ThreadLocal的核心思想很简单：为每个独立的线程提供一个变量的副本。
ThreadLocal则使用了“拷贝副本”的方式，人人有份，你用你的，我用我的，大家互不影响，是“以空间换时间”。每个线程修改变量时，实际上修改的是变量的副本，不怕影响到其它线程。
为了加深对ThreadLocal的理解，下面我使用一个例子来演示ThreadLocal如何隔离线程间的变量访问和修改：
【1】SerialNum类
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("线程 " + Thread.currentThread().getName()
+ " 获取到的序列号是" + mySerialNum);
System.out.println("线程 " + Thread.currentThread().getName()
+ " 修改了序列号为" + (mySerialNum * 3));
setSerialNum(mySerialNum * 3);
System.out.println("线程 " + Thread.currentThread().getName()
+ " 再次获得的序列号是" + getSerialNum());
}
private int getSerialNum() {
return SerialNum.get();
}
private void setSerialNum(int newSerialNum) {
SerialNum.set(new Integer(newSerialNum));
}
}
运行的结果如下：
线程 Thread A 获取到的序列号是1
线程 Thread A 修改了序列号为3
线程 Thread A 再次获得的序列号是3
线程 Thread B 获取到的序列号是2
线程 Thread B 修改了序列号为6
线程 Thread B 再次获得的序列号是6
可见第一个线程在调用SerialNum.set(int)方法修改static变量时，其实修改的是它自己的副本，而不是修改本地变量，第二个线程在初始化的时候拿到的序列号是2而不是7。
为什么会这样呢？明明serialNum是静态变量啊？其实我们只需要看看ThreadLocal的内部构造就知道了：
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在内部维护了一个Map，将变量的值和线程绑定起来，get/set方法都是对该线程对应的value进行操作，所以不会影响到其它线程。
posted on 2010-12-09 19:27
Daniel 阅读(189)
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