大部分頭次接觸註解的人,應該都是從框架開始的吧,之前感覺註解很神祕,也沒有了解過如何基於註解的開發,最近忙裏偷閒,學習了一下註解的原理及應用。簡單地說,註解就是在類、字段、方法上打一個標記,在之後的代碼中,可以通過反射獲取到被打上標記的類、字段、方法,方便做一些邏輯處理,而這些處理內容是你自己編寫的,所以註解提供的僅僅是一個標記而已,下面貼上一個註解的應用,場景是通過jdbc獲取數據庫中的行數據Map<String,String>,將該Map轉換爲相應的POJO
首先是註解的聲明,
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 name();
}
這裏name爲該註解的屬性,使用時按照格式:@Column(name="xxx")
public class UserDO {
@Column(name = "name")
private String userName;
@Column(name = "title")
private String userTitle;
private int loginTimes;
private String empId;
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getUserTitle() {
return userTitle;
}
public void setUserTitle(String userTitle) {
this.userTitle = userTitle;
}
public int getLoginTimes() {
return loginTimes;
}
public void setLoginTimes(int loginTimes) {
this.loginTimes = loginTimes;
}
public String getEmpId() {
return empId;
}
public void setEmpId(String empId) {
this.empId = empId;
}
}
之後是POJO,只有2個屬性被打上了註解。
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import org.apache.commons.lang.StringUtils;
public class ConvertionService {
private static List<Field> findFields(Class <?>clazz) {
List<Field> fieldList = new ArrayList<Field>();
Set<String>fieldNames = new HashSet<String>();
Class <?>clazzTemp = clazz;
while(clazzTemp != Object.class) {
Field []fields = clazzTemp.getDeclaredFields();
for(Field field : fields) {
if(!fieldNames.contains(field.getName())) {//同名屬性子類覆蓋
fieldList.add(field);
fieldNames.add(field.getName());
}
}
clazzTemp = clazzTemp.getSuperclass();
}
return fieldList;
}
@SuppressWarnings("unchecked")
public static <T> T convertMapToBean(Map<String , String>row , Class<T>clazz)
throws InstantiationException, IllegalAccessException {
Object object = clazz.newInstance();
List<Field>list = findFields(clazz);
for(Field field : list) {
if (!Modifier.isStatic(field.getModifiers()) && !Modifier.isFinal(field.getModifiers())) {//過濾static和final修飾的變量
if(!field.isAccessible()) field.setAccessible(true);//如果是private變量,放開訪問的權限
Column columnAnnotation = field.getAnnotation(Column.class);
if(columnAnnotation != null) {
String value = row.get(columnAnnotation.name());//通過annotation註解的名稱來獲取對應的值
Class <?>fieldType = field.getType();
if(fieldType == String.class) {
field.set(object, value);
}else if(fieldType == Integer.class) {
field.set(object, getInteger(value));
}else if(fieldType == int.class) {
field.setInt(object, getInt(value));
}else if(fieldType == Long.class) {
field.set(object, getLongWrapper(value));
}else if(fieldType == long.class) {
field.setLong(object , getLong(value));
}/*
繼續的數據類型大家可以自己補充
*/
}else {
/*沒有annotation的代碼交給大家自己去完成了*/
}
}
}
return (T)object;
}
public static Integer getInteger(String value) {
if (StringUtils.isEmpty(value)) return null;
return Integer.valueOf(value);
}
public static int getInt(String value) {
if (StringUtils.isEmpty(value)) return 0;
return Integer.valueOf(value);
}
public static Long getLongWrapper(String value) {
if (StringUtils.isEmpty(value)) return null;
return Long.valueOf(value);
}
public static long getLong(String value) {
if (StringUtils.isEmpty(value)) return 0;
return Long.valueOf(value);
}
}
通過註解及反射,將Map<String,String>轉化爲相應的POJO的方法
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
public class TestMain {
public static void main(String []args)
throws InstantiationException, IllegalAccessException {
@SuppressWarnings({ "unchecked", "serial" })
List<HashMap<String , String>>list = Arrays.asList(
new HashMap<String , String>() {
{
put("name" , "xieyuooo");
put("title" , "小胖");
}
},
new HashMap<String , String>() {
{
put("name" , "ffff");
put("title" , "標題2");
}
}
);
List<UserDO>users = new ArrayList<UserDO>(list.size());
for(HashMap<String , String> row : list) {
users.add(ConvertionService.convertMapToBean(row, UserDO.class));
}
System.out.println();
//這裏大家可以將users的列表進行輸出
}
}
測試類的入口方法