出處：https://juejin.im/post/5a0045ef5188254de169968e
ThreadLocal
ThreadLocal是一個爲線程提供線程局部變量的工具類。它的思想也十分簡單，就是爲線程提供一個線程私有的變量副本，這樣多個線程都可以隨意更改自己線程局部的變量，不會影響到其他線程。不過需要注意的是，ThreadLocal提供的只是一個淺拷貝，如果變量是一個引用類型，那麼就要考慮它內部的狀態是否會被改變，想要解決這個問題可以通過重寫ThreadLocal的initialValue()函數來自己實現深拷貝，建議在使用ThreadLocal時一開始就重寫該函數。
ThreadLocal與像synchronized這樣的鎖機制是不同的。首先，它們的應用場景與實現思路就不一樣，鎖更強調的是如何同步多個線程去正確地共享一個變量，ThreadLocal則是爲了解決同一個變量如何不被多個線程共享。從性能開銷的角度上來講，如果鎖機制是用時間換空間的話，那麼ThreadLocal就是用空間換時間。
ThreadLocal中含有一個叫做ThreadLocalMap的內部類，該類爲一個採用線性探測法實現的HashMap。它的key爲ThreadLocal對象而且還使用了WeakReference，ThreadLocalMap正是用來存儲變量副本的。
/**
* ThreadLocalMap is a customized hash map suitable only for
* maintaining thread local values. No operations are exported
* outside of the ThreadLocal class. The class is package private to
* allow declaration of fields in class Thread. To help deal with
* very large and long-lived usages, the hash table entries use
* WeakReferences for keys. However, since reference queues are not
* used, stale entries are guaranteed to be removed only when
* the table starts running out of space.
*/
static class ThreadLocalMap {
/**
* The entries in this hash map extend WeakReference, using
* its main ref field as the key (which is always a
* ThreadLocal object). Note that null keys (i.e. entry.get()
* == null) mean that the key is no longer referenced, so the
* entry can be expunged from table. Such entries are referred to
* as "stale entries" in the code that follows.
*/
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
....
}ThreadLocal中只含有三個成員變量，這三個變量都是與ThreadLocalMap的hash策略相關的。
/**
* ThreadLocals rely on per-thread linear-probe hash maps attached
* to each thread (Thread.threadLocals and
* inheritableThreadLocals). The ThreadLocal objects act as keys,
* searched via threadLocalHashCode. This is a custom hash code
* (useful only within ThreadLocalMaps) that eliminates collisions
* in the common case where consecutively constructed ThreadLocals
* are used by the same threads, while remaining well-behaved in
* less common cases.
*/
private final int threadLocalHashCode = nextHashCode();
/**
* The next hash code to be given out. Updated atomically. Starts at
* zero.
*/
private static AtomicInteger nextHashCode =
new AtomicInteger();
/**
* The difference between successively generated hash codes - turns
* implicit sequential thread-local IDs into near-optimally spread
* multiplicative hash values for power-of-two-sized tables.
*/
private static final int HASH_INCREMENT = 0x61c88647;
/**
* Returns the next hash code.
*/
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}唯一的實例變量threadLocalHashCode是用來進行尋址的hashcode，它由函數nextHashCode()生成，該函數簡單地通過一個增量HASH_INCREMENT來生成hashcode。至於爲什麼這個增量爲0x61c88647，主要是因爲ThreadLocalMap的初始大小爲16，每次擴容都會爲原來的2倍，這樣它的容量永遠爲2的n次方，該增量選爲0x61c88647也是爲了儘可能均勻地分佈，減少碰撞衝突。
/**
* The initial capacity -- MUST be a power of two.
*/
private static final int INITIAL_CAPACITY = 16;
/**
* Construct a new map initially containing (firstKey, firstValue).
* ThreadLocalMaps are constructed lazily, so we only create
* one when we have at least one entry to put in it.
