Java注解(Annotation)详解(四)——注解反射生成SQL语句

时间:2022-10-11 20:15:22

(四)注解反射生成SQL语句

1.任务说明

        ①有一张用户表,字段包括:用户ID、用户名、昵称、年龄、性别、所在城市、邮箱、手机号;         ②使用java注解来对用户表的每个字段或字段的组合条件进行动态生成SQL查询语句。

2.代码实现

2.1 定义注解

定义一个描述用户表的注解:
package dao;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Table {

String value();
}

定义一个描述用户表属性字段的注解:
package dao;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Column {

String value();
}

2.2 定义映射到数据库的bean

用户表,字段包括:用户ID、用户名、昵称、年龄、性别、所在城市、邮箱、手机号:
package dao;

@Table("user")
public class User {

@Column("id")
private int id;

@Column("user_name")
private String userName;

@Column("nick_name")
private String nickName;

@Column("age")
private int age;

@Column("city")
private String city;

@Column("email")
private String email;

@Column("mobile")
private String mobile;

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;
}
public String getNickName() {
return nickName;
}
public void setNickName(String nickName) {
this.nickName = nickName;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getMobile() {
return mobile;
}
public void setMobile(String mobile) {
this.mobile = mobile;
}
}

2.3 返回SQL查询语句的实现

根据参数动态返回查询语句:
package dao;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class ReturnQuery {
public static String query(Object u){
StringBuilder sqlStrBuilder = new StringBuilder();
//1.获取到Class
Class c = u.getClass();
//判断是否包含Table类型的注解
if(!c.isAnnotationPresent(Table.class)){
return null;
}
//2.获取到table的名字
Table t = (Table) c.getAnnotation(Table.class);
String tableName = t.value();
//加入 where 1=1 ,是为了防止未来如果没有条件的情况下也不会报错
sqlStrBuilder.append("select * from ").append(tableName).append(" where 1=1");

Field[] fArray = c.getDeclaredFields(); //获取类属性的所有字段,
//3.遍历所有的字段
for (Field field : fArray) {
//4.处理每个字段对应的sql
//判断是否包含Column类型的注解
if(!field.isAnnotationPresent(Column.class)){
continue;
}
//4.1.拿到字段上面注解的值,即Column注解的值
Column column = field.getAnnotation(Column.class);
String columnName = column.value();
//4.2.拿到字段的名
String filedName = field.getName();
//获取相应字段的getXXX()方法
String getMethodName = "get" + filedName.substring(0, 1).toUpperCase()
+ filedName.substring(1);
//字段的值
Object fieldValue = null;//属性有int、String等,所以定义为Object类
try {
Method getMethod = c.getMethod(getMethodName);
fieldValue = getMethod.invoke(u);
} catch (Exception e) {
e.printStackTrace();
}

//4.3.拼接sql
if(fieldValue==null || (fieldValue instanceof Integer && (Integer)fieldValue==0)){
continue;
}
sqlStrBuilder.append(" and ").append(filedName);
if(fieldValue instanceof String){
if(((String)fieldValue).contains(",")){
String[] values = ((String)fieldValue).split(",");
sqlStrBuilder.append(" in(");
for (String v : values) {
sqlStrBuilder.append("'").append(v).append("'").append(",");
}
sqlStrBuilder.deleteCharAt(sqlStrBuilder.length()-1);
sqlStrBuilder.append(")");
}
else{
sqlStrBuilder.append("=").append("'").append(fieldValue).append("'");
}
}
else if(fieldValue instanceof Integer){
sqlStrBuilder.append("=").append(fieldValue);
}
}
return sqlStrBuilder.toString();

}
}

2.4 返回SQL语句的测试类

package dao;
public class Test {
public static void main(String[] args) {
User u1 = new User();
u1.setId(9); //查询id为9的用户

User u2 = new User();
u2.setUserName("JTZeng"); //模糊查询用户名为JTZeng的用户
u2.setAge(21);

User u3 = new User();
u3.setEmail("123@163.com,123@qq.com"); //查询邮箱有任意一个的用户

String sql1 = ReturnQuery.query(u1); //查询id为9的用户
String sql2 = ReturnQuery.query(u2);//查询用户名为JTZeng和年龄为21的用户
String sql3 = ReturnQuery.query(u3);//查询邮箱中有任意一个的用户

System.out.println(sql1);
System.out.println(sql2);
System.out.println(sql3);
}
}

输出结果如下:
Java注解(Annotation)详解(四)——注解反射生成SQL语句



Java注解(Annotation)详解(四)——注解反射生成SQL语句