ThreadLocal是什麼
ThreadLocal是一個能創建線程局部變量的類。通過ThreadLocal提供的get和set方法,可以爲每一個使用該變量的線程保存一份數據副本,且線程之間是不能相互訪問的,從而達到變量在線程間隔離、封閉的效果。
使用例子
public static void main(String[] args) throws InterruptedException {
final ThreadLocal<String> threadLocal = new ThreadLocal<>();
threadLocal.set("AAA");
new Thread(new Runnable() {
@Override
public void run() {
threadLocal.set("BBB");
System.out.println("get in " + Thread.currentThread().getName() + " " + threadLocal.get());
}
}).start();
Thread.sleep(1000);
System.out.println("get in main thread " + threadLocal.get());
}
執行結果:
get in Thread-0 BBB
get in main thread AAA
首先,在主線程中初始化了ThreadLocal,並且操作的變量是String類型,在主線程中設置該變量爲"AAA",主線程等待1秒鐘,同時啓動了一個子線程也調用ThreadLocal設置該變量爲"BBB"並輸出,1秒之後通過get輸出主線程的結果,發現子線程設置的值並沒有影響主線程中設置的值,即通過ThreadLocal修飾的變量可以實現在各個線程之間互不干擾,相互隔離的效果。
源碼解析
初始化
//1
final ThreadLocal<String> threadLocal = new ThreadLocal<>();
threadLocal.set("AAA");
//2
final ThreadLocal<String> threadLocal = new ThreadLocal<String>() {
@Override
protected String initialValue() {
return "AAA";
}
};
對應的源碼:
protected T initialValue() {
return null;
}
public ThreadLocal() {
}
ThreadLocal的初始化可以有上面1、2兩種方式,一種是先初始化然後通過set設置值,一種直接重寫initialValue並設置值。既然ThreadLocal可以做到變量的線程封閉,我們有理由猜想是不是ThreadLocal是通過Map<Thread,T>來實現的呢?其中key是當前Thread,value是通過set或者initialValue設置的,看似是這樣,但ThreadLocal內部並不是這麼實現的,接着往下分析。
set值
public void set(T value) {
//獲取當前線程
Thread t = Thread.currentThread();
//根據當前線程獲取ThreadLocalMap,注:ThreadLocalMap內部並不是通過map來存儲value,而是通過數組存儲的
ThreadLocalMap map = getMap(t);
if (map != null)
//不爲空,內部直接通過數組設置Entry元素(Entry中包裝了ThreadLocal及value,其中key=ThreadLocal,value=傳入值value)
map.set(this, value);
else
//爲空,則初始化一個ThreadLocalMap,並將ThreadLocal及value包裝成Entry放入數組中。
createMap(t, value);
}
ThreadLocalMap getMap(Thread t) {
//threadLocals是Thread類中的成員變量
return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
Thread類:
public class Thread implements Runnable {
/* ThreadLocal values pertaining to this thread. This map is maintained by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
}
所以set方法首先根據當前線程獲取線程中的threadLocals變量(ThreadLocalMap類型),並將ThreadLocal及value包裝成Entry放入數組中,因爲threadLocals是Thread中的局部變量(存放在棧空間中),所以只有當前線程能訪問,其他線程無法訪問。這裏有個問題:爲什麼還需要將ThreadLocal作爲key傳入到ThreadLocalMap呢?因爲一個線程中可以初始化多個ThreadLocal,是一對多的關係,所以需要傳入ThreadLocal,如果初始化了多個ThreadLocal,根據不同的ThreadLocal可以獲得對應的value。那麼ThreadLocalMap內部到底是怎麼存儲的呢?
ThreadLocal靜態內部類ThreadLocalMap:
static class ThreadLocalMap {
//內部類Entry,繼承了弱引用WeakReference,使用ThreadLocal作爲鍵值
static class Entry extends WeakReference<ThreadLocal<?>> {
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
//初始容量 必須是2個倍數
private static final int INITIAL_CAPACITY = 16;
//Entry數組,必要時可以擴容,
private Entry[] table;
//數組大小
private int size = 0;
//初始化ThreadLocalMap,並將ThreadLocal、firstValue封裝成Entry並放入Entry數組中
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
//根據key(ThreadLocal類型)的hash獲取Entry在數組中的位置,有數據的話直接返回該數據
private Entry getEntry(ThreadLocal<?> key) {
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}
private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
while (e != null) {
ThreadLocal<?> k = e.get();
if (k == key)
return e;
if (k == null)
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
//根據key(ThreadLocal類型)設置value值
private void set(ThreadLocal<?> key, Object value) {
Entry[] tab = table;
int len = tab.length;
//獲取數組中存取位置
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
//如果key值在Entry中存在,那麼直接覆蓋之前的值
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
//移除key對應的value
private void remove(ThreadLocal<?> key) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
if (e.get() == key) {
e.clear();
expungeStaleEntry(i);
return;
}
}
}
}
get值
public T get() {
//獲取當前thread
Thread t = Thread.currentThread();
//根據當前線程獲取ThreadLocalMap,注:ThreadLocalMap內部並不是通過map來存儲value,而是通過數組存儲的
ThreadLocalMap map = getMap(t);
if (map != null) {
//根據this(ThreeadLocal)獲取數組中對應的Entry,不爲空直接取出value
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
T result = (T)e.value;
return result;
}
}
//如果線程中的ThreadLocalMap爲空,則進行初始化
return setInitialValue();
}
private T setInitialValue() {
//初始化值 默認是null
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
ThreadLocal在Handler中的使用
Handler機制:
Handler構造函數:
public Handler(Callback callback, boolean async) {
......其他代碼......
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Looper.prepare初始化:
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
Android中Handler機制在項目中使用的很頻繁,Handler底層通過MessageQueue和Looper來實現消息的線程間通信。其中Handler來發送及接收並處理消息,MessageQueue接收Handler發來的消息,並在Looper循環中根據msg.target(handler)來分發消息。一個線程只對應一個Lopper,一個Looper對應一個MessageQueue,但是一個線程中可以有多個Handler。因爲一個線程只能對應一個Looper,且Looper跟線程是一一綁定關係,此時用ThreadLocal再合適不過。
Looper中使用ThreadLocal關聯Looper,使得Looper只能在各自線程使用,並且不管handler從哪個線程傳來消息,ThreadLocal保證了最終消息在Looper初始化時所在的線程處理。
總結
- ThreadLocal存儲變量副本實際是保存在每個線程的threadLocals(ThreadLocal.ThreadLocalMap類型)變量中。
- ThreadLocal包含的對象(指的是ThreadLocal中的T對象)在不同的線程中有不同的副本(實際上也是不同的實例)
- ThreadLocalMap中的Entry弱引用於ThreadLocal,同時也會回收key爲null的Entry,從而避免了Entry無法釋放導致內存泄漏
畫一個簡易圖:
參考
【1】https://droidyue.com/blog/2016/03/13/learning-threadlocal-in-java/
【2】https://www.cnblogs.com/dolphin0520/p/3920407.html
【3】http://www.jasongj.com/java/threadlocal/
【4】https://juejin.im/post/5ba64dcee51d4543e609656d