阿里连接池Druid源码中的部分配置研究

Druid是一个JDBC组件,它包括三部分:

  • DruidDriver 代理Driver,能够提供基于Filter-Chain模式的插件体系。
  • DruidDataSource 高效可管理的数据库连接池。
  • SQLParser

常见配置说明

配置
默认值
说明
name
配置这个属性的意义在于,如果存在多个数据源,监控的时候可以通过名字来区分开来。如果没有配置,将会生成一个名字,格式是:”DataSource-” + System.identityHashCode(this)
jdbcUrl
连接数据库的url,不同数据库不一样。例如:mysql : jdbc:mysql://10.20.153.104:3306/druid2oracle : jdbc:oracle:thin:@10.20.149.85:1521:ocnauto
username
连接数据库的用户名
password
连接数据库的密码。如果你不希望密码直接写在配置文件中,可以使用ConfigFilter。详细看这里:https://github.com/alibaba/druid/wiki/%E4%BD%BF%E7%94%A8ConfigFilter
driverClassName
根据url自动识别
这一项可配可不配,如果不配置druid会根据url自动识别dbType,然后选择相应的driverClassName
initialSize
0
初始化时建立物理连接的个数。初始化发生在显示调用init方法,或者第一次getConnection时
maxActive
8
连接池中最大连接数量
maxIdle
8
连接池中最大连接数量,已经不再使用,配置了也没效果
minIdle
0
连接池中最小连接数量
maxWait
-1
获取连接时最大等待时间,单位毫秒。配置了maxWait之后,缺省启用公平锁,并发效率会有所下降,如果需要可以通过配置useUnfairLock属性为true使用非公平锁。
poolPreparedStatements
false
是否缓存preparedStatement,也就是PSCache。PSCache对支持游标的数据库性能提升巨大,比如说oracle。在mysql5.5以下的版本中没有PSCache功能,建议关闭掉。5.5及以上版本有PSCache,建议开启。
maxOpenPreparedStatements
-1
要启用PSCache,必须配置大于0,当大于0时,poolPreparedStatements自动触发修改为true。在Druid中,不会存在Oracle下PSCache占用内存过多的问题,可以把这个数值配置大一些,比如说100
maxPoolPreparedStatementsPerConnectionSize
10
为每个连接缓存的preparedStatement的最大数量
validationQuery
null
用来检测连接是否有效的sql,要求是一个查询语句。如果validationQuery为null,testOnBorrow、testOnReturn、testWhileIdle都不会起作用
validationQueryTimeout
-1
执行validationQuery的超时时间
testOnBorrow
false
申请连接时执行validationQuery检测连接是否有效,做了这个配置会降低性能
testOnReturn
false
归还连接时执行validationQuery检测连接是否有效,做了这个配置会降低性能
testWhileIdle
true
建议配置为true,不影响性能,并且保证安全性。申请连接的时候检测,如果空闲时间大于timeBetweenEvictionRunsMillis,执行validationQuery检测连接是否有效。
timeBetweenEvictionRunsMillis
-1L(最新版本1.0.17中已改为60 * 1000L)
有两个含义:
1) Destroy线程检测连接的间隔时间,此时如果未配置,则间隔1000毫秒
2) testWhileIdle的判断依据,详细看testWhileIdle属性的说明
minEvictableIdleTimeMillis
1000L * 60L * 30L
空闲连接回收的最小空闲时间
numTestsPerEvictionRun
3
不再使用,一个DruidDataSource只支持一个EvictionRun
connectionInitSqls
物理连接初始化的时候执行的sql
exceptionSorter
null,根据dbType自动识别
当数据库抛出一些不可恢复的异常时,抛弃连接
filters
属性类型是字符串,通过别名的方式配置扩展插件,常用的插件有:
监控统计用的filter:stat
日志用的filter:log4j
防御sql注入的filter:wall
proxyFilters
类型是List<com.alibaba.druid.filter.Filter>,如果同时配置了filters和proxyFilters,是组合关系,并非替换关系
removeAbandoned
false
是否强制关闭连接时长大于removeAbandonedTimeoutMillis的连接
removeAbandonedTimeoutMillis
300 * 1000
一个连接从被连接到被关闭之间的最大生命周期
logAbandoned
false
强制关闭连接时是否记录日志
queryTimeout
执行查询的超时时间(秒),执行Statement对象时如果超过此时间,则抛出SQLException
transactionQueryTimeout
执行一个事务的超时时间(秒),执行Statement对象 时判断是否为事务,如果是且此项未设置,则使用queryTimeout

