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This article introduces the relevant knowledge of "what is the cause of memory OOM caused by ThreadLocal in Java". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Cause analysis

What is the cause of memory OOM caused by ThreadLocal?

The underlying layer of ThreadLocal stores data through ThreadLocalMap

The source code is as follows

When we use ThreadLocal.set (), the value and key of set (that is, the ThreadLocal class defined by the business itself) are stored in the Entry [] array of ThreadLocalMap

The source code is as follows

Where Entry is a key that implements a weak reference to WeakReference,Entry (that is, the ThreadLocal class defined by the business side) will be packaged as a weak reference as a key of Entry. Java's definition of weak reference is that when JVM performs a garbage collection scan, when an object with only a weak reference is found, it will be recycled immediately, which is a means to prevent memory overflow when ThreadLocal was originally designed.

The source code is as follows

Although key is packaged as a weak reference to be reclaimed by the garbage collection mechanism, value may have a strong reference chain when the thread (Thread) does not die.

Because value is a strong reference, as long as Thread does not die, such as thread pool, this strong reference chain will exist, then value will not be recycled, which may cause memory overflow

The reference relationship is as follows: Thread = > ThreadLocalMap = = > Entry = > value

However, this action to eliminate the strong reference chain needs to be triggered by the business side in the case of get. The business side may not get, or the get may be key is not empty, and the expungeStaleEntry class will not be triggered. So developers should form a good habit and remember to call the ThreadLocal.remove () method or ThreadLocal.set (null) once after using ThreadLocal.

Correct use of public class ThreadLocalTest {/ * uninitialized local thread variables * / private static ThreadLocal userThreadLocal = new ThreadLocal (); public static void main (String [] args) throws InterruptedException {int threadNum = 10; List threadList = Lists.newArrayList (); for (int I = 0; I

{try {/ / set the variable value userThreadLocal.set (new User (userId, "lanxing" + userId, "2x"); / / use the variable doSomething () } finally {/ / remove variable userThreadLocal.remove (); / / remove ThreadLocal variable}}, "T" + I); threadList.add (t); t.start ();} for (int I = 0; I < threadNum) + + I) {threadList.get (I) .join ();}} private static void doSomething () {log.info ("Use ThreadLocal variable: {}", JSON.toJSONString (userThreadLocal.get ();} @ Data@NoArgsConstructor@AllArgsConstructorclass User {private Long id; private String name; private String level;}

Print the results:

14 id 30 level 26.790 [T2] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 2, "level": "2x", "name": "lanxing2"}

14 id 30 level 26.789 [T5] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 5, "level": "2x", "name": "lanxing5"}

14 id 30 level 26.792 [T0] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 0, "level": "2x", "name": "lanxing0"}

14 id 30 level 26.792 [T4] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 4, "level": "2x", "name": "lanxing4"}

14 id 30 level 26.792 [T8] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 8, "level": "2x", "name": "lanxing8"}

14 id 30 level 26.791 [T1] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 1, "level": "2x", "name": "lanxing1"}

14 id 30 level 26.792 [T7] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 7, "level": "2x", "name": "lanxing7"}

14 id 30 level 26.792 [T6] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 6, "level": "2x", "name": "lanxing6"}

14 id 30 level 26.791 [T9] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 9, "level": "2x", "name": "lanxing9"}

14 id 30 level 26.790 [T3] INFO io.zhengsh.order.tool.ThreadLocalTest-Use ThreadLocal variable: {"id": 3, "level": "2x", "name": "lanxing3"}

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