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再说JDBC

时间:2021-07-01 10:21:17 帮助过:39人阅读

上篇文章《再说Java EE》说明了一下什么是规范,有什么作用,这篇文章来细说一下JDBC。 JDBC JDBC(Java Database Connection)也是Java EE中的一个规范,所谓规范是一组接口,如JDBC接口包含在java.sql及javax.sql包中,其中java.sql属于JavaSE,javax.sql

上篇文章《再说Java EE》说明了一下什么是规范,有什么作用,这篇文章来细说一下JDBC。

JDBC

JDBC(Java Database Connection)也是Java EE中的一个规范,所谓规范是一组接口,如JDBC接口包含在java.sql及javax.sql包中,其中java.sql属于JavaSE,javax.sql属于JavaEE,部分如下图:

\

以上来自jdk中的src/java/sql。

因为提倡面向接口编程,所以建议仅使用JDBC规范中的类,规范与实现的关系如下:

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使用

核心API

JDBC中核心的API有: DriverManager:工厂类,用来生产Driver对象Driver:驱动程序对象的接口Connection:数据库连接对象Statement:执行静态的SQL语句的接口Resultset:结果集对象的接口

操作流程

加载数据库驱动创建数据库连接执行SQL语句,得到结果集对结果集进行CRUD处理释放资源

如图:

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源码分析

java.sql下有48个类,javax.sql下有45个类,展开分析不太现实,本文仅分析两个类,DriverManager和Driver。不知大家注意过这个问题没有,JDBC是接口,数据库驱动是实现,那么你编写的项目是如何找到实现的呢?

控制台输出

为了可以看到驱动加载过程中输出的日志,在加载驱动Class.forName("com.mysql.jdbc.Driver")之前,加上一句:

    DriverManager.setLogWriter(new java.io.PrintWriter(System.out));
即可在控制台中看到输出。

加载驱动

驱动使用很简单,将数据库驱动放到项目的lib中,在代码中写入:

    Class.forName("com.mysql.jdbc.Driver");
如果使用框架,如Hebernate配置文件中写入:
    com.mysql.jdbc.Driver
很明显,这两种方式都是使用反射加载驱动程序,我们来看一下驱动程序Driver的源代码,以mysql-connector-java-3.1.13为例:
package com.mysql.jdbc;

import java.sql.SQLException;

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
	//
	// Register ourselves with the DriverManager
	//
	static {
		try {
			java.sql.DriverManager.registerDriver(new Driver());
		} catch (SQLException E) {
			throw new RuntimeException("Can't register driver!");
		}
	}

	/**
	 * Construct a new driver and register it with DriverManager
	 * @throws SQLException
	 *             if a database error occurs.
	 */
	public Driver() throws SQLException {
		// Required for Class.forName().newInstance()
	}
}
核心代码就是那段静态代码块(static{}的意思是在类加载时执行一次,并且仅此一次),可以看到静态代码断的意思是将此Driver类实例化后注册到JDBC的java.sql.DriverManager类中,所以再来看一下JDBC的DriverManager.registerDriver:
/**
     * Registers the given driver with the DriverManager.
     * A newly-loaded driver class should call
     * the method registerDriver to make itself
     * known to the DriverManager.
     *
     * @param driver the new JDBC Driver that is to be registered with the
     *               DriverManager
     * @exception SQLException if a database access error occurs
     */
    public static synchronized void registerDriver(java.sql.Driver driver)
	throws SQLException {
	if (!initialized) {
	    initialize();
	}
      
	DriverInfo di = new DriverInfo();

	di.driver = driver;
	di.driverClass = driver.getClass();
	di.driverClassName = di.driverClass.getName();

	// Not Required -- drivers.addElement(di);

	writeDrivers.addElement(di); 
	println("registerDriver: " + di);
	
	/* update the read copy of drivers vector */
	readDrivers = (java.util.Vector) writeDrivers.clone();

    }
即可将com.mysql.jdbc.Driver添加到DriverManager的成员变量readDrivers中,以后获取数据库连接需要这个变量的帮助。

看上面的代码发现,还调用了initialize(),查看initialize()的源码看到它调用loadInitialDrivers(),这个函数的主要作用是加载JDBC默认驱动,registerDriver执行完,控制台的输出语句为:

JdbcOdbcDriver class loaded
registerDriver: driver[className=sun.jdbc.odbc.JdbcOdbcDriver,sun.jdbc.odbc.JdbcOdbcDriver@134e4fb]
DriverManager.initialize: jdbc.drivers = null
JDBC DriverManager initialized
registerDriver: driver[className=com.mysql.jdbc.Driver,com.mysql.jdbc.Driver@157c2bd]
可以看到先加载JdbcOdbcDriver,再加载我们加入的MySQL的driver。

获取链接

加载驱动完毕后,如何获取连接,继续看DriverManager的getConnection():
    //  Worker method called by the public getConnection() methods.
    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);
		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");
    }
这个函数代码比较多,但是我们关注的核心代码就一句:
    Connection result = di.driver.connect(url, info);

其中di就是我们前面加载驱动后DriverManager的成员变量readDrivers包含的一个对象,也就是调用com.mysql.jdbc.driver的connect函数,但是从上面该类代码可知,它只包含一个构造函数和静态代码段,connect函数从何而来?

别忘了com.mysql.jdbc.driver继承自NonRegisteringDriver,这也是MySQL驱动下的一个类,进入该类,找到connect函数:

package com.mysql.jdbc;
/***省略引用和注释***/
public class NonRegisteringDriver implements java.sql.Driver {
	
	/***省略其他函数和注释***/
	public java.sql.Connection connect(String url, Properties info)
			throws SQLException {
		Properties props = null;

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

		try {
			Connection newConn = new com.mysql.jdbc.Connection(host(props),
					port(props), props, database(props), url, this);

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

}

因为NonRegisteringDriver也是java.sql.Driver的实现,返回的也是JDBC中Connection的实现,所以如上面向接口编程,即可从DriverManager中得到MySQL的Connection。

总结

JDBC的分析介绍到此结束,如果有兴趣大家可以看一下其他数据库驱动的源码,因为都是根据JDBC而来,所以大都大同小异。

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