JAVA WEB项目中开启流量控制Filter

Flow Control：控流的概念

• 主要是用来限定server所能承载的最大（高并发）流量峰值，以免在峰值是Server过载而宕机，对于WEB系统而言
• 通常是分布式部署，如果请求并发量很大，会导致整个集群崩溃，也就是通常所说的“雪崩效应”。
• 所以，我们不仅在网络代理层面（比如nginx）设置流量控制以抵抗、拒止溢出流量，
• 我们还应该在application server层面有一定的自我保护策略，确保当前JVM的负载应该在可控范围之内，对于JVM承载能力之外的请求，应该被合理管理。

本文开发了一个分布式流量控制Filter，来限定application的并发量：

1）对于过量的请求，首先将请求buffer在队列中。

2）当buffer队列满时，多余的请求将会被直接拒绝。（过载请求量）

3）那些buffer中被阻塞的请求，等待一定时间后任然无法被执行，则直接返回错误URL。（溢出请求量）

4）我们设定一个允许的并发量，通过java中Semaphore控制。只有获取“锁”的请求，才能继续执行。

web.xml配置

<filter>
    <filter-name>flowControlFilter</filter-name>
    <filter-class>com.demo.security.FlowControlFilter</filter-class>
    <init-param>
        <param-name>permits</param-name>
        <param-value>128</param-value>
    </init-param>
    <init-param>
        <param-name>timeout</param-name>
        <param-value>15000</param-value>
    </init-param>
    <init-param>
        <param-name>bufferSize</param-name>
        <param-value>500</param-value>
    </init-param>
    <init-param>
        <param-name>errorUrl</param-name>
        <param-value>/error.html</param-value>
    </init-param>
</filter>  

<filter-mapping>
    <filter-name>flowControlFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>  

Java代码：

package com.src.java.filter;

import java.io.IOException;
import java.util.LinkedList;
import java.util.Queue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.LockSupport;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletResponse;

/**
 *
 * @ClassName: FlowControlFilter
 * @Description: 分布式系统流量控制
 * @author chinasoft_liuhanlin
 * @date 2017年6月1日 下午3:57:08
 */
public class FlowControlFilter implements Filter {

    /**
     * 最大并发量 默认为500
     */
    private int permits = Runtime.getRuntime().availableProcessors() + 1;

    /**
     * 当并发量达到permits后，新的请求将会被buffer，buffer最大尺寸 如果buffer已满，则直接拒绝
     */
    private int bufferSize = 500;
    /**
     * buffer中的请求被阻塞，此值用于控制最大阻塞时间 默认阻塞时间
     */
    private long timeout = 30000;
    /**
     * 跳转的错误页面
     */
    private String errorUrl;

    private BlockingQueue<Node> waitingQueue;

    private Thread selectorThread;
    private Semaphore semaphore;

    private Object lock = new Object();

    @Override
    public void destroy() {

    }

    /**
     * <p>
     * Title: doFilter
     * </p>
     * <p>
     * Description:
     * </p>
     *
     * @param request
     * @param response
     * @param chain
     * @throws IOException
     * @throws ServletException
     * @see javax.servlet.Filter#doFilter(javax.servlet.ServletRequest,
     *      javax.servlet.ServletResponse, javax.servlet.FilterChain)
     */
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        checkSelector();
        Thread t = Thread.currentThread();
        HttpServletResponse httpServletResponse = (HttpServletResponse) response;

        Node node = new Node(t, false);
        boolean buffered = waitingQueue.offer(node);
        // 如果buffer已满
        if (!buffered) {
            if (errorUrl != null) {
                httpServletResponse.sendRedirect(errorUrl);
            }
            return;
        }
        long deadline = System.currentTimeMillis() + timeout;
        // 进入等待队列后，当前线程阻塞
        LockSupport.parkNanos(this, TimeUnit.MICROSECONDS.toNanos(timeout));
        if (t.isInterrupted()) {
            // 如果线程是中断返回
            t.interrupted();// clear status

        }
        // 如果等待过期，则直接返回
        if (deadline >= System.currentTimeMillis()) {
            if (errorUrl != null) {
                httpServletResponse.sendRedirect(errorUrl);
            }
            // 对信号量进行补充
            synchronized (lock) {
                if (node.dequeued) {
                    semaphore.release();
                } else {
                    node.dequeued = true;
                }
            }
            return;
        }
        // 继续执行
        try {
            chain.doFilter(request, response);
        } finally {
            semaphore.release();
            checkSelector();
        }
    }

    /**
     * <p>
     * Title: init
     * </p>
     * <p>
     * Description:
     * </p>
     *
     * @param filterConfig
     * @throws ServletException
     * @see javax.servlet.Filter#init(javax.servlet.FilterConfig)
     */
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String p = filterConfig.getInitParameter("permits");
        if (p != null) {
            permits = Integer.parseInt(p);
            if (permits < 0) {
                throw new IllegalArgumentException("FlowControlFilter,permits parameter should be greater than 0 !");
            }
        }

        String t = filterConfig.getInitParameter("timeout");
        if (t != null) {
            timeout = Long.parseLong(t);
            if (timeout < 1) {
                throw new IllegalArgumentException("FlowControlFilter,timeout parameter should be greater than 0 !");
            }
        }

        String b = filterConfig.getInitParameter("bufferSize");
        if (b != null) {
            bufferSize = Integer.parseInt(b);
            if (bufferSize < 0) {
                throw new IllegalArgumentException("FlowControlFilter,bufferSize parameter should be greater than 0 !");
            }
        }

        errorUrl = filterConfig.getInitParameter("errorUrl");

        waitingQueue = new LinkedBlockingQueue<>(bufferSize);
        semaphore = new Semaphore(permits);

        selectorThread = new Thread(new SelectorRunner());
        selectorThread.setDaemon(true);
        selectorThread.start();

    }

    /**
     * @Title: checkSelector
     * @Description: TODO
     * @param:
     * @return: void
     * @throws
     */
    private void checkSelector() {
        if (selectorThread != null && selectorThread.isAlive()) {
            return;
        }
        synchronized (lock) {
            if (selectorThread != null && selectorThread.isAlive()) {
                return;
            }
            selectorThread = new Thread(new SelectorRunner());
            selectorThread.setDaemon(true);
            selectorThread.start();
        }
    }

    /**
     *
     * @ClassName: SelectorRunner
     * @Description: TODO
     * @author chinasoft_liuhanlin
     * @date 2017年6月1日 下午3:59:11
     */
    private class SelectorRunner implements Runnable {

        @Override
        public void run() {
            try {
                while (true) {
                    Node node = waitingQueue.take();
                    // 如果t，阻塞逃逸，只能在pack超时后退出
                    synchronized (lock) {
                        if (node.dequeued) {
                            // 如果此线程已经park过期而退出了，则直接忽略
                            continue;
                        } else {
                            node.dequeued = true;
                        }

                    }
                    semaphore.acquire();
                    LockSupport.unpark(node.currentThread);
                }
            } catch (Exception e) {
                //
            } finally {
                // 全部释放阻塞
                Queue<Node> queue = new LinkedList<>();
                waitingQueue.drainTo(queue);
                for (Node n : queue) {
                    if (!n.dequeued) {
                        LockSupport.unpark(n.currentThread);
                    }
                }
            }
        }
    }

    private class Node {
        Thread currentThread;
        boolean dequeued;// 是否已经出队

        public Node(Thread t, boolean dequeued) {
            this.currentThread = t;
            this.dequeued = dequeued;
        }
    }
}