Mysql JDBC驱动源码分析(获取连接)二

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_STATE_UNABLE_TO_CONNECT_TO_DATASOURCE, null);
			
			sqlEx.initCause(ex);
			
			throw sqlEx;
		}
	}

 

 对com.mysql.jdbc.JDBC4Connection的初始化

 

protected static Connection getInstance(String hostToConnectTo,
			int portToConnectTo, Properties info, String databaseToConnectTo,
			String url) throws SQLException {
		if (!Util.isJdbc4()) {
			return new ConnectionImpl(hostToConnectTo, portToConnectTo, info,
					databaseToConnectTo, url);
		}
               //初始化
		return (Connection) Util.handleNewInstance(JDBC_4_CONNECTION_CTOR,
				new Object[] {
							hostToConnectTo, Integer.valueOf(portToConnectTo), info,
							databaseToConnectTo, url }, null);
	}

 进入Jdbc4Connection的代码跟踪

 

 

protected ConnectionImpl(String hostToConnectTo, int portToConnectTo, Properties info,
			String databaseToConnectTo, String url)
			throws SQLException {
		this.charsetToNumBytesMap = new HashMap();
	
		this.connectionCreationTimeMillis = System.currentTimeMillis();
		this.pointOfOrigin = new Throwable();

      if (databaseToConnectTo == null) {
			databaseToConnectTo = "";
		}

		// Stash away for later, used to clone this connection for Statement.cancel
		// and Statement.setQueryTimeout().
		//
		
		this.origHostToConnectTo = hostToConnectTo;
		this.origPortToConnectTo = portToConnectTo;
		this.origDatabaseToConnectTo = databaseToConnectTo;

		try {
			Blob.class.getMethod("truncate", new Class[] {Long.TYPE});
			
			this.isRunningOnJDK13 = false;
		} catch (NoSuchMethodException nsme) {
			this.isRunningOnJDK13 = true;
		}
		
		this.sessionCalendar = new GregorianCalendar();
		this.utcCalendar = new GregorianCalendar();
		this.utcCalendar.setTimeZone(TimeZone.getTimeZone("GMT"));

		this.log = LogFactory.getLogger(getLogger(), LOGGER_INSTANCE_NAME, getExceptionInterceptor());

		// We store this per-connection, due to static synchronization
		// issues in Java's built-in TimeZone class...
		this.defaultTimeZone = Util.getDefaultTimeZone();
		
		if ("GMT".equalsIgnoreCase(this.defaultTimeZone.getID())) {
			this.isClientTzUTC = true;
		} else {
			this.isClientTzUTC = false;
		}

		this.openStatements = new HashMap();
		
		if (NonRegisteringDriver.isHostPropertiesList(hostToConnectTo)) {
			Properties hostSpecificProps = NonRegisteringDriver.expandHostKeyValues(hostToConnectTo);
			
			Enumeration<?> propertyNames = hostSpecificProps.propertyNames();
			
			while (propertyNames.hasMoreElements()) {
				String propertyName = propertyNames.nextElement().toString();
				String propertyValue = hostSpecificProps.getProperty(propertyName);
				
				info.setProperty(propertyName, propertyValue);
			}
		} else {
		
			if (hostToConnectTo == null) {
				this.host = "localhost";
				this.hostPortPair = this.host + ":" + portToConnectTo;
			} else {
				this.host = hostToConnectTo;
				
				if (hostToConnectTo.indexOf(":") == -1) {
					this.hostPortPair = this.host + ":" + portToConnectTo;
				} else {
					this.hostPortPair = this.host;
				}
			}
		}

		this.port = portToConnectTo;

		this.database = databaseToConnectTo;
		this.myURL = url;
		this.user = info.getProperty(NonRegisteringDriver.USER_PROPERTY_KEY);
		this.password = info
				.getProperty(NonRegisteringDriver.PASSWORD_PROPERTY_KEY);

		if ((this.user == null) || this.user.equals("")) {
			this.user = "";
		}

		if (this.password == null) {
			this.password = "";
		}

		this.props = info;
		
		
		
		initializeDriverProperties(info);
		
		
		try {   //获取数据库的信息
			this.dbmd = getMetaData(false, false);
			initializeSafeStatementInterceptors();
                        //======================================创建远程IO连接
			createNewIO(false);
			unSafeStatementInterceptors();
		} catch (SQLException ex) {
			cleanup(ex);

			// don't clobber SQL exceptions
			throw ex;
		} catch (Exception ex) {
			cleanup(ex);

			StringBuffer mesg = new StringBuffer(128);

			if (!getParanoid()) {
				mesg.append("Cannot connect to MySQL server on ");
				mesg.append(this.host);
				mesg.append(":");
				mesg.append(this.port);
				mesg.append(".\n\n");
				mesg.append("Make sure that there is a MySQL server ");
				mesg.append("running on the machine/port you are trying ");
				mesg
						.append("to connect to and that the machine this software is "
								+ "running on ");
				mesg.append("is able to connect to this host/port "
						+ "(i.e. not firewalled). ");
				mesg
						.append("Also make sure that the server has not been started "
								+ "with the --skip-networking ");
				mesg.append("flag.\n\n");
			} else {
				mesg.append("Unable to connect to database.");
			}

			SQLException sqlEx = SQLError.createSQLException(mesg.toString(),
					SQLError.SQL_STATE_COMMUNICATION_LINK_FAILURE, getExceptionInterceptor());
			
			sqlEx.initCause(ex);
			
			throw sqlEx;
		}
	}