滑动窗口计数有很多使用场景，比如说限流防止系统雪崩。相比计数实现，滑动窗口实现会更加平滑，能自动消除毛刺。

概念上可以参考TCP的滑窗算法，可以看一下这篇文章(http://go12345.iteye.com/blog/1744728)。在实现上，滑动窗口算法需要循环队列和线程安全保障。

下面的实现有几个点

1， 支持滑窗大小运行时动态调整

2， 基于 java8 编译器

3， DEMO实现只支持一个窗口对象，如果要支持多个，需要修改 SlotBaseCounter 类

package slidingwindow;

import java.util.Arrays;

import java.util.concurrent.atomic.AtomicInteger;

/**

* Created by admin on 2016/02/20.

*/

public class SlotBaseCounter {

private int slotSize;

private AtomicInteger[] slotCounter;

public SlotBaseCounter(int slotSize) {

slotSize = slotSize < 1 ? 1 : slotSize;

this.slotSize = slotSize;

this.slotCounter = new AtomicInteger[slotSize];

for (int i = 0; i < this.slotSize; i++) {

slotCounter[i] = new AtomicInteger(0);

}

}

public void increaseSlot(int slotSize) {

slotCounter[slotSize].incrementAndGet();

}

public void wipeSlot(int slotSize) {

slotCounter[slotSize].set(0);

}

public int totalCount() {

return Arrays.stream(slotCounter).mapToInt(slotCounter -> slotCounter.get()).sum();

}

@Override

public String toString() {

return Arrays.toString(slotCounter);

}

}

package slidingwindow;

/**

* Created by admin on 2016/02/20.

*/

public class SlidingWindowCounter {

private volatile SlotBaseCounter slotBaseCounter;

private volatile int windowSize;

private volatile int head;

public SlidingWindowCounter(int windowSize) {

resizeWindow(windowSize);

}

public synchronized void resizeWindow(int windowSize) {

this.windowSize = windowSize;

this.slotBaseCounter = new SlotBaseCounter(windowSize);

this.head = 0;

}

public void increase() {

slotBaseCounter.increaseSlot(head);

}

public int totalAndAdvance() {

int total = totalCount();

advance();

return total;

}

public void advance() {

int tail = (head + 1) % windowSize;

slotBaseCounter.wipeSlot(tail);

head = tail;

}

public int totalCount() {

return slotBaseCounter.totalCount();

}

@Override

public String toString() {

return "total = " + totalCount() + " head = " + head + " >> " + slotBaseCounter;

}

}

package slidingwindow;

import java.util.concurrent.TimeUnit;

/**

* Created by admin on 2016/02/20.

*/

public class Loops {

public static void dieLoop(Runnable runnable) {

while (true) {

run(runnable);

}

}

public static void rateLoop(Runnable runnable, int mills) {

while (true) {

try {

TimeUnit.MILLISECONDS.sleep(mills);

} catch (InterruptedException e) {

}

run(runnable);

}

}

public static void fixLoop(Runnable runnable, int loop) {

for (int i = 0; i < loop; i++) {

run(runnable);

}

}

private static void run(Runnable runnable) {

try {

runnable.run();

} catch (Exception e) {

e.printStackTrace();

}

}

}

package slidingwindow;

import org.junit.Test;

import java.io.IOException;

import java.util.Random;

import java.util.Scanner;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.ScheduledExecutorService;

import java.util.concurrent.TimeUnit;

/**

* Created by admin on 2016/02/20.

*/

public class SlidingWindowCounterTest {

private ExecutorService es = Executors.newCachedThreadPool();

private ScheduledExecutorService ses = Executors.newSingleThreadScheduledExecutor();

@Test

public void testNWindow() throws IOException {

SlidingWindowCounter swc = new SlidingWindowCounter(3);

ses.scheduleAtFixedRate(() -> {

Loops.fixLoop(swc::increase, new Random().nextInt(10));

}, 10, 2, TimeUnit.MILLISECONDS);

ses.scheduleAtFixedRate(() -> {

System.out.println(swc);

swc.advance();

}, 1, 1, TimeUnit.SECONDS);

ses.scheduleAtFixedRate(() -> {

swc.resizeWindow(new Random().nextInt(10));

}, 1, 10, TimeUnit.SECONDS);

System.in.read();

}

@Test

public void test1Window() {

SlidingWindowCounter swc = new SlidingWindowCounter(1);

System.out.println(swc);

swc.increase();

swc.increase();

System.out.println(swc);

swc.advance();

System.out.println(swc);

swc.increase();

swc.increase();

System.out.println(swc);

}

@Test

public void test3Window() {

SlidingWindowCounter swc = new SlidingWindowCounter(3);

System.out.println(swc);

swc.increase();

