今天我们分析的是ThreadLocal这个类（JDK1.8中。安卓中对这个类进行了改写，方法和原理都差不多，但是具体实现上有区别，对数据存储以及获取的方式进行了更改）
/** * Implements a thread-local storage, that is, a variable for which each thread * has its own value. All threads share the same {@codeThreadLocal} object, * but each sees a different value when accessing it, and changes made by one * thread do not affect the other threads. The implementation supports * {@codenull} values. * *@seejava.lang.Thread *@authorBob Lee */这是JDK中的注释，英文好的可以翻译一下。我Google了一下，意思大概是：
实现一个线程本地的存储，也就是说，每个线程都有自己的局部变量。所有线程都共享一个ThreadLocal对象，但是每个线程在访问这些变量的时候能得到不同的值，每个线程可以更改这些变量并且不会影响其他的线程，并且支持null值。
我们先来看一下ThreadLocal类中的方法：
其中有四个很重要的方法：
public void set(T value)设置当前线程的线程局部变量的值。public T get()该方法返回当前线程所对应的线程局部变量。public void remove()将当前线程局部变量的值删除，目的是为了减少内存的占用，该方法是JDK 5.0新增的方法。需要指出的是，当线程结束后，对应该线程的局部变量将自动被垃圾回收，所以显式调用该方法清除线程的局部变量并不是必须的操作，但它可以加快内存回收的速度。protected T initialValue()返回该线程局部变量的初始值，该方法是一个protected的方法，显然是为了让子类覆盖而设计的。这个方法是一个延迟调用方法，在线程第1次调用get()或set(Object)时才执行，并且仅执行1次。ThreadLocal中的缺省实现直接返回一个null。在这个方法内部我们看到，首先通过getMap(Thread t)方法获取一个和当前线程相关的ThreadLocalMap，然后将变量的值设置到这个ThreadLocalMap对象中，当然如果获取到的ThreadLocalMap对象为空，就通过createMap方法创建。
线程隔离的秘密，就在于ThreadLocalMap(android中是Values这个内部类)这个类。ThreadLocalMap是ThreadLocal类的一个静态内部类，它实现了键值对的设置和获取（对比Map对象来理解），每个线程中都有一个独立的ThreadLocalMap副本，它所存储的值，只能被当前线程读取和修改。ThreadLocal类通过操作每一个线程特有的ThreadLocalMap副本，从而实现了变量访问在不同线程中的隔离。因为每个线程的变量都是自己特有的，完全不会有并发错误。还有一点就是，ThreadLocalMap存储的键值对中的键是this对象指向的ThreadLocal对象，而值就是你所设置的对象了。
/** * 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; } }其实里面就是一个Entry，当前线程为键，存入的对象为值。
get() /** * 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(); }第一句是取得当前线程，然后通过getMap(t)方法获取到一个map，map的类型为ThreadLocalMap。然后接着下面获取到
/** * 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; }在getMap中，是调用当期线程t，返回当前线程t中的一个成员变量threadLocals。 那么我们继续取Thread类中取看一下成员变量threadLocals是什么：
/* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ ThreadLocal.ThreadLocalMap threadLocals = null;实际上就是一个ThreadLocalMap，这个类型是ThreadLocal类的一个内部类，我们继续去看ThreadLocalMap的实现：
/** * 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; } } ......可以看到ThreadLocalMap的Entry继承了WeakReference，并且使用ThreadLocal作为键值。 然后再继续看setInitialValue方法的具体实现：
/** * 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; }很容易了解，就是如果map不为空，就设置键值对，为空，再创建Map，看一下createMap的实现：
/** * 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 */ void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); }至此，可能大部分朋友已经明白了ThreadLocal是如何为每个线程创建变量的副本的： 首先，在每个线程Thread内部有一个ThreadLocal.ThreadLocalMap类型的成员变量threadLocals，这个threadLocals就是用来存储实际的变量副本的，键值为当前ThreadLocal变量，value为变量副本（即T类型的变量）。 初始时，在Thread里面，threadLocals为空，当通过ThreadLocal变量调用get()方法或者set()方法，就会对Thread类中的threadLocals进行初始化，并且以当前ThreadLocal变量为键值，以ThreadLocal要保存的副本变量为value，存到threadLocals。
然后在当前线程里面，如果要使用副本变量，就可以通过get方法在threadLocals里面查找。
public class Test { ThreadLocal<Long> longLocal = new ThreadLocal<Long>(); ThreadLocal<String> stringLocal = new ThreadLocal<String>(); public void set() { longLocal.set(Thread.currentThread().getId()); stringLocal.set(Thread.currentThread().getName()); } public long getLong() { return longLocal.get(); } public String getString() { return stringLocal.get(); } public static void main(String[] args) throws InterruptedException { final Test test = new Test(); test.set(); System.out.println(test.getLong()); System.out.println(test.getString()); Thread thread1 = new Thread(){ public void run() { test.set(); System.out.println(test.getLong()); System.out.println(test.getString()); }; }; thread1.start(); thread1.join(); System.out.println(test.getLong()); System.out.println(test.getString()); } }运行结果为:
这个方法在主线程如果没有调用test.set()方法会报出一个空指针异常的的错误,经过分析,原因为Map使用问题,请看我另一篇Map分析
本文部分内容参考Java并发编程：深入剖析ThreadLocal
