注解

元注解

@Target   用来约束注解可以应用的地方

@Retention   用来约束注解的生命周期

@Documented   可以被javadoc等工具文档化@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Table {
    String name() default "";
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface PrimaryKey {
    String name() default "";
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Column {
    // 列名 默认为""
	String name() default "";

    // 长度 默认为255
    int length() default 255;

    // 是否为varchar 默认为true
    boolean varchar() default true;
    
	// 是否为空 默认可为空
    boolean isNull() default true;
}

@Table
public class Person {
    @PrimaryKey
    private int id;

    @Column(isNull = false, length = 20)
    private String username;

    public int getId() {
	    return id;
    }
	
    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }
	
    public void setUsername(String username) {
        this.username = username;
    }
}

注解与反射

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Insert {
    String value();
}

public interface UserMapper {
    @Insert("insert into user (name,pass) values (?,?) ")
    void addUser(String name, String pass);
}

// sql执行类
public class DaoOperatorFactory {
    public static Object handle(Method method, Object[] parameters) throws SQLException {
        String sql = null;
        if (method.isAnnotationPresent(Insert.class)) {
            sql = checkSql(method.getAnnotation(Insert.class).value(),
                    Insert.class.getSimpleName());
            insert(sql, parameters);
            return null;
        }
        return null;
    }

    private static void insert(String sql, Object[] parameters) throws SQLException {
        System.out.println("---sql---" + sql);
        for (int i = 0; parameters != null && i < parameters.length; i++) {
            System.out.println("---parameters---" + parameters[i]);
        }
        Connection connection = DBUtil.getConnection();
        PreparedStatement prepareStatement = connection.prepareStatement(sql);
	for (int i = 0; parameters != null && i < parameters.length; i++) {
		prepareStatement.setObject(i + 1, parameters[i]);
	}
	prepareStatement.execute();
	connection.close();
    }

    private static String checkSql(String sql, String type) throws SQLException {
        String type2 = sql.split(" ")[0];
        if (type2 == null || !type2.equalsIgnoreCase(type)) {
            throw new SQLException("Incorrect SQL.");
        }
        return sql;
    }
}

public class MethodProxy implements InvocationHandler{
    @Override
    public Object invoke(Object proxy, Method method, Object[] parameters) throws Throwable {
        return DaoOperatorFactory.handle(method, parameters);
    }
}

public class MethodProxyFactory {
    public static <T> T getBean(Class<T> clazz) {
        final MethodProxy methodProxy = new MethodProxy();
        return (T) Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(), new Class[]{clazz},
                methodProxy);
    }
}

通过反射获取注解信息，达到代理的目的