How is jvm implemented

System class loader delegate load request to extension class loader and extension class loader delegate request to the bootstrap class loader.

If a class found in the boot-strap path, the class is loaded otherwise request again transfers to the extension class loader and then to the system class loader. At last, if the system class loader fails to load class, then we get run-time exception java. JVM Memory Method area: In the method area, all class level information like class name, immediate parent class name, methods and variables information etc.

There is only one method area per JVM, and it is a shared resource. Heap area: Information of all objects is stored in the heap area. It is also a shared resource. Stack area: For every thread, JVM creates one run-time stack which is stored here.

All local variables of that method are stored in their corresponding frame. After a thread terminates, its run-time stack will be destroyed by JVM. It is not a shared resource. PC Registers: Store address of current execution instruction of a thread. Obviously, each thread has separate PC Registers.

Native method stacks: For every thread, a separate native stack is created. It stores native method information. It reads the byte-code line by line, uses data and information present in various memory area and executes instructions. It can be classified into three parts: Interpreter : It interprets the bytecode line by line and then executes.

The disadvantage here is that when one method is called multiple times, every time interpretation is required. It compiles the entire bytecode and changes it to native code so whenever the interpreter sees repeated method calls, JIT provides direct native code for that part so re-interpretation is not required, thus efficiency is improved. Garbage Collector : It destroys un-referenced objects. For more on Garbage Collector, refer Garbage Collector.

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Skip to content. Change Language. Related Articles. In a somewhat circular fashion, the JVM spec highlights that its implementation details are not defined within the spec, in order to allow for maximum creativity in its realization:.

So, all the JVM has to do is run Java programs correctly. Sounds simple, might even look simple from outside, but it is a massive undertaking, especially given the power and flexibility of the Java language.

In fact, there are many JVM implementations , both open source and proprietary. HotSpot is also the most commonly used JVM. Developers creating a licensed fork off the OpenJDK are often motivated by the desire to add OS-specific performance improvements. After the JVM spec has been implemented and released as a software product, you may download and run it as a program. That downloaded program is an instance or instantiated version of the JVM.

Most of the time, when developers talk about "the JVM," we are referring to a JVM instance running in a software development or production environment. What a newbie mistake! We've talked about the JVM's role in running Java applications, but how does it perform its function?

Everything in Java is a class, and all Java applications are built from classes. An application could consist of one class or thousands. In order to run a Java application, a JVM must load compiled.

A JVM depends on its class loader to perform this function. The Java class loader is the part of the JVM that loads classes into memory and makes them available for execution. Class loaders use techniques like lazy-loading and caching to make class loading as efficient as it can be. That said, class loading isn't the epic brain-teaser that say portable runtime memory management is, so the techniques are comparatively simple. Every Java Virtual Machine includes a class loader.

The JVM spec describes standard methods for querying and manipulating the class loader at runtime, but JVM implementations are responsible for fulfilling these capabilities. From the developer's perspective, the underlying class loader mechanisms are typically a black box.

Once the class loader has done its work of loading classes, the JVM begins executing the code in each class. The execution engine is the JVM component that handles this function.

The execution engine is essential to the running JVM. In fact, for all practical purposes, it is the JVM instance. Sign Out View Profile. Over 2 million developers have joined DZone. Click here to check out this overview of the different components of the JVM, along with a very useful diagram! Like Join the DZone community and get the full member experience.

Join For Free. What Is the JVM? ClassLoader Subsystem Java's dynamic class loading functionality is handled by the ClassLoader subsystem. BootStrap ClassLoader — Responsible for loading classes from the bootstrap classpath, nothing but rt.

Highest priority will be given to this loader. Prepare — For all static variables memory will be allocated and assigned with default values.

Resolve — All symbolic memory references are replaced with the original references from Method Area. There is only one method area per JVM, and it is a shared resource. Heap Area — All the Objects and their corresponding instance variables and arrays will be stored here. Since the Method and Heap areas share memory for multiple threads, the data stored is not thread-safe.

Stack Area — For every thread, a separate runtime stack will be created.