.

General Questions

16. Why is Java called a ‘platform-independent’ language?
    • Java is called platform-independent because of its ability to run the same compiled bytecode on any operating system that has a compatible JVM, achieving “write once, run anywhere” (WORA).
17. How do you set the classpath in Java?
    • The classpath in Java can be set using the -classpath or -cp command-line option when running the java command, or by setting the CLASSPATH environment variable.
18. What is the purpose of the java command?
    • The java command is used to launch the Java interpreter. It starts the JVM, loads the specified class, and calls that class’s main() method.
19. What is the difference between JIT compiler and AOT compiler in Java?
    • JIT (Just-In-Time) compiler compiles bytecode to machine code at runtime, improving performance, while AOT (Ahead-Of-Time) compiler compiles bytecode to machine code when the application is built, reducing runtime and startup time.
20. How does garbage collection work in Java?
    • Garbage collection in Java automatically removes objects that are no longer being used by a program, freeing up memory resources. It is handled by the JVM.
21. What is bytecode in Java?
    • Bytecode is an intermediate code between the Java source and the machine code, which is executed by the JVM. It is platform-independent.
22. How can you monitor the performance of Java applications?
    • Performance can be monitored using tools like VisualVM, JConsole, and Java Mission Control which provide insights into memory usage, CPU usage, and thread behavior.
23. What is the purpose of the javac command?
    • The javac command is used to compile Java source files into bytecode, which can then be executed by the JVM.
24. Can Java run on mobile devices?
    • Yes, Java can run on mobile devices. Android apps are developed using Java (though now Kotlin is also used), and Java ME (Micro Edition) is used for other mobile platforms.
25. What is a ClassLoader in Java?
    • A ClassLoader in Java is a part of the JVM that loads classes at runtime, converting them into Class objects to be used by the JVM.
26. Explain the concept of Java Virtual Machine Stacks.
    • Java Virtual Machine Stacks are used for storing frames, which are data and partial results needed by Java Virtual Machine during the execution of Java programs. Each thread has its own JVM stack.
27. What role does the JIT compiler play in JVM?
    • The JIT compiler improves performance by compiling bytecode into native machine code at runtime, rather than interpreting one instruction at a time.
28. How do you ensure a Java application’s compatibility across different JVM versions?
    • Ensuring compatibility involves using features supported by targeted JVM versions, avoiding deprecated APIs, and thorough testing on different versions.
29. What is native code in the context of Java applications?
    • Native code refers to machine code that is directly executed by the host CPU. In Java, native code is typically used when interfacing with system libraries or for performance-critical sections through the Java Native Interface (JNI).
30. How can you run a Java program without the JDK or JRE installed on a machine?
    • You can use Java Web Start or a standalone Java application packaged with a runtime (like an executable for Windows or a bundled app for macOS), which includes its own JVM, to run a Java program without requiring JDK or JRE to be installed separately.