Java中自定义注解的应用

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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classClass 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 blocke.printStackTrace();} catch (SecurityException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalAccessException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalArgumentException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (InvocationTargetException e) {// TODO Auto-generated catch blocke.printStackTrace();}//4.3 拼装sqlif(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：

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