java注解实例-反射生成sql

 定义描述用户表的注解：

package dao;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

// 作用域
@Target({ ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
public @interface Table {
String value();
}

 定义描述用户属性的注解：

package dao;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

// 作用域
@Target({ ElementType.FIELD })
@Retention(RetentionPolicy.RUNTIME)
public @interface Column {
String value();
}

定义映射Bean类User：

package dao;

/**
*
* 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：
*
* @author  */
@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;
}
}

根据参数动态返回查询语句：

package dao;

import java.lang.reflect.Field;
import java.lang.reflect.Method;

/**
* 根据参数动态返回查询语句
*
* @author  */
public class ReturnQuery {

public static String query(User u1) {
StringBuilder str = new StringBuilder();
// 1.获取一个类class
Class c = u1.getClass();
// 2.获取Table的名字
boolean exists = c.isAnnotationPresent(Table.class);
if (!exists) {
return null;
}
Table t = (Table) c.getAnnotation(Table.class);
String tableName = t.value();
str.append("select * from ").append(tableName).append("where 1=1");
// 3.遍历所有的 字段
Field fArray[] = c.getDeclaredFields();
for (Field field : fArray) {
// 4.处理每个字段对应的sql
// 4.1取到字段名
boolean fExists = field.isAnnotationPresent(Column.class);// 判断是否包含Column类型的注解
if (!fExists) {
continue;
}
Column column = field.getAnnotation(Column.class);
String columnName = column.value();
// 4.2取到字段的值
String fieldName = field.getName();
// 获取相应字段的getXXX()方法
String getMethodName = "get" + fieldName.substring(0, 1).toUpperCase() + fieldName.substring(1);
Object fieldValue=null;
try {
Method getMethod = c.getMethod(getMethodName);
fieldValue = getMethod.invoke(u1);
} catch (Exception e) {
e.printStackTrace();
}
//4.3拼接Sql
if (fieldValue==null||fieldValue instanceof Integer &&(Integer)fieldValue==0) {
continue;
}
str.append(" and ").append(fieldName);
if (fieldValue instanceof String) {
if (((String) fieldValue).contains(",")) {
String[] values=((String) fieldValue).split(",");
str.append(" in (");
for (String s : values) {
str.append("'").append(s).append("'").append(",");
}
str.deleteCharAt(str.length()-1);
str.append(")");
}else{
str.append("=").append("'").append(fieldValue).append("' ");
}
}else {
str.append("=").append(fieldValue);
}
}
return str.toString();
}

}

测试类：

package dao;

public class Test {
public static void main(String[] args) {
User u1 = new User();
u1.setId(10); // 查询id

User u2 = new User();
u2.setUserName("JSFei"); // 模糊查询用户名
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);
String sql2 = ReturnQuery.query(u2);
String sql3 = ReturnQuery.query(u3);

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

输出结果：

...............................................