JVM Internals

Question 1

What is JRE composed of?

Answer 1

Java API and JVM

Question 2

What is role of JVM?

Answer 2

To read the Java application through the class loader and execute it along with the Java API

Question 3

What is Virtual Machine (VM) in JVM?

Answer 3

A virual machine is a software implementation of a machine that executes program like a physical machine. Basically an abstraction on top of machine.

Question 4

Is JVM hardware dependent?

Answer 4

Yes, JVM is hardware dependent portion. It executes the Java bytecode spit out by compiler. Java execution code does not need to change with changing hardware.

Question 5

JVM is register based or stack based?

Answer 5

Even though most popular computer architectures like x86 or ARM run on register based implementation, JVM runs based on stack.

Question 6

Platform independence and primitive data type

Answer 6

C/C++ have different int type size based on platform. JVM clearly defines primitives to maintain compatibility & guarantee platform independence.

Question 7

Is JVM little endian or big endian?

Answer 7

Java class file uses network byte order which is big endian. This is done to maintain platform independence between Intel x86 uses little and RISC uses big endian.

Question 8

Are there multiple JVM implementations?

Answer 8

Yes, any vendor can develop and provide JVM. JRockit, Hotspot, Azul, IBM, OpenJDK etc.

Question 9

What is Java Bytecode?

Answer 9

Middle language between Java (user language) and the machine language. This is what helps in achieving WORA.

Question 10

What is javap?

Answer 10

javap is disassembler. The result of javap is called Java assembly.

Question 11

Which is the opcode to invoke a method in java?

Answer 11

invokevirtual is the OpCode used to run method in Java bytecode. 4 OpCodes are invokeinterface, invokespecial, invokestatic, invokevirtual.

Question 12

4 OpCodes used to invoke method

Answer 12

invokeinterface - invokes an interface method
invokespecial - invokes an initializer, private method or superclass method
invokestatic - invokes static methods
invokevirutal - invokes instance methods

Question 13

Show sample bytecode

Answer 13

public void add(java.lang.String);
Code:
 0: aload_0
 1: getfield #15;
 4: aload_1
 5: invokevirtual #23;
 8: pop
 9: return

Question 14

What does number in front of the code mean? Ex

0: aload_0
1: getfield #15;
4: aload_1

Answer 14

It is the byte number. This is the reason why code executed by JVM is called “Byte code”.

Question 15

How many bytes is OpCode size?

Answer 15

1 byte. Therefore maximum number of Java Bytecode instruction OpCodes is 256.

Question 16

What is the maximum size of java method?

Answer 16

65536 bytes. It is because the branch/jump instructions used in Byte code take 2 byte argument. So maximum value that can be expressed is 65535.

Question 17

What happens if you run code compiled with higher version compiler and executed in lower version runtime?

Answer 17

You get runtime exception called java.lang.UnsupportedClassVersionError

Question 18

What happens if you run code compiled in lower version compiler and run in higher version runtime?

Answer 18

It just runs! Java maintains backward compatibility.

Question 19

JVM structure?

Answer 19

Java code (.java) is compiled using javac and the output is (.class). Then at execution time using java command, Classloader loads the classes in the Runtime data areas and executes the code using the execution engine.

Question 20

Which classloader is parent of all class loaders?

Answer 20

Bootstrap class loader is parent of all classloaders

Question 21

How classloading delegation works?

Answer 21

During class loading, first parent class loader is checked. If parent loads the class, it is used and if not the current classloader loads the class.

Question 22

Visibility limit of class in classloader?

Answer 22

A child class loader can find the class in parent class loader but parent cannot find class in child classloader.

Question 23

Can class be unloaded once loaded?

Answer 23

No. Instead the classloader can be deleted and a new one can be created.

Question 24

Can two .class files of same java code be different?

Answer 24

Yes. If they are loaded with two different classloaders. Classloaders has its namespace that stores the loaded classes.

Question 25

Show hierarchy of classloaders

Answer 25

Bootstrap classloader
   ^
   |
Extension classloader
   ^
   |
System classloader
   ^
   |
------------
User defined classloader
   ^
   |
User defined classloader

Question 26

What is Bootstrap classloader?

Answer 26

This is created when running the JVM. It loads java api, including Object classes. It is implemented in native code instead of Java.

Question 27

What is Extension class loader?

Answer 27

It loads the extension classes excluding basic java apis. It also loads various security extension functions.

Question 28

What is system class loader?

Answer 28

It loads application classes. It loads class in $CLASSPATH specified by user.

Question 29

What is User defined classloader?

Answer 29

Classloader that a user creates using code is User defined classloader.

Question 30

What are the steps in classloading?

Answer 30

1 - Loading
2 - Verifying
3 - Preparing
4 - Resolving
5 - Initializing

Question 31

Define Loading stage of classloading?

