Java Concurrent之 ThreadLocal

   之前的ThreadLocal的实现方式是声明一个HashTable，以Thread.currentThread()为key,变量的拷贝为value。而现在ThreadLocal的实现方式有点儿改头换面了。

   

public class ThreadLocal<T> {
    /**
     * ThreadLocals rely on per-thread linear-probe hash maps attached
     * to each thread (Thread.threadLocals and
     * inheritableThreadLocals).  The ThreadLocal objects act as keys,
     * searched via threadLocalHashCode.  This is a custom hash code
     * (useful only within ThreadLocalMaps) that eliminates collisions
     * in the common case where consecutively constructed ThreadLocals
     * are used by the same threads, while remaining well-behaved in
     * less common cases.
     */
    private final int threadLocalHashCode = nextHashCode();

    /**
     * The next hash code to be given out. Updated atomically. Starts at
     * zero.
     */
    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;

    /**
     * Returns the next hash code.
     */
    private static int nextHashCode() {
	return nextHashCode.getAndAdd(HASH_INCREMENT); 
    }

    /**
     * Returns the current thread's "initial value" for this
     * thread-local variable.  This method will be invoked the first
     * time a thread accesses the variable with the {@link #get}
     * method, unless the thread previously invoked the {@link #set}
     * method, in which case the <tt>initialValue</tt> method will not
     * be invoked for the thread.  Normally, this method is invoked at
     * most once per thread, but it may be invoked again in case of
     * subsequent invocations of {@link #remove} followed by {@link #get}.
     *
     * <p>This implementation simply returns <tt>null</tt>; if the
     * programmer desires thread-local variables to have an initial
     * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be
     * subclassed, and this method overridden.  Typically, an
     * anonymous inner class will be used.
     *
     * @return the initial value for this thread-local
     */
    protected T initialValue() {
        return null;
    }

    /**
     * Creates a thread local variable.
     */
    public ThreadLocal() {
    }

    从上述源码可以看出，ThreadLocal现在只有3个变量，当我们实例化一个ThreadLocal对象时，会调用nextHashCode()方法将nextHashCode的值赋值给实例的成员变量threadLocalHashCode;与此同时也会将nextHashCode的值增加HASH_INCREMENT。ThreadLocal就可利用这个变量来区分不同的ThreadLocal实例。

 

 

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

/**
     * Atomically adds the given value to the current value.
     *
     * @param delta the value to add
     * @return the previous value
     */
    public final int getAndAdd(int delta) {
        for (;;) {
            int current = get();
            int next = current + delta;
//如果当前的值(private static AtomicInteger nextHashCode)的值与current相等，               则将next赋值给当前值。
            if (compareAndSet(current, next))
                return current;
        }
    }

/**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * @param expect the expected value
     * @param update the new value
     * @return true if successful. False return indicates that
     * the actual value was not equal to the expected value.
     */
    public final boolean compareAndSet(int expect, int update) {
	return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }

    接下来我们来看看ThreadLocal的get()方法:

 

 /**
     * 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
     */
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);//获取ThreadLocal内部类ThreadLocalMap对象 （在Thread中定义的threadLocals）ThreadLocal中的值其实交给了当前Thread保管，自己只保管自己的ThreadLocalHashCode变量。这样一旦线程退出，ThreadLocal将无法访问该线程。
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();//返回初始值
    }
/**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

 ThreadLocal内部类ThreadLocalMap：

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;

        /**
         * 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

   我们通过源码会发现ThreadLocalMap的Entry继承了WeakReference，众所周知，weakReference是一种比软引用还要弱的引用类型。在系统发生GC时，不管系统的堆空间是否富余，只要发现弱引用，都会将其回收。ThreadLocalMap中以数组的形式来存储Entry实例，并且Entry是以ThreadLocal为key，初始大小为16.

