用Java的同學可能在自己使用或者面試的時候經常遇到這麼一個問題, 哪些資料結構或者容器是同步的, 是怎麼實作的同步?
其實很多的資料同步原理都比較簡單, 我把目前知道的資料容器的同步方式稍微梳理了一下
1. 線程安全容器
StringBuffer(太明顯,synchronized關鍵字)
@Override
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}
HashTable(額,一樣,synchronized關鍵字)
public synchronized V put(K key, V value) {
...
}
Vector(額, 還是一樣,synchronized關鍵字)
public synchronized boolean add(E e) {
...
}
ConcurrentHashMap (相對高效,針對某個節點加鎖,高端了點點)
public V put(K key, V value) {
return putVal(key, value, false);
}
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode());
int binCount = 0;
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
//看這裡,這個叫分片鎖
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) {
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) {
Node<K,V> p;
binCount = 2;
if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
CopyOnWriteArrayList (使用ReentrantLock, 進行加鎖)
public boolean add(E e) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] elements = getArray();
int len = elements.length;
Object[] newElements = Arrays.copyOf(elements, len + 1);
newElements[len] = e;
setArray(newElements);
return true;
} finally {
lock.unlock();
}
}
BlockingQueue 阻塞隊列
//資料插入
public boolean offer(E e) {
if (e == null) throw new NullPointerException();
final AtomicInteger count = this.count;
if (count.get() == capacity)
//超過容量,插入失敗
return false;
int c = -1;
Node<E> node = new Node<E>(e);
final ReentrantLock putLock = this.putLock;
//資料插入鎖,保證資料插入的同步
putLock.lock();
try {
if (count.get() < capacity) {
enqueue(node);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
}
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return c >= 0;
}
//資料擷取,類pop操作
public E poll() {
final AtomicInteger count = this.count;
if (count.get() == 0)
return null;
E x = null;
int c = -1;
final ReentrantLock takeLock = this.takeLock;
//資料擷取鎖,防止資料擷取異常
takeLock.lock();
try {
if (count.get() > 0) {
x = dequeue();
//隊列元素減少,AtomicInteger原子操作
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
}
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
2. 容器同步化
Collections.synchronizedXXX(<資料容器>)
...
final Object mutex; //使用者控制同步的對象鎖
//構造方法
SynchronizedCollection(Collection<E> c) {
this.c = Objects.requireNonNull(c);
mutex = this;
}
//接上面的操作,看出來沒有沒?就是以目前容器作為對象鎖, 在操作的時候通過鎖定對象實作線程安全
...
public boolean add(E e) {
synchronized (mutex) {return c.add(e);}
}
public boolean remove(Object o) {
synchronized (mutex) {return c.remove(o);}
}
...
關于線程部分的代碼目前還沒看到,後續補上
你們還知道哪些資料容器的加鎖方式, 可以給我評論
未完,有機會再續