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Java / JVM Architecture (Java21) Interview questions

What is the Runtime Data Area in JVM?

The Runtime Data Areas are the memory regions the JVM sets up to run a program, created when the JVM starts (or per thread) and torn down when the JVM, or that thread, exits.

AreaShared or per-thread?
Method AreaShared across all threads
HeapShared across all threads
JVM StackPer-thread
PC RegisterPer-thread
Native Method StackPer-thread

The shared areas (Method Area and Heap) hold class metadata and objects that every thread can see, while the per-thread areas track each thread's own call stack and execution position.

Which two runtime data areas are shared across all threads?
A per-thread runtime data area is created and destroyed:

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What is the Java Virtual Machine (JVM)? What are the main components of JVM architecture? What is the Class Loader subsystem in JVM? What are the types of class loaders in JVM? What is the Runtime Data Area in JVM? What is the Method Area in JVM? What is the Heap memory in JVM? What is the JVM Stack? What is the PC (Program Counter) Register? What are Native Method Stacks? What is the Execution Engine in JVM? What is the Interpreter in the execution engine? Define JVM, JRE, and JDK? What is bytecode in Java? What is the purpose of the Java Native Interface (JNI)? What are the types of garbage collectors available in Java 21? What is the purpose of the Just-In-Time (JIT) compiler? What are the types of JIT compilers in HotSpot JVM? What is Metaspace, and how does it differ from PermGen? Why is the Metaspace stored in native memory instead of the heap? How does class loading follow the delegation model? Why do we use the parent delegation model in class loading? What is the difference between Class.forName() and ClassLoader.loadClass()? What happens when a NoClassDefFoundError occurs versus ClassNotFoundException? Explain the lifecycle of a class in JVM (loading, linking, initialization)? Explain the internal working of the linking phase (verification, preparation, resolution)? What is the difference between Young Generation and Old Generation in heap memory? Why does the JVM use generational garbage collection? Explain the execution flow of a Minor GC vs Major GC? What is the difference between Serial GC and Parallel GC? How does G1 GC organize heap memory into regions? What is Generational ZGC introduced in Java 21, and how does it differ from G1? Why should you choose ZGC over G1 for low-latency applications? What is the difference between stop-the-world pauses and concurrent GC phases? How does escape analysis enable stack allocation optimizations? What is the JVM Code Cache, and why can it become a problem? How does tiered compilation work in HotSpot JVM? What are OSR (On-Stack Replacement) compilations, and when do they happen? Explain the internal working of the JVM stack frame during method invocation? What is the difference between StackOverflowError and OutOfMemoryError? How do Virtual Threads (Project Loom) change the JVM's threading model in Java 21? Why doesn't a blocked virtual thread block its carrier platform thread? What is the difference between platform threads and virtual threads at the JVM level? Explain the execution flow of a virtual thread when it performs a blocking I/O call? How does Class Data Sharing (CDS) improve JVM startup performance? What is Application Class Data Sharing (AppCDS), and how does it differ from default CDS? When should you choose AOT-style CDS archives versus JIT warm-up in a production deployment? How can you optimize JVM heap tuning using ergonomics vs manual flags? How do you troubleshoot a memory leak caused by classloader references (classloader leaks)? Explain the internal working of safepoints and why the JVM needs them?
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