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JVM memory and memory overflow exceptions (personal summary)

2022-04-23 05:23:00 【Cx_ Xuan】

JVM Memory overflow exception

    Want to know Java The memory overflow exception of the virtual machine needs to be understood first JVM What's in memory , What do they all do , In this way, we can have a deeper understanding of what memory overflow is .
stay JAVA The runtime memory area in the virtual machine memory PC（ Thread private -- Program counter ） Not going to happen OutOfMemoryError abnormal , Several other areas [｛ Thread sharing area ： Method area 、 Pile up ｝,｛ Thread private ： Native Method Stack 、 Virtual machine stack ｝].
Thread sharing area ----------------
----------- Method area ：
It is used to store the class information that has been loaded by the virtual machine （object header info）、 Constant and JIT Compiler compiled code and other data .
The goal of memory recycling in this area is to recycle the constant pool and unload the type .( Runtime constant pool -------Class In addition to including the version of the class in the file 、 Field 、 Method 、 Interface and other description information , Another piece of information is constant pool information , This includes various literal and symbolic references generated by the compiler , When the class is loaded, it is stored in the runtime constant pool of the method area ）.
----------- Pile up ：
For most applications, heap is the largest piece of memory management , It mainly stores object instance information .
It comes into being as the virtual machine starts , It is the main area of garbage collection management .
Thread private area -------------------
---------- Virtual machine stack ：
It has the same life cycle as threads . The virtual stack describes JAVA Memory model for method execution ：
When each method is executed, a stack frame will be created to store the local variable table 、 The stack of operands 、 Dynamic links 、 Methods the export . Each method is called until the execution of the completed process , All correspond to the process of stack frame entering and exiting the stack in the virtual machine .
---------- Native Method Stack
Stack plays a very similar role to virtual machine ,
The virtual machine stack executes for the virtual machine JAVA Method service , The local method stack is used for virtual machines Native Method services .
---------- Program counter
The program counter is a smaller area of memory , It can be seen as a line number indicator of bytecode executed by the current thread .
When the bytecode interpreter works, it selects the next byte instruction to be executed by changing the value of the counter , Branch 、 loop 、 Jump 、 exception handling 、 Basic functions such as thread recovery rely on this counter .

JVM Memory overflow exception ：
------------------------------------JAVA Stack overflow
StackOverFlowError：
whenever JAVA When the sequence starts a new thread ,JAVA The virtual opportunity assigns it a JAVA Stack .
JAVA The stack stores the running state of the thread in frames . Whenever a thread calls a method , The virtual machine pushes a stack frame into JAVA In the stack . As long as this method does not return , Then it will always exist . If thread nested methods （ No return ） Too much , As the number of stack frames increases ,JAVA The sum of all stack frame sizes in the stack is greater than -Xss Set the value of the , Will appear StackOverFlowError abnormal .

eg:
public class Cx{
public void method(){
System.out.print("cx");
method();
}
main(String[] args){
new Cx().method();
}
}
Method recursion , When there are too many methods, there will be exceptions .

OutOfMemoryError:
When JAVA When the program starts a new thread , There is not enough memory space allocated for this thread JAVA Stack （ One JAVA The size of the stack is determined by -Xss Parameter determination ）,JVM Throw out OutOfMemoryError abnormal .
eg:
public class Cx{
main(String[] args){
new Thread(new runable(){
public void run(){
System.out.print("....cx");
}
}).start();
}
}
-------------------------------------JAVA Heap overflow
We know JAVA Heap is used to store object instances . that JAVA Heap overflow , It shows that the space for storing object instances is insufficient .
Insufficient memory allocation to store object instances , Will appear OutOfMemoryError abnormal .
 
-------------------------------------JAVA Method area overflow
The method area is used to store JAVA Type information , Such as class name 、 Access modifier 、 Methods described 、 Field description, etc .
Load... In the class loader class File to memory process , Virtual opportunity extracts the information , And save it in the method area . The runtime produces a large number of classes , When the set memory value is reached , Will overflow .
I hope you will get something , If there is something wrong, I would like to leave a message to point out . thank you

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本文为[Cx_ Xuan]所创，转载请带上原文链接，感谢
https://yzsam.com/2022/04/202204220545385544.html

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