Fast crosstalk -- partition of JVM memory

Purpose

• Rapid positioning JVM Memory leak or overflow, etc .
• Basic questions of interview , pluses .

Program counter （Program Counter Register）

• Record the line number of bytecode executed by the current thread .
• Rely on it to implement branches 、 loop 、 Jump 、 exception handling 、 Thread recovery .
• Natvie When the method is used , Its value is empty .
• Java The only area in the virtual machine specification that does not specify memory overflow .
• “ Thread private ” The memory area of .

Virtual machine stack （Virtual Machine Stacks）

• Its life cycle is the same as thread .
• Store local variable table 、 The stack of operands 、 Dynamic links 、 Method exit information .
• When the method is executed, a stack frame is created and merged into the stack .
• When the execution of the method is completed, the corresponding stack frame is out of the stack .
• When the stack depth is too deep , Throw out StackOverflowError .
• Too many threads , Throw out OutOfMemoryError .
• “ Thread private ” The memory area of .

Native Method Stack （Native Method Stacks）

• Used for virtual machines Native Method service .
• When the stack depth is too deep , Throw out StackOverflowError .
• Too many threads , Throw out OutOfMemoryError .
• “ Thread private ” The memory area of .

Pile up （Java Heap）

• For storing object instances , yes JVM The largest area of memory .
• The main area of garbage collector Management .
• When using the generational collection algorithm , Heaps are subdivided into Cenozoic and older generations , The new generation is subdivided into Eden Space 、 From Survivor Space 、 To Survivor Space .
• stay HotSpot In the virtual machine , adopt -Xmx and -Xms Control the size of the heap , adopt -Xmn Control the size of the Cenozoic .
• The heap can be in a physically discontinuous memory space , As long as it's logically continuous .
• When there is no memory to complete the instance allocation , Throw out OutOfMemoryError .
• “ Thread sharing ” The memory area of , The heap shared by threads may be divided into private allocation buffers of multiple threads （Thread Local Allocation Buffer,TLAB）.

Method area （Method Area）

• Storage has been JVM Class information loaded 、 Constant 、 Static variables 、 Real time compiler compiled code .
• The goal of recycling is mainly to recycle the constant pool and unload the type .
• You can choose not to implement garbage collection .
• When the method area cannot meet the memory allocation requirements , Will throw out OutOfMemoryError .
• JDK6 Use the permanent generation to implement the method area .
• JDK7 Yongkudai's string constant pool has been removed .
• JDK8 Use the meta space to implement the method area .
• “ Thread sharing ” The memory area of .

summary

• Program counter ： The line number indicator of bytecode executed by the current thread .
• Virtual machine stack ：Java Memory model for method execution , Used to store local variables 、 The stack of operands 、 Dynamic links 、 Method exit information .
• Native Method Stack ： Memory model for local method execution , Very similar to the virtual machine stack , The difference is that the local method stack is JVM Used Native Method service .
• Pile up ： Used to store object instances , Is the main area of garbage collector Management .
• Method area ： Used to store has been JVM Class information loaded 、 Constant 、 Static variables 、 Real time compiler compiled code and other data .