Introduction to the basic components of java virtual machine

The basic composition of the java virtual machine:

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Class loading subsystem, Runtime data area (JVM memory model), execution engine, local method interface

Class loader:

is mainly responsible for the compiled .class word The section code file is loaded into the runtime data area for use by the runtime data area.

Runtime data area (JVM memory model):

1. Heap: mainly stores objects and is shared by multiple threads (the main area for garbage collection).

2. Method area: mainly stores constants, static variables, class metadata (class name, methods, fields, versions, etc.).

3. Thread stack: stores method parameters, local variables, intermediate operation results, object references, and provides some data needed for other modules to work, etc.

4. Local method stack: The local method stack serves the native methods used by the virtual machine.

5. Program counter: The line number indicator of the bytecode executed by the current thread. It is null when executing a local method.

Execution engine:

The execution engine is the core of JVM execution of Java bytecode. The execution methods are mainly divided into interpretation execution, compilation execution, adaptive optimization execution, and hardware chip execution.

Local method interface:

Method interface implemented in non-java language.

jdk1.8 jvm memory structure diagram:

The difference between JVM memory model before 1.8 and 1.8:

Main differences:

1. The metadata area replaces the permanent generation. The essence of the metaspace is similar to that of the permanent generation. They are both implementations of the method area in the JVM specification (ps: the method area is a specification in the Java virtual machine, and both the permanent generation and the metaspace are an implementation of the method area).

2. The metadata area is moved from the virtual machine to the local memory.

3. The constant pool and static variables in jdk1.8 have been moved to the heap for storage (logically they still belong to the method area)

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