setReadonly(true)

?概念：从这一点设置的时间点开始到这个事务结束的过程中，其他事务所提交的数据，该事务将看不见！（查询中不会出现别人在时间点a之后提交的数据）

应用场合：

如果你一次执行单条查询语句，则没有必要启用事务支持，数据库默认支持SQL执行期间的读一致性；?

如果你一次执行多条查询语句，例如统计查询，报表查询，在这种场景下，多条查询SQL必须保证整体的读一致性，否则，在前条SQL查询之后，后条SQL查询之前，数据被其他用户改变，则该次整体的统计查询将会出现读数据不一致的状态，此时，应该启用事务支持。

【注意是一次执行多次查询来统计某些信息，这时为了保证数据整体的一致性，要用只读事务】

怎样设置：

对于只读查询，可以指定事务类型为readonly，即只读事务。

由于只读事务不存在数据的修改，因此数据库将会为只读事务提供一些优化手段，例如Oracle对于只读事务，不启动回滚段，不记录回滚log。

?

?

当驱动加载完毕后，接下来就是怎样去获取一个连接

?

Drivermanager.getConnection(url,name,password)

?跟踪代码执行到以下一步

?

 private static Connection getConnection(
	String url, java.util.Properties info, ClassLoader callerCL) throws SQLException {
	java.util.Vector drivers = null;
        /*
	 * When callerCl is null, we should check the application's
	 * (which is invoking this class indirectly)
	 * classloader, so that the JDBC driver class outside rt.jar
	 * can be loaded from here.
	 */
	synchronized(DriverManager.class) {	 
	  // synchronize loading of the correct classloader.
	  if(callerCL == null) {
	      callerCL = Thread.currentThread().getContextClassLoader();
	   }    
	} 
	 
	if(url == null) {
	    throw new SQLException("The url cannot be null", "08001");
	}
    
	println("DriverManager.getConnection(\"" + url + "\")");
    
	if (!initialized) {
	    initialize();
	}

	synchronized (DriverManager.class){ 
            // use the readcopy of drivers
	    drivers = readDrivers;  
        }

	// Walk through the loaded drivers attempting to make a connection.
	// Remember the first exception that gets raised so we can reraise it.
	SQLException reason = null;
	for (int i = 0; i < drivers.size(); i++) {
	    DriverInfo di = (DriverInfo)drivers.elementAt(i);
      
	    // If the caller does not have permission to load the driver then 
	    // skip it.
	    if ( getCallerClass(callerCL, di.driverClassName ) != di.driverClass ) {
		println("    skipping: " + di);
		continue;
	    }
	    try {
		println("    trying " + di);
                //在注册的驱动中得到相应的连接
		Connection result = di.driver.connect(url, info);
		if (result != null) {
		    // Success!
		    println("getConnection returning " + di);
		    return (result);
		}
	    } catch (SQLException ex) {
		if (reason == null) {
		    reason = ex;
		}
	    }
	}
    
	// if we got here nobody could connect.
	if (reason != null)    {
	    println("getConnection failed: " + reason);
	    throw reason;
	}
    
	println("getConnection: no suitable driver found for "+ url);
	throw new SQLException("No suitable driver found for "+ url, "08001");
    }

?进入com.mysql.jdbc.Driver下connect(url,info)方法NonRegisteringDriver

?

?

public java.sql.Connection connect(String url, Properties info)
			throws SQLException {
		if (url != null) {
			if (StringUtils.startsWithIgnoreCase(url, LOADBALANCE_URL_PREFIX)) {//负载均衡的配置
				return connectLoadBalanced(url, info);
			} else if (StringUtils.startsWithIgnoreCase(url,
					REPLICATION_URL_PREFIX)) {//复制
				return connectReplicationConnection(url, info);
			}
		}

		Properties props = null;

		if ((props = parseURL(url, info)) == null) {
			return null;
		}

		if (!"1".equals(props.getProperty(NUM_HOSTS_PROPERTY_KEY))) {
			return connectFailover(url, info);
		}
		
		try {//初始化链接
			Connection newConn = com.mysql.jdbc.ConnectionImpl.getInstance(
					host(props), port(props), props, database(props), url);

			return newConn;
		} catch (SQLException sqlEx) {
			// Don't wrap SQLExceptions, throw
			// them un-changed.
			throw sqlEx;
		} catch (Exception ex) {
			SQLException sqlEx = SQLError.createSQLException(Messages
					.getString("NonRegisteringDriver.17") //$NON-NLS-1$
					+ ex.toString()
					+ Messages.getString("NonRegisteringDriver.18"), //$NON-NLS-1$
					SQLError.SQL_STAT