大家好，又见面了，我是你们的朋友全栈君。

用过spring的，应该都知道@transactional这个注解。这个注解给我们日常开发带来了很大便利，让我们无需为了数据的提交和回滚操心，只需要关注自己的业务逻辑即可。这里我模拟一下spring的注解是如何执行的。

这个是项目结构

自定义事务注解[通俗易懂]

首先我们需要一个数据库的连接

为了保证事务的完整性，这里我们用ThreadLocal保存当前线程的数据库连接

/**
 * Created by zhoum on 2019-06-28.
 */
public class ConnectionUtils {
    private static DruidDataSource dataSource = new DruidDataSource();
    private static ThreadLocal<Connection> threadLocal = new ThreadLocal<>();

    //静态代码块,设置连接数据库的参数
    static {
        dataSource.setDriverClassName("com.mysql.jdbc.Driver");
        dataSource.setUrl("jdbc:mysql://localhost:3306/web");
        dataSource.setUsername("root");
        dataSource.setPassword("root");
    }


    public static Connection getConection() throws SQLException {
        Connection conn = threadLocal.get();
        if (conn == null) {
            conn = dataSource.getConnection();
            threadLocal.set(conn);
        }
        return conn;
    }
}

说到自定义事务注解，肯定是要用自己写的注解

新建一个自己的事务注解

/**
 * Created by zhoum on 2019-06-28.
 */
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Transactional {

}

接下来，新建一个业务类，也就是我们常常写service

我在类上打上刚刚新建的自定义事务注解

/**
 * Created by zhoum on 2019-06-28.
 */
@Transactional
public class UserServiceImpl implements UserService {
    @Override
    public void add(User user) throws SQLException {
        Connection conn = ConnectionUtils.getConection();
        PreparedStatement preparedStatement = conn.prepareStatement("insert into USER (name,age)" +
                " VALUES (?,?)");
        preparedStatement.setObject(1, user.getName());
        preparedStatement.setObject(2, user.getAge());
        preparedStatement.execute();
        preparedStatement = conn.prepareStatement("insert into USER (name,age)" +
                " VALUES (?,?)");
        preparedStatement.setObject(1, user.getName());
        preparedStatement.setObject(2, user.getAge());
        preparedStatement.execute();
    }
}

接下来新建一个bean的工厂类，这就像我们找spring要bean一样，我们在xml或是注解上定义这个bean的基本信息，spring为我们创建这些bean并保存，当我需要时，只要打上autowire就可以拿到了

/**
 * Created by zhoum on 2019-07-02.
 */
public class BeanFactory {
    public static Object getBean(Class clazz) throws IllegalAccessException, InstantiationException, SQLException, NoSuchMethodException, InvocationTargetException {
        Object o = clazz.newInstance();
        if (o.getClass().isAnnotationPresent(Transactional.class)) {
            Connection conection = ConnectionUtils.getConection();
            ProxyBean proxyBean = new ProxyBean(o, conection);
            return proxyBean.getBean();
        }
        return o;
    }
}

代码中有一行 ProxyBean ，这个类是专门用来获取代理对象的。

target是当前目标执行类，connection是当前数据库连接

我们将UserServiceImpl进行代理，将connection开启事务，提交，回滚的代码把代理对象的add方法夹住


/**
 * Created by zhoum on 2019-07-02.
 */
public class ProxyBean {

    private Object target;

    private Connection connection;

    public ProxyBean(Object target, Connection connection) {
        this.target = target;
        this.connection = connection;
    }

    public Object getBean() throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
        Object o = Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                connection.setAutoCommit(false);
                try {
                    method.invoke(target, args);
                    connection.commit();
                } catch (Exception e) {
                    connection.rollback();
                } finally {
                    connection.close();
                }
                return null;
            }
        });
        return o;
    }
}

最后一步就是测试


/**
 * Created by zhoum on 2019-06-28.
 */
public class Main {
    public static void main(String[] args) throws SQLException, InvocationTargetException, NoSuchMethodException, InstantiationException, IllegalAccessException {
        UserService bean = (UserService) BeanFactory.getBean(UserServiceImpl.class);
        User user = new User();
        user.setName("伟哥");
        user.setAge(1);
        bean.add(user);
    }
}