Garbage collector and memory allocation strategy

1. Reference counting algorithm

Add the counter that should be referenced to the object , Whenever there's a place to quote it , The counter value adds 1; When the reference fails, the counter value is decremented by one ; When the counter is 0 It is impossible to be used again

• The problem is ： It can't solve the problem of circular reference between objects

objA,instance=objB;
objB.instance=objA;
objA=null;
objB=null;

2. Reachability analysis algorithm

adopt GC Roots As the starting point , Start with these nodes and drill down , The search path is called the reference chain , When an object arrives GC Roots There's no chain of references connected , Then prove that this object is not available

It can be used as GC Roots The object of ：

1. Objects referenced in the virtual machine stack
2. Class static attribute references in the method area
3. The object referenced by a constant in the method area
4. Objects referenced in the local method stack

3. Quote categories

1. Strong citation ： Object obj=new Object(), As long as strong quotes still exist ,GC Objects are not recycled
2. Soft citation ： Describe some useful but not necessary objects , When the memory is insufficient, the object pointed to by the soft reference is recycled
3. Weak reference ： Describe nonessential objects , The next time GC At the beginning, whether the memory is sufficient or not , It's all recycled
4. Virtual reference ： The weakest kind of reference relationship , Whether an object has a virtual reference , It doesn't affect their survival time at all

4.GC Roots Is the object bound to die when unreachable

• not always
• GC Mark for the first time and filter for the first time when it is not reachable
• If the object is not overridden finalize Methods or finalize The method has already been executed , The object has no chance of survival
• If the object overrides finalize Method and finalize The party has not implemented , stay fianlize Method to re-establish the relationship with the objects on the reference chain , The object will not be recycled
• Of an object fianlize Method will only be executed once

package test.JVM;

public class FinalizeEscapeGC {
    
    public static FinalizeEscapeGC SAVE_HOOK=null;
    public void isAlive()
    {
    
        System.out.println("yes,i am still alive");
    }
    @Override
    protected void finalize() throws Throwable
    {
    
        super.finalize();
        System.out.println("finalize method executed");
        FinalizeEscapeGC.SAVE_HOOK=this;
    }
    public static void main(String[] args) throws InterruptedException {
    
        SAVE_HOOK=new FinalizeEscapeGC();

        SAVE_HOOK=null;
        System.gc();
        Thread.sleep(500);
        if(SAVE_HOOK!=null)
        {
    
            SAVE_HOOK.isAlive();
        }
        else
        {
    
            System.out.println("no,i am dead");
        }


        SAVE_HOOK=null;
        System.gc();
        Thread.sleep(500);
        if(SAVE_HOOK!=null)
        {
    
            SAVE_HOOK.isAlive();
        }
        else
        {
    
            System.out.println("no,i ma dead");
        }
        
    }
    
}
finalize method executed
yes,i am still alive
no,i ma dead

5. Recovery method area

• The garbage collection of the permanent generation is divided into two parts ： Discard constants and useless classes
• Determine whether a constant is an obsolete constant ：
  1. No object references constants in the constant pool
• Determine whether a class is useless ：
  1. All instances of this class have been recycled
  2. Load the class ClassLoader Has been recovered
  3. Corresponding to this class java.lang.Class Object has no references

6. Garbage collection algorithm

6.1 Tag clearing algorithm

• Mark all the objects that need to be recycled first
• After marking, all marked objects will be recycled
• Low efficiency
• A large number of discontinuous memory fragments will be generated , As a result, there is not enough memory to allocate large objects later

6.2 Copy algorithm

• Divide the available memory into two equal sized blocks according to capacity , Use only one piece at a time
• A block of memory is used up , Just copy the living object to another piece , Then clean up the used memory space at one time
• Memory costs are high , waste 50% Of memory space

6.3 Mark - Sorting algorithm

• For the old age
• Marking process and “ Mark - eliminate ” Algorithm is the same , However, objects are not directly recycled later , Instead, let the living object move to one side , Then directly clean up the memory outside the end boundary

6.4 Generational collection algorithm

• The memory is divided into several blocks according to the life cycle of the object
• Java The heap is divided into The new generation and Old age
• The new generation ： A lot of people die , Little survival , Copy algorithm
• Old age ： The survival rate is high , There is no additional space to guarantee its distribution , Mark - Clean up algorithms or tags - Sorting algorithm

7. Enumerated root node

• The sensitivity of reachability analysis to execution time is reflected in GC Pause on , During reachability analysis, the whole execution system looks as if it is frozen at a certain point in time
• GC There must be a pause in the progress of all Java Execute thread

8. safer

• The program doesn't stop and start everywhere GC, Only when the safety point is reached can it pause
• Safety points can't be too few , Too little will lead to GC The wait is too long
• Don't be too safe , Too much will increase the load at run time
• The thread is in GC There are two ways to interrupt ： Preemptive interrupt and Active interrupt
  1. Preemptive interrupt ：GC when , First, interrupt all threads , If the place where the thread is interrupted is not found at the safe point , Just restore the thread , Let the thread run to the safe point , But few virtual machines do this
  2. Active interrupt ： Set a flag , This flag is actively rotated during thread execution , When the interrupt flag bit is found to be true, it suspends its own interrupt

9. The safety area

• For being in Sleep perhaps Blocked Thread in state , It's unresponsive JVM Interrupt request for , You can't go to a safe place to suspend , A safe area is required
• A security zone is defined as a code segment , The quoting relationship doesn't change , Anywhere in the area is GC Safe , It can be seen that the safety zone is an expanded safety point

10. Memory allocation and recycling strategy

10.1 Priority is given to Eden Distribute

• Most of the time , The object is the new generation Eden Regional distribution , When Eden There is not enough space to allocate , The virtual machine will do it once GC

10.2 Large objects go directly into the old generation

• Large objects are those that require a large amount of continuous memory space Java object

10.3 Long term survivors will enter the old age

• The virtual machine sets an object age counter for each object , If the object is Eden Born and passed for the first time Minor GC Still alive after , Then set the age of the object to 1
• The age of the subject increases to 15 Will be promoted to the old generation

10.4 Dynamic object age determination

• The virtual machine does not necessarily judge whether the object has entered the old generation according to the age counter
• If in Survivor The sum of all object sizes of the same age in the space is greater than Survivor Half of the space , Those who are older than or equal to this age can directly enter the elderly generation

10.5 Space allocation guarantee

• In the event of a Minor GC Before , Virtual opportunity first checks whether the largest available continuous space of the old generation is larger than the total space of all objects of the new generation , Greater than is safe , Otherwise it's not safe
• Virtual opportunity viewing when not secure HandlePromotionFailure Whether the set value allows the guarantee to fail
• If allowed , Continue to check whether the maximum available continuous space of the elderly generation is larger than the average size of objects promoted to the old age
• If it is greater than , Conduct Minor GC
• If less than or not allowed , Conduct Full GC

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Bibliography ：《 Deepen understanding Java virtual machine ：JVM Advanced features and best practices （ The first 2 edition ）》