JAVA Programming Language

What is Java programming language?

     Java is a general-purpose computer programming language that is concurrent, class-based, object-oriented, and specifically designed to have as few implementation dependencies as possible. It is intended to let application developers “write once, run anywhere” (WORA), meaning that compiled Java code can run on all platforms that support Java without the need for recompilation.

     For example, you can write and compile a Java program on UNIX and run it on Microsoft Windows, Macintosh, or UNIX machine without any modifications to the source code. WORA is achieved by compiling a Java program into an intermediate language called bytecode. The format of bytecode is platform-independent. A virtual machine, called the Java Virtual Machine (JVM), is used to run the bytecode on each platform.

JDK vs JRE vs JVM

History of Java

      Java was originally developed by James Gosling at Sun Microsystems (which has since been acquired by Oracle Corporation) and released in 1995 as a core component of Sun Microsystems’ Java platform. The language derives much of its syntax from C and C++, but it has fewer low-level facilities than either of them.

      Oracle Corporation is the current owner of the official implementation of the Java SE platform, following their acquisition of Sun Microsystems on January 27, 2010. This implementation is based on the original implementation of Java by Sun. The Oracle implementation is available for Microsoft Windows, Mac OS X, Linux and Solaris.

The Oracle implementation is packaged into two different distributions:

1. Java Runtime Environment (JRE) which contains the parts of the Java SE platform required to run Java programs and is intended for end users.
2. Java Development Kit (JDK) which is intended for software developers and includes development tools such as the Java compiler, Javadoc, Jar, and a debugger.

Why Choose Java?

Java was designed with a few key principles in mind:

• Ease of Use: The fundamentals of Java came from a programming language called C++. Although C++ is a powerful language, it is complex in its syntax and inadequate for some of Java's requirements. Java built on and improved the ideas of C++ to provide a programming language that was powerful and simple to use.
• Reliability: Java needed to reduce the likelihood of fatal errors from programmer mistakes. With this in mind, object-oriented programming was introduced. When data and its manipulation were packaged together in one place, Java was robust.
• Security: Because Java was originally targeting mobile devices that would be exchanging data over networks, it was built to include a high level of security. Java is probably the most secure programming language to date.
• Platform Independence: Programs need to work regardless of the machines they're being executed on. Java was written to be a portable and cross-platform language that doesn't care about the operating system, hardware, or devices that it's running on.

      The team at Sun Microsystems was successful in combining these key principles, and Java's popularity can be traced to it being a robust, secure, easy to use, and portable programming language.

What is Java JDK, JRE and JVM – In-depth Analysis

      Learn the differences between JDK, JRE and JVM. How JVM works inside? What are class loaders, interpreter and JIT compilers. Also checkout some interview questions.

1. Execution of a Java Program

Before jumping into the internals of Java, let’s understand how a Java source file is executed.

1. We write the Java source code in Simple.Java file using an editor or IDE (integrated development environment) e.g. Eclipse or IntelliJ Idea.
2. Program has to be compiled into bytecode. Java compiler (javac) compiles the sourcecode to Simple.class file.
3. This class file can be executed in any platform/OS by JVM (Java virtual machine).
4. JVM translates bytecode into native machine code which machines can execute.

Java Execution Flow

2. What is JVM?

      Java Virtual machine (JVM) is the virtual machine that runs the Java bytecodes. You get this bytecode by compiling the .java files into .class files. .class files contain the bytecodes understood by the JVM.

      In the real world, JVM is a specification that provides a runtime environment in which Java bytecode can be executed. Different vendors provide different implementations of this specification. For example, this wiki page lists down different JVM implementations.

      Most popular implementation of JVM is Hotspot which is owned and provided by Oracle Corporation. (Previously by Sun Microsystems, Inc.).

      JVM delivers the optimal performance for Java applications using many advanced techniques, incorporating a state-of-the-art memory model, garbage collector, and adaptive optimizer.

      JVM comes in two different flavors – client and server. Although the Server and the Client VMs are similar, the Server VM has been specially tuned to maximize peak operating speed. It is intended for executing long-running server applications, which need the fastest possible operating speed more than a fast start-up time or smaller runtime memory footprint. Developers can choose which system they want by specifying -client or -server.

      The JVM is called virtual because it provides a machine interface that does not depend on the underlying operating system and machine hardware architecture. This independence from hardware and the operating system is a cornerstone of the write-once-run-anywhere value of Java programs.

2.1. JVM Architecture

JVM Architecture

2.1.1. Class Loader

      The class loader is a subsystem used for loading class files. It performs three major functions i.e. class loading, linking, and initialization.

1. Loading

   • To load classes, JVM has 3 kind of class loaders. Bootstrap, extension and applicationclass loader.
   • When loading a class file, JVM finds out a dependency for some arbitrary class XYZ.class.
   • First bootstrap class loader tries to find the class. It scans the rt.jar file in JRE lib folder.
   • If class is not found then extension class loader searches the class file in inside jre\lib\ext folder.
   • Again if class is not found then application classloader searches all the Jar files and classes in CLASSPATH environment variable of system.
   • If class is found by any loader then class is loaded by class loader; else ClassNotFoundException is thrown.

