并发 | 等待超时模式构造简单的数据库连接池

等待超时模式构造简单的数据库连接池

ConnectionPool.java

package concurrency.connetionPool;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.sql.Connection;
import java.util.LinkedList;
import java.util.concurrent.TimeUnit;

class ConnectionDriver{
    static class ConnectionHandler implements InvocationHandler{
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            if(method.getName().equals("commit")){
                TimeUnit.MILLISECONDS.sleep(100);
            }
            return null;
        }
    }
    
    //创建一个Connection的代理，在commit时休眠100毫秒
    public static final Connection createConnection(){
        return (Connection) Proxy.newProxyInstance(ConnectionDriver.class.getClassLoader(), new Class<?>[] { Connection.class }, new ConnectionHandler());
    }
}

public class ConnectionPool {
    private LinkedList<Connection> pool = new LinkedList<Connection>();
    
    public ConnectionPool(int initialSize){
        if(initialSize > 0){
            for(int i = 0; i < initialSize; i++){
                pool.addLast(ConnectionDriver.createConnection());
            }
        }
    }
    
    public void releaseConnection(Connection connection){
        if(connection != null){
            synchronized(pool){
                //连接释放后需要进行通知，这样其他消费者能够感知到连接池中已经归还了一个连接
                pool.addLast(connection);
                pool.notifyAll();
            }
        }
    }
    
    //在mills内无法获取到连接，将会返回null
    public Connection fetchConnection(long mills) throws InterruptedException{
        synchronized(pool){
            //完全超时
            if(mills < 0){
                while(pool.isEmpty()){
                    pool.wait();
                }
                return pool.removeFirst();//Removes and returns the first element from this list.
            }else{
                long future = System.currentTimeMillis() + mills;
                long remaining = mills;
                while(pool.isEmpty() && remaining > 0){
                    pool.wait(remaining);
                    remaining = future - System.currentTimeMillis();
                }
                Connection result = null;
                if(!pool.isEmpty()){
                    result = pool.removeFirst();
                }
                return result;
            }
        }
    }
}

ConnectionPoolTest.java

package concurrency.connetionPool;

import java.sql.Connection;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;

public class ConnectionPoolTest {
    static ConnectionPool pool = new ConnectionPool(10);
    
    //保证所有ConnectionRunner能够同时开始
    static CountDownLatch start = new CountDownLatch(1);
    
    //main线程将会等待所有ConnectionRunner结束后才能继续执行
    static CountDownLatch end;
    
    public static void main(String[] args) throws InterruptedException{
        //线程数量，可以修改线程数量进行观察
        int threadCount = 50;
        end = new CountDownLatch(threadCount);
        int count = 20;
        AtomicInteger got = new AtomicInteger();
        AtomicInteger notGot = new AtomicInteger();
        for(int i = 0; i < threadCount; i++){
            Thread thread = new Thread(new ConnectionRunner(count, got, notGot), "ConnectionRunnerThread");
            thread.start();
        }
        start.countDown();//使latch的值减1，如果减到了0，则会唤醒所有等待在这个latch上的线程。
        end.await();//使当前线程进入同步队列进行等待，直到latch的值被减到0或者当前线程被中断，当前线程就会被唤醒。
        System.out.println("total invoke: " + (threadCount * count));
        System.out.println("got connection: " + got);
        System.out.println("not got connection: " + notGot);
    }
    static class ConnectionRunner implements Runnable{
        int count;
        AtomicInteger got;
        AtomicInteger notGot;
        
        public ConnectionRunner(int count, AtomicInteger got, AtomicInteger notGot){
            this.count = count;
            this.got = got;
            this.notGot = notGot;
        }
        
        public void run(){
            try{
                start.await();
            }catch(Exception ex){
                
            }
            while(count > 0){
                try{
                    //从线程池中获取连接，如果1000ms内无法获取到，将会返回null
                    //分别统计连接获取的数量got和未获取到的数量notGot
                    Connection connection = pool.fetchConnection(1000);
                    if(connection != null){
                        try{
                            connection.createStatement();
                            connection.commit();
                        }finally{
                            pool.releaseConnection(connection);
                            got.incrementAndGet();
                        }
                    }else{
                        notGot.incrementAndGet();
                    }
                }catch(Exception ex){
                    
                }finally{
                    count--;
                }
                
            }
            end.countDown();
        }
    }
}