JVM heap

2.4 Pile up

One JVM There is only one heap in the instance , So is heap Java The core area of memory management .

The only purpose of the heap is to store object instances , Almost all object instances and data are allocated memory here .

Java The heap is the main area managed by the garbage collector , So it's also called GC Pile up .

（1） Memory partition of heap

• The new generation
  • Eden
  • Survivor
    • from
    • to
• Old age
• Meta space （ Forever ）

（2） Set heap memory size

• -Xmx Used to represent the starting memory of the heap , By default , The initial heap memory size is ： Computer memory size / 64
• -Xms Used to represent the maximum memory of the heap , By default , The maximum heap memory size is ： Computer memory size / 4

（3） Common in the stack area OOM

• java.lang.OutOfMemoryError: GC Overhead Limit Exceeded ： When JVM When it takes too much time to perform garbage collection and only a small amount of heap space can be recycled , This error will happen .
• java.lang.OutOfMemoryError: Java heap space : If you are creating a new object , There is not enough space in the heap memory to hold the newly created objects , It will cause this error .
• Permgen space: Permanent generation is full , Usually loaded class Too many or too big 、

Object allocation process

• new The object of is stored in eden District , But Eden Park has a size limit
• When eden When the space is full , The program needs to create objects ,JVM Our garbage collector will recycle the garbage of Eden Park （Minor GC）, Destroy the objects in Eden Park that are no longer referenced by other objects , Then load the new object and put it in the garden of Eden
• Then move the remaining objects in Eden to the survivors 0 District
• If garbage collection is triggered again , At this point, the last survivor is placed in the survivor 0 District , without i Recycling , It's going to put the survivors 1 District
• The default is 15 The recycling mark will enter the old generation
• When the older generation runs out of memory , Trigger again Major GC, Clean up the old memory
• If the old age carried out Major GC After that, it is found that the object cannot be saved , It will produce OOM abnormal

Memory allocation policy

• Give priority to Eden Park
• Large objects are directly allocated to the old age
• Long lived objects are assigned to old age
• If survivor The sum of the sizes of all objects of the same age in the region is greater than survivor Half of the space , Objects older than or equal to this age can directly enter the elderly generation
• Space allocation guarantee