部分配置项生效过程理解
配置项中指定了各个参数后,在连接池内部是这么使用这些参数的。数据库连接池在初始化的时候会创建initialSize个连接,当有数据库操作时,会从池中取出一个连接。如果当前池中正在使用的连接数等于maxActive,则会等待一段时间,等待其他操作释放掉某一个连接,如果这个等待时间超过了maxWait,则会报错;如果当前正在使用的连接数没有达到maxActive,则判断当前是否有空闲连接,如果有则直接使用空闲连接,如果没有则新建立一个连接。在连接使用完毕后,不是将其物理连接关闭,而是将其放入池中等待其他操作复用。

    public DruidPooledConnection getConnection(long maxWaitMillis) throws SQLException {
        init();

        if (filters.size() > 0) {
            FilterChainImpl filterChain = new FilterChainImpl(this);
            return filterChain.dataSource_connect(this, maxWaitMillis);
        } else {
            return getConnectionDirect(maxWaitMillis);
        }
    }

    public DruidPooledConnection getConnectionDirect(long maxWaitMillis) throws SQLException {
        int notFullTimeoutRetryCnt = 0;
        for (;;) {
            // handle notFullTimeoutRetry
            DruidPooledConnection poolableConnection;
            try {
                poolableConnection = getConnectionInternal(maxWaitMillis);
            } catch (GetConnectionTimeoutException ex) {
                if (notFullTimeoutRetryCnt <= this.notFullTimeoutRetryCount && !isFull()) {
                    notFullTimeoutRetryCnt++;
                    if (LOG.isWarnEnabled()) {
                        LOG.warn("not full timeout retry : " + notFullTimeoutRetryCnt);
                    }
                    continue;
                }
                throw ex;
            }

            if (isTestOnBorrow()) {
                boolean validate = testConnectionInternal(poolableConnection.getConnection());
                if (!validate) {
                    if (LOG.isDebugEnabled()) {
                        LOG.debug("skip not validate connection.");
                    }

                    Connection realConnection = poolableConnection.getConnection();
                    discardConnection(realConnection);
                    continue;
                }
            } else {
                Connection realConnection = poolableConnection.getConnection();
                if (realConnection.isClosed()) {
                    discardConnection(null); // 传入null,避免重复关闭
                    continue;
                }

                if (isTestWhileIdle()) {
                    final long currentTimeMillis = System.currentTimeMillis();
                    final long lastActiveTimeMillis = poolableConnection.getConnectionHolder().getLastActiveTimeMillis();
                    final long idleMillis = currentTimeMillis - lastActiveTimeMillis;
                    long timeBetweenEvictionRunsMillis = this.getTimeBetweenEvictionRunsMillis();
                    if (timeBetweenEvictionRunsMillis <= 0) {
                        timeBetweenEvictionRunsMillis = DEFAULT_TIME_BETWEEN_EVICTION_RUNS_MILLIS;
                    }

                    if (idleMillis >= timeBetweenEvictionRunsMillis) {
                        boolean validate = testConnectionInternal(poolableConnection.getConnection());
                        if (!validate) {
                            if (LOG.isDebugEnabled()) {
                                LOG.debug("skip not validate connection.");
                            }

