Java中的引用類型：強(qiáng)軟弱虛
1）強(qiáng)引用：我們平時(shí)用到的引用 Object o = new Object();
2）軟引用：是一個(gè)由SoftReference定義的數(shù)據(jù)，當(dāng)強(qiáng)引用對象所需內(nèi)存不足的時(shí)候，GC會回收掉軟引用對象兵睛。
軟引用非常適合做緩存偷仿。
//new byte[1024 * 1024 * 10]這個(gè)是軟引用
SoftReference<byte[]> m = new SoftReference<>(new byte[1024 * 1024 * 10]);
3）弱引用：是一個(gè)由WeakReference定義的數(shù)據(jù)颖侄，只要GC就會回收
WeakReference<Tl> m_weak = new WeakReference<>(new Tl());
4）虛引用：
PhantomReference<Tl> m_ph = new PhantomReference<>(new Tl(), new ReferenceQueue<>());
//作用：防止堆外內(nèi)存泄漏萄凤。沒太懂
ThreadLocal：線程本地管理
在當(dāng)前線程取值丹皱，也只有當(dāng)前線程可以取值
每個(gè)線程都有一個(gè)ThreadLocalMap類型的threadLocals妒穴，這個(gè)ThreadLocalMap結(jié)構(gòu)類似
HashMap的結(jié)構(gòu)，里面有個(gè)table數(shù)據(jù)結(jié)構(gòu)摊崭，table里面存放的是Entry/Node節(jié)點(diǎn)讼油。通過ThreadLocal.set()將當(dāng)前ThreadLocal放到當(dāng)前線程的threadLocals中去
class Thread {
...
ThreadLocal.ThreadLocalMap threadLocals = null;
...
}
ThreadLocal<Object> tl = new ThreadLocal<>();
tl.set(new Object());
public void set(T value) {
Thread t = Thread.currentThread();//獲取當(dāng)前線程
ThreadLocalMap map = getMap(t);//獲取當(dāng)前線程的threadLocals
if (map != null)
map.set(this, value);//將當(dāng)前ThreadLocal和value通過鍵值對形式放到threadLocals
else
createMap(t, value);
}
/**
* ThreadLocalMap.set方法
*/
private void set(ThreadLocal<?> key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
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)]) {
ThreadLocal<?> k = e.get();
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();
}
ThreadLocalMap.set方法，就是將key(ThreadLocal對象)與value（業(yè)務(wù)對象）封裝成
Entry節(jié)點(diǎn)然后放到table中去呢簸。注意：new Entry()里面的代碼矮台，涉及到了內(nèi)存泄漏的設(shè)計(jì)，如下：
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
注意根时，Entry繼承了WeakReference弱引用瘦赫，通過super(k)，將k(ThreadLocal對象)放到
用弱引用指向蛤迎。這樣确虱，當(dāng)前線程中，k被兩個(gè)地方引用：1替裆、定義ThreadLocal的引用 tl校辩；
2窘问、Entry中的弱引用k。當(dāng)tl = null之后宜咒，ThreadLocal還剩下k這一個(gè)弱引用惠赫，下次垃圾回收的時(shí)候就會將
它回收掉，從而不會造成內(nèi)存泄漏故黑；但是GC掉弱引用對象后儿咱，value還存在，所以正確的用法是：
ThreadLocal<Object> tl = new ThreadLocal<>();
tl.set(new Object());
tl.remove(); //Remove the entry for key.
待續(xù)...
