ThreadLocal源码

孤者浪人 提交于 2020-01-15 21:23:13
  • public class ThreadId {
  • // Atomic integer containing the next thread ID to be assigned
    
  • private static final AtomicInteger nextId = new AtomicInteger(0);
    
  • // Thread local variable containing each thread's ID
    
  • private static final ThreadLocal<Integer> threadId =
    
  •     new ThreadLocal<Integer>() {
    
  •         @Override protected Integer initialValue() {
    
  •             return nextId.getAndIncrement();
    
  •     }
    
  • };
    
  • // Returns the current thread's unique ID, assigning it if necessary
    
  • public static int get() {
    
  •     return threadId.get();
    
  • }
    
  • }

public class ThreadLocal {

private final int threadLocalHashCode = nextHashCode();


private static AtomicInteger nextHashCode =
    new AtomicInteger();

private static final int HASH_INCREMENT = 0x61c88647;


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

protected T initialValue() {
    return null;
}

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


public ThreadLocal() {
}


public T get() {
    Thread t = Thread.currentThread();
    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);
}


 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<T> extends ThreadLocal<T> {

    private final Supplier<? extends T> supplier;

    SuppliedThreadLocal(Supplier<? extends T> supplier) {
        this.supplier = Objects.requireNonNull(supplier);
    }

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


static class ThreadLocalMap {

 
    static class Entry extends WeakReference<ThreadLocal<?>> {
        /** The value associated with this ThreadLocal. */
        Object value;

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

   
    private static final int INITIAL_CAPACITY = 16;

   
    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<Object> key = (ThreadLocal<Object>) e.get();
                if (key != null) {
                    Object value = key.childValue(e.value);
                    Entry c = new Entry(key, value);
                    int h = key.threadLocalHashCode & (len - 1);
                    while (table[h] != null)
                        h = nextIndex(h, len);
                    table[h] = c;
                    size++;
                }
            }
        }
    }

    private Entry getEntry(ThreadLocal<?> key) {
        int i = key.threadLocalHashCode & (table.length - 1);
        Entry e = table[i];
        if (e != null && e.get() == key)
            return e;
        else
            return getEntryAfterMiss(key, i, e);
    }

    private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
        Entry[] tab = table;
        int len = tab.length;

        while (e != null) {
            ThreadLocal<?> k = e.get();
            if (k == key)
                return e;
            if (k == null)
                expungeStaleEntry(i);
            else
                i = nextIndex(i, len);
            e = tab[i];
        }
        return null;
    }

    private void set(ThreadLocal<?> key, Object value) {

        // We don't use a fast path as with get() because it is at
        // least as common to use set() to create new entries as
        // it is to replace existing ones, in which case, a fast
        // path would fail more often than not.

        Entry[] tab = table;
        int len = tab.length;
        int i = key.threadLocalHashCode & (len-1);

        for (Entry e = tab[i];
             e != null;
             e = tab[i = nextIndex(i, len)]) {
            ThreadLocal<?> k = e.get();

            if (k == key) {
                e.value = value;
                return;
            }

            if (k == null) {
                replaceStaleEntry(key, value, i);
                return;
            }
        }

        tab[i] = new Entry(key, value);
        int sz = ++size;
        if (!cleanSomeSlots(i, sz) && sz >= threshold)
            rehash();
    }

   
    private void remove(ThreadLocal<?> key) {
        Entry[] tab = table;
        int len = tab.length;
        int i = key.threadLocalHashCode & (len-1);
        for (Entry e = tab[i];
             e != null;
             e = tab[i = nextIndex(i, len)]) {
            if (e.get() == key) {
                e.clear();
                expungeStaleEntry(i);
                return;
            }
        }
    }


    private void replaceStaleEntry(ThreadLocal<?> key, Object value,
                                   int staleSlot) {
        Entry[] tab = table;
        int len = tab.length;
        Entry e;

