Difference between interpreter and JIT compiler?
The interpreter interprets the bytecode line by line and
executes it sequentially. It results in poor performance. JIT compiler add
optimization to this process by analyzing the code in blocks and then prepare
more optimized machine code.
Difference between JRE and JVM?
JVM is the specification for runtime environment which
executes the Java applications. Hotspot JVM is such one implementation of the
specification. It loads the class files and uses interpreter and JIT compiler
to convert bytecode into machine code and execute it.
Difference Between JVM & HotSpot VM
JVM : is a
Specification, HotSpot : is a
implementation of JVM.
HotSpot is an implementation of the JVM concept, originally developed
by Sun and now owned by Oracle. There
are other implementations of the JVM specification, like JRockit, IBM J9, among many others.
How does WeakHashMap work?
WeakHashMap operates like a normal HashMap but uses
WeakReference for keys. Meaning if the key object does not devise any reference
then both key/value mapping will become appropriate for garbage collection.
How do you locate memory usage from a Java program?
You can use
memory related methods from java.lang.Runtime
class to get the free memory, total memory and maximum heap memory in Java.
public static Runtime getRuntime()
|
returns the instance of
Runtime class.
|
public void exit(int status)
|
terminates the current
virtual machine.
|
public void addShutdownHook(Thread hook)
|
registers new hook thread.
|
public Process exec(String command)
|
executes given command in a
separate process.
|
public int availableProcessors()
|
returns no. of available
processors.
|
public long freeMemory()
|
returns amount of free
memory in JVM.
|
public long totalMemory()
|
returns amount of total
memory in JVM.
|
public class TestApp {
public static void
main(String[] args) {
Runtime
r =
Runtime.getRuntime();
System.out.println(r.totalMemory()); //16252928
System.out.println(r.freeMemory()); //15709576
System.out.println(r.availableProcessors());//24
r.gc();
}
}
What is ClassLoader in Java?
When a Java
program is converted into .class file
by Java compiler which is collection of byte code. ClassLoader is responsible to load that class file from file
system, network or any other location
·
Bootstrap ClassLoader - JRE/lib/rt.jar
·
Extension ClassLoader - JRE/lib/ext or any directory denoted by java.ext.dirs
·
Application ClassLoader - CLASSPATH environment variable, -classpath or -cp option,
Class-Path attribute of Manifest inside JAR file.
Java heap memory
When a Java program started Java Virtual Machine gets some
memory from Operating System.
whenever we create an object using new operator or by any
another means the object is allocated memory from Heap and When object dies or
garbage collected, memory goes back to Heap space.
How to
increase heap size in Java
Default size of Heap space in Java is 128MB on most of 32
bit Sun's JVM but
its highly varies from JVM to JVM. change size of heap space by using JVM options -Xms and -Xmx.
Xms denotes starting size of Heap while -Xmx denotes maximum size of Heap in
Java.
Java
Heap and Garbage Collection
As we know objects are created inside heap memory and
Garbage Collection is a process which removes dead objects from Java Heap space
and returns memory back to Heap in Java.
For the sake of
Garbage collection Heap is divided into three main regions named as New Generation, Old Generation, and Perm
space
·
New
Generation of Java Heap is part of Java Heap memory where a newly created
object is stored,
·
Old
Generation During the course of application many objects created and died
but those remain live they got moved to Old Generation by Java Garbage
collector thread
·
Perm
space of Java Heap is where JVM stores Metadata about classes and methods,
String pool and Class level details.
·
Perm Gen stands for permanent generation which
holds the meta-data information about the classes.
·
Suppose if you create a class name A, it's
instance variable will be stored in heap memory and class A along with static
classloaders will be stored in permanent generation.
·
Garbage collectors will find it difficult to
clear or free the memory space stored in permanent generation memory. Hence it
is always recommended to keep the permgen memory settings to the advisable
limit.
·
JAVA8 has introduced the concept called
meta-space generation, hence permgen is no longer needed when you use jdk 1.8
versions.
- Garbage collection is performed by a daemon thread called Garbage Collector(GC). This thread calls the finalize() method before object is garbage collected.
- The Garbage collector of JVM collects only those objects that are created by new keyword. So if you have created any object without new, you can use finalize method to perform cleanup processing (destroying remaining objects).
- Neither finalization nor garbage collection is guaranteed.
0 comments:
Post a Comment