MySQL Fabric和MyBatis的整合过程中遇到的问题

这是我昨天在整合MySQL Fabric和MyBatis时遇到的问题，花了大半天才解决的问题，解决的过程中在网上查找了很久，都没有找到解决的方案。现在记下来，希望能够帮助有同样问题的朋友。如果各位朋友有更好的解决方案，也请告诉我。

1. 问题描述

这个问题是在整合MySQL和MyBatis的时候遇到的。

首先说一下我使用的jar包的版本，MySQL Connector用的是5.1.36，myBatis用的是3.2.8。我也试过将MySQL Connector升级到5.1.40，然并卵。

看看MyBatis中数据源的设置如下：

    <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"
        destroy-method="close">
        <property name="driverClassName" value="${mybatis.driverClassName}" />
        <property name="url" value="${mybatis.url}" />
        <property name="username" value="${mybatis.username}" />
        <property name="password" value="${mybatis.password}" />
        <property name="minIdle" value="${mybatis.minIdle}" />        <!-- 队列中的最小等待数 -->
        <property name="maxIdle" value="${mybatis.maxIdle}" />        <!-- 队列中的最大等待数 -->
        <property name="maxWait" value="${mybatis.maxWait}" />        <!-- 最长等待时间，单位毫秒 -->
        <property name="maxActive" value="${mybatis.maxActive}" />    <!-- 最大活跃数 -->
        <property name="initialSize" value="${mybatis.initialSize}" /><!-- 初始大小 -->
        <property name="validationQuery" value="${mybatis.validationQuery}" />
        <property name="testWhileIdle" value="${mybatis.testWhileIdle}" />
        <property name="timeBetweenEvictionRunsMillis" value="${mybatis.timeBetweenEvictionRunsMillis}" />
        <property name="numTestsPerEvictionRun" value="${mybatis.numTestsPerEvictionRun}" />
        <property name="minEvictableIdleTimeMillis" value="${mybatis.minEvictableIdleTimeMillis}" />
    </bean>

对应的属性值定义如下：

mybatis.driverClassName=com.mysql.fabric.jdbc.FabricMySQLDriver
mybatis.url=jdbc:mysql:fabric://10.8.48.230:32274/basicservice?fabricServerGroup=spfood&fabricUsername=admin&fabricPassword=admin123#
mybatis.username=spfood
mybatis.password=spfood123#
mybatis.initialSize=5
mybatis.minIdle=1
mybatis.maxIdle=10
mybatis.maxWait=3000
mybatis.maxActive=50
#SQL查询,用来验证从连接池取出的连接
mybatis.validationQuery=SELECT 1
#指明连接是否被空闲连接回收器(如果有)进行检验，如果检测失败，则连接将被从池中去除
mybatis.testWhileIdle=true
#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，一般比minEvictableIdleTimeMillis小
mybatis.timeBetweenEvictionRunsMillis=300000
#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，最好和maxActive一致
mybatis.numTestsPerEvictionRun=50
#连接池中连接，在时间段内一直空闲，被逐出连接池的时间(1000*60*60 = 1 hour)，以毫秒为单位
mybatis.minEvictableIdleTimeMillis=3600000

在查询的时候，一直遇到如下的错误信息：

org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException:
### Error updating database.  Cause: java.lang.NullPointerException
### The error may involve com.spfood.basicservice.idgeneration.domain.IDGenIDInfo.updateByPrimaryKey-Inline
### The error occurred while setting parameters
### SQL: update IDGen_IDInfo     set Next_Value = ?,       Step_Length = ?     where Id_Type = ?
### Cause: java.lang.NullPointerException
	at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:76)
	at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:421)
	at com.sun.proxy.$Proxy19.update(Unknown Source)
	at org.mybatis.spring.SqlSessionTemplate.update(SqlSessionTemplate.java:270)
	at com.spfood.kernel.dao.impl.BaseDaoImpl.updateById(BaseDaoImpl.java:273)
	... 43 more
Caused by: org.apache.ibatis.exceptions.PersistenceException:
### Error updating database.  Cause: java.lang.NullPointerException
### The error may involve com.spfood.basicservice.idgeneration.domain.IDGenIDInfo.updateByPrimaryKey-Inline
### The error occurred while setting parameters
### SQL: update IDGen_IDInfo     set Next_Value = ?,       Step_Length = ?     where Id_Type = ?
### Cause: java.lang.NullPointerException
	at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:26)
	at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:154)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:408)
	... 46 more
Caused by: java.lang.NullPointerException
	at com.mysql.jdbc.StatementImpl$CancelTask.<init>(StatementImpl.java:86)
	at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1893)
	at com.mysql.jdbc.PreparedStatement.execute(PreparedStatement.java:1193)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at com.mysql.jdbc.MultiHostConnectionProxy$JdbcInterfaceProxy.invoke(MultiHostConnectionProxy.java:91)
	at com.sun.proxy.$Proxy27.execute(Unknown Source)
	at org.apache.commons.dbcp.DelegatingPreparedStatement.execute(DelegatingPreparedStatement.java:172)
	at org.apache.commons.dbcp.DelegatingPreparedStatement.execute(DelegatingPreparedStatement.java:172)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at org.apache.ibatis.logging.jdbc.PreparedStatementLogger.invoke(PreparedStatementLogger.java:62)
	at com.sun.proxy.$Proxy28.execute(Unknown Source)
	at org.apache.ibatis.executor.statement.PreparedStatementHandler.update(PreparedStatementHandler.java:44)
	at org.apache.ibatis.executor.statement.RoutingStatementHandler.update(RoutingStatementHandler.java:69)
	at org.apache.ibatis.executor.ReuseExecutor.doUpdate(ReuseExecutor.java:50)
	at org.apache.ibatis.executor.BaseExecutor.update(BaseExecutor.java:105)
	at org.apache.ibatis.executor.CachingExecutor.update(CachingExecutor.java:71)
	at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:152)
	... 51 more

2. 查找问题根源

可以看到，具体的错误来自PreparedStatement.java:1193 和 StatementImpl.java:86， 查找到这两个地方的代码如下：

StatementImpl.java:84-86行

            Properties props = StatementImpl.this.connection.getProperties();

            Enumeration<?> keys = props.propertyNames();

PreparedStatement.java:1192-1895

            if (locallyScopedConnection.getEnableQueryTimeouts() && this.timeoutInMillis != 0 && locallyScopedConnection.versionMeetsMinimum(5, 0, 0)) {
               timeoutTask = new CancelTask(this);
               locallyScopedConnection.getCancelTimer().schedule(timeoutTask, this.timeoutInMillis);
            }

Debug一下，可以看到，在StatementImpl.java中的第84行，getProperties()返回的值为null，跟踪进去，可以看到connection的具体类是FabricMySQLConnectionProxy，该类的getProperties方法实现如下：

    public Properties getProperties() {
        return null;
    }

这个错误就是在创建CancelTask的时候失败。关于CancelTask，网上的资料大把，请自行搜索。

3. 解决方法

那么，有两种方式解决这个问题。

3.1 修改myBatis配置

我们可以看到在 PreparedStatement.java:1192这一行中的条件，如果this.timeoutInMillis为0，那么就不会创建CancelTask。我们只需要在MyBatis的配置中将这个值设置为0就可以了。

<configuration>
    <settings>
        <setting name="cacheEnabled" value="true" />
        <setting name="lazyLoadingEnabled" value="false" />
        <setting name="multipleResultSetsEnabled" value="true" />
        <setting name="useColumnLabel" value="true" />
        <setting name="defaultExecutorType" value="REUSE" />
        <setting name="defaultStatementTimeout" value="0" />
        <setting name="logImpl" value="LOG4J"/>
    </settings>  

</configuration>

就是上面的配置中的defaultStatementTimeout这一项。

3.2 扩展MySQL Connector的代码

如果还想继续使用CancelTask的功能，那就只有自行扩展MySQL Connector的代码了。

首先想到的是修改FabricMySQLConnectionProxy。我们编写一个自己的ConnectionProxy，继承FabricMySQLConnectionProxy，覆盖它的getProperties和getCancelTimer方法就可以了，如下：

public class AfragFabricMySQLConnectionProxy extends FabricMySQLConnectionProxy
      implements FabricMySQLConnection, FabricMySQLConnectionProperties{
    /**
     *
     */
    private static final long serialVersionUID = 8626818655234189033L;

    private transient Timer cancelTimer;

    public AfragFabricMySQLConnectionProxy(Properties props) throws SQLException {
        super(props);
    }

    public Timer getCancelTimer() {
        synchronized (getConnectionMutex()) {
            if (this.cancelTimer == null) {
                boolean createdNamedTimer = false;
                // Use reflection magic to try this on JDK's 1.5 and newer, fallback to non-named timer on older VMs.
                try {
                    Constructor<Timer> ctr = Timer.class.getConstructor(new Class[] { String.class, Boolean.TYPE });
                    this.cancelTimer = ctr.newInstance(new Object[] { "MySQL Statement Cancellation Timer", Boolean.TRUE });
                    createdNamedTimer = true;
                } catch (Throwable t) {
                    createdNamedTimer = false;
                }
                if (!createdNamedTimer) {
                    this.cancelTimer = new Timer(true);
                }
            }
            return this.cancelTimer;
        }
    }  

    public Properties getProperties() {
        return new Properties();
    }
}

那么，怎么让driver知道要使用我们的ConnectionProxy呢？那就需要修改FabricMySQLDriver类了。同样的，我们创建自己的Driver类，继承FabricMySQLDriver类。如下,主要修改的地方有两个：

1. 修改connection方法，使其返回我们的AfragFabricMySQLConnectionProxy，而不是原来的FabricMySQLConnectionProxy。
2. 在类的静态块中，首先Deregister FabricMySQLDriver，同时注册自己。不然DriverManager在解析url的时候，还是会使用老的FabricMySQLDriver类。

这样修改后，应该没有问题了吧？但是有时候还是会报错，在跟踪一下，可以看到在connection方法中，有时候会返回JDBC4FabricMySQLConnectionProxy，该类继承了FabricMySQLConnectionProxy。因此，我们需要创建自己的JDBC4 ConnectionProxy，实现getCancelTimer和getProperties方法。

实现后的Driver类和JDBC4 ConnectionProxy类如下：

Driver类：

public class AfragFabricMySQLDriver extends FabricMySQLDriver{

    // Deregister FabricMySQLDriver and Register ourselves with the DriverManager
    static {
        try {
            deregisterFabricMySQLDriver();

            DriverManager.registerDriver(new AfragFabricMySQLDriver());
        } catch (SQLException ex) {
            throw new RuntimeException("Can't register driver", ex);
        }
    }

    public static void deregisterFabricMySQLDriver() throws SQLException{
        Enumeration<Driver> drivers = DriverManager.getDrivers();
        while (drivers.hasMoreElements()){
            Driver driver = drivers.nextElement();
            if (driver instanceof FabricMySQLDriver){
                DriverManager.deregisterDriver(driver);
            }
        }
    }
    /**
     * @throws SQLException
     */
    public AfragFabricMySQLDriver() throws SQLException {
        super();
    }

    public Connection connect(String url, Properties info) throws SQLException {
        Properties parsedProps = parseFabricURL(url, info);
        if (parsedProps == null) {
            return null;
        }
        parsedProps.setProperty(FABRIC_PROTOCOL_PROPERTY_KEY, "http");
        if (com.mysql.jdbc.Util.isJdbc4()) {
            try {
                Constructor<?> jdbc4proxy = Class.forName("personal.afrag.AfragJDBC4FabricMySQLConnectionProxy").getConstructor(
                        new Class[] { Properties.class });
                return (Connection) com.mysql.jdbc.Util.handleNewInstance(jdbc4proxy, new Object[] { parsedProps }, null);
            } catch (Exception e) {
                throw (SQLException) new SQLException(e.getMessage()).initCause(e);
            }
        }
        return new AfragFabricMySQLConnectionProxy(parsedProps);
    }

    Properties parseFabricURL(String url, Properties defaults) throws SQLException {
        if (!url.startsWith("jdbc:mysql:fabric://")) {
            return null;
        }
        // We have to fudge the URL here to get NonRegisteringDriver.parseURL() to parse it for us.
        // It actually checks the prefix and bails if it's not recognized.
        // jdbc:mysql:fabric:// => jdbc:mysql://
        return super.parseURL(url.replaceAll("fabric:", ""), defaults);
    }
}

JDBC4 Connection Proxy:

public class AfragJDBC4FabricMySQLConnectionProxy extends JDBC4FabricMySQLConnectionProxy implements JDBC4FabricMySQLConnection, FabricMySQLConnectionProperties {
    /**
     *
     */
    private static final long serialVersionUID = 6404998348296596764L;
    /**
     * @param props
     * @throws SQLException
     */
    public AfragJDBC4FabricMySQLConnectionProxy (Properties props) throws SQLException {
        super(props);
    }

    private transient Timer cancelTimer;
    public Timer getCancelTimer() {
        synchronized (getConnectionMutex()) {
            if (this.cancelTimer == null) {
                boolean createdNamedTimer = false;
                // Use reflection magic to try this on JDK's 1.5 and newer, fallback to non-named timer on older VMs.
                try {
                    Constructor<Timer> ctr = Timer.class.getConstructor(new Class[] { String.class, Boolean.TYPE });
                    this.cancelTimer = ctr.newInstance(new Object[] { "MySQL Statement Cancellation Timer", Boolean.TRUE });
                    createdNamedTimer = true;
                } catch (Throwable t) {
                    createdNamedTimer = false;
                }
                if (!createdNamedTimer) {
                    this.cancelTimer = new Timer(true);
                }
            }
            return this.cancelTimer;
        }
    }
}