Revisiting Java -- ThreadLocal terminal

Selfless heart, wide world 2020-11-08 23:46:29
revisiting java threadlocal terminal


ThreadLocal What is it?

effect

ThreadLocal Used to store private variables between threads

data structure

image.png

Memory leak ?

Before explaining the problem , Need to see first JAVA Object Strong citation 、 Soft citation 、 Weak reference 、 Virtual reference

Four reference types of objects

  • Strong citation
    new Or objects created by reflection are called strong references , As long as strong quotes still exist , It won't be recycled
  • Soft citation
    Use SoftReference Decorated objects are called soft references , When out of memory , Soft reference objects are recycled
  • Weak reference
    Use WeakReference Decorated objects are called weak references , When an object has only weak references ,GC when , The object will be recycled
  • Virtual reference
    Use PhantomReference Decorated objects are called virtual references , When an object is recycled, it is notified by the system

WeakReference Case introduction

public class WeakReferenceObj extends WeakReference<Object> {
public WeakReferenceObj(Object referent) {
super(referent);
}
public static void main(String[] args) {
WeakReferenceObj weak = new WeakReferenceObj(new Object());
int i = 0;
while(true){
if((weak.get()) != null){
i++;
System.out.println("Object is alive for "+i+" loops - "+weak);
}else{
System.out.println("Object has been collected.");
break;
}
}
}
}

When the above program has been running for a while ,WeakReference The object you point to will be referenced only by a weak reference , And recycling objects
image.png

If you change the appeal code to the following code

public class WeakReferenceObj extends WeakReference<Object> {
public WeakReferenceObj(Object referent) {
super(referent);
}
public static void main(String[] args) {
Object o = new Object();
WeakReferenceObj weak = new WeakReferenceObj(o);
int i = 0;
while(true){
System.out.println(o);
if((weak.get()) != null){
i++;
System.out.println("Object is alive for "+i+" loops - "+weak);
}else{
System.out.println("Object has been collected.");
break;
}
}
}
}

You'll find that , No matter how long it runs , Objects pointed to by weak references are not recycled . Because the o It's also pointed to by a strong reference . namely Print stream .

ThreadLocal Memory leaks in

ThreadLocal As a weak reference exists in ThreadLocalMap key in . Because it's a weak quote , When ThreadLocal When pointed to only by a weak reference , It's triggering GC after ,ThreadLocal Will be recycled , namely ThreadLocalMap key Would be null. value By ThreadLocalMap Strong references point to , Lead to value Cannot be recycled .ThreadLocalMap again Thread A property of , So unless Thread The destruction ,ThreadLocalMap To be released , thus ,Entry Not for null ,key = null, value It's worth it again ( It's not shit to occupy the manger ),ThreadLocalMap If there is no effective clear Entry Not for null, key = null The mechanism of , Then it will be because of value Cannot be recycled , Resulting in memory leaks .

ThreadLocal Cleaning mechanism

stay ThreadLocal Memory leak analysis , We know , If ThreadLocal There is no effective clean-up mechanism , Then it will inevitably lead to a memory leak . So what follows is ThreadLocal in 2 Sort of clean-up mechanism , Prevent memory leaks

Probe cleaning

The logic of the code is :ThreadLocalMap.expungeStaleEntry
from key = null To clear the position forward , Then traverse ThreadLocalMap until Entry != null. If you encounter key = null Will Entry、value Set to null, If key != null, Then re hash Put the Entry Put in ThreadLocalMap in .

ThreadLocal Of get(), set(T t), It's not the same where to start cleaning up , But in the end it all calls expungeStaleEntry Method , Clean up .

get()

The cleaning point is : In from x Subscript Can't get the corresponding key Of value when , From x The subscript starts cleaning up

set(T t)

The cleaning point is : If you set the value , Found in x Subscript key = null. Will follow x Look forward to key = null, until Entry = null, If I find , x Will be given the subscript of the element . Finally, from x Start cleaning up .

Heuristic cleanup

The logic of the code is :ThreadLocalMap.cleanSomeSlots

from i Position start , Until now ThreadLocalMap in Entry Number n >>> 1 != 0.
If Entry != null, but key = null, Would call expungeStaleEntry Clean up .

How to prevent

After use , call remove Method , Clean up .

Conclusion

get、set Methods will be expired internally key Clean up . But just in case , After use , You also need to manually call remove Methods to clean up

ThreadLocal Hash Algorithm

ThreadLocal There's an attribute in HASH_INCREMENT = 0x61c88647, It is called Golden section number ,hash The increment is the number , therefore , Produced hash The numbers are very uniform .

private static final int HASH_INCREMENT = 0x61c88647;
public static void main(String[] args) {
for (int i = 0; i < 16; i++) {
int hash = HASH_INCREMENT * i + HASH_INCREMENT;
System.out.print(hash & (16 - 1));
System.out.print(",");
}
}

The results are as follows :

7,14,5,12,3,10,1,8,15,6,13,4,11,2,9,0,

ThreadLocal Hash Conflict

ThreadLocal Hash The conflict uses Open addressing with linear probe and hashing .
The so-called linear detection algorithm is as follows : From the current location of the conflict , Look back , Until you find one null Insert at .

Capacity expansion

When doing set after , Will execute cleanSomeSlots If there are cleaning elements , And the array size reaches the threshold of array expansion thresholdlen * 2 / 3) It's going to do exploratory cleaning . If after cleaning , Array size greater than treshold * 3 / 4 Then expand the capacity .

Add capacity , The array becomes the original 2 times , And the whole thing ThreadLocalMap Of key again hash Put in table in

Soul torture , Why? ThreadLocalMap key Is a weak reference ?

key Is a strong quote

ThreadLocalMap Life cycle and Thread Agreement . If Thread It's been too long , Added a lot of ThreadLocal. In this case, if the ThreadLocal Set to null, It should be recycled . however , because ThreadLocalMap There is still ThreadLocal A strong reference to , And it can't be recycled , Resulting in memory leaks . And in the code , It's hard to judge ThreadLocal Is there any quotation in other places .

key Is a weak reference

ThreadLocal Is a weak reference , The code is set to null after , Because there are only weak references , therefore , stay GC It will be recycled normally . however key = null There are also value Memory leak . although value There will be a memory leak , But it can be judged key = null To judge ,ThreadLocal There are no other quotations .

Conclusion

I think the core reason is :ThreadLocalMap Life cycle and Thread Agreement . It's too hard to judge ThreadLocal Only in ThreadLocalMap There is a quotation in . So set to weak reference , Give Way GC Recycling ThreadLocal after , use null To judge

Reference resources

interviewer : I heard you've seen ThreadLocal Source code ? Let me have a look ?

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本文为[Selfless heart, wide world]所创,转载请带上原文链接,感谢

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