RetentionPolicy.RUNTIME Java运用注解处理器接口注解生成SQL表

文章插图
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();}2.创建Column字段注解，用于标明该类中属性与数据库表字段的映射
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();}3.创建实体类，并使用注解Table、Column
/** * 数据库 user表，对应表字段为 userid/username/age/addredd */@Table("user")public class User { @Column("userid") private String userid; @Column("username") private String username; @Column("age") private int age; @Column("sl") private double sl; @Column("address") private String address; // 添加一个不是数据库的字段: 用于测试 private String noField; public double getSl() { return sl; } public void setSl(double sl) { this.sl = sl; } public String getNoField() { return noField; } public void setNoField(String noField) { this.noField = noField; } public String getUserid() { return userid; } public void setUserid(String userid) { this.userid = userid; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getAddress() { return address; } public void setAddress(String address) { this.address = address; }}4.创建工具类，根据传入该对象，来生成对应的查询sql
import java.lang.annotation.Annotation;import java.lang.reflect.Field;import java.lang.reflect.Method;import java.util.Objects;/** * 注解 */public class AnnotationUtil { /** * 根据传入的对象来生成query sql * * @param object * @return */ public static String getSqlByAnnotation(Object object) throws Exception { // 使用StringBuilder 线程安全 StringBuilder sb = new StringBuilder(); // 检查参数是否为空 Objects.requireNonNull(object, "该对象不能为空!"); // 1.通过反射获取该对象的类类型 Class c = object.getClass(); // 2.获取该对象的类注解：标明表名 boolean table_flag = c.isAnnotationPresent(Table.class); if (!table_flag) {throw new NullPointerException(); } Table table = (Table) c.getDeclaredAnnotation(Table.class); sb.append("SELECT * FROM ").append(table.value()).append(" WHERE 1 = 1"); // 3.获取该对象的字段注解：显示查询字段 Field[] fields = c.getDeclaredFields(); boolean field_flag; Column column; String field_name; Method menthod; Object val; for (Field field : fields) {// 首先得拿到字段名称：通过字段上的注解标明该指定是数据库表对应的字段field_flag = field.isAnnotationPresent(Column.class);if (!field_flag) {continue;}column = field.getDeclaredAnnotation(Column.class);// 其次得拿到字段值 : 通过反射调用类的getXxxx方法field_name = field.getName();menthod = c.getDeclaredMethod("get" + Character.toUpperCase(field_name.charAt(0)) + field_name.substring(1));val = menthod.invoke(object);// 判断该字段是否为空，不为空，才进行append// 这里要注意：有些属性不赋值时候，java会自动生成默认值，如Double默认为0.0// 还有其他类型，小伙伴们可以自行测试if (!Objects.isNull(val)) {if((val instanceof Double) && (double) val == 0.0){continue;}sb.append(" AND ").append(column.value()).append(" = ");sb.append(val);} } return sb.toString(); } public static void main(String[] args) { User user = new User(); user.setUserid("0001"); user.setAge(23); // 用于测试没有带Column注解的字段 user.setNoField("field"); try {System.out.println(AnnotationUtil.getSqlByAnnotation(user)); } catch (Exception e) {e.printStackTrace(); } }}