2. Linking

         After class is loaded by the classloader, linking is performed. A bytecode verifier will verify whether the generated bytecode is proper or not if verification fails we will get a verification error. It also performs the memory allocation to static variables and methods found in the class.

3. Initialization

         This is the final phase of class loading, here all static variable will be assigned with the original values and the static blocks will be executed.

2.1.2. JVM Memory Areas

Memory area inside JVM is divided into multiple parts to store specific parts of application data.

• Method Area stores class structures like metadata, the constant runtime pool, and the code for methods.
• Heap stores all objects that are created during application execution.
• Stacks store local variables, and intermediate results. All such variables are local to the thread by which they are created. Each thread has its own JVM stack, created simultaneously as the thread is created. So all such local variable are called thread-local variables.
• PC register store the physical memory address of the statements which is currently executing. In Java, each thread has its separate PC register.
• Java supports and uses native code as well. Many low level code is written in languages like C and C++. Native method stacks hold the instruction of native code.

2.2. JVM Execution Engine

      All code assigned to JVM is executed by an execution engine. The execution engine reads the byte code and executes one by one. It uses two inbuilt interpreter and JIT compiler to convert the bytecode to machine code and execute it.

Platform Specific Interpreters

      With JVM, both interpreter and compiler produce native code. The difference is in how they generate the native code, how optimized it is as well how costly the optimization is.

2.2.1. Interpreter

      A JVM interpreter pretty much converts each byte-code instruction to corresponding native instruction by looking up a predefined JVM-instruction to machine instruction mapping. It directly executes the bytecode and does not perform any optimization.

2.2.2. JIT Compiler

      To improve performance, JIT compilers interact with the JVM at runtime and compile appropriate bytecode sequences into native machine code. Typically, JIT compiler takes a block of code (not one statement at a time as interpreter), optimize the code and then translate it to optimized machine code.

      The JIT compiler is enabled by default. You can disable the JIT compiler, in which case the entire Java program will be interpreted. Disabling the JIT compiler is not recommended except to diagnose or work around JIT compilation problems.

3. What is JRE?

      The Java Runtime Environment (JRE) is a software package which bundles the libraries (jars) and the Java Virtual Machine, and other components to run applications written in the Java. JVM is just a part of JRE distributions.

      To execute any Java application, you need JRE installed in the machine. It’s minimum requirement to execute Java applications on any machine.

JRE bundles the following components –

1. DLL files used by the Java HotSpot Client Virtual Machine.
2. DLL files used by the Java HotSpot Server Virtual Machine.
3. Code libraries, property settings, and resource files used by the Java runtime environment. e.g. rt.jar and charsets.jar.
4. Java extension files such as localedata.jar.
5. Contains files used for security management. These include the security policy (java.policy) and security properties (java.security) files.
6. Jar files containing support classes for applets.
7. Contains TrueType font files for use by the platform.

      JREs can be downloaded as part of JDKs or you can download them separately. JREs are platform dependent. It means that based on the type of machine (OS and architecture), you will have to select the JRE bundle to import and install.

      For example, you cannot install a 64-bit JRE distribution on 32-bit machine. Similarly, JRE distribution for Windows will not work in Linux; and vice-versa.

4. What is JDK?

      JDK is a superset of JRE. JDK contains everything that JRE has along with development tools for developing, debugging, and monitoring Java applications. You need JDK when you need to develop Java applications.

Few important components shipped with JDKs are as follows:

• appletviewer – this tool can be used to run and debug Java applets without a web browser
• apt – the annotation-processing tool
• extcheck – a utility that detects JAR file conflicts
• javadoc – the documentation generator, which automatically generates documentation from source code comments
• jar – the archiver, which packages related class libraries into a single JAR file. This tool also helps manage JAR files
• jarsigner – the jar signing and verification tool
• javap – the class file disassembler
• javaws – the Java Web Start launcher for JNLP applications
• JConsole – Java Monitoring and Management Console
• jhat – Java Heap Analysis Tool
• jrunscript – Java command-line script shell
• jstack – utility that prints Java stack traces of Java threads
• keytool – tool for manipulating the keystore
• policytool – the policy creation and management tool
• xjc – Part of the Java API for XML Binding (JAXB) API. It accepts an XML schema and generates Java classes

      Same as JREs, JDKs are also platform dependent. So take care when you download the JDK package for your machine.

5. Differences between JDK, JRE and JVM

Based on the above discussions, we can draw a relationship between these three as below –

JRE = JVM + libraries to run Java application.

JDK = JRE + tools to develop Java Application.

JDK vs JRE vs JVM

      In short, if you are a Java application developer who writes code, you will need JDK installed in your machine. But, if you only want to run applications built in Java, you only need JRE installed into your machine.