guava RateLimiter uniform current limit

guava的pom依赖：

        <dependency>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
            <version>18.0</version>
        </dependency>

guava采用了令牌桶算法,That is, put tokens into the bucket at a constant rate,需要通过RateLimiter的acquireMethod blocking after getting the token,to continue the task（Such as request processing and other code）,So as to achieve the purpose of uniform current limiting.

guava的RateLimiterCurrent limiter creation method：

RateLimiter.create(5);

其中createThe parameters of the method indicate yes1The number of tokens allowed to be acquired in seconds,即QPS.
createIt is created by default inside the methodSmoothBursty类型的RateLimiter,It will be relatively evenQPS分散到1time periods in seconds.
比如这儿的create(5),表示1Only available in seconds5个令牌,并且前0.2Only available in seconds1个,0.2-0.4It can only be obtained in seconds1个,0.4-0.6秒,0.6-0.8秒,0.8-1.0Seconds can also be obtained separately1个令牌.The benefit of this is to prevent uneven requests,如果在1At the beginning of the second, the tokens are all used up,Then the token is also obtained at the end of the previous second,The number of tokens acquired in the second at the end of the previous second and the beginning of this second is exceededQPSThe defined number of tokens allowed to be obtained.

下面看一段代码：

public class RateLimiterDemo {
    

    public static void main(String[] args) throws Exception{
    
        RateLimiter rateLimiter = RateLimiter.create(5);

        for (int i = 0; i < 10; i++) {
    
            System.out.println(rateLimiter.tryAcquire());
        }
    }
}

执行后输出

true
false
false
false
false
false
false
false
false
false

只有第一次tryAcquire返回true,Returned several times laterfalse,That is, only the first attempt to obtain the token succeeds,后边的9have not been obtained,因为for循环执行很快,Basically all ahead0.2seconds to try to get the token,It can only be obtained1个（5tokens are scattered1秒内,前0.2Only one token can be obtained per second）.
将forAdd sleep to the loop：

public class RateLimiterDemo {
    

    public static void main(String[] args) throws Exception{
    
        RateLimiter rateLimiter = RateLimiter.create(5);

        for (int i = 0; i < 10; i++) {
    
            Thread.sleep(200);
            System.out.println(rateLimiter.tryAcquire());
        }

    }
}

输出的结果是10个true：

true
true
true
true
true
true
true
true
true
true

That is to say, the token is obtained every time.Because it sleeps after each fetch200毫秒,1秒除以200毫秒等于5,So each time period can get the token（我们通过create(5)The number of tokens that can be obtained in one second is limited5,会分散到5个时间段内）.