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Hi there this is they don't want to know.

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And welcome back.

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It's not time to learn about the under the hood the days or the way the movie industry folks call it

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behind the scenes didas.

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So we're going to learn about what goes on within JVM.

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And to be a complete engineer that's very important to a lot of programmers even though they have been

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programming for years.

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Do not have enough understanding of how JVM works.

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It is relatively rare for Djala courses or even giallo books to cover this topic.

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So it's a special chapter and I hope you like it.

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Let's just briefly review the control in this program.

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It would serve as a motivation for investing over time in learning about JVM internals.

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So here we have three courses class initialization D-Mont. subclass and superclass subclasses extending

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extending the superclass and class initialization is just an independent class which is run from the

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command line.

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And so it has a main method here.

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But we will not do that.

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The more I know the more will be done later but we'll just look at the concurrent floor right here within

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this slide.

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And it has a static initializer block and an instance initializer Rob or here on both of them Halprin

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statements saying where we are within the program.

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Here we are saying that we are inside the static initializers block within the class initialization

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Demel and within the subclass There is also the static initial as a Glock undercharge or defining.

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You'll start to underscore final Antarctic underscore final two.

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So both of them are static final views on one of them as a compile time constant whereas the other one

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is initialized via a random function within the math class.

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And here we have the main function.

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Also the boot subgraphs on superclass have constructors here with respect to print statements and within

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the main statement we have these two print statements both of them are printing these statements that

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not to underscore final so they are just accessing the static underscore final.

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Unstarted underscored final two undervalues are getting rendered here.

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And finally we are creating an instance of the subclass.

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Now when we run this class class initialization DeMaio.

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So there are these put in statements that get printed because most of them aren't.

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The interesting thing would be to observe the order in which these print statements get printed.

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So that would be the interesting thing.

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And from that we can look at couple of questions and that word so as motivation for why we are going

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to spend some time on this chapter learning the GBM internals.

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So let's assume that we have run this program using the Java command and the first statement that gets

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executed.

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So the print statement within the static initializer block is getting executed.

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Next is the print statement.

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The statement in the main method is getting a good or so the instance initializer is not getting executed

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.

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OK so that's one question that we can have raised or not getting printer.

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And the third statement is actually the static initializer group within subclass is getting initialiser

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is getting executed.

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So this print statement is getting executed rather than the second statement in the main method.

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So that's another question why is this thing getting is why is this thing getting printed.

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And then the second statement is getting executed.

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So so that's the question.

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Another question is what exactly is happening with the subclass itself when it is being accessed here

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when we are accessing the static final fields in the subclass.

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What is going to happen to subclass behind the scenes.

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So that's another question and the next treatment would be the superclass constructor would get executed

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and then the sapless constructor would get executed.

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So these are the kind of questions that this chapter will help us answer.

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And next we will do will cover something called as lifetime offer type and once we do that we will clearly

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understand what is happening here.

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So we will revisit this program later after completing that topic.

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The lifetime offer on a duck pond or family will be a real I'm offered.

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So next let's look at the agenda.

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We will first understand the lifetime of type within Gibeon.

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That is a lifetime book classes and interfaces within GBM.

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Next we will look at runtime data areas which will help us understand the different memory areas within

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GBM and how pipes and objects are handled within these memory areas.

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Finally we will look at the anatomy of a Djala classified which would help us understand a Java program

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at bytecode level.

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That is how Reems instructions work.

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So that's the agenda that we have for this chapter.

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Before we get started with GBM internals let's just do a very brief recap of what we discussed about

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GBM in the first chapter GBM is a cornerstone of Jalopnik foam.

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It hurts Djala it use some of its goals like platform independence security and very fast execution

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.

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It's an incredibly bright Thisted and very optimized runtime environment with excellent concurrency

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support.

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But just critical and overlapping large scale systems.

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Sometimes people say that even if joggles Abi GBM would still remain as JVM can't support many other

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popular languages like Scarlet groovey and closure that is source code in these other languages can

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be compiled into Java bytecode which can be run on JVM java what your mission is Calder's virtual because

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it's an abstract computing machine that is like a real computing machine.

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It has an instruction set that it executes which we know is a job by code.

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So for a Sebille machine code so says the instruction set well for a JVM Java bytecode so says the instruction

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set.

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Like a real computing machine GBM also Manipal its memory at runtime.

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That is it uses memory to run its programs and to manipulate their data.

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JVM has few core responsibilities and they include loading and interpreting of Java bytecode security

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an automatic memory management.

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Now this slide demonstrates the platform independence capability of Jawa platform independence is possible

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because JVM is platform dependent which can execute the platform independent job bytecode.

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This whole process is pretty similar to interpretation.

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It is just a JVM interprets bytecode and sort of source code.

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When we discuss JVM there are three things but Ingrid Hostetter a GBM specification.

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It is simply a specification that describes JVM features on how it should work.

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It also specifies the instruction set.

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Anyone can use a specification to implement their own Gibeon nesting related or JVM is a concrete implementation

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of the JVM specification popular JVM implementations include Oracle's hotspot and IBM GBM.

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Finally the third notion pertaining any good JVM is a runtime instance which is simply an instance of

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a concrete JVM implementation.

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When we're on a Java program from command line at runtime instance of JVM is created and then loaded

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into memory before that program can be run.

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Finally with regards to performance JVM plays a critical role in ensuring that Djala programs run very

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fast.

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One reason is to do with job bytecode as interpreting Java bytecode is much faster than interpreting

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source code.

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This is because bytecode has a compact format and is already compiled and optimized.

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Additionally most JVM implementations also perform just in time or GRB compilation to further speed

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up execution Geddie compiler basically caches frequently use bytecode in the form of machine code on

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when needed.

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Good stuff.

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Gosh the machine code to speed up performance.

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So that's a quick recap I and it's time to get started now on learning about JVM maintenance