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Java中自定义注解的应用


来自 慕课网 的学习

       我们可以使用自定义注解,实现ORM,即对象/关系的映射。通过自定义注解,定义对象对应数据表的属性,如表名,表字段等。

       Table.java(Table注解)


package com.ann.demo;

import 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();
}

        Column.java (Column注解)


package com.ann.demo;

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();
}

       Filter.java(使用到Table和Column注解,定义该类对应的数据表和数据表的字段)

package com.ann.demo;

@Table("user")
public class Filter {

@Column("id")
private int id;

@Column("user_name")
private String username;

@Column("nickname")
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;
}
}


       Filter2.java

package com.ann.demo;

@Table("department")
public class Filter2 {

@Column("id")
private int id;

@Column("name")
private String name;

@Column("leader")
private String leader;

@Column("amount")
private int amount;

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public String getLeader() {
return leader;
}

public void setLeader(String leader) {
this.leader = leader;
}

public int getAmount() {
return amount;
}

public void setAmount(int amount) {
this.amount = amount;
}
}


       Test.java(解析类和属性上的注解,可以得到对应数据库上表名和字段名,并进行相应操作)


package com.ann.demo;

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

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"); //查询用户名为lucy的用户
f2.setAge(18);

Filter f3 = new Filter();
f3.setEmail("liu@sina.com, zh@163.com, 77777@qq.com"); //查询邮箱为其中任意一个的用户

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

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


Filter2 filter2 = new Filter2();
filter2.setAmount(10);
filter2.setName("技术部");
System.out.println(query(filter2));

}

private static String query(Object 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");
//3.遍历所有字段
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 columnName = column.value();
//4.2 拿到字段的值
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 (NoSuchMethodException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (SecurityException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalAccessException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalArgumentException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (InvocationTargetException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

//4.3 拼装sql
if(fieldValue == null || ((fieldValue instanceof Integer) && (Integer)fieldValue == 0)) {
continue;
}

sb.append(" and ").append(columnName);
if(fieldValue instanceof String) {
if(((String)fieldValue).contains(",")) {
String[] values = ((String)fieldValue).split(",");
sb.append(" in(");
for(String v : values) {
sb.append("'").append(v.trim()).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();
}
}


       运行结果如下:

Java中自定义注解的应用_ORM


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