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Chapter 001

JXX

Positions:

  • Java full stack developer (UI testing, ...)
  • Java Web developer (JSP, Servlet, ...)
  • Java mobile app developer (Android, ...)
  • Java EE developer (EJB, Spring, Hibernate, REST, ...)
  • Java API developer (Spring Boot, ...)

Java program execution flow

Java execution flow

Key files in Java program

  • .java
  • .class
  • OS executable file

JRE (Java Runtime Environment)

  • Definition:

    • JRE is typically a program that can communicate with the class file provided to it.
    • A runtime environment is typically a kind of physical operating system on which the software runs traditionally.
    • JRE accepts your code, merges the different dependencies via a Java class loader, and passes it to JVM
    • implementation of JVM
    • The main job of JRE is to provide a runtime environment to Java

  • Components:

    1. Class loader

      • searching and loading classes in memory

      • classes are not loaded at once in memory by the class loader; it is done on a demand basis

      • Types (where they load the classes dynamically?):

        • Bootstrap class loader

          • loads jre/lib/rt.jar file which is a repository of all the system or internal classes in Java
          • in the lib directory of JRE
          • primordial class loader
          • implemented by C or C++ libraries

        • Extension class loader

          • child of the bootstrap class loader
          • in the jre/lib/ext directory
          • system property java.ext.dirs

        • System class loader (application)

          • child of the extension class loader
          • developer sets up the class path
          • loads the classes from the class path
          • CLASSPATH

      • Principles of functionality of a Java ClassLoader

        • Delegation Hierarchy Algorithm (*reference)

          Principles of functionality of a Java ClassLoader

        • Visibility Principle
        • Uniqueness Property

      • Custom Class Loaders (*reference)

        • Use-Cases
          • modify the existing bytecode -> e.g. weaving agents
          • dynamic class loading -> e.g. in JDBC, switching between different driver
          • implementing a class versioning mechanism -> URL class loader (load jars via URLs)
        • CustomClassLoader -> Code

    2. Bytecode verifier

      • JVM has a gatekeeper

      • verifications:

        • The operand stack present in the frame should not get overflowed, and it should not be underflow or underutilized.
        • The variables must be initialized before the usage.
        • Private data from the class is not accessed from outside of the class.
        • The proper references must be used for the method calls.
        • The stack should not get overflowed.
        • The final classes are not extended, and the final methods are not overridden.
        • If the verifier finds that something is missing from the preceding list, the code execution is blocked for that file.

      • bytecode verifier along with compiler?

    3. Interpreter

JVM

  • the heart of the entire Java ecosystem

  • a runtime environment provided by Java

  • The main task of JVM is to execute the bytecode, which is the platform-independent file format

  • platform-dependant

  • Architecture

    Java execution flow

    *reference

  • Memory management in JVM

    Java execution flow

    • all these segments get stored temporarily as long as the program is executing
    • do not get stored at the same location in JVM
    • get stored in to different logical compartments

  • types of memory areas:

    • Method Area or Class
      • class information like variables, method data, or runtime constant pool
    • Heap
      • fixed size memory
      • store the object or arrays
    • Stack
      • stores the frames created during execution for each thread
      • expandable at runtime
      • local variable and references to object or arrays
    • PC
      • Every task that is being executed by JVM is executed within a thread
    • Native Stack
      • utilized by the native method if used in an application

  • Exception

    • OutOfMemoryException?
      • heap memory size
      • java Sample -Xms512m -Xmx1024m
      • _JAVA_OPTIONS = -Xms512m -Xmx1024m
    • java.lang.StackOverflowError
      • recursive calls

Just-in-time compiler

  • JIT

    • converts the bytecode to native code
    • performance
    • contrast to normal compilers
    • dynamic in nature
    • JIT and JVM are integrated together
    • JIT comes bundled with JVM (not possible to separate)

  • JIT vs AoT(Ahead-of-Time Compilation)

  • Interpreter Story:

    • jdk 1.0 -> C, infinite while loop
    • jdk 1.2 -> C++, Hotspot VM
    • zero-assembler
    • template interpreter -> codelet, dispatch table

Truffle interpreter for GraalVM...

Hotspot

  • one implementation of JIT technology that starts by running interpreted, and watches the actual performance of the app
  • an implementation of the part of the JVM concept, originally developed by Sun, and now it is owned by Oracle

List of Java Virtual Machine Implementations

  • OpenJ9
    • is an alternative
  • ...

Java SE > Java SE Specifications

  • The Java Language Specification
  • The Java Virtual Machine Specification

JDK

Java Development Kit

this is a developer’s bundle

OpenJDK, Oracle OpenJDK, ...

is an open-source project that fully implements

  • the Java Specifications,
  • JSRs, -> Java Specification Requests
  • and JEPs -> JDK Enhancement Proposals

that define the Java platform

Differences Between Oracle JDK and OpenJDK

Create Project

install JDKs or SDKs

IDEs (Integrated development environment Software/java):

  • IntelliJ IDEA
  • Eclipse
  • NetBeans

  1. pure java project

    • additional libs or frameworks:

      • Apache Groovy

        • grails.org (an Open Source, full stack, web application framework for the JVM)
        • Spock
        • GPars
        • Ratpack
        • Griffon
        • Geb
        • Micronaut
        • GraalVM (high-performance JDK distribution)
        • Grape

      • Kotlin

        • KotlinTest
        • Spek
        • Hamkrest
        • Mockito-Kotlin
        • Kodein
        • jackson-module-kotlin
        • GSON, Kotson
        • Klaxon
        • Retrofit
        • OkHttp
        • Spring Kotlin project
        • vert.x
        • Ktor
        • Kara
        • Wasabi
        • Kovert
        • Exposed
        • RxJava
        • funKTionale
        • Kovenant
        • TornadoFX

      • SQL support

        • Amazon Redshift
        • Apache
          • Cassandra (NoSQL distributed database)
          • Derby
          • Hive
          • Spark (multi-language engine for executing data engineering, data science, and machine learning on single-node machines or clusters)
        • Azure SQL Database
        • GCP BigQuery
        • CockroachDB
        • couchbase
        • Exasol
        • Greenplum
        • ...

  2. Maven

  • project management
  • comprehension tool
  • Based on the concept of a project object model (POM), Maven can manage a project's build, reporting and documentation from a central piece of information.
  • builds a project using its project object model (POM) and a set of plugins
  • multi module project (example)
    • EAR (Enterprise ARchive),
    • WAR (Web ARchive),
    • JAR (Java ARchive)

  1. Gradle

    • DSL (Domain Specific Language)

  2. Java Enterprise

  3. Spring Initializer

  4. Quarkus

  5. Micronaut

  6. MicroProfile

  7. Ktor

  8. Groovy

  9. Grails App Forge

  10. Kotlin

  11. Web

  12. Multi-module Project

  13. JavaScript

  14. Empty Project

Test

Other Concepts