JVM foundation review

Run time data area

Thread sharing ： Heap and method area
Thread exclusive ： Virtual machine stack , Native Method Stack , Program counter

The distribution of objects in memory

Object structure includes object header and object body
Object location in memory includes direct pointer and handle pool

How to allocate object memory

Pointer collision : Memory is regular , The pointer moves the distance of the object size to complete the allocation ;
Free list ： Memory is not regular , Need to maintain a free list , Record the free location and location size , Find the right space to allocate ;
tlab： Thread allocates buffer locally , In the case of concurrency, each thread pre allocates a small amount of memory in the heap , Which thread wants to allocate memory is allocated in its local buffer , Synchronization locking is not required until a new cache is allocated ;

Object to recycle
Judge life and death first
Reference counting ： Object records the reference of other objects , Update when the reference is invalid , Circular references may occur , Cannot be recycled
Accessibility analysis ：GCroot Root node enumeration , Using three color marking method , Objects that are not marked are recycled

Recovery algorithm
Mark clear ： Clear after marking , Memory fragmentation will occur, but fast , Can't STW
Mark copy ： After marking, copy the living object to the unused space , Clean the original space at one time , Copy objects to STW, And waste memory space
Tag to sort out ： After marking, move the living object to one place , Clean up all the space behind the pointer of the area where the living object is located , Moving objects also requires STW
Generational collection : Divide the memory area into the new generation and the old generation , Collect according to different strategies , The new generation generally adopts the label Replication Algorithm , It is divided into eden and suivivor( Two ), Default scale 8:1:1, Only enable at a time eden And a piece of survivor, take eden Qu Heyuan survivor Live objects are copied to idle survivor, And clean it at one time ;
In the old days, mark sorting algorithm was generally used （ There are many surviving objects in the old age ）

Cenozoic collector （ Are copied with marks ）
serial: Single thread
ParNew: Multithreaded collection
Parallel Scavenge The collector ： Same multithreading , Controllable throughput

Old age collector
Serial Old The collector ： Single thread
Parallel Old The collector ：Parallel Scavenge Old age version of collector

CMS: Concurrent low pause , Adopt mark clearing algorithm （ fragmentation ）
Initial marker Objects that the root node can be directly associated with
Concurrent Tags Root node enumeration
Final marker Use incremental update to solve the problem of concurrent marking , Record the reference of black to white in the process of concurrent marking , Black turns gray and traverses again
Concurrent elimination Recycling
The most time-consuming concurrent marking and concurrent cleanup are

G1: be based on region Of , Can manage the heap independently , Manage in different ways region, On the whole, it is the tag sorting algorithm , Locally, it is a marker Replication Algorithm , No memory fragmentation , Low pause and high throughput , Pause time model can be established （ Maintain a priority list ）
Initial marker
Concurrent Tags
Re label Use the original snapshot to solve the problem of concurrent marking , Record gray objects with white objects disconnected , And rescan
Screening and recovery

Object's memory allocation strategy
Priority is given to eden District Distribution
Large objects directly enter the era （ High cost of replication ）
Dynamic age determination ： The new generation of objects will be recorded in the object header every time they survive the collection , arrive 15 Backward into old age , Or objects of the same age exceed survivor Half of , Those who are older than or equal to this age are promoted to the elderly generation
Space allocation guarantee mechanism ： If risk is allowed , Based on historical experience, try to collect only the Cenozoic （ The old age may be able to accommodate promotion objects ）, If it fails, it will still trigger full gc

GC technological process
The new generation GC Under the space allocation guarantee mechanism , If it fails, it will trigger fullGC

Class loading

load verification Get ready analysis initialization
Get the binary stream according to the class name , And load the class information into the method area , create a class object
Verify file formats and specifications
Initialize the class variable to 0
Resolve symbolic references to direct references
Of the execution class init Method , Initialize class variables according to predetermined rules

Parent delegate mechanism

Start class loader
Extended class loader
Application class loader

When performing class loading, delegate to the parent class to load , Don't let the subclass load until the parent class can't load , Only one... Can be loaded through the class loader Object class , Have uniqueness

Break three times
1. Implement the class loader by yourself and inherit classloader, rewrite loadclass Method
2.SPI（ Service provider interface ）, Such as JDBC, Use thread context loader , The parent class entrusts the child class to load
3.OSGI Achieve modular hot deployment , Reticular , There are horizontal dependencies