使用Java注解实现拼接sql语句的功能

使用到的两个注解类：

<span style="font-size:18px;">package dao;

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

/**
 * Created by raid on 2016/5/14.
 */
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Table {
    String value();
}
</span>

package dao;

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

/**
 * Created by raid on 2016/5/14.
 */
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Column {
    String value();
}

package dao;

/**
 * Created by raid on 2016/5/14.
 */
@Table("user")
public class Filter {

    @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;
    }
}

demo类：

package dao;

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

/**
 * Created by raid on 2016/5/14.
 */
public class Test {

    public static void main(String[] args) {
        Filter f1 = new Filter();
        f1.setId(10);//查询ID为10的用户

        Filter f2 = new Filter();
        f2.setUserName("lucy");

        Filter f3 = new Filter();
        f3.setEmail("[e-mail],[e-mail],[e-mail]");//查询邮箱为其中任意一个

        String sql1 = query(f1);
        String sql2 = query(f2);
        String sql3 = query(f3);

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

    private static String query(Filter f) {
        StringBuilder sb = new StringBuilder();
        //1.获取到class
        Class c = f.getClass();
        //2.获取到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();

        sb.append("select * from ").append(tableName).append(" where 1=1");

        //2.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拿到字段名
            boolean fExists = field.isAnnotationPresent(Column.class);
            if (!fExists) {
                continue;
            }
            Column column = field.getAnnotation(Column.class);
            String columName = column.value();
            //4.2拿到字段值
            String fieldName = field.getName();
            String getMethodName = "get" + fieldName.substring(0,1).toUpperCase() + fieldName.substring(1);
            Object fieldValue = null;
            try {
                Method getMethod = c.getMethod(getMethodName);
                fieldValue = getMethod.invoke(f);
            } catch (Exception e) {
                e.printStackTrace();
            }
            //4.3拼装sql
            if (fieldValue == null || (fieldValue instanceof Integer && (Integer)fieldValue == 0)) {
                continue;
            }
            if (fieldValue instanceof String) {
                if (((String)fieldValue).contains(",")) {
                    String[] values = ((String)fieldValue).split(",");
                    fieldValue = " in(";
                    for (String v : values) {
                        fieldValue += "'" + v + "'" + ",";
                    }
                    fieldValue = ((String)fieldValue).substring(0, ((String) fieldValue).length()-1) + ")";
                } else {
                    fieldValue = "'" + fieldValue + "'";
                }

            }

            sb.append(" and ").append(fieldName).append("=").append(fieldValue);
        }

        return sb.toString();
    }

}