java自定义注解-生成sql语句

叁歲伎倆 2022-05-23 09:14 293阅读 0赞

一、自定义Table注解

@Target(value=ElementType.TYPE)//使用在类或接口上
    @Retention(value=RetentionPolicy.RUNTIME)//在运行期间可以获取
    @Documented//文档含有该注解
    public @interface Table {
    	String value();
    }

二、自定义Column注解

@Target(value=ElementType.FIELD)//使用在类或接口上
    @Retention(value=RetentionPolicy.RUNTIME)//在运行期间可以获取
    @Documented//文档含有该注解
    public @interface Column {
    	String value();
    }

三、实体类

@Table(value = "tbl_user")
    public class UserEntity {
    
    	@Column("id")
        private int id;
    	
        @Column("username")
        private String username;
    
    	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;
    	}
    
    	@Override
    	public String toString() {
    		return "UserEntity [id=" + id + ", username=" + username + "]";
    	}
        
    }

四、测试

public class AnnotationTest {
    
    	public static void main(String[] args) {
    
    		UserEntity userEntity1 = new UserEntity();
    		userEntity1.setId(1);
    		UserEntity userEntity2 = new UserEntity();
    		userEntity2.setUsername("zhangsan");
    		
    		System.out.println(query(userEntity1));
    		System.out.println(query(userEntity2));
    	}
    	
    	@SuppressWarnings({ "unchecked", "rawtypes" })
    	private static String query(Object object) {
    		StringBuffer sql = new StringBuffer();
    		Class c = object.getClass();//获取对象的类对象
    		boolean existsTableAnno = c.isAnnotationPresent(Table.class);
    		if(existsTableAnno){//该类是否存在自定义Table注解
    			//获取注解和注解的value字段值
    			Annotation anno = c.getAnnotation(Table.class);
    			Table t = (Table)anno;
    			String tableName = t.value();
    			sql.append("select * from ").append(tableName).append(" where 1=1 ");
    			
    			//根据类对象获取该类的所有字段集
    			Field[] fields = c.getDeclaredFields();
    			for(Field field : fields){
    				//字段是否标注有Column注解
    				if(field.isAnnotationPresent(Column.class)){
    					Column column = (Column)field.getAnnotation(Column.class);
    					String columnName = column.value();//注解Column的值
    					
    					//获取字段值（通过getter方法）
    					String fieldName = field.getName();
    					String getMethodName = "get" + fieldName.substring(0,1).toUpperCase() + fieldName.substring(1);
    					Method method = null;
    					Object columnValue = null;
    					try {
    						method = c.getMethod(getMethodName);//通过类和方法名找到指定的方法
    						columnValue = method.invoke(object);//通过对象用指定的方法获取返回结果
    					} catch (Exception e) {
    						e.printStackTrace();
    					}
    					if(columnValue instanceof Integer && (int)columnValue != 0){
    						sql.append("and " + columnName + " = " + columnValue + " ");
    					}
    					if(columnValue instanceof String && (String)columnValue != null){
    						sql.append("and " + columnName + " = '" + columnValue + "' ");
    					}
    				}
    			}
    			
    		}
    		
    		return sql.toString();
    	}
    
    }

结果：

select \* from tbl\_user where 1=1 and id = 1   
    select \* from tbl\_user where 1=1 and username = 'zhangsan'