Java自定义注解入门举例

转载请注明出处：http://blog.csdn.net/Hello_Chillax/article/details/48805387

学Android也有一年多了，打算分出一半的精力来学习JavaWeb的开发。
JavaWeb中第一个接触的框架就是SpringMVC了，其中用到了“注解”这个东西。作为入门，自己写了个例子，记录一下。

我们打算写一个quary(Filter filter) 函数来根据model生成一个SQL语句。

首先，Filter.java

/** * Created by Xiao on 2015/9/29. */@Table("User")public class Filter {    @Column("name")    private String name;    @Column("pwd")    private String pwd;    @Column("age")    private String age;    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public String getPwd() {        return pwd;    }    public void setPwd(String pwd) {        this.pwd = pwd;    }    public String getAge() {        return age;    }    public void setAge(String age) {        this.age = age;    }}

接下来，Table.java && Column.java

/** * Created by Xiao on 2015/9/29. */@Target({ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@Inherited@Documentedpublic @interface Table {    String value();}

/**
* Created by Xiao on 2015/9/29.
*/
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface Column {
String value();
}

最后，测试类 Test.java

/** * Created by Xiao on 2015/9/29. */public class Test {    public static void main(String[] args) {        Filter filter = new Filter();        filter.setName("肖哥");        filter.setAge("23");        System.out.println(quary(filter));    }    public static String quary(Filter filter) {        StringBuilder sb = new StringBuilder();        Class c = filter.getClass();        if (!c.isAnnotationPresent(Table.class)) {            return null;        }        sb.append("select * from " + ((Table) c.getAnnotation(Table.class)).value()).append(" where ");        for (Field field : c.getDeclaredFields()) {            Column column = field.getDeclaredAnnotation(Column.class);            field.setAccessible(true);            try {                Object value = field.get(filter);                if (value == null || (value.getClass() == Integer.class && (Integer) value == 0)) continue;                sb.append(column.value()).append("=").append(field.get(filter)).append(" and ");            } catch (Exception e) {                e.printStackTrace();            }            field.setAccessible(false);        }        sb.replace(sb.lastIndexOf(" and "),sb.length(),"");        return sb.toString();    }}

输出结果：

select * from User where name=肖哥 and age=23

完成。

总结：这个小例子说明了如何自定义注解。以及提供了一个应用场景。
其中涉及到了Reflect的知识，还算比较有参考价值。