Java internals

• Java internals

  • Links

  • compiler

  • JVM

    • links

    • Byte code

    • Class Loader

    • Crashing the JVM

  • JDK

  • JRE

  • JDK/JRE/JVM

  • Just In Time Compiler

  • 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.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.youtube.com/watch?v=iDypTyErl78

• https://www.javacodegeeks.com/2017/10/introduction-java-virtual-machine-jvm.html

compiler

• 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.

    • 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

links

• https://www.quora.com/How-does-JVM-works-internally

• 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/2018/04/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.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/

Class Loader

• 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.journaldev.com/349/java-classloader

• https://en.wikipedia.org/wiki/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

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/

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://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

Linker?????

• https://www.developer.com/java/data/understand-jvm-loading-jvm-linking-and-jvm-initialization.html