ThreadLocal類別可以理解為ThreadLocalVariable(線程局部變數)，提供了get與set等存取介面或方法，這些方法為每個使用該變數的執行緒都存有一份獨立的副本，因此get總是傳回目前執行緒在呼叫set時設定的最新值。可以將ThreadLocal<T>視為包含了Map<Thread,T>對象，並保存了特定於該線程的值。
概括起來說，對於多執行緒資源共享的問題，同步機制採用了「以時間換空間」的方式，而ThreadLocal則採用了「以空間換時間」的方式。前者僅提供一份變量，讓不同的線程排隊訪問，而後者為每一個線程都提供了一份變量，因此可以同時訪問而互不影響。
模擬ThreadLocal
複製代碼代碼如下:
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
public class SimpleThreadLocal<T> {
private Map<Thread, T> valueMap = Collections
.synchronizedMap(new HashMap<Thread, T>());
public void set(T newValue) {
valueMap.put(Thread.currentThread(), newValue); // ①鍵為執行緒對象，值為本執行緒的變數副本
}
public T get() {
Thread currentThread = Thread.currentThread();
T o = valueMap.get(currentThread); // ②傳回本線程對應的變數
if (o == null && !valueMap.containsKey(currentThread)) { // ③如果在Map中不存在，放到Map中保存。
o = initialValue();
valueMap.put(currentThread, o);
}
return o;
}
public void remove() {
valueMap.remove(Thread.currentThread());
}
protected T initialValue() {
return null;
}
}
實用ThreadLocal
複製代碼代碼如下:
class Count {
private SimpleThreadLocal<Integer> count = new SimpleThreadLocal<Integer>() {
@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
+ "th/t" + 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();
}
}
輸出複製程式碼如下:
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