System.out.println(swc);

swc.advance();

System.out.println(swc);

swc.increase();

swc.increase();

System.out.println(swc);

swc.advance();

System.out.println(swc);

swc.increase();

swc.increase();

swc.increase();

System.out.println(swc);

swc.advance();

System.out.println(swc);

swc.increase();

swc.increase();

swc.increase();

swc.increase();

System.out.println(swc);

swc.advance();

System.out.println(swc);

}

}

这是部分测试结果输出：

total = 2245 head = 0 >> [2245, 0, 0, 0, 0, 0]

total = 4561 head = 1 >> [2245, 2316, 0, 0, 0, 0]

total = 6840 head = 2 >> [2245, 2316, 2279, 0, 0, 0]

total = 8994 head = 3 >> [2245, 2316, 2279, 2154, 0, 0]

total = 11219 head = 4 >> [2245, 2316, 2279, 2154, 2225, 0]

total = 13508 head = 5 >> [2245, 2316, 2279, 2154, 2225, 2289]

total = 13602 head = 0 >> [2339, 2316, 2279, 2154, 2225, 2289]

total = 13465 head = 1 >> [2339, 2179, 2279, 2154, 2225, 2289]

total = 13474 head = 2 >> [2339, 2179, 2288, 2154, 2225, 2289]

total = 13551 head = 3 >> [2339, 2179, 2288, 2231, 2225, 2289]

total = 2192 head = 0 >> [2192]

total = 2207 head = 0 >> [2207]

total = 2291 head = 0 >> [2291]

total = 2257 head = 0 >> [2257]

total = 2250 head = 0 >> [2250]

total = 2201 head = 0 >> [2201]

total = 2299 head = 0 >> [2299]

total = 2223 head = 0 >> [2223]

total = 2190 head = 0 >> [2190]

total = 2306 head = 0 >> [2306]

total = 2290 head = 0 >> [2290, 0, 0, 0, 0, 0, 0, 0, 0]

total = 4474 head = 1 >> [2290, 2184, 0, 0, 0, 0, 0, 0, 0]

total = 6632 head = 2 >> [2290, 2184, 2158, 0, 0, 0, 0, 0, 0]

total = 8744 head = 3 >> [2290, 2184, 2158, 2112, 0, 0, 0, 0, 0]

total = 11008 head = 4 >> [2290, 2184, 2158, 2112, 2264, 0, 0, 0, 0]

total = 13277 head = 5 >> [2290, 2184, 2158, 2112, 2264, 2269, 0, 0, 0]

total = 15446 head = 6 >> [2290, 2184, 2158, 2112, 2264, 2269, 2169, 0, 0]

total = 17617 head = 7 >> [2290, 2184, 2158, 2112, 2264, 2269, 2169, 2171, 0]

total = 19749 head = 8 >> [2290, 2184, 2158, 2112, 2264, 2269, 2169, 2171, 2132]

total = 19608 head = 0 >> [2149, 2184, 2158, 2112, 2264, 2269, 2169, 2171, 2132]

total = 2256 head = 0 >> [2256, 0, 0, 0]

total = 4624 head = 1 >> [2256, 2368, 0, 0]

total = 6811 head = 2 >> [2256, 2368, 2187, 0]

total = 8973 head = 3 >> [2256, 2368, 2187, 2162]

total = 8934 head = 0 >> [2217, 2368, 2187, 2162]

total = 8798 head = 1 >> [2217, 2232, 2187, 2162]

total = 8912 head = 2 >> [2217, 2232, 2301, 2162]

total = 8940 head = 3 >> [2217, 2232, 2301, 2190]

total = 8987 head = 0 >> [2264, 2232, 2301, 2190]

total = 9049 head = 1 >> [2264, 2294, 2301, 2190]

total = 2220 head = 0 >> [2220, 0, 0, 0, 0, 0, 0]

total = 4477 head = 1 >> [2220, 2257, 0, 0, 0, 0, 0]

total = 6718 head = 2 >> [2220, 2257, 2241, 0, 0, 0, 0]

total = 8939 head = 3 >> [2220, 2257, 2241, 2221, 0, 0, 0]

total = 11174 head = 4 >> [2220, 2257, 2241, 2221, 2235, 0, 0]

total = 13420 head = 5 >> [2220, 2257, 2241, 2221, 2235, 2246, 0]

total = 15673 head = 6 >> [2220, 2257, 2241, 2221, 2235, 2246, 2253]

total = 15779 head = 0 >> [2326, 2257, 2241, 2221, 2235, 2246, 2253]

total = 15796 head = 1 >> [2326, 2274, 2241, 2221, 2235, 2246, 2253]

total = 15802 head = 2 >> [2326, 2274, 2247, 2221, 2235, 2246, 2253]