ThreadLocal 源码分析

使用

static ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>() {
    @Override
    protected Integer initialValue() {
        return 0;
    }
};
public static void main(String[] args) {
    for (int i=0; i<5; i++) {
        new Thread(() -> {
            Integer integer = threadLocal.get();
            System.out.println(Thread.currentThread().getName() + " " + integer);
            threadLocal.set(integer+5);
            System.out.println(Thread.currentThread().getName() + " " + threadLocal.get());
        }).start();
    }
}

ThreadLocal可以为线程绑定变量，该变量只与其绑定的线程有关，不会产生线程安全问题

源码分析

public class ThreadLocal<T> {

	private static AtomicInteger nextHashCode = new AtomicInteger();
	//0110 0001 1200 0100 0100 0110 0100 0111	threadlocal的hash步进
	private static final int HASH_INCREMENT = 0x61c88647;
	//获取hashcode
	private final int threadLocalHashCode = nextHashCode();
	
	private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }

	//构造方法
	public ThreadLocal() {
    }
	
	protected T initialValue() {
        return null;
    }
	//set方法
	public void set(T value) {
		//获取当前线程
        Thread t = Thread.currentThread();
		//获取当前线程的ThreadLocalMap
        ThreadLocalMap map = getMap(t);
		//如果map初始化过，直接设值
        if (map != null)
            map.set(this, value);
        else//创建map
            createMap(t, value);
    }
	//获取map
	ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
	//创建map
	void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }
	
	//get方法
	public T get() {
		//获取当前线程
        Thread t = Thread.currentThread();
		//获取当前线程的 threadLocalMap
        ThreadLocalMap map = getMap(t);
        //如果当前线程的ThreadLocalMap不为null
		if (map != null) {
			//获取当前线程map的entry
            ThreadLocalMap.Entry e = map.getEntry(this);
            //如果buweinull，返回entry的value
			if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
		//如果没找到则初始化
        return setInitialValue();
    }
	

	
	private T setInitialValue() {
		//获取初始值，默认为null，重写后会返回重写的值
        T value = initialValue();
		//获取当前线程
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
		//如果map不为null，将初始值放入
        if (map != null)
            map.set(this, value);
        else//创建map
            createMap(t, value);
        return value;
    }
	
	static class ThreadLocalMap {
		//entry的key是弱引用，每次发生gc时，如果对象只有弱引用，将会被回收
		//key是ThreadLocal
        static class Entry extends WeakReference<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
		//阈值为数组长度的三分之二
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }
		//获取下一个下标
        private static int nextIndex(int i, int len) {
            return ((i + 1 < len) ? i + 1 : 0);
        }
		//获取上一个下标
        private static int prevIndex(int i, int len) {
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }
		//构造方法，初始容量为16，将entry放入
        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++;
                    }
                }
            }
        }

		//获取map的entry
        private Entry getEntry(ThreadLocal<?> key) {
			//获取hash并计算下标	
            int i = key.threadLocalHashCode & (table.length - 1);
            //获取下标元素
			Entry e = table[i];
			//下标元素的key是当前的threadLocal，并且值不为空，返回
            if (e != null && e.get() == key)
                return e;
            else//可能因为hash碰撞，entry被放到其他下标了，ThreadLocalMap解决hash碰撞的方法是开放寻址法，hashmap使用拉链法
                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)
					//删掉元素，hash碰撞后，会在碰撞地址后面寻找空余地方放入
                    expungeStaleEntry(i);
                else//向后遍历数组，直到找到目标
                    i = nextIndex(i, len);
                e = tab[i];
            }
			//还是找不到，真不存在，返回null
            return null;
        }
		//给ThreadLocalMap设值	key是ThreadLocal
        private void set(ThreadLocal<?> key, Object value) {

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

            for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) {
                //获取下标元素的key，如果找不到key相同或者下标元素为空的，就一直向后寻找
				ThreadLocal<?> k = e.get();
				//覆盖原值
                if (k == key) {
                    e.value = value;
                    return;
                }
				//下标元素不为空，元素的key为空，坏值，替换
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
			//将entry放入
            tab[i] = new Entry(key, value);
            int sz = ++size;//增加元素个数
			//清除坏值，如果达到扩容阈值，扩容
            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;
                }
            }
        }
		//替换下标的元素
        private void replaceStaleEntry(ThreadLocal<?> key, Object value,
                                       int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            Entry e;

            int slotToExpunge = staleSlot;
			//找到下标元素不为空，元素的key为空的最小下标
            for (int i = prevIndex(staleSlot, len); (e = tab[i]) != null; i = prevIndex(i, len))
                if (e.get() == null)
                    slotToExpunge = i;

            for (int i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) {
                ThreadLocal<?> k = e.get();
				//覆盖旧值 
                if (k == key) {
                    e.value = value;
					//坏值向后移动
                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;

                    // 坏值的最小下标和传入值相同，说明只有这一个坏值，因为上面向后移动了一位，所以把再赋值为i
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
					//删除旧值，清除一些下标
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                    return;
                }

                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }

            tab[staleSlot].value = null;
            tab[staleSlot] = new Entry(key, value);

            if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }

        private int expungeStaleEntry(int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
			//清除下标的元素
            tab[staleSlot].value = null;
            tab[staleSlot] = null;
            //元素个数减一
			size--;

            Entry e;
            int i;
			//再次向后遍历，清除坏值，如果后面的不是坏值，重新确定正常元素的下标
            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;

                        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;
        }

		//如果清除坏值后，元素数量仍旧达到阈值，扩容，阈值为0.75
        private void rehash() {
            expungeStaleEntries();
			//数组长度2/3的0.75     0.5
            if (size >= threshold - threshold / 4)
                resize();
        }
		//扩容
        private void resize() {
            Entry[] oldTab = table;
            int oldLen = oldTab.length;
            int newLen = oldLen * 2;//扩容为原来的两倍
            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;
        }

        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);
            }
        }
    }
}

public class Thread implements Runnable {
	ThreadLocal.ThreadLocalMap threadLocals = null;
}