Timeout: Pool empty. Unable to fetch a connection in 30 seconds, none available[size:5; busy:5; idle

在一次线上做数据库主备倒换演练操作的时候，突然发现其中一个数据量巨大的服务出现大量这个错误：

Timeout: Pool empty. Unable to fetch a connection in 30 seconds, none available[size:5; busy:5; idle.

看网友的博文，是来自org.apache.tomcat.jdbc.pool.ConnectionPool,源码如下：

private PooledConnection borrowConnection(int wait, String username, String password) throws SQLException {

        if (isClosed()) {
            throw new SQLException("Connection pool closed.");
        } //end if

        //get the current time stamp
        long now = System.currentTimeMillis();
        //see if there is one available immediately
        PooledConnection con = idle.poll();

        while (true) {
            if (con!=null) {
                //configure the connection and return it
                PooledConnection result = borrowConnection(now, con, username, password);
                borrowedCount.incrementAndGet();
                if (result!=null) return result;
            }

            //if we get here, see if we need to create one
            //this is not 100% accurate since it doesn't use a shared
            //atomic variable - a connection can become idle while we are creating
            //a new connection
            if (size.get() < getPoolProperties().getMaxActive()) {
                //atomic duplicate check
                if (size.addAndGet(1) > getPoolProperties().getMaxActive()) {
                    //if we got here, two threads passed through the first if
                    size.decrementAndGet();
                } else {
                    //create a connection, we're below the limit
                    return createConnection(now, con, username, password);
                }
            } //end if

            //calculate wait time for this iteration
            long maxWait = wait;
            //if the passed in wait time is -1, means we should use the pool property value
            if (wait==-1) {
                maxWait = (getPoolProperties().getMaxWait()<=0)?Long.MAX_VALUE:getPoolProperties().getMaxWait();
            }

            long timetowait = Math.max(0, maxWait - (System.currentTimeMillis() - now));
            waitcount.incrementAndGet();
            try {
                //retrieve an existing connection
                con = idle.poll(timetowait, TimeUnit.MILLISECONDS);
            } catch (InterruptedException ex) {
                if (getPoolProperties().getPropagateInterruptState()) {
                    Thread.currentThread().interrupt();
                }
                SQLException sx = new SQLException("Pool wait interrupted.");
                sx.initCause(ex);
                throw sx;
            } finally {
                waitcount.decrementAndGet();
            }
            if (maxWait==0 && con == null) { //no wait, return one if we have one
                if (jmxPool!=null) {
                    jmxPool.notify(org.apache.tomcat.jdbc.pool.jmx.ConnectionPool.POOL_EMPTY, "Pool empty - no wait.");
                }
                throw new PoolExhaustedException("[" + Thread.currentThread().getName()+"] " +
                        "NoWait: Pool empty. Unable to fetch a connection, none available["+busy.size()+" in use].");
            }
            //we didn't get a connection, lets see if we timed out
            if (con == null) {
                if ((System.currentTimeMillis() - now) >= maxWait) {
                    if (jmxPool!=null) {
                        jmxPool.notify(org.apache.tomcat.jdbc.pool.jmx.ConnectionPool.POOL_EMPTY, "Pool empty - timeout.");
                    }
                    throw new PoolExhaustedException("[" + Thread.currentThread().getName()+"] " +
                        "Timeout: Pool empty. Unable to fetch a connection in " + (maxWait / 1000) +
                        " seconds, none available[size:"+size.get() +"; busy:"+busy.size()+"; idle:"+idle.size()+"; lastwait:"+timetowait+"].");
                } else {
                    //no timeout, lets try again
                    continue;
                }
            }
        } //while
    }

Probably, you are holding connection for too long.

Make sure that you do not open DB connection when you start processing request and then release it when you finally committed the response.

可能你持有太久的连接。

确保在开始处理请求时不打开数据库连接，然后在最终提交响应时将其释放。

 

当时也没有想到处理的方案，直接重启了服务，置空了数据库连接池的连接，服务正常。