# 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 * * * * * ## compiler * ## 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 * ### links * * * * * * ### Byte code * ### Class Loader * * * * * * ### 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 * ## 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 * * ## Just In Time Compiler * ## Linker????? *