JVM architecture

Understanding JVM internals

Articles:
1. Introduction level : Geeks4geeks and Dzone
2. Little bit deep : Understanding JVM internals
3. JVM specifications : JSE 8

Lessons learned 

* Stack-based virtual machine: The most popular computer architectures such as Intel x86 Architecture and ARM Architecture run based on a register. However, JVM runs based on a stack

* Symbolic reference: All types (class and interface) except for primitive data types are referred to through symbolic reference, instead of through explicit memory address-based reference.

* Network byte order: The Java class file uses the network byte order. To maintain platform independence between the little endian used by Intel x86 Architecture and the big endian used by the RISC Series Architecture, a fixed byte order must be kept. Therefore, JVM uses the network byte order, which is used for network transfer. The network byte order is the big endian.

* The class file itself is a binary file that cannot be understood by a human. To manage this file, JVM vendors provide javap, the disassembler. The result of using javap is called Java assembly.

* JVM follow Delegation-Hierarchy principle to load classes. System class loader delegate load request to extension class loader and extension class loader delegate request to boot-strap class loader. If class found in boot-strap path, class is loaded otherwise request again transfers to extension class loader and then to system class loader. At last if system class loader fails to load class, then we get run-time exception java.lang.ClassNotFoundException.

*The method area can be implemented in various formats by JVM vendor. Oracle Hotspot JVM calls it Permanent Area or Permanent Generation (PermGen). The garbage collection for the method area is optional for each JVM vendor.

* The bytecode that is assigned to the runtime data areas in the JVM via class loader is executed by the execution engine. The execution engine reads the Java Bytecode in the unit of instruction. It is like a CPU executing the machine command one by one

* Interpreter: Reads, interprets and executes the bytecode instructions one by one. As it interprets and executes instructions one by one, it can quickly interpret one bytecode, but slowly executes the interpreted result. This is the disadvantage of the interpret language. The 'language' called Bytecode basically runs like an interpreter.

* JIT (Just-In-Time) compiler: The JIT compiler has been introduced to compensate for the disadvantages of the interpreter. The execution engine runs as an interpreter first, and at the appropriate time, the JIT compiler compiles the entire bytecode to change it to native code. After that, the execution engine no longer interprets the method, but directly executes using native code. Execution in native code is much faster than interpreting instructions one by one. The compiled code can be executed quickly since the native code is stored in the cache.

*It takes more time for JIT compiler to compile the code than for the interpreter to interpret the code one by one. Therefore, if the code is to be executed just once, it is better to interpret it instead of compiling. Therefore, the JVMs that use the JIT compiler internally check how frequently the method is executed and compile the method only when the frequency is higher than a certain level.

* The Hotspot VM is divided into the Server VM and the Client VM, and the two VMs use different JIT compilers.