Multithreading and high concurrency IV: varhandle, strong weak virtual reference and ThreadLocal

1：VarHandle

The last one said AQS Source code , There is also a knowledge point VarHandle, Look at the code below

Everyone will ask. ：VarHandle What is it? ？ What's the role ？
Actually ：VarHandle yes jdk1.9 After that , Represents a reference to a variable , Atomic thread - safe operations can be performed through cas Method to compare the set value . Faster than reflection , Direct manipulation of binary code , Reflection should be checked before each operation

2： Strong weak virtual reference

Part of the content of the blog quotes ：http://blog.csdn.net/liuxian13183

2.1. Strong citation

Object object=new Object（）; that object Is a strong reference . If an object has Strong citation , It's similar to the necessities , The garbage collector will never recycle it . When memory runs out of space ,Java The virtual machine would rather throw OutOfMemoryError error , Causes the program to terminate abnormally , It will not solve the problem of insufficient memory by randomly recycling objects with strong references .

2.2. Soft citation （SoftReference）

If an object has only Soft citation , It's similar to the things you can have for daily use . If there's enough memory , The garbage collector won't recycle it , If there's not enough memory , It will reclaim the memory of these objects . As long as the garbage collector doesn't recycle it , This object can be used by the program . Soft citation It can be used to implement memory sensitive cache . Soft references can be combined with a reference queue （ReferenceQueue） A combination of , If the object referenced by the soft reference is garbage collected ,Java The virtual machine adds the soft reference to the reference queue associated with it .

2.3. Weak reference （WeakReference）

Usually used in containers WeakHashMap also ThreadLocal . If an object has only weak references , It's similar to the things you can have for daily use . The difference between weak quotation and soft quotation is ： Objects with only weak references have a shorter life cycle . In the process of the garbage collector thread scanning the memory area it governs , Once you find an object with only weak references , Whether the current memory space is enough or not , Will reclaim its memory . however , Because the garbage collector is a low priority thread , So it's not necessarily easy to find objects with only weak references . Weak references can be associated with a reference queue （ReferenceQueue） A combination of , If the object referenced by the weak reference is garbage collected ,Java The virtual machine will add this weak reference to the reference queue associated with it .

2.4. Virtual reference （PhantomReference）： Manage off heap memory

" Virtual reference " seeing the name of a thing one thinks of its function , It's the same thing , Different from other quotations , Virtual references do not determine the life cycle of an object . If an object only holds virtual references , Then it's the same as without any reference , Can be recycled at any time . Virtual references are mainly used to track the garbage collection activities of objects . One of the differences between virtual reference and soft reference and weak reference is ： Virtual references must be with reference queues （ReferenceQueue） A combination of . When the garbage collector is ready to recycle an object , If it's found to have virtual references , Before reclaiming the object's memory , Add this virtual reference to the reference queue associated with it . The program can judge whether a virtual reference has been added to the reference queue , To see if the referenced object is going to be garbage collected . If the program finds that a virtual reference has been added to the reference queue , Then you can take the necessary action before the memory of the referenced object is recycled .NIO Inside PirectByteBuffer Point to off heap memory , It is called direct memory , Managed by the operating system ,GC Cannot recycle directly . So we need to use virtual references , When direct memory needs to be reclaimed , By referencing queue input detection , And then clean it up .C or C++ virtual machine , use delete or free.Java What can be used is UnSafe Class freeMemory() To recycle .

3：ThreadLocal

3.1： Thread local

Other threads cannot access

3.2： principle

set（） Source code ：Set The current thread's map in , Other threads are unreadable
map Not empty ：Thrad.currentThread.map(ThreadLocal,valve)
Map It's empty ： initialization map

3.3：ThreadLocal Use with weak references , Solve the memory leak

ThreadLocal Of map Medium entry Is a weak reference

1: Solve the memory leak problem : If strong citation , Even if tl=null, but key The reference to still points to ThreadLocal object ,ThreadLocal Cannot be recycled , There will be Memory leak . Using weak references makes key Point to ThreadLocal Object becomes recyclable . Solved the memory leak problem .
2: There is a memory leak problem :ThreadLocal After being recycled ,key The value of the into null, Lead to value Cannot be accessed , There are also memory leaks . therefore , commonly Threadlocal Run out of , manual remove fall .

Be careful ： This article only represents the personal views of novice bloggers , If something is wrong or there is a better idea through the technology , Welcome to leave a message , Sharing and communication make us progress , thank you .