ThreadLocal

一：源码

 

package java.lang;
import java.lang.ref.*;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;
public class ThreadLocal {
    private final int threadLocalHashCode = nextHashCode();

   //避免多线程获取相同的hashcode
    private static AtomicInteger nextHashCode =
        new AtomicInteger();

    private static final int HASH_INCREMENT = 0x61c88647;

    private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }

    public static  ThreadLocal withInitial(Supplier supplier) {
        return new SuppliedThreadLocal<>(supplier);
    }
 
    public ThreadLocal() {
    }

    public T get() {
        Thread t = Thread.currentThread();
        //获取该线程的threadlocalMap对象
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

   
    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
     //删除threadlocal变量
     public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

  
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }


    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
    }

    T childValue(T parentValue) {
        throw new UnsupportedOperationException();
    }

    static final class SuppliedThreadLocal extends ThreadLocal {

        private final Supplier supplier;

        SuppliedThreadLocal(Supplier supplier) {
            this.supplier = Objects.requireNonNull(supplier);
        }

        @Override
        protected T initialValue() {
            return supplier.get();
        }
    }

   
    static class ThreadLocalMap {

    
        static class Entry extends WeakReference> {
            Object value;

            Entry(ThreadLocal k, Object v) {
                super(k);
                value = v;
            }
        }

        
        private static final int INITIAL_CAPACITY = 16;

       //存储多个threadlocal变量
        private Entry[] table;

        private int size = 0;

       
        private int threshold; // Default to 0

       
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }

        private static int nextIndex(int i, int len) {
            return ((i + 1 < len) ? i + 1 : 0);
        }

      
        private static int prevIndex(int i, int len) {
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }

        ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }

       
        private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];

            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal