实验背景

近一年来发生几起的数据库连接被打满的情况，初步分析是应用使用连接数量“超过了”连接池（DBCP1.4）的上限，导致数据库连接被打满，其中一个结论是连接池的bug导致

问题分析

1、DBCP连接池的Bug导致连接数超过大小设置，其根本原因是在连接池初始化的时候，有可能创建多个连接池导致

protected synchronized DataSource createDataSource()
        throws SQLException {
        if (closed) {
            throw new SQLException("Data source is closed");
        }

        // Return the pool if we have already created it
        if (dataSource != null) {
            return (dataSource);
        }

        // create factory which returns raw physical connections
        ConnectionFactory driverConnectionFactory = createConnectionFactory();

        // create a pool for our connections
        createConnectionPool();

        // Set up statement pool, if desired
        GenericKeyedObjectPoolFactory statementPoolFactory = null;
        if (isPoolPreparedStatements()) {
            statementPoolFactory = new GenericKeyedObjectPoolFactory(null,
                        -1, // unlimited maxActive (per key)
                        GenericKeyedObjectPool.WHEN_EXHAUSTED_FAIL,
                        0, // maxWait
                        1, // maxIdle (per key)
                        maxOpenPreparedStatements);
        }

        // Set up the poolable connection factory
        createPoolableConnectionFactory(driverConnectionFactory, statementPoolFactory, abandonedConfig);

        // Create and return the pooling data source to manage the connections
        createDataSourceInstance();

        try {
            for (int i = 0 ; i < initialSize ; i++) {
                connectionPool.addObject();
            }
        } catch (Exception e) {
            throw new SQLNestedException("Error preloading the connection pool", e);
        }

        return dataSource;
    }

createDataSource方法里边调用createConnectionPool方法，如果后面方式执行失败，比如createPoolableConnectionFactory，当再次调用createDataSource，又会再次调用createConnectionPool，从而初始化多次连接池，下面是createConnectionPool

 protected void createConnectionPool() {
        // Create an object pool to contain our active connections
        GenericObjectPool gop;
        if ((abandonedConfig != null) && (abandonedConfig.getRemoveAbandoned())) {
            gop = new AbandonedObjectPool(null,abandonedConfig);
        }
        else {
            gop = new GenericObjectPool();
        }
        gop.setMaxActive(maxActive);
        gop.setMaxIdle(maxIdle);
        gop.setMinIdle(minIdle);
        gop.setMaxWait(maxWait);
        gop.setTestOnBorrow(testOnBorrow);
        gop.setTestOnReturn(testOnReturn);
        gop.setTimeBetweenEvictionRunsMillis(timeBetweenEvictionRunsMillis);
        gop.setNumTestsPerEvictionRun(numTestsPerEvictionRun);
        gop.setMinEvictableIdleTimeMillis(minEvictableIdleTimeMillis);
        gop.setTestWhileIdle(testWhileIdle);
        connectionPool = gop;
    }

显然这个情况只能是模块启动初始化的时候产生，且连接都不会被使用，而线上出现的问题是连接都在执行sql，不是这种情况！在1.4x的这个bug已经修复
2、启动参数分析，从启动参数入手，看一下是不是连接池主动释放连接，导致正在使用的连接被释放，从而创建新的连接，给我们的现象是“连接数”超过了连接池的限制，下面是一个入库的连接池参数

partition1.driverClassName=com.MySQL.jdbc.Driver
partition1.initialSize=2
partition1.maxActive=25
partition1.minIdle=2
partition1.maxIdle=5
partition1.maxWait=3000
partition1.threadPoolSize=10
partition1.logAbandoned=true
partition1.testWhileIdle=true
partition1.testOnReturn=false
partition1.testOnBorrow=true
partition1.validationQuery=select now()
//在每次空闲连接回收器线程(如果有)运行时检查的连接数量
partition1.numTestsPerEvictionRun=5
//在空闲连接回收器线程运行期间休眠的时间值,以毫秒为单位
partition1.timeBetweenEvictionRunsMillis=30000
//连接在池中保持空闲而不被空闲连接回收器线程
partition1.minEvictableIdleTimeMillis=180000
//设置了rmoveAbandoned=true 那么在getNumactive()快要到getMaxActive()的时候，系统会进行无效的Connection的回收，回收的 Connection为removeAbandonedTimeout(默认300秒)中设置的秒数后没有使用的Connection
partition1.removeAbandoned=true
//强制回收连接的时间，单位秒
partition1.removeAbandonedTimeout=18

注意connectionProperties参数

<bean id="partition[j]" class="org.Apache.commons.dbcp.BasicDataSource"
          destroy-method="close">
        <property name="driverClassName" value="${partition[j].driverClassName}" ></property>
        <property name="url" value="${partition[j].url}" ></property>
        <property name="username" value="${partition[j].username}" ></property>
        <property name="password" value="${partition[j].password}" ></property>
        <property name="defaultAutoCommit" value="false" ></property>
        <property name="maxActive" value="${partition[j].maxActive}" ></property>
        <property name="maxIdle" value="${partition[j].maxIdle}" ></property>
        <property name="maxWait" value="${partition[j].maxWait}" ></property>
        <property name="initialSize" value="${partition[j].initialSize}" ></property>
        <property name="minIdle" value="${partition[j].minIdle}" ></property>
        <property name="logAbandoned" value="${partition[j].logAbandoned}" ></property>
        <property name="testWhileIdle" value="${partition[j].testWhileIdle}" ></property>
        <property name="testOnReturn" value="${partition[j].testOnReturn}" ></property>
        <property name="testOnBorrow" value="${partition[j].testOnBorrow}" ></property>
        <property name="validationQuery" value="${partition[j].validationQuery}" ></property>
        <property name="numTestsPerEvictionRun" value="${partition[j].numTestsPerEvictionRun}" ></property>
        <property name="timeBetweenEvictionRunsMillis" value="${partition[j].timeBetweenEvictionRunsMillis}" ></property>
        <property name="minEvictableIdleTimeMillis" value="${partition[j].minEvictableIdleTimeMillis}" ></property>
        <property name="removeAbandoned" value="${partition[j].removeAbandoned}" ></property>
        <property name="removeAbandonedTimeout" value="${partition[j].removeAbandonedTimeout}" ></property>
        <property name="connectionProperties" value="useUnicode=true;
   characterEncoding=utf8;initialTimeout=1;connectTimeout=1000;socketTimeout=6000;
   rewriteBatchedStatements=true;autoReconnectForPools=true;autoReconnect=true;maxReconnects=1;
   failOverReadOnly=false;roundRobinLoadBalance=true;allowMultiQueries=true"></property>
    </bean>

从上面参数我们重点关注removeAbandonedTimeout 这个参数的意义

  //创建连接
   public Object borrowObject() throws Exception {
        if (config != null
                && config.getRemoveAbandoned()
                && (getNumIdle() < 2)
                && (getNumActive() > getMaxActive() - 3) ) {
            removeAbandoned();
        }
        Object obj = super.borrowObject();
        if (obj instanceof AbandonedTrace) {
            ((AbandonedTrace) obj).setStackTrace();
        }
        if (obj != null && config != null && config.getRemoveAbandoned()) {
            synchronized (trace) {
                trace.add(obj);
            }
        }
    ...

    private void removeAbandoned() {
        // Generate a list of abandoned connections to remove
        long now = System.currentTimeMillis();
        long timeout = now - (config.getRemoveAbandonedTimeout() * 1000);
        ArrayList remove = new ArrayList();
        synchronized (trace) {
            Iterator it = trace.iterator();
            while (it.hasNext()) {
                AbandonedTrace pc = (AbandonedTrace) it.next();
                if (pc.getLastUsed() > timeout) {
                    continue;
                }
                if (pc.getLastUsed() > 0) {
                    remove.add(pc);
                }
            }
        }

        // Now remove the abandoned connections
        Iterator it = remove.iterator();
        while (it.hasNext()) {
            AbandonedTrace pc = (AbandonedTrace) it.next();
            if (config.getLogAbandoned()) {
                pc.printStackTrace();
            }             
            try {
                invalidateObject(pc);
            } catch (Exception e) {
                e.printStackTrace();
            }

        }
    }

代从代码可以看出，配置了removeAbandonedOnBorrow和removeAbandonedTimeout就是触发正在使用的连接remove，假如这个时候，数据库正在执行sql，就会出现连接池已经断开连接，而数据库的连接还在执行，造成数据库的连接超过连接池的显现，如果少量慢sql执行时间大于removeAbandonedTimeout，不会出现问题，出现大量的慢sql，就会导致数据库中的这种连接越来越多，最后把数据库打满

实验过程

创建测试库

mkdir /usr/local/data/mysql
Docker run -d -e MYSQL_ROOT_PASSWORD=root --name centos/mysql-57-centos7 -v /usr/local/data/mysql:/var/lib/mysql -p 3306:3306 mysql --character-set-server=utf8mb4 --collation-server=utf8mb4_unicode_ci --lower_case_table_names=1

创建测试库

create database test ;

