l的核心思想很簡單:為每個獨立的線程提供一個變量的副本。
Java提供的synchronized關鍵字使用了“同步鎖”的機制來阻止線程的競爭訪問,即“以時間換空間”。: " 10pt; FONT-SIZE:> 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“以空間換時間”。每個線程修改變量時,實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ThreadLocal的理解,下面我使用一個例子來演示ThreadLocal如何隔離線程間的變量訪問和修改:
【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()
+ " 獲取到的序列號是" + mySerialNum);
System.out.println("線程 " + Thread.currentThread().getName()
+ " 修改了序列號為" + (mySerialNum * 3));
setSerialNum(mySerialNum * 3);
System.out.println("線程 " + Thread.currentThread().getName()
+ " 再次獲得的序列號是" + getSerialNum());
}
private int getSerialNum() {
return SerialNum.get();
}
private void setSerialNum(int newSerialNum) {
SerialNum.set(new Integer(newSerialNum));
}
}
運行的結果如下:
線程 Thread A 獲取到的序列號是1
線程 Thread A 修改了序列號為3
線程 Thread A 再次獲得的序列號是3
線程 Thread B 獲取到的序列號是2
線程 Thread B 修改了序列號為6
線程 Thread B 再次獲得的序列號是6
可見第一個線程在調用SerialNum.set(int)方法修改static變量時,其實修改的是它自己的副本,而不是修改本地變量,第二個線程在初始化的時候拿到的序列號是2而不是7。
為什麼會這樣呢?明明serialNum是靜態變量啊?其實我們只需要看看ThreadLocal的內部構造就知道了:
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在內部維護了一個Map,將變量的值和線程綁定起來,get/set方法都是對該線程對應的value進行操作,所以不會影響到其它線程。