Java注解【五、注解实战】

需求：

1、表：用户ID，用户名，年龄，邮箱。

2、实现方法，传入实体，打印sql。

实现：

1、表：

package Annotation;
@Table("user")
public class User {
@Column("id")
public int UserId;
@Column("user_name")
public String Username;
@Column("age")
public int Age;
@Column("e_mail")
public String Email;

public int getUserId() {
	return UserId;
}
public void setUserId(int userId) {
	UserId = userId;
}
public String getUsername() {
	return Username;
}
public void setUsername(String username) {
	Username = username;
}
public int getAge() {
	return Age;
}
public void setAge(int age) {
	Age = age;
}
public String getEmail() {
	return Email;
}
public void setEmail(String email) {
	Email = email;
}
}

2、注解

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

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

3、调用

    public class QueryUser {

public static void main(String[] args) {
	User u = new User();
	u.setAge(18);
            u.setEmail("123@123.com,234@132.com");
	u.setUsername("amy");

	String sql = getQuerySql(u);
	System.out.println(sql);
}

private static String getQuerySql(Object o) {
	StringBuilder sb = new StringBuilder();
	// 1、获取类
	Class<? extends Object> c = o.getClass();

	// 2、获取表名
	String tableName = "";
	if (c.isAnnotationPresent(Table.class)) {
		Table t = (Table) c.getAnnotation(Table.class);
		tableName = t.value();
	}
	if (tableName == "") {
		return "no table name";
	}
	sb.append("select * from ").append(tableName).append(" where 1=1 ");

	// 3、获取字段名、字段值、拼装where条件
	Field[] fs = c.getFields();
	for (Field field : fs) {

		// 3.1获取字段名称
		String fieldName = field.getName();

		// 3.2用字段名称获取get方法
		String getMethodName = "get" + fieldName;
		Method getMethod = null;
		try {
			getMethod = c.getMethod(getMethodName);
		} catch (Exception e) {
			System.out.println(e.toString());
		}
		if (getMethod == null) {
			return "no method";
		}

		// 3.3用get方法获取字段值，当有值时才拼装
		Object fieldValue = null;
		try {
			fieldValue = getMethod.invoke(o);
		} catch (Exception e) {
			System.out.println(e.toString());
		}
		//无值跳过
		if(fieldValue==null ||
				(fieldValue instanceof Integer && (Integer)fieldValue==0)){
			continue;
		}

		//3.4 有值时获取表字段名
		String columnName = "";
		if(field.isAnnotationPresent(Column.class)){
			Column co = field.getAnnotation(Column.class);
			columnName=co.value();
		}
		if(columnName==""){
			return "no column name";
		}

		// 3.5获取字段类型根据类型进行语句处理
		Class<?> fieldType = field.getType();
		// string类型，有,分割，需要in
		if (fieldType.toString().toLowerCase().contains("string")
				&& fieldValue.toString().contains(",")) {
			String[] values = fieldValue.toString().split(",");
			sb.append("and ").append(columnName).append(" in (");
			for (String v : values) {
				sb.append("'").append(v).append("' ,");
			}
			// 删掉最后一个逗号
			sb.deleteCharAt(sb.length() - 1);
			sb.append(") \n");
		} else if (fieldType.toString().toLowerCase().contains("string")) {
			sb.append("and ").append(columnName).append(" = '")
					.append(fieldValue).append("' \n");
		} else {
			sb.append("and ").append(columnName).append(" = ")
					.append(fieldValue.toString()).append("\n");
		}
	}

	return sb.toString();
}
}