多线程进阶ThreadLocal

线程的私有变量是利用ThreadLocal来实现的，那么怎么来的实现的呢，简单来说，就是Thread包含一变量map,这个map里面的键值对存放的是ThreadLocal —- Value键值对，也就是线程若需要存放多个私有变量，需要new出多个ThreadLocal，如

public class Run {        public static void main(String[] args) {//          new ThreadImp().start();            new Thread(new Runnable() {                public void run() {                    ThreadLocal<String> t1 = new ThreadLocal<String>();                    t1.set("123");//到这里时线程A里面的map存放了t1--"123"键值对                    ThreadLocal<String> t2 = new ThreadLocal<String>();                    t2.set("345");//到这里时线程A里面的map存放了t1--"123"，t2--"345"两个键值对                    System.out.println(t1.get());                    System.out.println(t2.get());                }            },"线程A").start();            ThreadLocal<Date> t = new ThreadLocal<Date>();            //这里如果不set值的话，下面的get就会返回一个null,另外可以自己实现一            //个ThreadLocal重新定义这个初始值            t.set(new Date());            while(Thread.activeCount()>1){            }            System.out.println(t.get());        }}

输出
123
345
Fri Oct 20 15:42:41 CST 2017

先来分析下源码
分析源码前先看下类的成员的访问权限情况

主要的类有三个，Thread，ThreadLocal，ThreadLocalMap,
ThreadLocalMap是ThreadLocal的一个内部静态类，Thread中含有一个ThreadLocalMap的引用，ThreadLocalMap就相当于一个Map，其存放的键值对就是ThreadLocal—value.