                            discardConnection(realConnection);
                            continue;
                        }
                    }
                }
            }
            ……

            return poolableConnection;
        }
    }

同时连接池内部有机制判断,如果当前的总的连接数少于minIdle,则会建立新的空闲连接,以保证连接数达到minIdle。以上源码可以看出,当testWhileIdle参数为true时,在获取连接时将对得到的连接进行检测,如果空闲时间大于timeBetweenEvictionRunsMillis,则执行validationQuery验证连接是否有效。

    public class DestroyConnectionThread extends Thread {
        public DestroyConnectionThread(String name){
            super(name);
            this.setDaemon(true);
        }

        public void run() {
            initedLatch.countDown();

            for (;;) {
                // 从前面开始删除
                try {
                    if (closed) {
                        break;
                    }

                    if (timeBetweenEvictionRunsMillis > 0) {
                        Thread.sleep(timeBetweenEvictionRunsMillis);
                    } else {
                        Thread.sleep(1000); //
                    }

                    if (Thread.interrupted()) {
                        break;
                    }

                    destroyTask.run();
                } catch (InterruptedException e) {
                    break;
                }
            }
        }

    }

    public class DestroyTask implements Runnable {
        @Override
        public void run() {
            shrink(true);

            if (isRemoveAbandoned()) {
                removeAbandoned();
            }
        }

    }

    public void shrink(boolean checkTime) {
        final List evictList = new ArrayList();
        try {
            lock.lockInterruptibly();
        } catch (InterruptedException e) {
            return;
        }

        try {
            final int checkCount = poolingCount - minIdle;
            final long currentTimeMillis = System.currentTimeMillis();
            for (int i = 0; i < checkCount; ++i) {
                DruidConnectionHolder connection = connections[i];

                long phyConnectTimeMillis = connection.getTimeMillis() - currentTimeMillis;//physical connection connected time
                if( phyConnectTimeMillis  > phyTimeoutMillis  ){
                    evictList.add(connection);//if physical connection connected greater than phyTimeoutMillis, close the connection, for mysql 8 hours timeout
                    continue;
                }

                if (checkTime) {
                    long idleMillis = currentTimeMillis - connection.getLastActiveTimeMillis();
                    if (idleMillis >= minEvictableIdleTimeMillis) {
                        evictList.add(connection);
                    } else {
                        break;
                    }
                } else {
                    evictList.add(connection);
                }
            }

            int removeCount = evictList.size();
            if (removeCount > 0) {
                System.arraycopy(connections, removeCount, connections, 0, poolingCount - removeCount);
                Arrays.fill(connections, poolingCount - removeCount, poolingCount, null);
                poolingCount -= removeCount;
            }
        } finally {
            lock.unlock();
        }

        for (DruidConnectionHolder item : evictList) {
            Connection connection = item.getConnection();
            JdbcUtils.close(connection);
            destroyCount.incrementAndGet();
        }
    }

    public int removeAbandoned() {
        int removeCount = 0;

        long currrentNanos = System.nanoTime();

        List abandonedList = new ArrayList();

        synchronized (activeConnections) {
            Iterator iter = activeConnections.keySet().iterator();

            for (; iter.hasNext();) {
                DruidPooledConnection pooledConnection = iter.next();

                if (pooledConnection.isRunning()) {
                    continue;
                }

                long timeMillis = (currrentNanos - pooledConnection.getConnectedTimeNano()) / (1000 * 1000);

                if (timeMillis >= removeAbandonedTimeoutMillis) {
                    iter.remove();
                    pooledConnection.setTraceEnable(false);
                    abandonedList.add(pooledConnection);
                }
            }
        }

        if (abandonedList.size() > 0) {
            for (DruidPooledConnection pooledConnection : abandonedList) {
                synchronized (pooledConnection) {
                    if (pooledConnection.isDisable()) {
                        continue;
                    }
                }
                
                JdbcUtils.close(pooledConnection);
                pooledConnection.abandond();
                removeAbandonedCount++;
                removeCount++;

                if (isLogAbandoned()) {
                    StringBuilder buf = new StringBuilder();
                    buf.append("abandon connection, owner thread: ");
                    buf.append(pooledConnection.getOwnerThread().getName());
                    buf.append(", connected at : ");
                    buf.append(pooledConnection.getConnectedTimeMillis());
                    buf.append(", open stackTrace\n");

