Java 自定义注解及注解读取解析--模拟框架生成SQL语句
假设们使用一张简单的表,结构如下:
定义注解:
表注解:
package com.xzlf.annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* 对象对应表名注解
* @author xzlf
*
*/
@Target(value = {ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface StudentTable {
String value();
}
字段注解:
package com.xzlf.annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* 对象属性对应表字段注解
* @author xzlf
*
*/
@Target(value = {ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface StudentField {
String columnName();
String type();
int length();
}
写个对象类并使用自动以注解:
package com.xzlf.annotation;
@StudentTable(value="tb_student")
public class Student {
@StudentField(columnName = "id", type = "int", length = 10)
private int id;
@StudentField(columnName = "sname", type = "varchar", length = 10)
private String sname;
@StudentField(columnName = "age", type = "int", length = 3)
private int age;
public Student() {
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getSname() {
return sname;
}
public void setSname(String sname) {
this.sname = sname;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}
使用反射读取注解并生成SQL语句:
package com.xzlf.annotation;
import java.lang.reflect.Field;
/**
* 使用反射读取注解的信息,模拟框架生成SQL语句
*
* @author xzlf
*
*/
public class AnnotationApp {
public static void main(String[] args) {
// 需要拼接的SQL语句
StringBuilder sb = new StringBuilder("create table ");
try {
Class<Student> clz = (Class<Student>) Class.forName("com.xzlf.annotation.Student");
// 获取类的指定信息
StudentTable stable = clz.getAnnotation(StudentTable.class);
String tableName = stable.value();
sb.append(tableName);
// 获取类属性注解
Field[] declaredFields = clz.getDeclaredFields();
sb.append("(");
for (Field field : declaredFields) {
// 获取类属性注解
StudentField annotation = field.getAnnotation(StudentField.class);
String fieldSQLStr = annotation.columnName() + " "
+ annotation.type() + "("+annotation.length()+"),";
//System.out.println(fieldSQLStr);
sb.append(fieldSQLStr);
}
sb.setCharAt(sb.length() - 1, ')');
System.out.println(sb);
} catch (Exception e) {
e.printStackTrace();
}
}
}
运行测试:
SQL语句生成了。
重视基础,才能走的更远。