publicclass Thread implements Runnable {/* ThreadLocal values pertaining to this thread. This map is maintained     * by the ThreadLocal class. */     //Thread里面含有一个map<ThreadLocal,Object>,注意其为default的访问权限，同包下可以访问   ThreadLocal.ThreadLocalMap threadLocals = null;}package java.lang;import java.lang.ref.*;import java.util.concurrent.atomic.AtomicInteger;public class ThreadLocal<T> {    /**      * threadlocal的哈希值，下面的ThreadlocalMap中有个table数组，里面存放的就是ThreadLocal,      * 而其在table数组中的索引就是根据这个算出来的     */    private final int threadLocalHashCode = nextHashCode();    //注意这个是静态变量，类初始化的时候就已经显示初始化值了    private static AtomicInteger nextHashCode =        new AtomicInteger();    /**     * The difference between successively generated hash codes - turns     * implicit sequential thread-local IDs into near-optimally spread     * multiplicative hash values for power-of-two-sized tables.     */    private static final int HASH_INCREMENT = 0x61c88647;    /**     * 每new出一个ThreadLocal时都会产生一个唯一不同的hashcode值     */    private static int nextHashCode() {        return nextHashCode.getAndAdd(HASH_INCREMENT);    }    //线程的私有变量初始值为null    protected T initialValue() {        return null;    }    /**     * Creates a thread local variable.     */    public ThreadLocal() {    }    /**     * Returns the value in the current thread's copy of this     * thread-local variable.  If the variable has no value for the     * current thread, it is first initialized to the value returned     * by an invocation of the {@link #initialValue} method.     *     * @return the current thread's value of this thread-local     * 获取当前线程map中当前ThrteadLocal对应的私有属性     */    public T get() {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null) {            ThreadLocalMap.Entry e = map.getEntry(this);            if (e != null)                return (T)e.value;        }        return setInitialValue();    }    /**     * Variant of set() to establish initialValue. Used instead     * of set() in case user has overridden the set() method.     *设置当前线程map中当前ThrteadLocal对应的私有属性的初始值     * @return the initial value     */    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;    }    /**     * Sets the current thread's copy of this thread-local variable     * to the specified value.  Most subclasses will have no need to     * override this method, relying solely on the {@link #initialValue}     * method to set the values of thread-locals.     *设置当前线程map中当前ThreadLocal对应的私有属性     * @param value the value to be stored in the current thread's copy of     *        this thread-local.     */    public void set(T value) {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);    }    /**     * Removes the current thread's value for this thread-local     * variable.  If this thread-local variable is subsequently     * {@linkplain #get read} by the current thread, its value will be     * reinitialized by invoking its {@link #initialValue} method,     * unless its value is {@linkplain #set set} by the current thread     * in the interim.  This may result in multiple invocations of the     * <tt>initialValue</tt> method in the current thread.     *     * @since 1.5     */     public void remove() {         ThreadLocalMap m = getMap(Thread.currentThread());         if (m != null)             m.remove(this);     }    /**     * Get the map associated with a ThreadLocal. Overridden in     * InheritableThreadLocal.     *获取线程中的map     * @param  t the current thread     * @return the map     */    ThreadLocalMap getMap(Thread t) {        return t.threadLocals;    }    /**     * Create the map associated with a ThreadLocal. Overridden in     * InheritableThreadLocal.     *为t线程的map初始化，并存入一个私有属性值     * @param t the current thread     * @param firstValue value for the initial entry of the map     * @param map the map to store.     */    void createMap(Thread t, T firstValue) {        t.threadLocals = new ThreadLocalMap(this, firstValue);    }    /**     * Factory method to create map of inherited thread locals.     * Designed to be called only from Thread constructor.     *     * @param  parentMap the map associated with parent thread     * @return a map containing the parent's inheritable bindings     */    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {        return new ThreadLocalMap(parentMap);    }    /**     * Method childValue is visibly defined in subclass     * InheritableThreadLocal, but is internally defined here for the     * sake of providing createInheritedMap factory method without     * needing to subclass the map class in InheritableThreadLocal.     * This technique is preferable to the alternative of embedding     * instanceof tests in methods.     */    T childValue(T parentValue) {        throw new UnsupportedOperationException();    }    /**     *ThreadLocal的一个内部静态类，和HashMap的实现有点类似     */    static class ThreadLocalMap {        /**         * The entries in this hash map extend WeakReference, using         * its main ref field as the key (which is always a         * ThreadLocal object).  Note that null keys (i.e. entry.get()         * == null) mean that the key is no longer referenced, so the         * entry can be expunged from table.  Such entries are referred to         * as "stale entries" in the code that follows.         */        static class Entry extends WeakReference<ThreadLocal> {            /** The value associated with this ThreadLocal. */            Object value;            Entry(ThreadLocal k, Object v) {                super(k);                value = v;            }        }        /**         * The initial capacity -- MUST be a power of two.         */        private static final int INITIAL_CAPACITY = 16;        /**         *实体内容数组         * The table, resized as necessary.         * table.length MUST always be a power of two.         */        private Entry[] table;        /**         * The number of entries in the table.         */        private int size = 0;        /**         * The next size value at which to resize.         */        private int threshold; // Default to 0        /**         * Set the resize threshold to maintain at worst a 2/3 load factor.         */        private void setThreshold(int len) {            threshold = len * 2 / 3;        }        /**         * Increment i modulo len.         */        private static int nextIndex(int i, int len) {            return ((i + 1 < len) ? i + 1 : 0);        }        /**         * Decrement i modulo len.         */        private static int prevIndex(int i, int len) {            return ((i - 1 >= 0) ? i - 1 : len - 1);        }        /**         * Construct a new map initially containing (firstKey, firstValue).         * ThreadLocalMaps are constructed lazily, so we only create         * one when we have at least one entry to put in it.         */        ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {            table = new Entry[INITIAL_CAPACITY];            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);            //计算ThreadLocal--value对应的实体在table中的索引位置            table[i] = new Entry(firstKey, firstValue);            size = 1;            setThreshold(INITIAL_CAPACITY);        }        /**         * Construct a new map including all Inheritable ThreadLocals         * from given parent map. Called only by createInheritedMap.         *         * @param parentMap the map associated with parent thread.         */        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) {                    ThreadLocal key = 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++;                    }                }            }        }        /**         * Get the entry associated with key.  This method         * itself handles only the fast path: a direct hit of existing         * key. It otherwise relays to getEntryAfterMiss.  This is         * designed to maximize performance for direct hits, in part         * by making this method readily inlinable.         *         * @param  key the thread local object         * @return the entry associated with key, or null if no such         */        private Entry getEntry(ThreadLocal key) {            int i = key.threadLocalHashCode & (table.length - 1);            Entry e = table[i];            //如果能在table中找到key则返回，否则查找下一个位置            if (e != null && e.get() == key)                return e;            else                return getEntryAfterMiss(key, i, e);        }        /**         * Set the value associated with key.         *将ThreadLocal--value键值对存入线程的map中         * @param key the thread local object         * @param value the value to be set         */        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();        }        }    }}

