Java面試必問ThreadLocal捌浩，所以想必大家對ThreadLocal并不陌生右犹，從字面意思來看ThreadLocal很容易理解，但是想要真正理解并沒有那么容易，今天我們就來扒下它的外衣……
1.首先我們來看看ThreadLocal如何使用掉分，它能解決什么樣的問題
ThreadLocal赴恨，線程本地變量晤郑，它可以為變量在每個(gè)線程中都創(chuàng)建一個(gè)副本负蚊，但它本身能夠被多個(gè)線程共享使用，并且又能夠達(dá)到線程安全的目的浆兰，且絕對線程安全磕仅。
來來來珊豹，F(xiàn)or example：
ThreadLocal<String> strLocal1=new ThreadLocal<>();
public void setValue(){
strLocal1.set(Thread.currentThread().getName());
}
public String getStrLocal1(){
return strLocal1.get();
}
public void Run(){
setValue();
System.out.println("<<<<"+Thread.currentThread().getName()+":"+getStrLocal1());
Thread thread=new Thread(){
@Override
public void run() {
super.run();
setValue();
System.out.println("<<<<"+Thread.currentThread().getName()+":"+getStrLocal1());
}
};
thread.start();
System.out.println("<<<<"+Thread.currentThread().getName()+":"+getStrLocal1());
}
看看Log:
<<<<main:main
<<<<main:main
<<<<Thread-4:Thread-4
從這段代碼的輸出結(jié)果可以看出，在main線程中和Thread-4線程中榕订，strLocal1保存的副本值都不一樣平夜。最后一次在main線程再次打印副本值是為了證明在main線程中和Thread-4線程中的副本值確實(shí)是不同的。
ThreadLocal存儲(chǔ)的值卸亮，在每個(gè)線程中互不影響，是不是很容易就實(shí)現(xiàn)線程安全玩裙。
我們來看下ThreadLocal的源碼兼贸，扒下它的遮羞布
先來看看ThreadLocal提供的幾個(gè)方法：
/**
* 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();
}
get() 方法第一句是取得當(dāng)前線程，然后通過getMap(t)方法獲取到一個(gè)map吃溅，map的類型為ThreadLocalMap溶诞。然后接著下面獲取到<key,value>鍵值對，如果獲取成功决侈，則返回value值螺垢。如果map為空，則調(diào)用setInitialValue方法返回value赖歌。
注意：ThreadLocalMap.Entry e = map.getEntry(this);
這里傳進(jìn)去的是this,也就是ThreadLocal枉圃。
來看看getMap()方法中干了什么：
/**
* 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;
}
在getMap中，是調(diào)用當(dāng)期線程t庐冯，返回當(dāng)前線程t中的一個(gè)成員變量threadLocals孽亲。
那么我們繼續(xù)去Thread類中取看一下成員變量threadLocals是什么：
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
實(shí)際上就是一個(gè)ThreadLocalMap，這個(gè)類型是ThreadLocal類的一個(gè)內(nèi)部類展父，我們繼續(xù)取看ThreadLocalMap的實(shí)現(xiàn)：
/**
* 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;
}
}
可以看到ThreadLocalMap的Entry繼承了WeakReference返劲，并且使用ThreadLocal作為鍵值。
WeakReference是個(gè)什么東西呢栖茉？
它就是Java里面?zhèn)髡f的：弱引用篮绿，弱引用用來描述非必需對象的，當(dāng)JVM進(jìn)行垃圾回收時(shí)吕漂，無論內(nèi)存是否充足亲配，都會(huì)回收被弱引用關(guān)聯(lián)的對象。在java中痰娱，用java.lang.ref.WeakReference類來表示弃榨，對于弱引用我們這里就不過多的講解了，有時(shí)間我們也來扒下它神秘的外衣梨睁。
ThreadLocalMap中ThreadLocal做為key被保存在了WeakReference中鲸睛，這就說明ThreadLocal在沒有外部強(qiáng)引用時(shí)，發(fā)生GC時(shí)會(huì)被回收坡贺。
然后再繼續(xù)看setInitialValue方法的具體實(shí)現(xiàn)：
/**
* 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;
}
很容易了解官辈，就是如果map不為空箱舞，就設(shè)置鍵值對，為空拳亿，再創(chuàng)建Map晴股。
先來看一下 T value = initialValue()
/**
* Returns the current thread's "initial value" for this
* thread-local variable. This method will be invoked the first
* time a thread accesses the variable with the {@link #get}
* method, unless the thread previously invoked the {@link #set}
* method, in which case the {@code initialValue} method will not
* be invoked for the thread. Normally, this method is invoked at
* most once per thread, but it may be invoked again in case of
* subsequent invocations of {@link #remove} followed by {@link #get}.
