适配器设计模式实战2

一.目的

　　结合源码来深入理解适配器模式，本文通过java.sql的源码来讲解适配器模式的思想

　　适配器模式介绍可参考    适配器模式实战1

二.业务场景介绍

　　在编写服务端代码时，我们需要使用数据库，而数据库种类繁多，最常用的关系型数据库mysql,oracle等，不同的数据库由不同的组织和公司开发，所以api不可能完全做的一样，而jdk为了适配不同类型的数据库，提供了Driver接口和DriverManager类，为我们执行数据库业务提供了同一的接口.

三.角色划分

 

 

 

四.具体代码

java.sql.Driver代码

public interface Driver {

 
    Connection connect(String url, java.util.Properties info)
        throws SQLException;

    boolean acceptsURL(String url) throws SQLException;

    DriverPropertyInfo[] getPropertyInfo(String url, java.util.Properties info)
                         throws SQLException;

    int getMajorVersion();

    int getMinorVersion();

    boolean jdbcCompliant();

    public Logger getParentLogger() throws SQLFeatureNotSupportedException;
}

　　

com.mysql.jdbc.NonRegisteringDriver 代码

public class NonRegisteringDriver implements Driver{
    //...
    public Connection connect(String url, Properties info) throws SQLException {
        if (url != null) {
            if (StringUtils.startsWithIgnoreCase(url, "jdbc:mysql:loadbalance://")) {
                return this.connectLoadBalanced(url, info);
            }

            if (StringUtils.startsWithIgnoreCase(url, "jdbc:mysql:replication://")) {
                return this.connectReplicationConnection(url, info);
            }
        }

        Properties props = null;
        if ((props = this.parseURL(url, info)) == null) {
            return null;
        } else if (!"1".equals(props.getProperty("NUM_HOSTS"))) {
            return this.connectFailover(url, info);
        } else {
            try {
                com.mysql.jdbc.Connection newConn = ConnectionImpl.getInstance(this.host(props), this.port(props), props, this.database(props), url);
                return newConn;
            } catch (SQLException var6) {
                throw var6;
            } catch (Exception var7) {
                SQLException sqlEx = SQLError.createSQLException(Messages.getString("NonRegisteringDriver.17") + var7.toString() + Messages.getString("NonRegisteringDriver.18"), "08001", (ExceptionInterceptor)null);
                sqlEx.initCause(var7);
                throw sqlEx;
            }
        }
    }
//...
}

　　

com.mysql.jdbc.Driver 

 

 

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    public Driver() throws SQLException {
    }

    static {
        try {
            DriverManager.registerDriver(new Driver());
        } catch (SQLException var1) {
            throw new RuntimeException("Can't register driver!");
        }
    }
}

 

　　

当调用Class.forName("com.mysql.jdbc.Driver") 时，该驱动会被注册到DriverManager里

OracleDriver 

public class OracleDriver implements Driver{
//...
    public Connection connect(String var1, Properties var2) throws SQLException{
    //...
}
//...
}

　　

public interface Connection  extends Wrapper, AutoCloseable{
    Statement createStatement() throws SQLException;
    PreparedStatement prepareStatement(String sql) throws SQLException;
    CallableStatement prepareCall(String sql) throws SQLException;
    String nativeSQL(String sql) throws SQLException;
    void setAutoCommit(boolean autoCommit) throws SQLException;
    boolean getAutoCommit() throws SQLException;
    void commit() throws SQLException;
    void rollback() throws SQLException;
    void close() throws SQLException;
    boolean isClosed() throws SQLException;
//.....
}

　　

com.mysql.jdbc.Connection 

public interface Connection extends java.sql.Connection, ConnectionProperties{
//....
}

　　

oracle.jdbc.OracleConnection 

public interface OracleConnection extends Connection{
//...
}

　　

oracle.jdbc.OracleConnectionWrapper 

public class OracleConnectionWrapper implements OracleConnection{
//...
}

　　

五.具体调用

        //首先建立驱动 oracle
        Class.forName("oracle.jdbc.driver.OracleDriver");

        //mysql
        Class.forName("com.mysql.jdbc.driver");
　　　　　//数据库不同，接口都通用一个
        //驱动成功后进行连接
        conn=DriverManager.getConnection(url, user, password);    

        //准备SQL语句
        PreparedStatement pStatement = connection.prepareStatement(query);
	//执行SQL语句
       ResultSet resultSet = pStatement.executeQuery();

　　六.总结

　　由此可见，jdbc采用了接口适配的思想，为我们调用数据库提供了同一接口，而不用管具体不同类型数据库的内部实现，既方便了调用，也方便了用配置文档类配置数据库相关信息