一、前言
AtomicInteger基于CAS(Compare and Swap，比較并修改)的操作，主要實現樂觀鎖的思想。
對于傳統的悲觀鎖來說，會假設線程并發非常重，每次修改數據，一定先100%確保自己進入安全區，再安心修改目標值。進而出現線程在競爭鎖的過程中消耗大量時間在等待鎖、加鎖、解鎖等操作上。（注：鎖還可能涉及鎖自旋、公平鎖等知識點，而非簡單暴力競爭）
相比之下的樂觀鎖，會假設只有自己一個線程修改目標值，先比較修改前的值是不是和自己的預期的一致，一致就修改并返回，不一致就放棄這次修改（CAS），并發起下一次的嘗試，不會把時間用在加鎖和解鎖上。
雖然樂觀鎖看起來比悲觀鎖好很多，不過樂觀鎖主要用在單個值（Int，Float，Double）的并發修改上，而不是悲觀鎖對一個對象甚至是一個代碼塊的操作。
其次，若樂觀鎖對目標值修改操作次數遠多于讀取操作，那么CAS實際也會大量失敗抵消CAS的優點，并演變成成多次重試失效。
說到CAS，需要了解的是這個能力并非由操作系統或JVM提供，而是CPU原生支持的。如果一個CPU不能保證CAS能力，那這個CPU的完全沒有數據修改安全可言。
說回AtomicInteger，這類不能看見底層的運作，因為主要調用了Unsafe實現CAS。以后會寫關于Unsafe的文章，敬請期待。
二、類簽名
由于集成了Number，所以任何能接受Number類型的形參都能使用AtomicInteger
public class AtomicInteger extends Number implements java.io.Serializable
三、靜態初始化
在靜態初始化塊里面獲取value的內存地址，這時的value內存地址已經開辟，但是沒有被實例初始化。而靜態初始化塊是類初始化最早調用的，靜態初始化安全由JVM來保證。
static {
try {
valueOffset = unsafe.objectFieldOffset
(AtomicInteger.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}
四、數據成員
volatile保證value值的有序性和可見性，不保證原子性。原子性一般由synchronized或Lock來提供支持。
private volatile int value;
// setup to use Unsafe.compareAndSwapInt for updates
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long valueOffset;
五、構造方法
// 用一個給定的整形值初始化一個AtomicInteger實例
public AtomicInteger(int initialValue) {
value = initialValue;
}
// 初始化一個值為0的AtomicInteger實例
public AtomicInteger() {
}
六、成員方法
// 獲取當前的整形值，線程不安全
public final int get() {
return value;
}
// 設置新的整形值，線程不安全
public final void set(int newValue) {
value = newValue;
}
// 最終一定會把newValue設置成功
public final void lazySet(int newValue) {
unsafe.putOrderedInt(this, valueOffset, newValue);
}
// 設置新的整形值，把返回上一個保存的值
public final int getAndSet(int newValue) {
return unsafe.getAndSetInt(this, valueOffset, newValue);
}
如果待修改的值和期待值相同，那就把待修改的值設置為update的值
偽代碼： value == expect ? value = update; return isModified;
public final boolean compareAndSet(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}
/**
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
*
* <p><a href="package-summary.html#weakCompareAndSet">May fail
* spuriously and does not provide ordering guarantees</a>, so is
* only rarely an appropriate alternative to {@code compareAndSet}.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful
*/
public final boolean weakCompareAndSet(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}
// 先返回上一個值，然后再在原基礎上自增1
public final int getAndIncrement() {
return unsafe.getAndAddInt(this, valueOffset, 1);
}
// 先返回上一個值，然后再在原基礎上自減1
public final int getAndDecrement() {
return unsafe.getAndAddInt(this, valueOffset, -1);
}
// 返回上一個值，并在原基礎上加上指定值
// 偽代碼： oldValue = value; value += delta; return oldValue;
public final int getAndAdd(int delta) {
return unsafe.getAndAddInt(this, valueOffset, delta);
}
// 先自增，然后返回自增后的值
public final int incrementAndGet() {
return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
}
// 先自減，然后返回自減后的值
public final int decrementAndGet() {
return unsafe.getAndAddInt(this, valueOffset, -1) - 1;
}
// 先增加delta的值，然后返回增加后的值
public final int addAndGet(int delta) {
return unsafe.getAndAddInt(this, valueOffset, delta) + delta;
}
七、Java8 Lambda支持
IntUnaryOperator -> This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
/**
* Atomically updates the current value with the results of
* applying the given function, returning the previous value. The
* function should be side-effect-free, since it may be re-applied
* when attempted updates fail due to contention among threads.
*
* @param updateFunction a side-effect-free function
* @return the previous value
* @since 1.8
*/
public final int getAndUpdate(IntUnaryOperator updateFunction) {
int prev, next;
do {
prev = get();
next = updateFunction.applyAsInt(prev);
} while (!compareAndSet(prev, next));
return prev;
}
/**
* Atomically updates the current value with the results of
* applying the given function, returning the updated value. The
* function should be side-effect-free, since it may be re-applied
* when attempted updates fail due to contention among threads.
*
* @param updateFunction a side-effect-free function
* @return the updated value
* @since 1.8
*/
public final int updateAndGet(IntUnaryOperator updateFunction) {
int prev, next;
do {
prev = get();
next = updateFunction.applyAsInt(prev);
} while (!compareAndSet(prev, next));
return next;
}
/**
* Atomically updates the current value with the results of
* applying the given function to the current and given values,
* returning the previous value. The function should be
* side-effect-free, since it may be re-applied when attempted
* updates fail due to contention among threads. The function
* is applied with the current value as its first argument,
* and the given update as the second argument.
*
* @param x the update value
* @param accumulatorFunction a side-effect-free function of two arguments
* @return the previous value
* @since 1.8
*/
public final int getAndAccumulate(int x,
IntBinaryOperator accumulatorFunction) {
int prev, next;
do {
prev = get();
next = accumulatorFunction.applyAsInt(prev, x);
} while (!compareAndSet(prev, next));
return prev;
}
/**
* Atomically updates the current value with the results of
* applying the given function to the current and given values,
* returning the updated value. The function should be
* side-effect-free, since it may be re-applied when attempted
* updates fail due to contention among threads. The function
* is applied with the current value as its first argument,
* and the given update as the second argument.
*
* @param x the update value
* @param accumulatorFunction a side-effect-free function of two arguments
* @return the updated value
* @since 1.8
*/
public final int accumulateAndGet(int x,
IntBinaryOperator accumulatorFunction) {
int prev, next;
do {
prev = get();
next = accumulatorFunction.applyAsInt(prev, x);
} while (!compareAndSet(prev, next));
return next;
}
八、參考鏈接
https://docs.oracle.com/javase/8/docs/api/java/util/function/IntUnaryOperator.html
https://docs.oracle.com/javase/8/docs/api/java/util/function/IntBinaryOperator.html
