Talk about ThreadLocal

1. Chat ThreadLocal
   1. java Four kinds of quotations from
   2. Solution
   3. summary

Chat ThreadLocal

Let's talk today ThreadLocal, Although less is used in development , But it is really a point often asked in interviews .

Every thread has a ThreadLocalMap,ThreadLocalMap yes ThreadLocal The static inner class of , its key yes ThreadLocal Example ,value Is the object that needs to be stored , and ThreadLocalMap In fact, it is Entry Array storage ,Entry Inherit WeakReference, It is a weak reference .

static class Entry extends WeakReference<ThreadLocal<?>> {            /** The value associated with this ThreadLocal. */            Object value;            Entry(ThreadLocal<?> k, Object v) {                super(k);                value = v;            }        }

It needs to be popularized java Four kinds of quotations from

java Four kinds of quotations from

Strong citation ： Like through new Objects are strong references , This is generally not GC Recycling

Soft citation ： Only when the memory is insufficient will the memory be recycled ,

Weak reference ： Whether memory is enough or not , The garbage collector will collect memory when it collects , Generally, weak references are in map Use in

Virtual reference ： It can be recycled at any time

TheadLocal Threads are private , Threads are not affected , Therefore, thread safety can be guaranteed , When we transfer across layers, we can consider using ThreadLocal,spring The transaction information of is placed in ThreadLocal Medium

But everybody knows that ,ThreadLocal There is a memory leak problem , The so-called memory leak is that the memory occupied by objects that are no longer used cannot be recycled ,new Objects may cause memory leaks , When ThreadLocal When there is no strong reference ,ThreadLocalMap Of key It will be recycled , Now key by null, At this time value But strong citation , At this time, there is only the interruption or end of the current thread ,ThreadLocalMap The data in will be cleared , If the current thread has been running , obviously value Can't be recycled , A memory leak occurs . And when key When it's empty , call ThreadLocalMap Of get Methods or set Methods or remove Methods will be cleared value value

Solution

The solution is when we ThreadLocal After use , By calling ThreadLocal Of remove Method to clear data , Also can put the ThreadLocal Set to private static, such ThreadLocal It's a strong reference , such TheadLocalMap Of key Not for null, Thus, the data will be cleaned up at the appropriate time .

summary

This article mainly talks about hreadLocal Relevant knowledge points of , Including why memory leaks occur , There have been solutions to this problem , And incidentally java Four ways of reference .