JVM object lifecycle

jvm The life cycle of the object in

Create a stage

（1） Allocate storage space for objects 
（2） Start constructing objects 
（3） From superclass to subclass pair static Members are initialized 
（4） Superclass member variables are initialized in order , Recursively calls the constructor of the superclass 
（5） Subclass member variables are initialized in order , Subclass construct method call , And once the object is created , And assigned to some variables 
	 value , The shape of this object 		 State switches to the application stage 

Application stage

（1） The system maintains at least one strong reference to the object （Strong Reference）
（2） All references to this object are strong references （ Unless we explicitly use ： Soft citation （Soft Reference）、
	 Weak reference （Weak Reference） Or virtual reference （Phantom Reference））

 The definition of reference ：
1. Our data type must be reference type 
2. The data stored in our type of data must be the starting address of another piece of memory 

Invisible stage

Objects in the invisible phase can no longer find direct or indirect strong references in the object root reference collection of the virtual machine , The most common is temporary variables in threads or functions . Programs no longer hold strong references to objects .（ But static variables of some classes or JNI It is possible to hold ）

Unreachable stage

It means that the object is no longer held by any strong reference ,GC It is found that the object is unreachable

The difference from the invisible stage is ： Unreachable is after Algorithm
So what are the algorithms ？

 Reference count ：
	 Object has a reference to +1, Remove a reference and -1, When the number of references is 0 It is considered that there is no quotation , Recyclable 
	 problem ： Can't solve the problem of circular dependency 

 Accessibility analysis ： adopt GC Root The object of , Start looking down , See if an object can reach 
	 What can be regarded as GC ROOT： loader 、Thread、 Local variable table of virtual machine stack 、static member 、 const reference 、
		 Variables of the local method stack are essentially a set of active references 

Collection phase

 See whether the object needs to be executed finalize Method ：
	 If you don't need to ： Virtual machine has been called finalize Methods or objects are not overridden finalize Method , Then the object can be recycled 
	 need ： The virtual machine puts objects into F-queue, Create a low priority thread to execute the objects in the queue finalize Method 
		finalize Method to re establish the reference connection for the object 
			 yes ： The virtual machine moves the object out F-queue queue , Object returns to the application stage 
			 no ： Objects can be recycled 

finalize Methods can affect JVM The object of the allocation and the recycling speed
finalize Methods may revive objects

The end stage

Object's finalize() After the function is executed , The object is still unreachable , This object is the process termination stage .

Object memory space reallocation phase

GC Reclaim or reallocate the memory space occupied by this object , The object disappeared completely

out of memory

1、 Heap memory overflow ：java.lang.OutOfMemoryError: GC overhead limit exceeded
	-Xmx20M ： The maximum size that can be expanded when the program runs 
	-Xms20M： Initialize heap size 
2、 Method area memory overflow ：java.lang.OutOfMemoryError: Metaspace
	-XX:MetaspaceSize： Initialize method area size 
	-XX:MaxMetaspaceSize： The maximum size that can be expanded when the program runs 
3、 Occupy memory overflow ：java.lang.StackOverflowError
	-Xss： The size of the stack ( Default 1M)