Reflection + Custom Annotation Implementation (Generation of Database Statements)

Because when writing custom annotations in the front, it is really difficult to write a demo. Of course, after learning some reflections, I will write a demo. Then take advantage of this weekend's free time, write an incoming Class, and automatically generate a database A demo of the statement

1. Let's use it first 

String userSql = SqlSentence.create ( User.class ) ; //The calling instance created by the database statement

结果：create table UserTable(id integer primary key autoincrement,userName varchar(8),info text,password varchar(16))

2. Look at the User class

public class User {

    @SqlField (length = 8 )
     private String userName ; //The generated data statement is userName varchar(8)
 @SqlField
 private int password ; //The generated data statement is password varchar(16) because the default length is 16
 @SqlField (type = "text" )
     private String info ; //The generated data statement is info text
 private int age ; //There is no declaration that @SqlField annotation does not generate
 private String sex ; //There is no declaration that @SqlField annotation does not generate
 public String getUserName() {
         return userName ;                    
    
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public int getPassword() {
        return password;
    }

    public void setPassword(int password) {
        this.password = password;
    }

    public String getInfo() {
        return info;
    }

    public void setInfo(String info) {
        this.info = info;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }
}

3. Look at the custom annotation class SqlField

/**
 * Configure database statement fields
 */
 @Retention (RetentionPolicy. RUNTIME )
 @Target ({ElementType. FIELD })
 public @ interface SqlField {

    /**
     * 字段的类型
     * @return varchar
     */
    String type() default "varchar";

    /**
     * 字段的长度
     * @return 16
     */
    int length() default 16;
}

4.看SqlSentence类

public class SqlSentence {

    private SqlSentence() { /* cannot be instantiated */ }

    private final static String VARCHAR = "varchar";

    /**
     * @param clz class
     * @return sqlite sentence
     */
    public static <T> String create(Class<T> clz) {
        if (clz == null) {
            return null;
        }
        try {
            Object instance = Class.forName(clz.getName()).newInstance();
            Class<?> aClass = instance.getClass();
            Field[] fields = aClass.getDeclaredFields();
            StringBuilder sb = new StringBuilder();
            for (Field field : fields) {
                field.setAccessible(true); 
                if (field.isAnnotationPresent(SqlField.class)) { //判断是否存在此注解
                    SqlField sqlField = field.getAnnotation(SqlField.class); //获取此注解
                    String fieldName = field.getName();
                    String type = sqlField.type();
                    int length = sqlField.length();
                    if (type.contains(VARCHAR)) {
                        sb.append(fieldName).append(" " + VARCHAR + "(").append(length).append("),");
                    } else { //用户自定义类型
                        sb.append(fieldName).append(" ").append(type).append(",");
                    }
                }
            }
            int length = sb.toString().length();
            if (length > 1) { //使用生成的字符串长度判断是否存在注解 
                sb = sb.deleteCharAt(sb.toString().length() - 1); //删除字符串的最后一个字符
                return "create table " + clz.getSimpleName() + "Table" + "(id integer primary key autoincrement," + sb + ")";
            } else {
                throw new NullPointerException("No @SqlField annotations are configured on a field");
            }
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        return null;
    }

}

 好了，一个简单的反射+自定义注解实现生成数据库语句的demo就ok了，如果对Android开发有所了解的人，可以看一看我在

Android系列中所写的Sqlite数据库SDK,这是一个完全运用到反射+自定义注解去写的一个小框架，当然如果在Android系列中

没有这篇文章，说明我暂时还木有更新上去。