Lecture 3
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The document discusses Java programs, bytecode, and the Java Virtual Machine (JVM). It explains that Java source code is compiled into bytecode, an intermediate format that can run on any JVM. The JVM then executes the bytecode by parsing the instructions one by one. It describes how a Java program loads a class and executes its main method, and how classes must be linked and initialized before use. Finally, it provides an overview of how the JVM works, including just-in-time compilation to native machine code.
![Core Java
Debasish Pratihari
Hello world Program
File Name : Lakshya.java
class Lakshya
{
public static void main(String args[])
{
System.out.println(“Welcome to Lakshya..”);
}
}
To compile: javac Lakshya.java
To Run
: java Lakshya
Lakshya.java
Compilation
javac
Lakshya.class
Lecture/core/intro3/03
Source code
javac
Bytecode
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![Core Java
Debasish Pratihari
Bytecode:
An architecture neutral intermediate format
designed to transport code efficiently to multiple
hardware and software.
Bytecode is the intermediate representation of
Java programs
A bytecode program is normally executed by
parsing the instructions one at a time.
To see the Bytecode:
Javap –c Lakshya
.class file
Compiled from "Lakshya.java"
name
class Lakshya extends java.lang.Object{
Lakshya();
Code:
0:
aload_0
1:
invokespecial
#1; //Method java/lang/Object."<init>":()V
4:
return
public static void main(java.lang.String[]);
Code:
0:
getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
3:
ldc
#3; //String Welcome to Lakshya..
5:
invokevirtual
#4; //Method
java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
}
More about Bytecodes:
Bytecodes are the machine language of the JVM.
There is one stream of Bytecodes for each method in
the class.
The Bytecodes are stored in the method area of JVM.
The Bytecodes for a method are executed when that
method is invoked.
A Bytecode stream is a sequence of instruction for
JVM.
Each instruction consist of one-byte opcode followed
by zero or more operands.
Bytecodes can be executed by interpretation or by
Just-In-Time compiler.
Lecture/core/intro3/03
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Lecture 3
- 1. Core Java Debasish Pratihari Hello world Program File Name : Lakshya.java class Lakshya { public static void main(String args[]) { System.out.println(“Welcome to Lakshya..”); } } To compile: javac Lakshya.java To Run : java Lakshya Lakshya.java Compilation javac Lakshya.class Lecture/core/intro3/03 Source code javac Bytecode Page #1 feel the Technology…
- 2. Core Java Debasish Pratihari Bytecode: An architecture neutral intermediate format designed to transport code efficiently to multiple hardware and software. Bytecode is the intermediate representation of Java programs A bytecode program is normally executed by parsing the instructions one at a time. To see the Bytecode: Javap –c Lakshya .class file Compiled from "Lakshya.java" name class Lakshya extends java.lang.Object{ Lakshya(); Code: 0: aload_0 1: invokespecial #1; //Method java/lang/Object."<init>":()V 4: return public static void main(java.lang.String[]); Code: 0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3; //String Welcome to Lakshya.. 5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 8: return } More about Bytecodes: Bytecodes are the machine language of the JVM. There is one stream of Bytecodes for each method in the class. The Bytecodes are stored in the method area of JVM. The Bytecodes for a method are executed when that method is invoked. A Bytecode stream is a sequence of instruction for JVM. Each instruction consist of one-byte opcode followed by zero or more operands. Bytecodes can be executed by interpretation or by Just-In-Time compiler. Lecture/core/intro3/03 Page #2 feel the Technology…
- 3. Core Java Debasish Pratihari Java Program Execution: A JVM starts up by loading a specified class and then invoking the main method from the specified class, providing it a single parameter of array of string. A class must be initialize before it can be invoked. A class must be linked before it initialized. Linking: Verification Well-formed Proper Symbol Table semantics Preparation Allocation of Static Storage Create data structures required By JVM Resolution it is the process of checking symbolic references from a class to other classes and interface Initialize: Class variables initalizers and static initalizers are executed. Before a class gets initializes its direct super class must be initialized. Loading of Classes and Interfaces: Loading refers to the process of finding the binary form of a class or interface. The loading process is implemented by the class ClassLoader. Lecture/core/intro3/03 Page #3 feel the Technology…
- 4. Core Java Debasish Pratihari Introduction to JVM It is a machine out of software and exits in computer memory. It is responsible to convert Java Bytecodes to native code and execute it. A runtime instance of the JVM runs one java application. Every java application need a separate JVM for their execution. The JVM instance dies after execution of a Java program. How JVM Works : 25% Inside view of JVM : Lecture/core/intro3/03 Page #4 feel the Technology…
- 5. Core Java Debasish Pratihari JVM Platforms: IA-64 (Itanium) IA-32 Win 32 Linux X86 OS/2 S/390 Power/PowerPC AIX OS/400 Linux PPC Linux IA-64 Win 64 Linux 390 OS/39j Just-In-Time(JIT) Compiler: The Just-In-Time compiler is a component of Java Run-Time environment. It improves the performance of Java applications by compiling bytecodes to native machine code at run-time. It is also referred as dynamic compilation It can be disable by passing JVM parameter o Java –Djava.compiler=NONE Test Java Run-Time Enviro nme nt Class Lo ader Bytecode Verifier Java C lass Lib raries ByteCodes Java Interp reter Just-in-time Co mpiler Runtime S ystem Operating S ystem Hard ware Lecture/core/intro3/03 Page #5 feel the Technology…