Java源码阅读

前言：

java源码阅读，看看是怎么组织起来表达逻辑的，菜鸟笔记。

1.connection接口作数据库连接对象

1.1一阵土话

类a中有接口A作属性Field，接口A中定义了抽象方法aM，规定了返回值是接口B

调用：类a对象.接口A对象(接口实例化).aM(接口方法)，返回值接口B的实现类对象(aM的返回值是对象)

1.2原java语句在这里：

        //获取数据库的连接对象
        Connection conn=DriverManager.getConnection
                ("jdbc:mysql://localhost:3306/db3","root","root");

1.3解读：DriverManager.getConnection ---DriverManager的静态方法getConnection

getConnection方法原型：

    public static Connection getConnection(String url,
        String user, String password) throws SQLException {
        java.util.Properties info = new java.util.Properties();
 
        if (user != null) {
            info.put("user", user);
        }
        if (password != null) {
            info.put("password", password);
        }
 
        return (getConnection(url, info, Reflection.getCallerClass()));
    }

查看return内容： return (getConnection(url, info, Reflection.getCallerClass()));

继续查看getConnection()方法

    private static Connection getConnection(
        String url, java.util.Properties info, Class caller) throws SQLException {
        /*
         * 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.
         */
        ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
        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 + "\")");
 
        // Walk through the loaded registeredDrivers attempting to make a connection.
        // Remember the first exception that gets raised so we can reraise it.
        SQLException reason = null;
 
        for(DriverInfo aDriver : registeredDrivers) {
            // If the caller does not have permission to load the driver then
            // skip it.
            if(isDriverAllowed(aDriver.driver, callerCL)) {
                try {
                    println("    trying " + aDriver.driver.getClass().getName());
                    Connection con = aDriver.driver.connect(url, info);
                    if (con != null) {
                        // Success!
                        println("getConnection returning " + aDriver.driver.getClass().getName());
                        return (con);
                    }
                } catch (SQLException ex) {
                    if (reason == null) {
                        reason = ex;
                    }
                }
 
            } else {
                println("    skipping: " + aDriver.getClass().getName());
            }
 
        }
 
        // 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");
    }

找到返回值项return(con);

       for(DriverInfo aDriver : registeredDrivers) {
            // If the caller does not have permission to load the driver then
            // skip it.
            if(isDriverAllowed(aDriver.driver, callerCL)) {
                try {
                    println("    trying " + aDriver.driver.getClass().getName());
                    Connection con = aDriver.driver.connect(url, info);
                    if (con != null) {
                        // Success!
                        println("getConnection returning " + aDriver.driver.getClass().getName());
                        return (con);
                    }
                } catch (SQLException ex) {
                    if (reason == null) {
                        reason = ex;
                    }
                }
 
            } else {
                println("    skipping: " + aDriver.getClass().getName());
            }
 
        }

其中

 Connection con = aDriver.driver.connect(url, info);

就是需要的结果，而这一段可以衔接上1.1，分析如下：

DriverInfo类，包含了接口Drive

class DriverInfo {
 
    final Driver driver;
    DriverAction da;
    DriverInfo(Driver driver, DriverAction action) {
        this.driver = driver;
        da = action;
    }

 接口里有抽象方法connect，返回值是接口对象connection

public interface Driver {
 
 
    Connection connect(String url, java.util.Properties info)
        throws SQLException;

aDriver是DriverInfo对象，driver是Driver接口类型的属性，connect是Driver接口方法

  for(DriverInfo aDriver : registeredDrivers) {
            // If the caller does not have permission to load the driver then
            // skip it.
            if(isDriverAllowed(aDriver.driver, callerCL)) {
                try {
                    println("    trying " + aDriver.driver.getClass().getName());
                    Connection con = aDriver.driver.connect(url, info);
                    if (con != null) {
                        // Success!
                        println("getConnection returning " + aDriver.driver.getClass().getName());
                        return (con);
                    }

如此可以返回一个Connection接口的实现类对象

小结：java里常用接口来实现目的，而不一定在意其具体实现类是什么。

            Java里数据传递基本是以对象进行的。