*
* <p>This implementation simply returns {@code null}; if the
* programmer desires thread-local variables to have an initial
* value other than {@code null}, {@code ThreadLocal} must be
* subclassed, and this method overridden. Typically, an
* anonymous inner class will be used.
*
* @return the initial value for this thread-local
*/
protected T initialValue() {
return null;
}
這里會(huì)返回null,會(huì)報(bào)空指針，所以我么在get()的時(shí)候肺魁，必須先set()
再來电湘，看一下createMap的實(shí)現(xiàn)：
/**
* 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);
}
再來看看這一行：
t.threadLocals = new ThreadLocalMap(this, firstValue);
* 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);
}
可能大部分朋友已經(jīng)明白了ThreadLocal是如何為每個(gè)線程創(chuàng)建變量的副本的：
首先，在每個(gè)線程Thread內(nèi)部有一個(gè)ThreadLocal.ThreadLocalMap類型的成員變量threadLocals鹅经，這個(gè)threadLocals就是用來存儲(chǔ)實(shí)際的變量副本的寂呛，鍵值為當(dāng)前ThreadLocal變量，value為變量副本（即T類型的變量）瘾晃。
初始化時(shí)贷痪，在Thread里面，threadLocals為空蹦误，當(dāng)通過ThreadLocal變量調(diào)用get()方法或者set()方法劫拢，就會(huì)對Thread類中的threadLocals進(jìn)行初始化，并且以當(dāng)前ThreadLocal變量為鍵值强胰，以ThreadLocal要保存的副本變量為value舱沧，存到threadLocals。
再來看看ThreadLocal里面的set方法：
/**
* 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);
}
和setInitialValue()方法一樣偶洋，如果map不為空狗唉，就設(shè)置鍵值對，為空涡真，再創(chuàng)建Map分俯，createMap(t, value)我們上面已經(jīng)分析過了。
好了哆料，該到總結(jié)的時(shí)候了：
1.通過ThreadLocal創(chuàng)建的副本缸剪，存儲(chǔ)在每個(gè)線程自己的threadLocals中。
- threadLocals實(shí)際就是ThreadLocalMap东亦，ThreadLocalMap把ThreadLocal做為key杏节。
3.在進(jìn)行g(shù)et之前，必須先set典阵，否則會(huì)報(bào)空指針異常奋渔。
4.如果想在get之前不需要調(diào)用set就能正常訪問的話，必須重寫initialValue()方法壮啊。For example嫉鲸，修改一下上面的例子：
ThreadLocal<String> strLocal1 = new ThreadLocal<String>() {
@Override
protected String initialValue() {
return Thread.currentThread().getName();
}
};
// public void setValue() {
// strLocal1.set(Thread.currentThread().getName());
// strLocal1.set("dddddd");
// }
public String getStrLocal1() {
return strLocal1.get();
}
public void Run() {
// setValue();
System.out.println("<<<<" + Thread.currentThread().getName() + ":" + getStrLocal1());
Thread thread = new Thread() {
@Override
public void run() {
super.run();
// setValue();
System.out.println("<<<<" + Thread.currentThread().getName() + ":" + getStrLocal1());
}
};
thread.start();
System.out.println("<<<<" + Thread.currentThread().getName() + ":" + getStrLocal1());
}
貼上Log：
<<<<main:main
<<<<main:main
<<<<Thread-4:Thread-4
沒有報(bào)錯(cuò)哦，趕緊動(dòng)手試試吧歹啼。