Answer 31

In this stage, the class is read from the file and loaded into JVM memory.

Question 32

Define Verification stage of classloading?

Answer 32

In this stage, checks whether or not the class is configured as per JLS and JVM specifications. This is the most complicated test process of class loading process.

Question 33

Define prepare stage of classloading?

Answer 33

Prepare data structure that assigns the memory required by classes and indicates the fields, methods and interfaces defined in the class.

Question 34

Define resolving stage of classloading?

Answer 34

Change all symbolic references in the constant pool of the class to direct references.

Question 35

Define Initializing stage of classloading?

Answer 35

Initialize class variables to proper values. Execute static initializers and initialize static fields to configured values.

Question 36

Which three stages combined are called Linking?

Answer 36

Verification, Preparing and Resolving. Combined together are called Linking.

Question 37

Which are thread specific memory that JVM allocates?

Answer 37

PC register, Call Stack, Native method stack.

Question 38

Which memory areas are shared across all threads?

Answer 38

Heap, Method area, Constant pool

Question 39

What does method area contain?

Answer 39

Shared area, which stores runtime constant pool, field and method information, static variables and method bytecode for each of the classes and interfaces read by the JVM.

Question 40

What is method area called in Hotspot JVM?

Answer 40

Method area is called PermGen (Permanent Generation) in Hotspot JVM. The garbage collection is optional for JVM vendor.

Question 41

What is Runtime constant pool?

Answer 41

It plays important role in JVM operation. It is part of method area. When a method or field is referred to, JVM searches the actual address of method or field on memory by using the runtime constant pool.

Question 42

How execution engine runs bytecode?

Answer 42

Engine needs to convert bytecode into instructions that machine can understand. There are two ways - 1) Interpreter 2) JIT

Question 43

How interpreter executes bytecode?

Answer 43

Interpreter reads each instruction line one by one. Interpreter can quicky interpret one bytecode but slowly executes the interpreted result. That is disadvantage of interpreted language.

Question 44

How JIT compiler helps?

Answer 44

Compensate for disadvantages of Interpreter. After a method is used many times, JIT compiles the code to native code. So it eliminates the need to interpret. But it is slow process so only hotspot methods are compiled down to native code.

Question 45

Is compiled JITed native code also rolled back?

Answer 45

Yes, if a method is no longer a hotspot it is removed from cache and again interpreted.

Question 46

What does -server flag mean when starting JVM?

Answer 46

There are two flavours of JVM, Server VM and Client VM. The optimization level in both is different and both use different JIT compilers.

Question 47

What was the biggest change made in Java SE 7 in JVM?

Answer 47

A new instruction OpCode was added called “invokedynamic” to support new dynamically typed languages.

Question 48

public class A {
  public void doSomething() {
  B b = new B();
  b.doSomethingElse();
}
}
What is the line B b = new B() semantically equivalent to in terms of classloading?

Answer 48

It is equivalent to
B b = A.class.getClassLoader().loadClass("B").newInstance();

Question 49

Which is the default classloader of all classloaders?

Answer 49

It is System ClassLoader, the one which loads class from $CLASSPATH.

Question 50

Which classloader has no parent?

Answer 50

Its Bootstrap classloader. It is parent of all classloaders and does not have a parent. It is written in native code.

Question 51

Do web applications J2EE applications, follow same ordering in classloading?

Answer 51

It is not defnied. They also follow reversed order of classloading. Different containers can have different behaviors.

Question 52

What is NoClassDefFoundError?

Answer 52

NoClassDefFoundError is thrown if class loader tries to load the definition of class and none is found. Which means that the definition existed when the class was compiled but could not be found at runtime. So we cannot fully rely on IDE telling us problem.

Question 53

When does classloading happen?

Answer 53

Classloading happens at runtime and not compile time.

Question 54

Which are ways to see classpath of running application?

Answer 54

1 - Print it with code by casting classloader to URLClassLoader and use #getURLs() method and print it.
2 - Use JConsole and view the classpath MBean property.

Question 55

When does NoSuchMethodError occur?

Answer 55

The class that is being referred is present but incorrect verision is loaded at runtime, hence the required method is not found.

Question 56

class A {
 public void test() { ((Util)Factory.getUtil()).sayHello();
}
}
class Factory { 
  public static Object getUtil() {
    return new Util();
  } 
}
When can this code throw ClassCastException?

Answer 56

This can happen if two different Util class were loaded from different class contexts. Solution is to figure out the class path problem.

Question 57

How is a class identified in context of classloading?

Answer 57

A class is identified by combination of classloader and FQDN. It means same class loaded from two different classloaders will not be equal.

Question 58

Which is the reason why classcastexception occurs for same class?

Answer 58

It happens when different versions of the same library is bundled in different places. Typically happens with de facto libraries like log4j, hibernate etc.