*/
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);
}要獲得當前線程私有的變量副本需要調用get()函數。首先，它會調用getMap()函數去獲得當前線程的ThreadLocalMap，這個函數需要接收當前線程的實例作爲參數。如果得到的ThreadLocalMap爲null，那麼就去調用setInitialValue()函數來進行初始化，如果不爲null，就通過map來獲得變量副本並返回。
setInitialValue()函數會去先調用initialValue()函數來生成初始值，該函數默認返回null，我們可以通過重寫這個函數來返回我們想要在ThreadLocal中維護的變量。之後，去調用getMap()函數獲得ThreadLocalMap，如果該map已經存在，那麼就用新獲得value去覆蓋舊值，否則就調用createMap()函數來創建新的map。
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
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();
}
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
protected T initialValue() {
return null;
}ThreadLocal的set()與remove()函數要比get()的實現還要簡單，都只是通過getMap()來獲得ThreadLocalMap然後對其進行操作。
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
/**
* Removes the current thread's value for this thread-local
* variable. If this thread-local variable is subsequently
* {@linkplain #get read} by the current thread, its value will be
* reinitialized by invoking its {@link #initialValue} method,
* unless its value is {@linkplain #set set} by the current thread
* in the interim. This may result in multiple invocations of the
* {@code initialValue} method in the current thread.
*
* @since 1.5
*/
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
}getMap()函數與createMap()函數的實現也十分簡單，但是通過觀察這兩個函數可以發現一個祕密：ThreadLocalMap是存放在Thread中的。
/**
* Get the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @return the map
*/
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
/**
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
// Thread中的源碼
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;仔細想想其實就能夠理解這種設計的思想。有一種普遍的方法是通過一個全局的線程安全的Map來存儲各個線程的變量副本，但是這種做法已經完全違背了ThreadLocal的本意，設計ThreadLocal的初衷就是爲了避免多個線程去併發訪問同一個對象，儘管它是線程安全的。而在每個Thread中存放與它關聯的ThreadLocalMap是完全符合ThreadLocal的思想的，當想要對線程局部變量進行操作時，只需要把Thread作爲key來獲得Thread中的ThreadLocalMap即可。這種設計相比採用一個全局Map的方法會多佔用很多內存空間，但也因此不需要額外的採取鎖等線程同步方法而節省了時間上的消耗。
ThreadLocal中的內存泄漏
我們要考慮一種會發生內存泄漏的情況，如果ThreadLocal被設置爲null後，而且沒有任何強引用指向它，根據垃圾回收的可達性分析算法，ThreadLocal將會被回收。這樣一來，ThreadLocalMap中就會含有key爲null的Entry，而且ThreadLocalMap是在Thread中的，只要線程遲遲不結束，這些無法訪問到的value會形成內存泄漏。爲了解決這個問題，ThreadLocalMap中的getEntry()、set()和remove()函數都會清理key爲null的Entry，以下面的getEntry()函數的源碼爲例。
/**
* Get the entry associated with key. This method
* itself handles only the fast path: a direct hit of existing
* key. It otherwise relays to getEntryAfterMiss. This is
* designed to maximize performance for direct hits, in part
* by making this method readily inlinable.
*
* @param key the thread local object
* @return the entry associated with key, or null if no such
*/
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);
}
/**
* Version of getEntry method for use when key is not found in
* its direct hash slot.
*
* @param key the thread local object
* @param i the table index for key's hash code
* @param e the entry at table[i]
* @return the entry associated with key, or null if no such
*/
private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
// 清理key爲null的Entry
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;
}在上文中我們發現了ThreadLocalMap的key是一個弱引用，那麼爲什麼使用弱引用呢？使用強引用key與弱引用key的差別如下：
-
強引用key：ThreadLocal被設置爲null，由於ThreadLocalMap持有ThreadLocal的強引用，如果不手動刪除，那麼ThreadLocal將不會回收，產生內存泄漏。
-
弱引用key：ThreadLocal被設置爲null，由於ThreadLocalMap持有ThreadLocal的弱引用，即便不手動刪除，ThreadLocal仍會被回收，ThreadLocalMap在之後調用set()、getEntry()和remove()函數時會清除所有key爲null的Entry。
但要注意的是，ThreadLocalMap僅僅含有這些被動措施來補救內存泄漏問題。如果你在之後沒有調用ThreadLocalMap的set()、getEntry()和remove()函數的話，那麼仍然會存在內存泄漏問題。
在使用線程池的情況下，如果不及時進行清理，內存泄漏問題事小，甚至還會產生程序邏輯上的問題。所以，爲了安全地使用ThreadLocal，必須要像每次使用完鎖就解鎖一樣，在每次使用完ThreadLocal後都要調用remove()來清理無用的Entry。