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Java并發(fā)編程:深入剖析ThreadLocal
ThreadLocal類(lèi)可以理解為T(mén)hreadLocalVariable(線程局部變量)，提供了get與set等訪問(wèn)接口或方法，這些方法為每個(gè)使用該變量的線程都存有一份獨(dú)立的副本，因此get總是返回當(dāng)前執(zhí)行線程在調(diào)用set時(shí)設(shè)置的最新值�？梢詫hreadLocal
概括起來(lái)說(shuō)，對(duì)于多線程資源共享的問(wèn)題，同步機(jī)制采用了“以時(shí)間換空間”的方式，而ThreadLocal采用了“以空間換時(shí)間”的方式。前者僅提供一份變量，讓不同的線程排隊(duì)訪問(wèn)，而后者為每一個(gè)線程都提供了一份變量，因此可以同時(shí)訪問(wèn)而互不影響。
模擬ThreadLocal
復(fù)制代碼 代碼如下:
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
public class SimpleThreadLocal
private Map
.synchronizedMap(new HashMap
public void set(T newValue) {
valueMap.put(Thread.currentThread(), newValue); // ①鍵為線程對(duì)象，值為本線程的變量副本
}
public T get() {
Thread currentThread = Thread.currentThread();
T o = valueMap.get(currentThread); // ②返回本線程對(duì)應(yīng)的變量
if (o == null && !valueMap.containsKey(currentThread)) { // ③如果在Map中不存在，放到Map中保存起來(lái)。
o = initialValue();
valueMap.put(currentThread, o);
}
return o;
}
public void remove() {
valueMap.remove(Thread.currentThread());
}
protected T initialValue() {
return null;
}
}
實(shí)用ThreadLocal
復(fù)制代碼 代碼如下:
class Count {
private SimpleThreadLocal
@Override
protected Integer initialValue() {
return 0;
}
};
public Integer increase() {
count.set(count.get() + 1);
return count.get();
}
}
class TestThread implements Runnable {
private Count count;
public TestThread(Count count) {
this.count = count;
}
@Override
public void run() {
// TODO Auto-generated method stub
for (int i = 1; i <= 3; i++) {
System.out.println(Thread.currentThread().getName() + "t" + i
+ "tht" + count.increase());
}
}
}
public class TestThreadLocal {
public static void main(String[] args) {
Count count = new Count();
Thread t1 = new Thread(new TestThread(count));
Thread t2 = new Thread(new TestThread(count));
Thread t3 = new Thread(new TestThread(count));
Thread t4 = new Thread(new TestThread(count));
t1.start();
t2.start();
t3.start();
t4.start();
}
}
輸出
復(fù)制代碼 代碼如下:
Thread-0 1th 1
Thread-0 2th 2
Thread-0 3th 3
Thread-3 1th 1
Thread-1 1th 1
Thread-1 2th 2
Thread-2 1th 1
Thread-1 3th 3
Thread-3 2th 2
Thread-3 3th 3
Thread-2 2th 2
Thread-2 3th 3
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