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java 中ThreadLocal實(shí)例分析
從概念上理解，threadlocal使變量在多個(gè)線程中相互隔離實(shí)現(xiàn)線程安全，threadlocal包裝的變量最終都專屬于對(duì)應(yīng)的每個(gè)線程，線程之間相互獨(dú)立，用一個(gè)具體實(shí)現(xiàn)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(jié)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(gè)線程相互獨(dú)立，實(shí)現(xiàn)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(yùn)行的結(jié)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(gè)線程擁有自己的獨(dú)立變量，相互隔離實(shí)現(xiàn)線程安全。
那ThreadLocal是怎樣實(shí)現(xiàn)這種線程隔離的線程安全的呢？
從ThreadLocal源碼可以看到，真正實(shí)現(xiàn)線程隔離，與線程掛鉤的，其實(shí)是ThreadLocal.ThreadLocalMap這個(gè)實(shí)現(xiàn)類，最明顯的體現(xiàn)就在于Thread類源碼的這樣一個(gè)變量申明說明了ThreadLocal.ThreadLocalMap與Thread的關(guān)系：
ThreadLocal.ThreadLocalMap threadLocals, inheritableThreadLocals;
Thread類是包含threadLocals對(duì)象的，ThreadLocal的具體實(shí)現(xiàn)就是根據(jù)提供的get,set等接口，對(duì)當(dāng)前thread的threadLocals變量進(jìn)行相關(guān)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(dāng)前thread獲取對(duì)應(yīng)的threadLocals變量，然后從這個(gè)ThreadLocal.ThreadLocalMap類型的threadLocals變量中獲取對(duì)應(yīng)線程中該ThreadLocal對(duì)象對(duì)應(yīng)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(nèi)部類Entry的數(shù)組,Entry是繼承WeakReference實(shí)現(xiàn)，WeakReference的好處是保存對(duì)象引用，而又不干擾該對(duì)象被GC回收，線程執(zhí)行完回收threadLocals變量時(shí)不會(huì)受到Entry封裝的變量的干擾。
而且ThreadLocalMap中的key是ThreadLocal，所以一個(gè)ThreadLocal對(duì)象只能在一個(gè)Thread對(duì)象中保存一個(gè)ThreadLocal的value。
綜上，很多人說ThreadLocal的實(shí)現(xiàn)是ThreadLocalMap中存Thread對(duì)象為key，變量為value的map結(jié)構(gòu)，其實(shí)是錯(cuò)誤的。
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