不同的執行緒可以通過同一個 ThreadLocal 物件獲取只屬於自己的資料。
ThreadLocal.ThreadLocalMap
ThreadLocal的內部類。是以ThreadLocal的 hash 值為陣列下標，Entry元素為值的陣列。ThreadLocalMap 內部是實現了一個類似 Map 的對映關係，內部的 Entry 繼承自WeakReference<ThreadLocal<?>>，它持有ThreadLocal的弱引用，儲存ThreadLocal.set(value)傳入的value。
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
複製程式碼ThreadLocal
get 方法
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
複製程式碼set 方法
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
複製程式碼使用地方有:
- Android的訊息迴圈機制(Looper Handler MessageQueue)就是基於這個。
- ...
例項:
public class Main {
static final ThreadLocal<String> mThreadLocal = new ThreadLocal<>();
public static void main(String[] args) {
new Thread("thread1") {
@Override
public void run() {
mThreadLocal.set("value1");
try {
Thread.sleep(4000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(currentThread().getName() + " localValue:" + mThreadLocal.get());
}
}.start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
new Thread("thread2") {
@Override
public void run() {
mThreadLocal.set("value2");
System.out.println(currentThread().getName() + " localValue:" + mThreadLocal.get());
}
}.start();
}
}
複製程式碼輸出:
thread2 localValue:value2
thread1 localValue:value1
複製程式碼雖然是同一個ThreadLocal物件，而且都呼叫的同樣的set get方法，但是get方法返回的值，一定是與當前執行緒對應的。