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This article mainly explains the "JVM memory model and garbage collection knowledge points collation", the content of the article is simple and clear, easy to learn and understand, the following please follow the editor's ideas slowly in depth, together to study and learn "JVM memory model and garbage collection knowledge points collation" bar!

The main contents of this paper are as follows:

Memory model

Garbage collection

The function of each part

The main storage areas are the stack area and the heap area, so what is the stack and what is the heap? To put it simply, the stack stores basic data types and references, while the heap stores various object instances.

Why are heaps and stacks designed separately?

The stack stores processing logic and the heap stores specific data, so the isolation design is clearer.

The heap is separated from the stack so that the heap can be shared by multiple stacks.

The stack holds the context information, so it can only grow upwards, while the heap is dynamically allocated.

The size of the stack can be set by-XSs. If it is insufficient, it will cause an exception of java.lang.StackOverflowError.

Stack area

The thread is private and has the same life cycle as the thread. When each method is executed, a stack frame (stack frame) is created for storing local variables, operation stacks, dynamic links, and method exits.

Heap

Store object instances, where the memory of all objects is allocated. Garbage collection is mainly used here.

The heap memory is specified by-Xms, which defaults to 1x64 of physical memory, and the maximum memory is specified by-Xmx, which defaults to 1x4 of physical memory.

When the default free heap memory is less than 40%, it increases until the memory set by-Xmx. The specific scale can be specified by-XX:MinHeapFreeRatio

When the free memory is greater than 70%, the memory is reduced to the size set by-Xms. It is specified by-XX:MaxHeapFreeRatio.

Therefore, it is generally recommended that these two parameters be set to the same size to prevent JVM from constantly resizing.

Program counter

The line number of the bytecode executed by the thread is recorded here, which depends on this counter for branches, loops, jumps, exceptions, thread recovery, and so on.

Method area

Type information, field information, method information, other information

Summary

The thread in the stack area is private, using a continuous memory space to store local variable tables, operation stack, dynamic link, method exit-XSsStackOverflowError OutOfMemoryError heap thread sharing, life cycle the same as the virtual machine to save the object instance-Xms-Xmx-XmnOutOfMemoryError program counter thread private, memory bytecode line number no method area thread sharing storage class load information, constants, static variables, etc.-how to define garbage in XX:PermSize-XX:MaxPermSizeOutOfMemoryError

There are two ways, one is reference counting (but does not solve the problem of circular references), and the other is reachability analysis.

Determine the situation in which the object can be recycled:

Display to set a reference to NULL or point to another object

The object that the local reference points to

Weak reference to the associated object

The method of garbage collection Mark-Sweep marking-clearing algorithm

This approach has the advantage of reducing pause time, but it has the disadvantage of causing memory fragmentation.

Copying replication algorithm

This method does not involve the deletion of objects, but simply copies available objects from one place to another, so it is suitable for scenarios where a large number of objects are recycled, such as the new generation of recycling.

Mark-Compact marking-finishing algorithm

This approach solves the memory fragmentation problem, but increases the pause time.

Generational Collection generation collection

The last method is a combination of the previous several, that is, one of the main methods adopted by JVM at present, the idea is to divide the JVM into different regions. Each area uses a different garbage collection method.

As you can see above, the heap is divided into three areas:

New generation (Young Generation): used to store newly created objects, using replication recycling method, if replicated between S0 and S1 for a certain number of times, transfer to the older generation. The garbage collection here is called minor GC

Old Generation: these objects are garbage collected less frequently, using the tagging method, where garbage collection is called major GC.

Permanent Generation: stores some data from the Java itself, which is also recycled when the class is no longer in use.

Here you can talk about the algorithm flow of the new generation of replication and recycling in detail:

In the Cenozoic era, it is divided into three regions: Eden, from survivor and to survior.

When minor GC is triggered, the surviving objects in Eden will be copied to to Survivor first.

Then look at from survivor. If the number of times reaches the standard of the old generation, it will be copied to the old generation; if it does not reach it, it will be copied to the to survivor; if the to survivor is full, it will be copied to the old generation.

Then change the names of from survivor and to survivor to make sure that each time the to survivor is empty waiting for the object to be copied there.

Garbage collector

Serial collector Serial

This collector collects in a single-threaded manner, and other threads cannot work during garbage collection.

Parallel collector Parallel

Collect in a multithreaded manner

Concurrent tag cleanup collector Concurrent Mark Sweep Collector, CMS

The general process is as follows: initial marking-concurrent marking-relabeling-concurrent clearing

G1 Collector Garbage First Collector

The general process is as follows: initial marking-concurrent marking-final marking-filtering and recycling

Thank you for your reading, the above is the content of "JVM memory model and garbage collection knowledge points finishing". After the study of this article, I believe you have a deeper understanding of the JVM memory model and garbage collection knowledge points collation, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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