ThreadLocal源码分析

在阅读《Java Concurrency In Practice》时，书中提到ThreadLocal是一种更为规范常用的Thread Confine方式。于是想仔细分析一下ThreadLocal的实现方式。曾经转载了一篇关于ThreadLocal的文章：hi.baidu.com/gefforey520/blog/item/c3bb64fa4ad1779358ee902c.html，其中提到ThreadLocal的实现方式是声明一个Hashtable,然后以Thread.currentThread()为key，变量的拷贝为value。今天阅读源码才知道实现方式已经大为改变，下面来看代码。
/**
* 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);
}
/**
* Creates a thread local variable.
*/
public ThreadLocal() {
}
ThreadLocal只有三个变量，从构造函数知道，在创建一个ThreadLocal实例时，只是调用nextHashCode方法将nextHashCode的值赋给实例的threadLocalHashCode，然后nextHashCode的值增加HASH_INCREMENT这个值。 因此ThreadLocal实例的变量只有threadLocalHashCode，而且是final的，用来区分不同的ThreadLocal实例。
再来看其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);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T) e.value;
}
return setInitialValue();
}
其中调用getMap(Thread t)返回ThreadLocalMap，ThreadLocalMap是内部静态类，部分代码如下：
/**
* ThreadLocalMap is a customized hash map suitable only for maintaining
* thread local values. No operations are exported outside of the
* ThreadLocal class. The class is package private to allow declaration of
* fields in class Thread. To help deal with very large and long-lived
* usages, the hash table entries use WeakReferences for keys. However,
* since reference queues are not used, stale entries are guaranteed to be
* removed only when the table starts running out of space.
*/
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
Entry继承WeakReference，通过其注释并结合WeakReference的功能，我们知道：一旦没有指向 key 的强引用, ThreadLocalMap 在 GC 后将自动删除相关的 entry。ThreadLocalMap采用数组来保存Entry，并且Entry中以ThreadLocal为key,初始大小为16.
        接着看ThreadLocalMap的constructor:
/**
* 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++;
}
}
}
}
ThreadLocalMap有两个构造函数，可以直接传入ThreadLcoal-value对，也可以传入一个ThreadLocalMap,传入ThreadLocalMap的时候，会依次将其Entry存放在table中。接着来分析get方法：
/**
* 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];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}
通过实例变量threadLocalHashCode算出下标，然后返回其值。set和remove方法类似。
继续看ThreadLocal类的get方法，通过getMap(Thread t)返回ThreadLocalMap，然后从ThreadLocalMap中通过getEntry(ThreadLocal key) 取出值。下面继续看getMap(Thread t)方法：
/**
* 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;
}
可以看出其返回的是线程的一个实例变量。由此可知Thread类也持有ThreadLocalMap，这样每个线程的变量都存放在自己的ThreadLocalMap中，可谓名符其实。
继续看Thread类如何操作ThreadLocalMap：
/*
* 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;
Thread类中声明了两个ThreadLocalMap变量，
/**
* 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();
}
在init方法中，存在着对inheritableThreadLocals的操作：
if (parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
而ThreadLocal的createInheritedMap方法则是调用ThreadLocalMap类传入ThreadLocalMap参数的构造函数。
也就是说在Thread类中，当前线程会调用init方法去初始一个线程，而在init方法中，会将当前线程的inheritableThreadLocals拷贝给等待初始化的线程。这让我联想起unix/linux系统中，父线程会调用fork()函数生成一个子线程，而且会把父线程大部分的信息拷贝给子线程。
         最后来看Thread类的exit方法：
/**
* 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;
}
在线程真正终止前会执行这个方法，这个方法会把threadLocals和inheritableThreadLocals指向null。但我在Thread类中并没有看到对threadLocals的赋值，应该是通过ThreadLocal来设置的。
        写了个简单的Thread测试程序，只是想跟踪一下上述两个ThreadLocalMap变量的状态：
public class TimePrinter extends Thread {

public void run() {
while (true) {
try {
System.out.println(new Date(System.currentTimeMillis()));
} catch (Exception e) {
System.out.println(e);
}
}
}

static public void main(String args[]) {
TimePrinter tp1 = new TimePrinter();
tp1.start();
ThreadLocal t2 = new ThreadLocal();
t2.set("aaaaaaaaaaaaaaaaaaaaaaaa");
}
}
可以看到，启动一个线程，不停打印系统时间，然后通过ThreadLocal给当前线程添加一份字符串，观察有：




inheritableThreadLocals中有一个Entry,但value为null，threadLocals中有三个Entry,其中两个value不明，一个为ThreadLocal设置的值。不过我实在不知道其他三个Entry值是如何设置的，留个疑问。
总结：ThreadLocal实例只有一个threadLocalHashCode值，ThreadLocal给各个线程设置的值都是存在各个线程threadLocals里 。相比Hashtable的实现方式，现在的方式更为合理。当一个线程终止时，其inheritableThreadLocals和threadLocals均被置为null,于是通过TreadLocal也就无法访问这个线程；而当ThreadLocal被设置为null时，Thread里threadLocals就会移除key为ThreadLocal的Entry。Hashtable的实现方式则无法实现这一点。最为关键的是Hashtable的实现需要同步，所带来的性能损耗是很大的，而现在的方式则不需要同步。性能提升很大。

转自http://hi.baidu.com/gefforey520/blog/item/ee476de928908735b80e2d97.html