13. 计数for (;;) {
int current = get();
int next = current + 1;
if (compareAndSet(current, next))
return next;
}AtomicInteger max = new AtomicInteger();
for (;;) {
int current = max.get();
if (current > value)
return current;
if (max.compareAndSet(current, value))
return value;
}场景：当前值为1，并发写入2和3最大值：
int max = 0;
public int max(int value) {
if (max < value) max = value;
}计数：
int inc = 0;
public int increment() {
return inc ++;
}加锁？加锁？AtomicInteger. incrementAndGet();CAS (Lock-Free)ABA问题
14. 计数多线程完成计算：
int n = 10;
latch = new CountDownLatch(n);
for (int i = 0; i < n; i ++) {
new Thread() {
public void run() {
try {
// do something …
} finally {
latch.countDown();
}
}
}.start();
}
latch.await();多线程同时并发计数：
int n = 10;
barrier = new CyclicBarrier(n);
for (int i = 0; i < n; i ++) {
new Thread() {
public void run() {
barrier.await();
// do something …
}
}.start();
}
15. 缓存Map cache = new HashMap();
synchronized(cache) {
value = cache.get(key);
if (value == null) { value = load(key);
cache.put(key, value);
}
}// slowConcurrentMap cache = new ConcurrentHashMap();
value = cache.get(key);
if (value == null) { value = load(key);
cache.putIfAbsent(key, value);
}// repeat// waitMap cache = new HashMap();
value = cache.get(key);
if (value == null) {
synchronized(cache) {
value = cache.get(key);
if (value == null) { value = load(key);
cache.put(key, value);
}
}
}Map cache = new ConcurrentHashMap();
value = cache.get(key);
if (value == null) {
synchronized(cache) {
value = cache.get(key);
if (value == null) { value = load(key);
cache.put(key, value);
}
}
}// cpu100%// lock?
16. 缓存Map cache = new HashMap();
synchronized(cache) {
item = cache.get(key);
if (item == null) { item = new Item();
cache.put(key, item);
}
}
synchronized(item) {
value = item.get();
if (value == null) { value = load(key);
item.set(value);
}
}
ConcurrentMap cache = new ConcurrentHashMap();
item = cache.get(key);
if (item == null) { item = new Item();
oldItem = cache.putIfAbsent(key, item);
if (oldItem != null) {
item = oldItem;
}
}
value = item.get();
if (value == null) {
synchronized(item) {
value = item.get();
if (value == null) { value = load(key);
item.set(value);
}
}
}
class Item { volatile Object value; Object get() {…} set(Object value) {…} }// map lock// entry lock// quick// slow// low cost
