spring的设计思想

  // 1. 程序有强耦合关系: 使用mysql驱动的时候, 使用Driver类的对象
    @Test
    public void test1() {
        try {
            // 创建了一个Driver对象
            // 使用registerDriver来添加mysql驱动到驱动管理器中
            Driver driver = new com.mysql.jdbc.Driver();
            DriverManager.registerDriver(driver);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

   // 2. 把代码中的强关系, 转换为了弱关系
        String driver = "com.mysql.jdbc.Driver";
        try {
            Class.forName(driver);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

 /**
     * 通过配置文件来实现解耦
     */
    @Test
    public void test3() {
        // db.properties文件位置在src下
        // 编译完成后, src目录下的配置文件会拷贝一份到classes目录
        ClassLoader classLoader = TestMySQL.class.getClassLoader();
        InputStream resourceAsStream = classLoader.getResourceAsStream("db.properties");
        Properties properties = new Properties();
        try {
            properties.load(resourceAsStream);
            System.out.println(properties.get("driver"));
            String driver = (String)properties.get("driver");
            Class.forName(driver);
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

  // 使用bundle
    @Test
    public void test4() {
        // ResourceBundle是专门用来读取配置文件的工具类
        // bundle只能读取properties类型的文件, 读取的时候只需要文件名, 不需要后缀
        // bundle还提供了迭代的方法读取所有配置
        ResourceBundle db = ResourceBundle.getBundle("db");
        db.getString("driver");
        Enumeration<String> keys = db.getKeys();
        while(keys.hasMoreElements()) {
            String key = keys.nextElement();
            System.out.println(key +": " + db.getString(key));
        }
    }

public class BeanFactory {

    // 在bean的工厂中准备一个容器,将bean都反射后添加进来,随用随取
    private static Map<String,Object> beanMap = null;
    static {
        beanMap = new HashMap<>();
        ResourceBundle bean = ResourceBundle.getBundle("bean");
        Enumeration<String> keys = bean.getKeys();
        while(keys.hasMoreElements()) {
            String key = keys.nextElement();
            String classPath = bean.getString(key);
            try {
                Object object = Class.forName(classPath).newInstance();
                beanMap.put(key,object);
            } catch (InstantiationException | IllegalAccessException | ClassNotFoundException e) {
                e.printStackTrace();
            }
        }
    }
    /**
     * 使用Object的基础类来接受获取的对象
     * @param key
     * @return
     */
    public static Object getBean(String key) {
        Object o = null;
        ResourceBundle bean = ResourceBundle.getBundle("bean");
        String classPath = bean.getString(key);
        try {
            o = Class.forName(classPath).newInstance();
        } catch (InstantiationException | IllegalAccessException | ClassNotFoundException e) {
            e.printStackTrace();
        }
        return o;
    }

 /**
     * 使用泛型方法来获取对象(推荐)
     * @param key
     * @param clazz
     * @param <T>
     * @return
     */
    public static<T> T getBean(String key, Class<T> clazz) {
      return (T) beanMap.get(key);
    }

        UserService userService = BeanFactory.getBean("userService", UserService.class);
        userService.register(user);