构建测试类，核心代码

public void setUp() throws Exception {
        ds = createDataSource();
        ds.setDriverClassName("com.mysql.jdbc.Driver");
        ds.setUrl("jdbc:mysql://127.0.0.1:3306/test?allowMultiQueries=true&characterEncoding=UTF-8&autoReconnect=true&failOverReadOnly=false&useSSL=false&allowPublicKeyRetrieval=true");
        ds.setUsername("root");
        ds.setPassword("Htbuy@2016");
        ds.setMaxActive(1);
        ds.setMaxWait(1000);
        ds.setTestWhileIdle(true);
        ds.setTestOnBorrow(true);
        ds.setTestOnReturn(false);
        ds.setValidationQuery("select now()");
        ds.setNumTestsPerEvictionRun(5);
        ds.setMinEvictableIdleTimeMillis(2000);
        ds.setLogAbandoned(true);
        ds.setRemoveAbandoned(true);
        ds.setRemoveAbandonedTimeout(1);
}
public void testAbandoned() throws Exception {

        for (int i = 0; i < 20; i++) {
            Thread t = new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        Connection conn = ds.getConnection();
                        Statement statement = conn.createStatement();
                        ResultSet resultSet = null; 
    #模拟慢sql
                        resultSet = statement.executeQuery("select  sleep(100),now()");
                        while (resultSet.next()) {
                            System.out.println("result+" + resultSet.getString(1));
                        }
                        resultSet.close();
                        statement.close();
                        conn.close();
                    } catch (Exception ex) {
                        System.out.println(ex.getMessage());

                    }
                    System.out.println(Thread.currentThread().getName() + "---------------------- end----------------------");
                }
            });
            t.setName(i + "");
            t.start();
            Thread.sleep((i + 1) * 1000);
        }
        System.out.println(Thread.currentThread().getName() + "---------------------- end----------------------");
        Thread.sleep(1000000);
    }

通过运行上面代码，并没有像我们所期望那样，超过连接的最大限制1，通过RemoveAbandoned=true和RemoveAbandonedTimeout=1 应该已经触发了连接池的Abandoned机制，但是都阻塞到下面的一行代码，DelegatingStatement的close方法

/**
     * Close this DelegatingStatement, and close
     * any ResultSets that were not explicitly closed.
     */
    public void close() throws SQLException {
        try {
            try {
                if (_conn != null) {
                    _conn.removeTrace(this);
                    _conn = null;
                }

                // The JDBC spec requires that a statment close any open
                // ResultSet's when it is closed.
                // FIXME The PreparedStatement we're wrApping should handle this for us.
                // See bug 17301 for what could happen when ResultSets are closed twice.
                List resultSets = getTrace();
                if( resultSets != null) {
                    ResultSet[] set = (ResultSet[]) resultSets.toArray(new ResultSet[resultSets.size()]);
                    for (int i = 0; i < set.length; i++) {
                        set[i].close();
                    }
                    clearTrace();
                }
//阻塞地方
                _stmt.close();
            }
            catch (SQLException e) {
                handleException(e);
            }
        }
        finally {
            _closed = true;
        }
    }

当我们在url设置socketTimeout=1000，这时候阻塞的地方成功执行完成，超过数据库出现大量的连接数1，问题重现！同时出现典型的日志The last packet successfully received from the server was 1,001 milliseconds ago. The last packet sent successfully to the server was 1,001 milliseconds ago.

当设置socketTimeout=1000，RemoveAbandoned=false ，并没有重现问题，且都是大量的等待连接超时

原因是下面的代码差异，开启RemoveAbandoned，使用的是AbandonedObjectPool，而不开启是默认的GenericObjectPool，AbandonedObjectPool增加了Abandoned逻辑

 */
    protected void createConnectionPool() {
        // Create an object pool to contain our active connections
        GenericObjectPool gop;
        if ((abandonedConfig != null) && (abandonedConfig.getRemoveAbandoned())) {
            gop = new AbandonedObjectPool(null,abandonedConfig);
        }
        else {
            gop = new GenericObjectPool();
        }
        gop.setMaxActive(maxActive);
        gop.setMaxIdle(maxIdle);
        gop.setMinIdle(minIdle);
        gop.setMaxWait(maxWait);
        gop.setTestOnBorrow(testOnBorrow);
        gop.setTestOnReturn(testOnReturn);
        gop.setTimeBetweenEvictionRunsMillis(timeBetweenEvictionRunsMillis);
        gop.setNumTestsPerEvictionRun(numTestsPerEvictionRun);
        gop.setMinEvictableIdleTimeMillis(minEvictableIdleTimeMillis);
        gop.setTestWhileIdle(testWhileIdle);
        connectionPool = gop;
    }

总结

当RemoveAbandoned=true， 且执行时间超过socketTimeout ，达到RemoveAbandonedTimeout的触发点时，就会导致数据库连接数超过连接池的限制，注意这种情况关闭模块是没用的，sql还在数据库中执行，应该直接kill或者切库操作！！

优化方案

1、关闭RemoveAbandoned设置，目前wms系统的还是AP和TP混合型，有很大的几率触发这种情况
2、经过邱玉堃复核模块测试，需要关闭socketTimeout参数，引用的1.4.jar版本与源码的1.4版本存在差异
3、 注意当mysql-connector-JAVA 版本小于5.1.45就会出现这个bug