# Java internals

* [Java internals](#java-internals)
  * [Links](#links)
  * [compiler](#compiler)
  * [JVM](#jvm)
    * [links](#links)
    * [Byte code](#byte-code)
    * [Class Loader](#class-loader)
    * [Crashing the JVM](#crashing-the-jvm)
  * [JDK](#jdk)
  * [JRE](#jre)
  * [JDK/JRE/JVM](#jdkjrejvm)
  * [Just In Time Compiler](#just-in-time-compiler)
  * [Linker?????](#linker)
* Java is a programming language and a platform. Java is a high level, robust, object-oriented and secure programming language.
* Platform:
  * Any hardware or software environment in which a program runs, is known as a platform.
  * Since Java has a runtime environment (JRE) and API, it is called a platform.
* Java Platforms / Editions
  * Java SE (Java Standard Edition)
    * It is a Java programming platform.
    * It includes Java programming APIs such as java.lang, java.io, java.net, java.util, java.sql, java.math etc.
    * It includes core topics like OOPs, String, Regex, Exception, Inner classes, Multithreading, I/O Stream, Networking, AWT, Swing, Reflection, Collection, etc.
  * Java EE (Java Enterprise Edition)
    * It is an enterprise platform which is mainly used to develop web and enterprise applications.
    * It is built on the top of the Java SE platform. It includes topics like Servlet, JSP, Web Services, EJB, JPA, etc.
  * Java ME (Java Micro Edition)
    * It is a micro platform which is mainly used to develop mobile applications.
  * JavaFX
    * It is used to develop rich internet applications. It uses a light-weight user interface API.

## Links

* [https://www.javaworld.com/article/2076075/learn-java/core-java-learn-java-from-the-ground-up.html](https://www.javaworld.com/article/2076075/learn-java/core-java-learn-java-from-the-ground-up.html)
* [https://www.javacodegeeks.com/2018/04/jvm-architecture-execution-engine-in-jvm.html](https://www.javacodegeeks.com/2018/04/jvm-architecture-execution-engine-in-jvm.html)
* [https://www.javacodegeeks.com/2018/04/jvm-architecture-overview-of-jvm-and-jvm-architecture.html](https://www.javacodegeeks.com/2018/04/jvm-architecture-overview-of-jvm-and-jvm-architecture.html)
* [https://www.youtube.com/watch?v=iDypTyErl78](https://www.youtube.com/watch?v=iDypTyErl78)
* [https://www.javacodegeeks.com/2017/10/introduction-java-virtual-machine-jvm.html](https://www.javacodegeeks.com/2017/10/introduction-java-virtual-machine-jvm.html)

## compiler

* [https://www.quora.com/How-does-the-Java-compiler-work](https://www.quora.com/How-does-the-Java-compiler-work)

## JVM

* JVM (Java Virtual Machine) is an abstract machine.
* called a virtual machine because it doesn't physically exist. It is a specification that provides a runtime environment in which Java bytecode can be executed
* can run those programs which are written in other languages and compiled to Java bytecode.
* platform dependent because the configuration of each OS is different from each other. However, Java is platform independent.
* There are three notions of the JVM:
  * specification
    * where working of Java Virtual Machine is specified.
    * But implementation provider is independent to choose the algorithm.
    * Its implementation has been provided by Oracle and other companies.
  * implementation
    * Its implementation is known as JRE (Java Runtime Environment).
  * instance.
    * Whenever you write java command on the command prompt to run the java class, an instance of JVM is created.
* JVM performs the following main tasks:
  * Loads code
  * Verifies code
  * Executes code
  * Provides runtime environment
* JVM provides definitions for the:
  * Memory area
  * Class file format
  * Register set
  * Garbage-collected heap
  * Fatal error reporting etc

### Arguments

* [https://www.javacodegeeks.com/2020/03/7-jvm-arguments-of-highly-effective-applications.html](https://www.javacodegeeks.com/2020/03/7-jvm-arguments-of-highly-effective-applications.html)

### links

* [https://www.quora.com/How-does-JVM-works-internally](https://www.quora.com/How-does-JVM-works-internally)
* [https://www.cubrid.org/blog/understanding-jvm-internals/](https://www.cubrid.org/blog/understanding-jvm-internals/)
* [https://www.javacodegeeks.com/2018/05/jvm-architecture-101-get-to-know-your-virtual-machine.html](https://www.javacodegeeks.com/2018/05/jvm-architecture-101-get-to-know-your-virtual-machine.html)
* [https://www.javacodegeeks.com/2018/04/jvm-architecture-overview-of-jvm-and-jvm-architecture.html](https://www.javacodegeeks.com/2018/04/jvm-architecture-overview-of-jvm-and-jvm-architecture.html)
* [https://www.javacodegeeks.com/2018/04/jvm-architecture-jvm-class-loader-and-runtime-data-areas.html](https://www.javacodegeeks.com/2018/04/jvm-architecture-jvm-class-loader-and-runtime-data-areas.html)
* [https://www.freecodecamp.org/news/jvm-tutorial-java-virtual-machine-architecture-explained-for-beginners/](https://www.freecodecamp.org/news/jvm-tutorial-java-virtual-machine-architecture-explained-for-beginners/)

