All you need to know about JVM !
JVM(Java Virtual Machine) acts as a run-time engine to run Java applications. JVM is the one that actually calls the main method present in a java code. JVM is a part of JRE(Java Runtime Environment)..
The JVM has two primary functions: to allow Java programs to run on any device or operating system (known as the “Write once, run anywhere” principle), and to manage and optimize program memory. When Java was released in 1995, all computer programs were written to a specific operating system, and program memory was managed by the software developer. So the JVM was a revelation.
HOW JVM WORKS ?
When we compile a .java file, .class files(contains byte-code) with the same class names present in .java file are generated by the Java compiler. This .class file goes into various steps when we run it. These steps together describe the whole JVM.
Class Loader Subsystem
It is mainly responsible for three activities.
- Loading
- Linking
- Initialization
Loading: The Class loader reads the “.class” file, generate the corresponding binary data and save it in the method area. For each “.class” file, JVM stores the following information in the method area.
- The fully qualified name of the loaded class and its immediate parent class.
- Whether the “.class” file is related to Class or Interface or Enum.
- Modifier, Variables and Method information etc.
After loading the “.class” file, JVM creates an object of type Class to represent this file in the heap memory. Please note that this object is of type Class predefined in java.lang package. These Class object can be used by the programmer for getting class level information like the name of the class, parent name, methods and variable information etc. To get this object reference we can use getClass() method of Object class.
Linking: Performs verification, preparation, and (optionally) resolution.
- Verification: It ensures the correctness of the .class file i.e. it checks whether this file is properly formatted and generated by a valid compiler or not. If verification fails, we get run-time exception java.lang.VerifyError. This activity is done by the component ByteCodeVerifier. Once this activity is completed then the class file is ready for compilation.
- Preparation: JVM allocates memory for class static variables and initializing the memory to default values.
- Resolution: It is the process of replacing symbolic references from the type with direct references. It is done by searching into the method area to locate the referenced entity.
Initialization: In this phase, all static variables are assigned with their values defined in the code and static block(if any). This is executed from top to bottom in a class and from parent to child in the class hierarchy.
In general, there are three class loaders :
- Bootstrap class loader: Every JVM implementation must have a bootstrap class loader, capable of loading trusted classes. It loads core java API classes present in the “JAVA_HOME/jre/lib” directory. This path is popularly known as the bootstrap path. It is implemented in native languages like C, C++.
- Extension class loader: It is a child of the bootstrap class loader. It loads the classes present in the extensions directories “JAVA_HOME/jre/lib/ext”(Extension path) or any other directory specified by the java.ext.dirs system property. It is implemented in java by the sun.misc.Launcher$ExtClassLoader class.
- System/Application class loader: It is a child of the extension class loader. It is responsible to load classes from the application classpath. It internally uses Environment Variable which mapped to java.class.path. It is also implemented in Java by the sun.misc.Launcher$AppClassLoader class.
Note:
JVM follows the Delegation-Hierarchy principle to load classes. 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.lang.ClassNotFoundException.
JVM Memory
- Method area: In the method area, all class level information like class name, immediate parent class name, methods and variables information etc. are stored, including static variables. There is only one method area per JVM, and it is a shared resource. From java 8, static variables are now stored in Heap area.
- Heap area: Information of all objects is stored in the heap area. There is also one Heap Area per JVM. It is also a shared resource.
- Stack area: For every thread, JVM creates one run-time stack which is stored here. Every block of this stack is called activation record/stack frame which stores methods calls. 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.
Execution Engine
Execution engine executes the “.class” (bytecode). 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.
- Just-In-Time Compiler(JIT) : It is used to increase the efficiency of an interpreter. 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.
Java Native Interface (JNI) :
It is an interface that interacts with the Native Method Libraries and provides the native libraries(C, C++) required for the execution. It enables JVM to call C/C++ libraries and to be called by C/C++ libraries which may be specific to hardware.
Native Method Libraries :
It is a collection of the Native Libraries(C, C++) which are required by the Execution Engine.
JVM Threads
We discussed on how a Java program gets executed, but didn’t specifically mention about the executors. Actually to perform each task we discussed earlier, the JVM concurrently runs multiple threads. Some of these threads carry the programming logic and are created by the program (application threads), while the rest is created by JVM itself to undertake background tasks in the system (system threads).
The major application thread is the main thread which is created as part of invoking public static void main(String[]) and all other application threads are created by this main thread. Application threads perform tasks such as executing instructions starting with main() method, creating objects in Heap area if it finds new keyword in any method logic etc.
The major system threads are as follows.
- Compiler threads: At runtime, compilation of bytecode to native code is undertaken by these threads.
- GC threads: All the GC related activities are carried out by these threads.
- Periodic task thread: The timer events (i.e. interrupts) to schedule execution of periodic operations are performed by this thread.
- Signal dispatcher thread: This thread receives signals sent to the JVM process and handle them inside the JVM by calling the appropriate JVM methods.
- VM thread: As a pre-condition, some operations need the JVM to arrive at a safe point where modifications to the Heap area does no longer happen. Examples for such scenarios are “stop-the-world” garbage collections, thread stack dumps, thread suspension and biased locking revocation. These operations can be performed on a special thread called VM thread.
Bonus Points:
- Java is considered as both interpreted and compiled Language.
- By design, Java is slow due to dynamic linking and run-time interpreting.
- JIT compiler compensate for the disadvantages of the interpreter for repeating operations by keeping a native code instead of bytecode.
- The latest Java versions address performance bottlenecks in its original architecture.
- JVM is only a specification. Vendors are free to customize, innovate, and improve its performance during the implementation.
REFERENCES:
# https://en.wikipedia.org/wiki/Java_virtual_machine
# https://www.infoworld.com/article/3272244/what-is-the-jvm-introducing-the-java-virtual-machine.html
# https://www.geeksforgeeks.org/jvm-works-jvm-architecture/
# https://medium.com/platform-engineer/understanding-jvm-architecture-22c0ddf09722