对照着这个，自己实现了一个简洁的ThreadLocal，方便理解

public class MyThread extends Thread{    MyThreadLocal.MyThreadLocalMap mythreadlocals;}

public class MyThreadLocal<T> {    protected T initialValue() {        return (T) "初始值";    }    public void set(T value){        MyThread t = (MyThread) MyThread.currentThread();        MyThreadLocalMap map = getMap(t);         if (map != null)                map.set(this, value);            else                createMap(t, value);    }    public T get() {        MyThread t = (MyThread) MyThread.currentThread();        MyThreadLocalMap map = getMap(t);        if (map != null) {            MyThreadLocalMap.Entry e = map.getEntry(this);            if (e != null)                return (T)e.value;        }        return setInitialValue();    }    private void createMap(MyThread t,T value){        t.mythreadlocals = new MyThreadLocalMap(this, value);    }    MyThreadLocalMap getMap(MyThread t) {            return t.mythreadlocals;        }     private T setInitialValue() {            T value = initialValue();            MyThread t = (MyThread) MyThread.currentThread();            MyThreadLocalMap map = getMap(t);            if (map != null)                map.set(this, value);            else                createMap(t, value);            return value;        }    static class MyThreadLocalMap{        static class Entry{            MyThreadLocal key;            Object value;            Entry(MyThreadLocal key,Object value){                this.key = key;                this.value = value;            }            MyThreadLocal get(){                return key;            }        }        private static final int INITIAL_CAPACITY = 16;        Entry[] table;        MyThreadLocalMap(MyThreadLocal firstKey, Object firstValue) {            table = new Entry[INITIAL_CAPACITY];            table[0] = new Entry(firstKey, firstValue);        }        private static int prevIndex(int i, int len) {            return ((i - 1 >= 0) ? i - 1 : len - 1);        }        private static int nextIndex(int i, int len) {            return ((i + 1 < len) ? i + 1 : 0);        }        private Entry getEntry(MyThreadLocal key) {            Entry e = table[0];            for(int i=0;i<table.length;i++){                if(table[i]!=null){                    if(table[i].key == key){                        return table[i];                    }                }            }            return null;        }         private void set(MyThreadLocal key, Object value) {                Entry[] tab = table;                int len = tab.length;                int i = 0;                for (Entry e = tab[i];                     e != null;                     e = tab[i = nextIndex(i, len)]) {                    MyThreadLocal k = e.get();                    if (k == key) {                        e.value = value;                        return;                    }                    if (k == null) {                        tab[i].key = key;                        tab[i].value = value;                        return;                    }                }                tab[i] = new Entry(key, value);            }    }}public class ThreadImp extends MyThread{    @Override    public void run() {            MyThreadLocal<String> local1 = new MyThreadLocal<String>();            local1.set("新值"+getName());            System.out.println(local1.get());            MyThreadLocal<String> local2 = new MyThreadLocal<String>();            System.out.println(local2.get());    }}package com.lzzl.thread9;public class Run1 {        public static void main(String[] args) {            new ThreadImp().start();        }}

输出
新值Thread-0
初始值