        // Back up to check for prior stale entry in current run.
        // We clean out whole runs at a time to avoid continual
        // incremental rehashing due to garbage collector freeing
        // up refs in bunches (i.e., whenever the collector runs).
        int slotToExpunge = staleSlot;
        for (int i = prevIndex(staleSlot, len);
             (e = tab[i]) != null;
             i = prevIndex(i, len))
            if (e.get() == null)
                slotToExpunge = i;

        // Find either the key or trailing null slot of run, whichever
        // occurs first
        for (int i = nextIndex(staleSlot, len);
             (e = tab[i]) != null;
             i = nextIndex(i, len)) {
            ThreadLocal<?> k = e.get();

            // If we find key, then we need to swap it
            // with the stale entry to maintain hash table order.
            // The newly stale slot, or any other stale slot
            // encountered above it, can then be sent to expungeStaleEntry
            // to remove or rehash all of the other entries in run.
            if (k == key) {
                e.value = value;

                tab[i] = tab[staleSlot];
                tab[staleSlot] = e;

                // Start expunge at preceding stale entry if it exists
                if (slotToExpunge == staleSlot)
                    slotToExpunge = i;
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                return;
            }

            // If we didn't find stale entry on backward scan, the
            // first stale entry seen while scanning for key is the
            // first still present in the run.
            if (k == null && slotToExpunge == staleSlot)
                slotToExpunge = i;
        }

        // If key not found, put new entry in stale slot
        tab[staleSlot].value = null;
        tab[staleSlot] = new Entry(key, value);

        // If there are any other stale entries in run, expunge them
        if (slotToExpunge != staleSlot)
            cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
    }

    private int expungeStaleEntry(int staleSlot) {
        Entry[] tab = table;
        int len = tab.length;

        // expunge entry at staleSlot
        tab[staleSlot].value = null;
        tab[staleSlot] = null;
        size--;

        // Rehash until we encounter null
        Entry e;
        int i;
        for (i = nextIndex(staleSlot, len);
             (e = tab[i]) != null;
             i = nextIndex(i, len)) {
            ThreadLocal<?> k = e.get();
            if (k == null) {
                e.value = null;
                tab[i] = null;
                size--;
            } else {
                int h = k.threadLocalHashCode & (len - 1);
                if (h != i) {
                    tab[i] = null;

                    // Unlike Knuth 6.4 Algorithm R, we must scan until
                    // null because multiple entries could have been stale.
                    while (tab[h] != null)
                        h = nextIndex(h, len);
                    tab[h] = e;
                }
            }
        }
        return i;
    }

    private boolean cleanSomeSlots(int i, int n) {
        boolean removed = false;
        Entry[] tab = table;
        int len = tab.length;
        do {
            i = nextIndex(i, len);
            Entry e = tab[i];
            if (e != null && e.get() == null) {
                n = len;
                removed = true;
                i = expungeStaleEntry(i);
            }
        } while ( (n >>>= 1) != 0);
        return removed;
    }

  
    private void rehash() {
        expungeStaleEntries();

        // Use lower threshold for doubling to avoid hysteresis
        if (size >= threshold - threshold / 4)
            resize();
    }

    private void resize() {
        Entry[] oldTab = table;
        int oldLen = oldTab.length;
        int newLen = oldLen * 2;
        Entry[] newTab = new Entry[newLen];
        int count = 0;

        for (int j = 0; j < oldLen; ++j) {
            Entry e = oldTab[j];
            if (e != null) {
                ThreadLocal<?> k = e.get();
                if (k == null) {
                    e.value = null; // Help the GC
                } else {
                    int h = k.threadLocalHashCode & (newLen - 1);
                    while (newTab[h] != null)
                        h = nextIndex(h, newLen);
                    newTab[h] = e;
                    count++;
                }
            }
        }

        setThreshold(newLen);
        size = count;
        table = newTab;
    }

    /**
     * Expunge all stale entries in the table.
     */
    private void expungeStaleEntries() {
        Entry[] tab = table;
        int len = tab.length;
        for (int j = 0; j < len; j++) {
            Entry e = tab[j];
            if (e != null && e.get() == null)
                expungeStaleEntry(j);
        }
    }
}

}


来自:' . https://blog.csdn.net/qq_42773076/article/details/103992971 . '