Java注解之实战

1.需求描述
有两张表(用户表和部门表)，字段包括用户ID、用户名、年龄、邮箱、部门领导和部门人数等字段，现在需要对每个字段或字段的组合条件进行检索，并打印SQL。

2.代码示例
(1).表定义

@Table("user")
public class User {
    @Column("id")
    private int id;

    @Column("user_name")
    private String userName;
、、、、、
    @Column("age")
    private int age;

    @Column("email")
    private String email;

    //get和set方法
}

@Table("department")
public class Department {
    @Column("id")
    private int id;

    @Column("name")
    private String name;

    @Column("leader_name")
    private String leader_name;

    @Column("amount")
    private int amount;

    //get和set方法
}

(2).自定义Table和Column注解

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

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

(3).测试代码

public class App {
    public static void main(String[] args) {
        //查询id为10的用户
        User query1 = new User();
        query1.setId(10);

        //查询name为lucy的用户
        User query2 = new User();
        query2.setUserName("lucy");
        query2.setAge(18);

        //查询邮箱为12******[e-mail]的用户
        User query3 = new User();
        query3.setEmail("12******[e-mail],188****[e-mail]");

        System.out.println(query(query1));
        System.out.println(query(query2));
        System.out.println(query(query3));

        Department department = new Department();
        department.setName("技术部");
        department.setAmount(10);
        System.out.println(query(department));
    }

    private static String query(Object object) {
        StringBuilder sb = new StringBuilder();
        //1.获取到class
        Class objectClass = object.getClass();
        //2.获取到table的名字
        boolean tableIsExists = objectClass.isAnnotationPresent(Table.class);
        if (!tableIsExists) {
            return null;
        }
        Table table = (Table) objectClass.getAnnotation(Table.class);
        String tableName = table.value();
        sb.append("select * from ").append(tableName).append(" where 1=1");
        //3.遍历所有字段,处理每个字段对应的sql
        Field[] fields = objectClass.getDeclaredFields();
        for (Field field : fields) {
            //4.1拿到字段名
            boolean fieldIsExists = field.isAnnotationPresent(Column.class);
            if (!fieldIsExists) {
                continue;
            }
            //4.2拿到字段的值
            String filedName = field.getName();
            String getMethodName = "get" + filedName.substring(0, 1).toUpperCase() + filedName.substring(1);
            Object fieldValue = null;
            try {
                Method getMethod = objectClass.getMethod(getMethodName);
                fieldValue = getMethod.invoke(object, null);
            } catch (Exception e) {
                e.printStackTrace();
            }
            //4.3拼装sql
            if (fieldValue == null || (fieldValue instanceof Integer && (Integer) fieldValue == 0)) {
                continue;
            }
            sb.append(" and ").append(filedName);
            if (fieldValue instanceof String) {
                if (((String) fieldValue).contains(",")) {
                    String[] values = ((String) fieldValue).split(",");
                    sb.append(" in(");
                    for (String v : values) {
                        sb.append("'").append(v).append("'").append(",");
                    }
                    sb.deleteCharAt(sb.length() - 1);
                    sb.append(")");
                } else {
                    sb.append("=").append("'").append(fieldValue).append("'");
                }
            } else if (fieldValue instanceof Integer) {
                sb.append("=").append(fieldValue);
            }
        }
        return sb.toString();
    }
}

select * from user where 1=1 and id=10
select * from user where 1=1 and userName='lucy' and age=18
select * from user where 1=1 and email in('12******[e-mail]','188****[e-mail]')
select * from department where 1=1 and name='技术部' and amount=10