[Java知识库]Mybatis连接数据库部分源码分析

1、JDBC连接数据库的四部曲

 public static void main(String[] args) throws Exception {
        //1. 加载驱动
        Class.forName("com.mysql.jdbc.Driver");

        String url = "jdbc:mysql://localhost:3306/test";
        String user = "root";
        String password = "123456";

        //2. 获取连接
        Connection connection = DriverManager.getConnection(url,user,password);

        String sql = " select id,name from user  ";

        Statement statement = connection.createStatement();

     	//3. 执行sql
        ResultSet rs = statement.executeQuery(sql);
		//4. 解析结果
        while(rs.next()){
            int id = rs.getInt(1);
            String name = rs.getString(2);
            System.out.println("id:"+id+",name:"+name);
        }

        rs.close();
        statement.close();
        connection.close();


    }

2、mybatis连接数据库源码分析

mybatis基于jdbc，本质上不过是对jdbc进行封装，这里分析一下源码是如何连接到数据库的，即对应jdbc的加载驱动和获取连接；

   public static void main(String[] args) throws InterruptedException {
         Reader reader = Resources.getResourceAsReader("config/mybatis-config.xml");
         SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
       	 SqlSession sqlSession = sqlSessionFactory.openSession();
    }

这里简单测试获取到sqlSession，拿到它就可以进行数据库操作。

调试时注意在第2行打上断点，一路debug下去发现build方法主要是在解析mybatis的xml文件，当解析执行到到org.apache.ibatis.builder.xml.XMLConfigBuilder#environmentsElement要注意，下图284行是解析数据源配置，

跟踪进入org.apache.ibatis.builder.xml.XMLConfigBuilder#dataSourceElement，注意330行

此处明显是用了反射来实例化对象，这里直接在idea调试工具中看不出来resolveClass后得到的是什么类型，此处可以用idea自带的调试工具打印出来看看就知道了，如下图：

打印结果：class org.apache.ibatis.datasource.pooled.PooledDataSourceFactory，此处的PooledDataSourceFactory对应配置文件中的type=“pooled”

<dataSource type="pooled">
    <!-- 配置与数据库交互的4个必要属性 -->
    <property name="driver" value="${oracle.driver}"/>
    <property name="url" value="${oracle.url}"/>
    <property name="username" value="${oracle.username}"/>
    <property name="password" value="${oracle.password}"/>
</dataSource>

知道反射结果的类型后，直接找到这个类，在他的构造函数打个断点，注意public class PooledDataSourceFactory extends UnpooledDataSourceFactory，构造函数执行时需要先调用其父类的构造方法，所以我们要在其父类构造方法也打断点观察，

其父类构造函数

public UnpooledDataSourceFactory() {
  this.dataSource = new UnpooledDataSource();
}

调用org.apache.ibatis.datasource.unpooled.UnpooledDataSource的构造方法，所以进入UnpooledDataSource类，此类就比较重点了，它有个静态代码块：

  static {
    Enumeration<Driver> drivers = DriverManager.getDrivers();
    while (drivers.hasMoreElements()) {
      Driver driver = drivers.nextElement();
      registeredDrivers.put(driver.getClass().getName(), driver);
    }
  }

静态代码快有个DriverManager.getDrivers();，这里就是加载驱动，此函数中的操作就类似我们在jdbc中的Class.forName(xxxx)，跟踪进入java.sql.DriverManager看看，此类也有个静态代码块：

   /**
     * Load the initial JDBC drivers by checking the System property
     * jdbc.properties and then use the {@code ServiceLoader} mechanism
     */
    static {
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }

重点来了，有个loadInitialDrivers()，这里就是经典的java的spi实现，第20行会读取mysql驱动包下的META-INF/services/java.sql.Driver文件，此文件中只有一行com.mysql.cj.jdbc.Driver，代表java.sql.Driver接口的实现类是com.mysql.cj.jdbc.Driver，此处需要了解java的SPI机制

 private static void loadInitialDrivers() {
        String drivers;
        try {
            drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
                public String run() {
                    return System.getProperty("jdbc.drivers");
                }
            });
        } catch (Exception ex) {
            drivers = null;
        }
        // If the driver is packaged as a Service Provider, load it.
        // Get all the drivers through the classloader
        // exposed as a java.sql.Driver.class service.
        // ServiceLoader.load() replaces the sun.misc.Providers()

        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {

                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                Iterator<Driver> driversIterator = loadedDrivers.iterator();

                /* Load these drivers, so that they can be instantiated.
                 * It may be the case that the driver class may not be there
                 * i.e. there may be a packaged driver with the service class
                 * as implementation of java.sql.Driver but the actual class
                 * may be missing. In that case a java.util.ServiceConfigurationError
                 * will be thrown at runtime by the VM trying to locate
                 * and load the service.
                 *
                 * Adding a try catch block to catch those runtime errors
                 * if driver not available in classpath but it's
                 * packaged as service and that service is there in classpath.
                 */
                try{
                    while(driversIterator.hasNext()) {
                        //进入此方法继续跟踪
                        driversIterator.next();
                    }
                } catch(Throwable t) {
                // Do nothing
                }
                return null;
            }
        });

继续跟踪进入driversIterator.next();会到达java.util.Iterator#next，最后肯定会到java.util.ServiceLoader.LazyIterator#nextService，如下代码，第8行调用了反射来加载之前读取到的com.mysql.cj.jdbc.Driver，这里就完成了SPI,就加载了驱动

  private S nextService() {
            if (!hasNextService())
                throw new NoSuchElementException();
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {
                c = Class.forName(cn, false, loader);
            } catch (ClassNotFoundException x) {
                fail(service,
                     "Provider " + cn + " not found");
            }
            if (!service.isAssignableFrom(c)) {
                fail(service,
                     "Provider " + cn  + " not a subtype");
            }
            try {
                S p = service.cast(c.newInstance());
                providers.put(cn, p);
                return p;
            } catch (Throwable x) {
                fail(service,
                     "Provider " + cn + " could not be instantiated",
                     x);
            }
            throw new Error();          // This cannot happen
        }

加载驱动后，需要注册驱动，加载com.mysql.cj.jdbc.Driver同样会执行静态代码块，同时完成驱动注册

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

后面不再分析了。。。。