                    StackTraceElement[] trace = pooledConnection.getConnectStackTrace();
                    for (int i = 0; i < trace.length; i++) {
                        buf.append("\tat ");
                        buf.append(trace[i].toString());
                        buf.append("\n");
                    }

                    buf.append("ownerThread current state is "+pooledConnection.getOwnerThread().getState() + ", current stackTrace\n");
                    trace = pooledConnection.getOwnerThread().getStackTrace();
                    for (int i = 0; i < trace.length; i++) {
                        buf.append("\tat ");
                        buf.append(trace[i].toString());
                        buf.append("\n");
                    }

                    LOG.error(buf.toString());
                }
            }
        }

        return removeCount;
    }

以上源码可以看出每隔timeBetweenEvictionRunsMillis进行一次shrink(连接池大小收缩)检测,如果当前连接池中的连接数量大于minIdle,则对超出minIdle的较早的连接进行空闲时间检测,如果某个连接在空闲了minEvictableIdleTimeMillis时间后仍然没有使用,则被物理性的关闭掉。除了定时检测空闲连接以外,Druid还有一个removeAbandoned机制,如果removeAbandoned为true,则在执行shrink后执行removeAbandoned(),如果某个连接从被连接到当前时间超过removeAbandonedTimeoutMillis,则无论是否被使用都被强制物理性的关闭掉,同时将该连接的abandoned设置为true。

    public int getTransactionQueryTimeout() {
        if (transactionQueryTimeout <= 0) {
            return queryTimeout;
        }

        return transactionQueryTimeout;
    }

    public void setTransactionQueryTimeout(int transactionQueryTimeout) {
        this.transactionQueryTimeout = transactionQueryTimeout;
    }

    /**
     * Retrieves the number of seconds the driver will wait for a Statement object to execute. If the limit
     * is exceeded, a SQLException is thrown.
     * 
     * @return the current query timeout limit in seconds; zero means there is no limit
     * @exception SQLException if a database access error occurs or this method is called on a closed
     * Statement
     * @see #setQueryTimeout
     */
    public int getQueryTimeout() {
        return queryTimeout;
    }

    /**
     * Sets the number of seconds the driver will wait for a Statement object to execute to the given
     * number of seconds. If the limit is exceeded, an SQLException is thrown. A JDBC driver must apply
     * this limit to the execute, executeQuery and executeUpdate methods. JDBC
     * driver implementations may also apply this limit to ResultSet methods (consult your driver vendor
     * documentation for details).
     * 
     * @param seconds the new query timeout limit in seconds; zero means there is no limit
     * @exception SQLException if a database access error occurs, this method is called on a closed
     * Statement or the condition seconds >= 0 is not satisfied
     * @see #getQueryTimeout
     */
    public void setQueryTimeout(int seconds) {
        this.queryTimeout = seconds;
    }

    void initStatement(DruidPooledConnection conn, Statement stmt) throws SQLException {
        boolean transaction = !conn.getConnectionHolder().isUnderlyingAutoCommit();

        int queryTimeout = transaction ? getTransactionQueryTimeout() : getQueryTimeout();

        if (queryTimeout > 0) {
            stmt.setQueryTimeout(queryTimeout);
        }
    }

以上源码中可以看出在初始化Statement对象时,会设置其queryTimeout,如果conn开启了事务,则使用transactionQueryTimeout,没开启就直接用queryTimeout。使用transactionQueryTimeout的时候,如果该项没有设置,依然直接使用queryTimeout。此参数的作用是执行Statement对象时如果超过此时间,就抛出SQLException。

有些数据库连接的时候有超时限制(mysql连接在8小时后断开),或者由于网络中断等原因,连接池的连接会出现失效的情况,这时候设置一个testWhileIdle参数为true,可以保证连接池内部定时检测连接的可用性,不可用的连接会被抛弃或者重建,最大情况的保证从连接池中得到的Connection对象是可用的。当然,为了保证绝对的可用性,你也可以使用testOnBorrow为true(即在获取Connection对象时检测其可用性),不过这样会影响性能。

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