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java 中ThreadLocal實例分析
從概念上理解，threadlocal使變量在多個線程中相互隔離實現線程安全，threadlocal包裝的變量最終都專屬于對應的每個線程，線程之間相互獨立，用一個具體實現來說明：
public interface Consumer {
int consume();
}
public class ComsumeThread implements Runnable {
private Consumer consumer;
public ComsumeThread(Consumer consumer) {
this.consumer = consumer;
}
@Override
public void run() {
for(int i=0;i<10;i++){
System.out.println(Thread.currentThread().getName()+" After Consume left:"+consumer.consume());
}
}
}
public class ConsumeClientA implements Consumer {
private static int leftNum = 30;
@Override
public int consume() {
int orgLeftNum = leftNum;
Random random = new Random(System.currentTimeMillis());
try {
Thread.sleep(random.nextInt(3));
} catch (InterruptedException e) {
e.printStackTrace();
}
orgLeftNum = orgLeftNum -1;
leftNum = orgLeftNum;
return leftNum;
}
public static void main(String[] args){
Consumer consumer = new ConsumeClientA();
Thread thread1 = new Thread(new ComsumeThread(consumer));
Thread thread2 = new Thread(new ComsumeThread(consumer));
Thread thread3 = new Thread(new ComsumeThread(consumer));
thread1.start();
thread2.start();
thread3.start();
}
}
ConsumeClientA是在沒有做任何線程安全處理，結果如下：
Thread-2 After Consume left:29 Thread-1 After Consume left:29 Thread-3 After Consume left:29 Thread-2 After Consume left:28 Thread-1 After Consume left:28 Thread-3 After Consume left:28 Thread-2 After Consume left:27 Thread-1 After Consume left:27 Thread-2 After Consume left:26 Thread-3 After Consume left:27 Thread-1 After Consume left:25 Thread-2 After Consume left:25 Thread-3 After Consume left:25 Thread-1 After Consume left:24 Thread-2 After Consume left:24 Thread-3 After Consume left:24 Thread-1 After Consume left:23 Thread-2 After Consume left:23 Thread-3 After Consume left:23 Thread-1 After Consume left:22 Thread-2 After Consume left:22 Thread-3 After Consume left:22 Thread-1 After Consume left:21 Thread-2 After Consume left:21 Thread-3 After Consume left:21 Thread-1 After Consume left:20 Thread-2 After Consume left:20 Thread-3 After Consume left:20 Thread-1 After Consume left:19 Thread-3 After Consume left:18
增加threadlocal處理，每個線程相互獨立，實現如下：
public class ConsumeClientB implements Consumer {
private ThreadLocal<Integer> leftNumThreadLocal = new ThreadLocal<Integer>(){
@Override
protected Integer initialValue() {
return 30;
}
};
@Override
public int consume() {
int orgLeftNum = leftNumThreadLocal.get();
Random random = new Random(System.currentTimeMillis());
try {
Thread.sleep(random.nextInt(3));
} catch (InterruptedException e) {
e.printStackTrace();
}
orgLeftNum = orgLeftNum -1;
leftNumThreadLocal.set(orgLeftNum);
return leftNumThreadLocal.get();
}
public static void main(String[] args){
Consumer consumer = new ConsumeClientB();
Thread thread1 = new Thread(new ComsumeThread(consumer));
Thread thread2 = new Thread(new ComsumeThread(consumer));
Thread thread3 = new Thread(new ComsumeThread(consumer));
thread1.start();
thread2.start();
thread3.start();
}
}
運行的結果如下：
Thread-1 After Consume left:29 Thread-3 After Consume left:29 Thread-2 After Consume left:29 Thread-1 After Consume left:28 Thread-3 After Consume left:28 Thread-2 After Consume left:28 Thread-1 After Consume left:27 Thread-3 After Consume left:27 Thread-2 After Consume left:27 Thread-1 After Consume left:26 Thread-3 After Consume left:26 Thread-2 After Consume left:26 Thread-1 After Consume left:25 Thread-3 After Consume left:25 Thread-2 After Consume left:25 Thread-1 After Consume left:24 Thread-3 After Consume left:24 Thread-2 After Consume left:24 Thread-1 After Consume left:23 Thread-3 After Consume left:23 Thread-2 After Consume left:23 Thread-1 After Consume left:22 Thread-3 After Consume left:22 Thread-2 After Consume left:22 Thread-1 After Consume left:21 Thread-3 After Consume left:21 Thread-2 After Consume left:21 Thread-1 After Consume left:20 Thread-3 After Consume left:20 Thread-2 After Consume left:20
每個線程擁有自己的獨立變量，相互隔離實現線程安全。
那ThreadLocal是怎樣實現這種線程隔離的線程安全的呢？
從ThreadLocal源碼可以看到，真正實現線程隔離，與線程掛鉤的，其實是ThreadLocal.ThreadLocalMap這個實現類，最明顯的體現就在于Thread類源碼的這樣一個變量申明說明了ThreadLocal.ThreadLocalMap與Thread的關系：
ThreadLocal.ThreadLocalMap threadLocals, inheritableThreadLocals;
Thread類是包含threadLocals對象的，ThreadLocal的具體實現就是根據提供的get,set等接口，對當前thread的threadLocals變量進行相關操作的，如get操作代碼如下：
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();
}
ThreadLocal.ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
可以看到，getMap()方法就是從當前thread獲取對應的threadLocals變量，然后從這個ThreadLocal.ThreadLocalMap類型的threadLocals變量中獲取對應線程中該ThreadLocal對象對應的變量值。
set方法的操作也是一樣：
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocal.ThreadLocalMap map = getMap(t);
if(map != null) {
map.set(this, value);
} else {
this.createMap(t, value);
}
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
static class Entry extends WeakReference<ThreadLocal> {
Object value;
Entry(ThreadLocal var1, Object var2) {
super(var1);
this.value = var2;
}
}
ThreadLocalMap中存的是內部類Entry的數組,Entry是繼承WeakReference實現，WeakReference的好處是保存對象引用，而又不干擾該對象被GC回收，線程執行完回收threadLocals變量時不會受到Entry封裝的變量的干擾。
而且ThreadLocalMap中的key是ThreadLocal，所以一個ThreadLocal對象只能在一個Thread對象中保存一個ThreadLocal的value。
綜上，很多人說ThreadLocal的實現是ThreadLocalMap中存Thread對象為key，變量為value的map結構，其實是錯誤的。
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