反射+自定义注解 实现（数据库语句的生成）

由于在前面写自定义注解的时候，实在不好写demo演示，当然也说了学完一些反射之后，再来写一个demo，那就趁着今天周末有空，写一个 传入Class,自动生成数据库语句的demo吧1.首先看使用吧 String userSql = SqlSentence.create(User.class); //数据库语句创建的调用实例

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

2.看User类public class User {@SqlField(length = 8)private String userName; //生成的数据语句是 userName varchar(8)@SqlFieldprivate int password; //生成的数据语句是 password varchar(16) 因为默认长度是16@SqlField(type = "text")private String info; //生成的数据语句是 info textprivate int age; //没有申明@SqlField注解 不生成private String sex; //没有申明@SqlField注解 不生成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.看自定义注解类SqlField/*** 配置数据库语句字段*/@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。