Simple understanding of ThreadLocal

One 、 background

Recently someone asked me ThreadLocal How to make the values in each thread isolated , Write an article here to briefly record .

Two 、ThreadLocal Problem solved

1. The data belongs to the thread Thread Oneself , Other threads cannot affect it ._{( We need to pay attention to ： Need to call ThreadLocal Of remove Method )}
2. There is no thread safety issue ._{（ because ThreadLocal A variable of type can only be accessed by its own thread , So this is true .）}

such as ：

After the user logs in successfully , Need to put Login user information Save up , So that you can use... Anywhere in the system , Then you can use ThreadLocal To achieve . for example ：Spring Security Medium ThreadLocalSecurityContextHolderStrategy class .

3、 ... and 、 How to create a ThreadLocal example

private static final ThreadLocal<String> USER_NAME = new ThreadLocal<>();

ThreadLocal For example, it is recommended to use private static final To modify .

Four 、ThreadLocal How to achieve thread variable isolation

1、 understand 3 Classes

1. ThreadLocal: This class provides a simple set,get,remove Method , Used for setting up , Get or remove Bound to the value in the thread local variable .

2. ThreadLocalMap: This is ThreadLocal A class defined in , It can be simply understood as a Map, But its key yes WeakReference Weak reference type , So when the value is not referenced elsewhere , In case of garbage collection , This map Of key Will be automatically recycled , However, its value will not be automatically recycled .

   static class Entry extends WeakReference<ThreadLocal<?>> {
         
   	Object value;
   	Entry(ThreadLocal<?> k, Object v) {
         
   		// key  Weak reference 
   		super(k);
   		//  Value strong reference 
   		value = v;
   	}
   }
   

3. Thread： This is the thread class , There is a in this class threadLocals Variable , The specific type is ThreadLocal.ThreadLocalMap.

2、 look down set How are methods implemented

public void set(T value) {
    
	//  Get the current thread 
	Thread t = Thread.currentThread();
	//  Get the... Bound to the thread itself  ThreadLocalMap, This ThreadLocalMap It's from Thread Class `threadLocals` Variable 
	ThreadLocalMap map = getMap(t);
	if (map != null) {
    
		//  towards map Set the value of ,key by  ThreadLocal  Instance of object .
		map.set(this, value);
	} else {
    
		//  If map non-existent , Then create .
		createMap(t, value);
	}
}

Through the code above , We know ： When we turn to ThreadLocal Set a value in the , It will go through the following steps ：

1. Get the current thread Thread
2. Gets the current thread's ThreadLocalMap object .
3. towards ThreadLocalMap Set the value of ,key by ThreadLocal object , The value is a specific value .

3、 have a look get How to achieve

public T get() {
    
	//  Get the current thread 
	Thread t = Thread.currentThread();
	//  Get the binding of the thread itself  ThreadLocalMap  object 
	ThreadLocalMap map = getMap(t);
	if (map != null) {
    
		// this yes ThreadLocal object , obtain Map Medium Entry object 
		ThreadLocalMap.Entry e = map.getEntry(this);
		if (e != null) {
    
			@SuppressWarnings("unchecked")
			//  Get specific values 
			T result = (T)e.value;
			return result;
		}
	}
	//  Set initial value 
	return setInitialValue();
}

From above get and set The way to know , Through to ThreadLocal Object to set or get a value , In fact, it is the final operation to Thread Object threadLocals Field , And this field is Thread Self , Therefore, isolation is achieved .

5、 ... and 、ThreadLocalMap Medium hash How the conflict is handled

1、ThreadLocal Object's hash What is the value

private final int threadLocalHashCode = nextHashCode();
	//  The  ThreadLocal  Of the object itself hash code value 
    private final int threadLocalHashCode = nextHashCode();
	//  from 0 Start 
    private static AtomicInteger nextHashCode = new AtomicInteger();
	//  Increment a fixed value each time 
    private static final int HASH_INCREMENT = 0x61c88647;
	// hash code  Value calculation 
    private static int nextHashCode() {
    
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }

From the code above, you can ,ThreadLocal Class after instantiation , its hash code value (threadLocalHashCode) It's fixed , Even if ThreadLocal Called set Method , Other values are set , its hash code value It won't change .

This field threadLocalHashCode by ThreadLocal Object's hash value , stay ThreadLocalMap This is needed in hash value .

2、 solve hash Conflict

ThreadLocalMap solve hash The solution to conflict is simple . Is through linear detection . If there is a conflict , Just look for the available position behind the array . See the picture above for details . The demonstration is A and B 2 individual ThreadLocal object , Then there was a conflict ,A and B The location of existence is in that place .

6、 ... and 、ThreadLocal Memory leak

ThreadLocal Why is there a memory leak ？

This is because ThreadLocalMap Medium key yes WeakReference type , That is, weak reference types , Data with weak reference type has no external strong reference type , In the event of a gc When , Will be automatically recycled . Be careful ： Now it's key It's recycled , however value There is no recycling . So in ThreadLocalMap Medium Entry[] There may be key yes null, however value Is an object of concrete value , So there is a memory leak .

Fix memory leaks ：

When we're done ThreadLocal After the object , Need to call... At the right time ThreadLocal#remove() Method . _{Otherwise, we have to wait Thread Auto exit to clear , If the thread pool is used ,Thread Reuse , The opportunity to clear is even more difficult .}