JAVA自定义注解使用

• RetentionPolicy.SOURCE 源码注解（注解将被编译器丢弃）
• RetentionPolicy.CLASS 编译时注解（注解在class文件可用，但会被VM丢弃）
• RetentionPolicy.RUNTIME 运行时注解（运行时保留注解，因此可以使用java反射机制读取注解信息）

package com.susu;

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

Column注解，作用域在字段名@Target(ElementType.FIELD)生命周期为@Retention(RetentionPolicy.RUNTIME)

package com.susu;import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.FIELD)public @interface Column {	String value();}

User实体类。

package com.susu;@Table("user")public class User {	@Column("id")	private int id;	@Column("username")	private String username;	@Column("nickname")	private String nickname;	@Column("age")	private int age;	@Column("gender")	private String gender;	@Column("city")	private String city;	@Column("email")	private String email;	@Column("phoneNumber")	private String phoneNumber;	public User() {		super();		// TODO Auto-generated constructor stub	}	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 getGender() {		return gender;	}	public void setGender(String gender) {		this.gender = gender;	}	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 getPhoneNumber() {		return phoneNumber;	}	public void setPhoneNumber(String phoneNumber) {		this.phoneNumber = phoneNumber;	}}

下面是测试类

package com.susu;import java.lang.reflect.Field;import java.lang.reflect.Method;public class Test {	public static void main(String[] args) {		User user1=new User();		user1.setNickname("小明");		user1.setAge(10);				User user2=new User();		user2.setUsername("张三");		User user3=new User();		user3.setEmail("1142819049@qq.com");		user3.setCity("北京");		query(user1);		query(user2);		query(user3);	}	@SuppressWarnings({ "rawtypes", "unchecked" })	private static void query(Object obj) {		StringBuilder sb=new StringBuilder();		Class c=obj.getClass();		//如果类不包含@Table注解 结束		if(!c.isAnnotationPresent(Table.class)){			return ;		}		//获得表名		Table table= (Table) c.getAnnotation(Table.class);		sb.append("select * from "+table.value()+" where 1 = 1");		Field fields[]=c.getDeclaredFields();		for (Field field : fields) {			//如果该字段不包含@Column注解  遍历下一个			if(!field.isAnnotationPresent(Column.class)){				continue;			}			//获得字段名						Column column=field.getAnnotation(Column.class);			String columnName=column.value();			String fieldName=field.getName();			//获取该字段的get方法			String methodName="get"+fieldName.substring(0, 1).toUpperCase()+fieldName.substring(1);			try {				Method method=c.getDeclaredMethod(methodName);				Object o =method.invoke(obj);				if(o==null||(o instanceof Integer&&(Integer)o==0)){					continue;				}				//拼装sql				if(o instanceof String){					sb.append(" and "+columnName+" = '"+o+"'");				}else if(o instanceof Integer){					sb.append(" and "+columnName+" = "+o);				}								} catch (Exception e) {				e.printStackTrace();			} 		}		System.out.println(sb.toString());	}}

运行结果：

package com.susu;@Table("Department")public class Department {	@Column("id")	private int id;	@Column("leader")	private String leader;	@Column("city")	private String name;	@Column("city")	private String city;	public Department() {		super();		// TODO Auto-generated constructor stub	}	public int getId() {		return id;	}	public void setId(int id) {		this.id = id;	}	public String getLeader() {		return leader;	}	public void setLeader(String leader) {		this.leader = leader;	}	public String getName() {		return name;	}	public void setName(String name) {		this.name = name;	}	public String getCity() {		return city;	}	public void setCity(String city) {		this.city = city;	}	}

在测试类里面继续调用

                Department dep1=new Department();
		dep1.setId(5);
		dep1.setLeader("张三");
		dep1.setCity("上海");
		Department dep2=new Department();
		dep2.setCity("成都");
		query(dep1);
		query(dep2);