如何创建，使用以及解析自定义注解

首先要想使用自定义注解，必须创建自己的注解类

右键项目，new -> Annotation

然后在注解里定义自己的方法，该方法是别的类使用注解时需要填的属性

package com.sy.demo.annotation;import java.lang.annotation.Documented;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)@Documentedpublic @interface Table {public String value();}

注：如果只有一个方法时，应该用value()来指定方法名，这样就可以直接简写@Table("xxx")而不是@Table(aaa="xxx");

其中注解类上的注解称为元注解

@Target(ElementType.TYPE)

@Target的意思是，该注解类是放在什么位置的，是放在类上、字段上还是方法上，ElementType.TYPE意思是只能放在类上或接口上，ElementType.FIELD意思是只能放在字段上等等。

如果有多个位置选择可以这么写：

@Target({ElementType.TYPE, ElementType.METHOD})

@Retention(RetentionPolicy.RUNTIME)

意思是作用域，一般写RUNTIME就行

@Documented

意思是是否在生成JavaDoc时加入该注解类，这个看情况写不写

还有其他元注解，想要研究的就自己研究吧

定义完自定义注解了，下面就是使用的时候了

package com.sy.demo.entity;import com.sy.demo.annotation.Column;import com.sy.demo.annotation.Table;@Table("tdb_user")public class User { @Column("id") private Long id; @Column("email") private String email; @Column("password") private String password; public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getEmail() { return email; } public void setEmail(String email) { this.email = email; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; }}

在这里我定义了一个实体类，用于表示用户信息，其中还是用了一个@Column类，代码如下

package com.sy.demo.annotation;import java.lang.annotation.Documented;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)@Documentedpublic @interface Column { public String value();}

由代码可知，@Column是放在field上的

使用也使用完了，下面该是解析的时候了。

package com.sy.demo.util;import java.lang.reflect.Field;import com.sy.demo.annotation.Column;import com.sy.demo.annotation.Table;public class SqlUtil { private static final String EMPTY = ""; @SuppressWarnings("unchecked") public static String getSql(Object object) { StringBuilder sb = new StringBuilder(); Class<Object> c; boolean isExist; Column column; String columnName; String getMethodName; Object columnValue; String[] strs; try { c = (Class<Object>) object.getClass(); isExist = c.isAnnotationPresent(Table.class); if (!isExist) { return EMPTY; } Table table = c.getAnnotation(Table.class); sb.append(" SELECT * FROM " + table.value() + " WHERE 1 = 1 " ); Field[] fields = c.getDeclaredFields(); for (Field field: fields) { isExist = field.isAnnotationPresent(Column.class); if (!isExist) { continue; } column = field.getAnnotation(Column.class); columnName = column.value(); getMethodName = "get" + columnName.substring(0, 1).toUpperCase() + columnName.substring(1); columnValue = c.getMethod(getMethodName, new Class[0]).invoke(object, new Object[0]); if (columnValue == null) { continue; } if (columnValue instanceof String) { columnValue = (String)columnValue; if(((String) columnValue).contains(",")) { sb.append("AND " + columnName + " IN ("); strs = ((String) columnValue).split(","); for(String str: strs) { sb.append(""" + str + "","); } sb.deleteCharAt(sb.length() - 1); sb.append(") "); } else { sb.append("AND " + columnName + " = "" + columnValue + "" "); } } else if (columnValue instanceof Integer || columnValue instanceof Long) { sb.append("AND " + columnName + " = " + columnValue + " "); } } } catch (Exception e) { e.printStackTrace(); } return sb.toString(); }}

解析的时候用的是反射机制，可能看着比较麻烦比较乱，而且也新手可能也不太理解，在用的时候会发现其实还是挺方便的。

原理解释根据反射找到User类，在判断是否有注解，接着拼接sql什么的

整个列子项目中完整的代码如下(有许多步骤测试用例，懒得删了，全贴出来吧）

Controller

package com.sy.demo.controller;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Controller;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.ResponseBody;import com.sy.demo.entity.User;import com.sy.demo.service.IUserService;@Controller@RequestMapping("hello")public class UserController { @Autowired private IUserService hService; @RequestMapping(value = "demo1") public String demo1() { return "demo1"; } @SuppressWarnings("deprecation") @RequestMapping(value = "demo2") public String demo2() { return hService.test(); } @RequestMapping(value = "demo3") @ResponseBody public String demo3() { User user = new User(); user.setId(1L); user.setEmail("mr_songyang1990@1*.com"); user.setPassword("1q2w3e4r,123456,aaaaa"); return hService.getUser(user); } @RequestMapping(value = "demo4") @ResponseBody public String demo4() { User user = new User(); user.setId(1L); user.setEmail("mr_songyang1990@1*.com"); user.setPassword("1q2w3e4r,123456,aaaaa"); return hService.getUser2(user); }}

service:

package com.sy.demo.service;import com.sy.demo.entity.User;public interface IUserService { @Deprecated public String test(); public String getUser(User user); public String getUser2(User user);}

package com.sy.demo.service.impl;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Service;import com.sy.demo.entity.User;import com.sy.demo.repository.IUserRepository;import com.sy.demo.service.IUserService;@Service("hService")public class UserServiceImpl implements IUserService { @Autowired private IUserRepository hRepository; @Deprecated @Override public String test() { return "demo2"; } @Override public String getUser(User user) { return hRepository.queryUser(user); } @Override public String getUser2(User user) { return hRepository.queryUser2(user); }}

