JVM initial

Java One of the most important characteristics of language is that it is platform independent . While using Java virtual machine （JVM） It's the key to this feature .JVM Is a specification for computing equipment , It's an imaginary computer , It is realized by simulating various computer functions on a real computer . Before, there were mainly three kinds of mainstream JVM,
Sun The company's HotSpot
BEA The company's JRockit
IBM The company's J9 JVM
later Sun Company and BEA Companies are oracle Acquisition , also oracle use Sun The company's HotSpot and BEA The company's JRockit Two JVM Essence quit jdk1.8 Of JVM.
The picture below is JVM Structure , It's very important , Especially the runtime area .

Run time data area

1、 Pile up

JVM The biggest piece in the world , It is mainly used to store object instances and arrays , Almost all object instances allocate memory here . Thread sharing , Internal will be divided into multiple threads private allocation buffer (TLAB). Can be in a physically Discontinuous Space , But logically, it should be continuous .
2、 Virtual machine stack

Each method is executed with a stack frame created at the same time (Stack Frame） Used to store local variables 、 Stack operation 、 Dynamic links 、 Method exit information . Each method is called until the procedure completes , This corresponds to the process of a stack frame in the virtual machine stack from push to push . Thread private , Life cycle is consistent with thread .
3、 Method area （ Non heap ）

It belongs to the shared memory area , Store class information that has been loaded by the virtual machine 、 Constant 、 Static variables 、 Real time compiler compiled code and other data .
4、 Native Method Stack

Native Method Stack （Native MethodStacks） It's very similar to what the virtual machine stack does , The difference is that the virtual machine stack executes for the virtual machine Java Method （ That's bytecode ） service , The local method stack is used by the virtual machine Native Method service .
5、 Program counter

Program counter （Program CounterRegister） It's a small amount of memory , It can be thought of as a line number indicator of the bytecode being executed by the current thread . This memory area is the only one in Java Nothing is specified in the virtual machine specification OutOfMemoryError Area of situation .
6、 Direct memory

Direct memory （Direct Memory） It's not part of the virtual machine runtime datazone , Neither Java Memory area defined in virtual machine specification , But this part of memory is also frequently used , And it can also lead to OutOfMemoryError Abnormal appearance . stay JDK 1.4 Newly added NIO（New Input/Output） class , A channel based approach is introduced （Channel） With the buffer （Buffer） Of I/O The way , It can be used Native Function library directly allocates out of heap memory , And then through a Java In the pile DirectByteBuffer Object operates as a reference to this memory . This can significantly improve performance in some scenarios , Because it avoids Java Heaps and Native Copy data back and forth in the heap .
Generational collection of heaps

JVM There are many garbage collection algorithms , One of the most classic algorithms currently in use is the generational collection algorithm , Here is the priority record .
Forever （Perm）： The main preservation is class,method,field Objects such as , The space size , Depending on the number of classes loaded by the system startup , Generally, the memory overflow in this area is overflow at startup .java.lang.OutOfMemoryError: PermGen space
Old age （Old）： Usually after multiple garbage collection （GC） Objects that have not been recycled .
The new generation （Eden）： Newly created objects .
The new generation （Survivor0）： After recycling （GC） after , Objects that have not been recycled .
The new generation （Survivor1）： Same as Survivor0 identical , The same size and space , At the same time Survivor0 and Survivor1 Only one is in use , One is empty .
Java The heap is the main area managed by the garbage collector , According to the division of generational collection algorithm , Heap memory space can continue to be subdivided into younger generations , Old age . The younger generation can be divided into larger Eden District , Two of the same size From Survivor,To Survivor District . default Eden Area and Survivor The area size ratio is 8:1:1. When allocating memory for the newly created object, allocate the object to Eden Area and From Survivor District , When recycling immediately , Will Eden Area and Survivor Objects that are still alive in the area are copied to To Survivor In the area , If To Survivor The size of the area cannot accommodate surviving objects , The surviving objects will be assigned to the elderly area . On the whole , Newly created small objects will be placed in the younger generation , Most of the objects of the younger generation will be recycled in the next garbage collection , Older generations store large objects and long-lived objects .
appendix ： Relevant concepts

1、 Basic types and reference types

JVM in , There are two types of data ： Basic types and reference types . Basic types of variables hold the original values , namely ： The value he represents is the value itself ; And variables of reference type hold reference values .“ Reference value ” Represents a reference to an object , Not the object itself , The object itself is stored in the location of the address represented by the reference value .
2、 Heap and stack

  A stack is a unit of runtime , And the heap is the unit of storage . stay Java There will be a thread stack corresponding to one thread in , It's easy to understand , Because different threads have different execution logic , So we need a separate thread stack . The heap is shared by all threads . The stack is the running unit , So the information stored in it is the same as the current thread （ Or a program ） Related information . Include local variables 、 Program running state 、 Method return value, etc ; The heap is only responsible for storing object information .
The stack represents the processing logic , And the heap represents the data . 
There are objects in the heap , The stack stores basic data types and references to objects in the heap .
stay Java in ,Main The function is the starting point of the stack , It's also the starting point of the program .
3、 Reference type

Object reference types are divided into ： Strong citation 、 Soft citation 、 Weak quotation and virtual quotation .
Strong citation ： Will not be recycled . It is the reference generated by the virtual machine when we generally declare the object to be , In a strong reference environment , When garbage collection, it is necessary to judge whether the current object is strongly referenced , If it is strongly quoted , It won't be recycled .
Soft citation ： Enough memory , It's not recycled ; If not enough, recycle . Soft references are generally used as a cache . The difference with strong quotes is , Soft quotes in garbage collection , The virtual machine determines whether to recycle the soft references according to the remaining memory of the current system . If the remaining memory is tight , The virtual opportunity reclaims the space referenced by the soft reference ; If the remaining memory is relatively rich , No recycling . let me put it another way , Virtual machines are happening OutOfMemory when , There must be no soft reference .
Weak reference ： Will be recycled . Weak quotation is similar to soft quotation , It's all used as a cache . But unlike soft quotes , Weak references in garbage collection , It must be recycled , So its life cycle only exists in one garbage collection cycle .