Java面經(jīng)之ThreadLocal底層原理

private static ThreadLocal<Connection> connectionHolder = new ThreadLocal<>(); connectionHolder.set(DriverManager.getConnection(DB_URL)); connectionHolder.get();

public class Thread implements Runnable {/* ThreadLocal values pertaining to this thread. This map is maintained     * by the ThreadLocal class. */    ThreadLocal.ThreadLocalMap threadLocals = null;}

static class ThreadLocalMap {    private static final int INITIAL_CAPACITY = 16;     private int size = 0;     private int threshold;     private Entry[] table;    //Thread的map容器     // 構(gòu)造方法，在第一次調(diào)用ThreadLocal.set()時(shí)會(huì)new一個(gè)ThreadLocalMap    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);    }     static class Entry extends WeakReference<ThreadLocal<?>> {            Object value;            Entry(ThreadLocal<?> k, Object v) {                super(k);                value = v;            }     }}

public abstract class Reference<T> {    private T referent;           // key。注入了傳入的ThreadLocal，被Entry繼承      //ThreadLocal最終傳給了參數(shù)referent    Reference(T referent, ReferenceQueue<? super T> queue) {        this.referent = referent;        this.queue = (queue == null) ? ReferenceQueue.NULL : queue;    }     // 獲取“key”    public T get() {        return this.referent;    }}

   private void set(ThreadLocal<?> key, Object value) {            Entry[] tab = table;            int len = tab.length;             //根據(jù)ThreadLocal的hash值計(jì)算應(yīng)該從table中哪個(gè)位置開始(跟HashMap一樣)            int i = key.threadLocalHashCode & (len-1);             //遍歷table中的每一個(gè)Entry            for (Entry e = tab[i];                 e != null;                 e = tab[i = nextIndex(i, len)]) {                ThreadLocal<?> k = e.get();                 //如果找到相同的key，覆蓋并返回                if (k == key) {                    e.value = value;                    return;                }                 //如果發(fā)現(xiàn)了空key為空，即ThreadLocal對(duì)象被GC了，就把數(shù)據(jù)放到該位置                if (k == null) {                    replaceStaleEntry(key, value, i);                    return;                }            }             //當(dāng)返現(xiàn)e==null時(shí)，執(zhí)行下面代碼，new再檢測(cè)是否擴(kuò)容            tab[i] = new Entry(key, value);            int sz = ++size;            if (!cleanSomeSlots(i, sz) && sz >= threshold)                rehash();        }

      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;        }

    public void set(T value) {        Thread t = Thread.currentThread();    //獲取當(dāng)前線程對(duì)象        ThreadLocalMap map = getMap(t);       //拿到t的map容器        if (map != null)                    map.set(this, value);            //map不空，根據(jù)key(調(diào)用set方法的ThreadLocal對(duì)象)設(shè)置值        else            createMap(t, value);             //map為空則創(chuàng)建一個(gè)map(第一次插入，有點(diǎn)怪怪的)    }     // 從當(dāng)前線程中獲取ThreadLocalMap     ThreadLocalMap getMap(Thread t) {        return t.threadLocals;    }     // 初始化當(dāng)前線程的ThreadLocalMap容器    void createMap(Thread t, T firstValue) {        t.threadLocals = new ThreadLocalMap(this, firstValue);    }

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;    } protected T initialValue() {        return null;    }

public class Test1 {     public static void main(String[] args) {        for (int i=0;i<5;++i){            new MyThread().start();        }    }} class MyThread extends Thread{     private static final ThreadLocal<String> threadLocal1 = new ThreadLocal<String>();    private static final ThreadLocal<String> threadLocal2 = new ThreadLocal<String>();     @Override    public void run(){        threadLocal1.set(getName()+"：調(diào)用了threadLocal1的set方法");        threadLocal2.set(getName()+"：調(diào)用了threadLocal2的set方法");        System.out.println(threadLocal1.get());        System.out.println(threadLocal2.get());    }} /*---------------打印結(jié)果-----------------Thread-0：調(diào)用了threadLocal1的set方法Thread-0：調(diào)用了threadLocal2的set方法Thread-2：調(diào)用了threadLocal1的set方法Thread-2：調(diào)用了threadLocal2的set方法Thread-3：調(diào)用了threadLocal1的set方法Thread-4：調(diào)用了threadLocal1的set方法Thread-3：調(diào)用了threadLocal2的set方法Thread-4：調(diào)用了threadLocal2的set方法Thread-1：調(diào)用了threadLocal1的set方法Thread-1：調(diào)用了threadLocal2的set方法*/