### Byte code

* [https://examples.javacodegeeks.com/core-java/bytecode-primer-java-class-files/](https://examples.javacodegeeks.com/core-java/bytecode-primer-java-class-files/)

### Class Loader

* [https://www.javaworld.com/article/2076075/learn-java/core-java-learn-java-from-the-ground-up.html](https://www.javaworld.com/article/2076075/learn-java/core-java-learn-java-from-the-ground-up.html)
* [https://www.javaworld.com/article/2077260/learn-java/learn-java-the-basics-of-java-class-loaders.html](https://www.javaworld.com/article/2077260/learn-java/learn-java-the-basics-of-java-class-loaders.html)
* [https://www.journaldev.com/349/java-classloader](https://www.journaldev.com/349/java-classloader)
* [https://en.wikipedia.org/wiki/Java\_Classloader](https://en.wikipedia.org/wiki/Java_Classloader)
* [https://stackoverflow.com/questions/2424604/what-is-a-java-classloader](https://stackoverflow.com/questions/2424604/what-is-a-java-classloader)
* [https://www.javacodegeeks.com/2018/04/jvm-architecture-jvm-class-loader-and-runtime-data-areas.html](https://www.javacodegeeks.com/2018/04/jvm-architecture-jvm-class-loader-and-runtime-data-areas.html)

### Crashing the JVM

* Understanding code outside of libraries/apis and IDEs can help prevent or solve problems
* Examples
  * Try to allocate as much memory as you can.
    * if more than what exists it will crash
  * Try to write data to your hard disk until it is full.
    * If not enough HDD space then fail
  * Try to open as many files as you can
    * Dependent on `maximum number of file descriptors for your environment`
  * Try to create as many threads as you can. On a Linux system,
    * you can look at /proc/sys/kernel/pid\_max and you will see how many processes may be running on your system.
    * How many threads are you allowed to create on your system?
  * Try to modify your own .class files in the filesystem
  * Try to find your own process ID, and then try to kill it by using Runtime.exec (e.g., by calling kill -9 on your process ID)
  * Try to create a class at runtime that only calls System.exit, load that class dynamically via the class loader, then call it.
  * Try to open as many socket connections as possible.
    * On a Unix system, the maximum number of possible socket connections equals the maximum number of file descriptors (often 2,048)
  * Try to hack your system.
    * Download an exploit via code or by using wget. Execute the exploit, and then call `shutdown -h` as root on a Unix system or `shutdown /s` as administrator on a Windows system.
  * Run your JVM with `-noverify` or `-Xverify:none`, which disables all bytecode verification, and write something that would otherwise not be allowed to run.
  * Try using Unsafe.
    * This backdoor class is used to get access to low-level facilities such as memory management. All the syntax of Java, all the safety of C!
  * Try going native. Write some native code. All the syntax of C, all the safety of C!

## JDK

* acronym for Java Development Kit
* a software development environment which is used to develop Java applications and applets.
* physically exists and contains JRE + development tools.
* JDK contains a
  * private Java Virtual Machine (JVM) and
  * a few other resources such as
  * an interpreter/loader (java)
  * a compiler (javac)
  * an archiver (jar)
  * a documentation generator (Javadoc)
  * jconsole

### Multple jdk

* [https://reflectoring.io/manage-jdks-with-sdkman/](https://reflectoring.io/manage-jdks-with-sdkman/)

## JRE

* acronym for Java Runtime Environment
* AKA Java RTE
* set of software tools which are used for developing Java applications.
* provide the runtime environment
* implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime

## JDK/JRE/JVM

* [https://www.journaldev.com/546/difference-jdk-vs-jre-vs-jvm](https://www.journaldev.com/546/difference-jdk-vs-jre-vs-jvm)
* [https://self-learning-java-tutorial.blogspot.co.uk/2014/03/difference-between-jre-and-jdk.html](https://self-learning-java-tutorial.blogspot.co.uk/2014/03/difference-between-jre-and-jdk.html)

## Just In Time Compiler

* [https://self-learning-java-tutorial.blogspot.co.uk/2014/03/jit-compiler.html](https://self-learning-java-tutorial.blogspot.co.uk/2014/03/jit-compiler.html)

## Linker?????

* [https://www.developer.com/java/data/understand-jvm-loading-jvm-linking-and-jvm-initialization.html](https://www.developer.com/java/data/understand-jvm-loading-jvm-linking-and-jvm-initialization.html)
