Detailed explanation of Java virtual machine

1. The main structure of JVM is as follows:

## 2. Introduction to the functions of each area

1), Method Area:

(1) Used to store virtual Machine-loaded class information,

constants, staticvariables, etc. are memory areas shared by each thread;

(2) The default minimum value is 16MB and the maximum value is 64MB. You can limit the size of the method area through the -XX:PermSize and -XX:MaxPermSize parameters;

(3) There is also a part of the method area that is

Runtime constant pool, mainly used to store literals and symbols generated during compilationReference, constants can also be generated at runtime, such as the intern method of String;

(4) Like the Java heap, it does not require continuous memory and can choose a fixed size or expandable. You can also choose not to implement garbage collection.

2), stack:

(1) The Java virtual machine stack is

thread private, life cycle and The threads are the same.

(2) When each method is executed, a stack frame is created at the same time, which is used to store local variable tables, operation stacks, dynamic links, method exits and other information. The execution process of each method corresponds to the stack frame

entering and exiting the stack.

(3) The local variable table in the JVM virtual machine stack stores various basic

data types that are known at compile time. The space required for the local variable table is allocated at compile time.

(4) Stack frame is the basic data structure during method runtime.

3), Native Method Stack:

(1) The local method stack serves the Native methods used by the JVM virtual machine.

4), Heap:

(1) The largest piece of memory managed by the Java virtual machine.

(2) The Java heap is an area shared by all threads and is created when the virtual machine starts.

(3) The only purpose of the memory area is to store

object instances. All object instances and arrays allocate memory on the heap.

(4) The Java heap is the memory area mainly managed by the JVM garbage collection mechanism. The Java heap can be in a discontinuous memory space in the memory.

5), program counter:

(1) A relatively small memory space, its function can be regarded as the line number indication of the bytecode executed by the current thread Device

(2) When the bytecode interpreter works, it selects the next bytecode instruction, branch, loop, jump, # by changing the value of this counter. ##Exception handling, thread recovery, etc. all need to rely on the program counter.

6), Runtime constant pool:

(1) The runtime constant pool is part of the method area.

(2) In addition to the class version, fields, methods,

Interface description and other information, the Class file also has a constant pool for storage. Various literals and symbol references generated during compilation will be stored in the runtime constant pool in the method area after the class is loaded.

7), direct memory:

Since Java introduced NIO and introduced the IO method of channel and buffer, you can call the Native method to directly allocate off-heap memory. This off-heap memory is local memory and will not affect the heap memory size.

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