一,实现原理
ThreadLocal,ThreadLocalMap(ThreadLocal的静态内部类),Thread三个类。
1,在每个Thread实例中,都有一个threadlocals成员属性,用于存储该线程内的数据。
public class Thread implements Runnable { /* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ ThreadLocal.ThreadLocalMap threadLocals = null; }
2,ThreadLocalMap是一个Map结构,key为ThreadLocal类型,value为Object类型。
public class ThreadLocal<T> { /** * 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(); } /** * Variant of set() to establish initialValue. Used instead * of set() in case user has overridden the set() method. * @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. * @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. * @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. * @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); } /** * 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; } } private static final int INITIAL_CAPACITY = 16; /** * The table, resized as necessary. * table.length MUST always be a power of two. */ private Entry[] table; } } }
在每个线程Thread实例中都有一个存放本线程内数据的ThreadLocalMap【Map类型】成员变量,其中key为ThreadLocal实例,value为Object类型,所以一个线程下可以保存多个"线程局部变量"。当调用ThreadLocal中的set(T value)方法时,实际上是将该ThreadLocal实例作为key,存入当前运行线程中的数据区ThreadLocalMap【Map类型】中了,这样就保证了每个线程都有自己的一个副本,不会相互影响。
二,应用场景
Struts2相对于Struts1的一个重要升级是对request,response两个对象的解耦,Struts2的Action方法中不再需要传递 request,response参数。Struts2不通过方法直接传入request,response对象,那么这两个值是如何传递的呢?Struts2采用的正是ThreadLocal变量。在每次接收到请求时,Struts2在调用拦截器和action前,通过将 request,response对象放入ActionContext实例中,而ActionContext实例则作为”线程局部变量”存入当前线程的ThreadLocalMap成员变量中key=ThreadLocal实例 ,value=actionContext。【作用:参数隐传,达到解耦的目的】将一个共用的ThreadLocal静态实例作为key,将不同对象的引用保存到不同线程的ThreadLocalMap中,然后在线程执行的各处通过这个静态ThreadLocal实例的get()方法取得自己线程保存的那个对象,避免了将这 个对象作为参数传递的麻烦。