   ThreadLocalMap包含有两个构造器，使得ThreadLocalMap既可以通过传入ThreadLocal-value对的形式、也可以通过传入ThreadLocalMap实例的方式实例化ThreadLocalMap。使用第二种方式初始化ThreadLocalMap时，会将传入的ThreadLocalMap中的Entry依次添加到当前的ThreadLocalMap实例的table变量中。

/**
         * 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);
            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
         */
//通过实例变量threadLocalHashCode计算下标，然后返回对应的值 
//set 、remove方法类似
        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);
        }

  从上述ThreadLocal源码加上Thread的源码，我们会发现每个线程都持有一个自己的ThreadLocal.ThreadLocalMap变量。这样就方便每个线程持有一份自己的变量拷贝而互不干扰。

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

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.  
     */ 
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
/**
     * Initializes a Thread.
     *
     * @param g the Thread group
     * @param target the object whose run() method gets called
     * @param name the name of the new Thread
     * @param stackSize the desired stack size for the new thread, or
     *        zero to indicate that this parameter is to be ignored.
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
	Thread parent = currentThread();
	SecurityManager security = System.getSecurityManager();
	if (g == null) {
	    /* Determine if it's an applet or not */
	    
	    /* If there is a security manager, ask the security manager
	       what to do. */
	    if (security != null) {
		g = security.getThreadGroup();
	    }

	    /* If the security doesn't have a strong opinion of the matter
	       use the parent thread group. */
	    if (g == null) {
		g = parent.getThreadGroup();
	    }
	}

	/* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
	g.checkAccess();

	/*
	 * Do we have the required permissions?
	 */
	if (security != null) {
	    if (isCCLOverridden(getClass())) {
	        security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
	    }
	}


        g.addUnstarted();

	this.group = g;
	this.daemon = parent.isDaemon();
	this.priority = parent.getPriority();
	this.name = name.toCharArray();
	if (security == null || isCCLOverridden(parent.getClass()))
	    this.contextClassLoader = parent.getContextClassLoader();
	else
	    this.contextClassLoader = parent.contextClassLoader;
	this.inheritedAccessControlContext = AccessController.getContext();
	this.target = target;
	setPriority(priority);
        if (parent.inheritableThreadLocals != null)
	    this.inheritableThreadLocals =
		ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();
    }

   /**
     * Allocates a new <code>Thread</code> object. This constructor has 
     * the same effect as <code>Thread(null, null,</code>
     * <i>gname</i><code>)</code>, where <b><i>gname</i></b> is 
     * a newly generated name. Automatically generated names are of the 
     * form <code>"Thread-"+</code><i>n</i>, where <i>n</i> is an integer. 
     *
     * @see     #Thread(ThreadGroup, Runnable, String)
     */
    public Thread() {
	init(null, null, "Thread-" + nextThreadNum(), 0);
    }

    /**
     * Allocates a new <code>Thread</code> object. This constructor has 
     * the same effect as <code>Thread(null, target,</code>
     * <i>gname</i><code>)</code>, where <i>gname</i> is 
     * a newly generated name. Automatically generated names are of the 
     * form <code>"Thread-"+</code><i>n</i>, where <i>n</i> is an integer. 
     *
     * @param   target   the object whose <code>run</code> method is called.
     * @see     #Thread(ThreadGroup, Runnable, String)
     */
    public Thread(Runnable target) {
	init(null, target, "Thread-" + nextThreadNum(), 0);
    }
... ...
}

    实际上，Thread里定义了两个ThreadLocalMap变量，在init()方法里我们会看到对于inheritableThreadLocals变量的操作。

if (parent.inheritableThreadLocals != null)
	    this.inheritableThreadLocals =
		ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);


//调用ThreadLocalMap的传入ThreadLocalMap参数的构造器
 static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
    }

   也就是说在Thread类中，当前线程回去调用init()方法去初始化一个线程，而在init方法调用过程中会执行将当前线程的inheritableThreadLocals的值拷贝给待初始化的线程。

   而在线程真正终止前会执行exit()方法，将threadLocals和inheritableThreadLocals变量都指向null。

 /**
     * This method is called by the system to give a Thread
     * a chance to clean up before it actually exits.
     */
    private void exit() {
	if (group != null) {
	    group.remove(this);
	    group = null;
	}
	/* Aggressively null out all reference fields: see bug 4006245 */
	target = null;
	/* Speed the release of some of these resources */
        threadLocals = null;
        inheritableThreadLocals = null;
        inheritedAccessControlContext = null;
        blocker = null;
        uncaughtExceptionHandler = null;
    }

  综述：

    ThreadLocal为各个线程管理的变量副本都放在各自线程中的threadLocals变量中。ThreadLocal变量中保管只是一个ThreadLocalHashCode变量，这样在线程退出后是无法通过ThreadLocal变量访问这个线程的。(因为线程退出时会调用exit()方法，将threadLocals和inheritableThreadLocals都指向了null；而当ThreadLocal为null时，thread里的threadLocals也会移除key为ThreadLocal的Entry)。这种实现方式相比之前的HashTable，无需同步，大大提升了性能。