ThreadLocal的核心思想很簡(jiǎn)單：為每個(gè)獨(dú)立的線程提供一個(gè)變量的副本。
ThreadLocal則使用了“拷貝副本”的方式，人人有份，你用你的，我用我的，大家互不影響，是“以空間換時(shí)間”。每個(gè)線程修改變量時(shí)，實(shí)際上修改的是變量的副本，不怕影響到其它線程。
為了加深對(duì)ThreadLocal的理解，下面我使用一個(gè)例子來(lái)演示ThreadLocal如何隔離線程間的變量訪問(wèn)和修改：
【1】SerialNum類
package example.thread.threadLocal;
public class SerialNum {
private static int nextSerialNum = 1;
@SuppressWarnings("unchecked")
private static ThreadLocal serialNum = new ThreadLocal() {
protected synchronized Object initialValue() {
return new Integer(nextSerialNum++);
}
};
public static int get() {
return ((Integer) (serialNum.get())).intValue();
}
@SuppressWarnings("unchecked")
public static void set(Integer newSerial){
serialNum.set(newSerial);
}
}
【2】GetSerialNumThread
package example.thread.threadLocal;
public class GetSerialNumThread implements Runnable {
public static void main(String args[]) {
GetSerialNumThread serialNumGetter = new GetSerialNumThread();
Thread t1 = new Thread(serialNumGetter, "Thread A");
Thread t2 = new Thread(serialNumGetter, "Thread B");
t1.start();
try {
t1.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
public void run() {
int mySerialNum = getSerialNum();
System.out.println("線程 " + Thread.currentThread().getName()
+ " 獲取到的序列號(hào)是" + mySerialNum);
System.out.println("線程 " + Thread.currentThread().getName()
+ " 修改了序列號(hào)為" + (mySerialNum * 3));
setSerialNum(mySerialNum * 3);
System.out.println("線程 " + Thread.currentThread().getName()
+ " 再次獲得的序列號(hào)是" + getSerialNum());
}
private int getSerialNum() {
return SerialNum.get();
}
private void setSerialNum(int newSerialNum) {
SerialNum.set(new Integer(newSerialNum));
}
}
運(yùn)行的結(jié)果如下：
線程 Thread A 獲取到的序列號(hào)是1
線程 Thread A 修改了序列號(hào)為3
線程 Thread A 再次獲得的序列號(hào)是3
線程 Thread B 獲取到的序列號(hào)是2
線程 Thread B 修改了序列號(hào)為6
線程 Thread B 再次獲得的序列號(hào)是6
可見(jiàn)第一個(gè)線程在調(diào)用SerialNum.set(int)方法修改static變量時(shí)，其實(shí)修改的是它自己的副本，而不是修改本地變量，第二個(gè)線程在初始化的時(shí)候拿到的序列號(hào)是2而不是7。
為什么會(huì)這樣呢？明明serialNum是靜態(tài)變量啊？其實(shí)我們只需要看看ThreadLocal的內(nèi)部構(gòu)造就知道了：
A. ThreadLocal的get()方法：
/**
* Returns the value in the current thread's copy of this thread-local
* variable. Creates and initializes the copy if this is the first time
* the thread has called this 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)
return (T)map.get(this);
// Maps are constructed lazily. if the map for this thread
// doesn't exist, create it, with this ThreadLocal and its
// initial value as its only entry.
T value = initialValue();
createMap(t, value);
return value;
}
B. ThreadLocal的set()方法：
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Many applications will have no need for
* this functionality, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current threads' 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);
}
可以看到ThreadLocal在內(nèi)部維護(hù)了一個(gè)Map，將變量的值和線程綁定起來(lái)，get/set方法都是對(duì)該線程對(duì)應(yīng)的value進(jìn)行操作，所以不會(huì)影響到其它線程。
posted on 2010-12-09 19:27
Daniel 閱讀(209)
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