Repository

package com.sy.demo.service.impl;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Service;import com.sy.demo.entity.User;import com.sy.demo.repository.IUserRepository;import com.sy.demo.service.IUserService;@Service("hService")public class UserServiceImpl implements IUserService { @Autowired private IUserRepository hRepository; @Deprecated @Override public String test() { return "demo2"; } @Override public String getUser(User user) { return hRepository.queryUser(user); } @Override public String getUser2(User user) { return hRepository.queryUser2(user); }}

package com.sy.demo.repository.impl;import java.sql.PreparedStatement;import java.sql.ResultSet;import java.sql.SQLException;import org.springframework.stereotype.Repository;import com.sy.demo.entity.User;import com.sy.demo.repository.IUserRepository;import com.sy.demo.util.DBUtil;import com.sy.demo.util.SqlUtil;@Repository("hRepository")public class UserRepositoryImpl implements IUserRepository { public String queryUser(User user) { String sql = SqlUtil.getSql(user); System.out.println(sql); return sql; } @Override public String queryUser2(User user) { StringBuilder sb = new StringBuilder(); String sql = SqlUtil.getSql(user); System.out.println(sql); PreparedStatement ps = DBUtil.getPreparedStatement(sql); Long id; String email; String password; try { ResultSet rs = ps.executeQuery(); while (rs.next()) { id = rs.getLong("id"); email = rs.getString("email"); password = rs.getString("password"); sb.append("ID:").append(id).append(", email:"). append(email).append(", password:").append(password).append("<br/>"); } } catch (SQLException e) { e.printStackTrace(); } return sb.toString(); }}

entity:

package com.sy.demo.entity;import com.sy.demo.annotation.Column;import com.sy.demo.annotation.Table;@Table("tdb_user")public class User { @Column("id") private Long id; @Column("email") private String email; @Column("password") private String password; public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getEmail() { return email; } public void setEmail(String email) { this.email = email; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; }}

annotation

package com.sy.demo.annotation;import java.lang.annotation.Documented;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)@Documentedpublic @interface Table { public String value();}

package com.sy.demo.annotation;import java.lang.annotation.Documented;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)@Documentedpublic @interface Column { public String value();}

util工具类

package com.sy.demo.util;import java.sql.Connection;import java.sql.PreparedStatement;import java.sql.SQLException;import com.mysql.jdbc.jdbc2.optional.MysqlDataSource;public class DBUtil { public static final String URL = "jdbc:mysql://localhost:3306/db_test"; public static final String USERNAME = "root"; public static final String PASSWORD = "root"; public static Connection conn = null; public static MysqlDataSource dataSource; static { dataSource = new MysqlDataSource(); dataSource.setUser(USERNAME); dataSource.setPassword(PASSWORD); dataSource.setURL(URL); } public static PreparedStatement getPreparedStatement(String sql) { try { conn = dataSource.getConnection(); return conn.prepareStatement(sql); } catch (SQLException e) { e.printStackTrace(); } return null; }}

package com.sy.demo.util;import java.lang.reflect.Field;import com.sy.demo.annotation.Column;import com.sy.demo.annotation.Table;public class SqlUtil { private static final String EMPTY = ""; @SuppressWarnings("unchecked") public static String getSql(Object object) { StringBuilder sb = new StringBuilder(); Class<Object> c; boolean isExist; Column column; String columnName; String getMethodName; Object columnValue; String[] strs; try { c = (Class<Object>) object.getClass(); isExist = c.isAnnotationPresent(Table.class); if (!isExist) { return EMPTY; } Table table = c.getAnnotation(Table.class); sb.append(" SELECT * FROM " + table.value() + " WHERE 1 = 1 " ); Field[] fields = c.getDeclaredFields(); for (Field field: fields) { isExist = field.isAnnotationPresent(Column.class); if (!isExist) { continue; } column = field.getAnnotation(Column.class); columnName = column.value(); getMethodName = "get" + columnName.substring(0, 1).toUpperCase() + columnName.substring(1); columnValue = c.getMethod(getMethodName, new Class[0]).invoke(object, new Object[0]); if (columnValue == null) { continue; } if (columnValue instanceof String) { columnValue = (String)columnValue; if(((String) columnValue).contains(",")) { sb.append("AND " + columnName + " IN ("); strs = ((String) columnValue).split(","); for(String str: strs) { sb.append(""" + str + "","); } sb.deleteCharAt(sb.length() - 1); sb.append(") "); } else { sb.append("AND " + columnName + " = "" + columnValue + "" "); } } else if (columnValue instanceof Integer || columnValue instanceof Long) { sb.append("AND " + columnName + " = " + columnValue + " "); } } } catch (Exception e) { e.printStackTrace(); } return sb.toString(); }}