本篇介紹利用Spring BeanUtils.copyProperties(Object source, Object target)方法把JavaBean/POJO物件的properties值複製到另一個物件。
例如下面範例把Member物件m1中的屬性值複製到同為Member的m2。
package com.abc.demo;
import java.util.Arrays;
import java.util.List;
import org.springframework.beans.BeanUtils;
public class Main {
public static void main(String[] args) {
List<Article> articleList = Arrays.asList(new Article(1L, "hello world"));
Member m1 = new Member("Eddy", articleList);
Member m2 = new Member();
BeanUtils.copyProperties(m1, m2);
System.out.println("m1:" + m1); // m1:name:Eddy, articleList:[id:1, content:hello world]
System.out.println("m2:" + m2); // m2:name:Eddy, articleList:[id:1, content:hello world]
}
}
class Member {
private String name;
private List<Article> articleList;
public Member() {
}
public Member(String name, List<Article> articleList) {
this.name = name;
this.articleList = articleList;
}
@Override
public String toString() {
return "name:" + name + ", " + "articleList:" + articleList;
}
// getters and setters
}
class Article {
private Long id;
private String content;
public Article(Long id, String content) {
this.id = id;
this.content = content;
}
@Override
public String toString() {
return "id:" + id + ", content:" + content;
}
// getters and setters
}
上面可以看到經由BeanUtils.copyProperties()複製後m2的屬性值確實與m1相同，裡面的List<Article>物件屬性也會被複製，屬於深拷貝(deep copy)。
下面範例則是把m1複製到另一個類別User，除了articlelist屬性名稱外幾乎相同。
package com.abc.demo;
import java.util.Arrays;
import java.util.List;
import org.springframework.beans.BeanUtils;
public class Main {
public static void main(String[] args) {
List<Article> articleList = Arrays.asList(new Article(1L, "hello world"));
Member m1 = new Member("Eddy", articleList);
User u1 = new User();
BeanUtils.copyProperties(m1, u1);
System.out.println("m1:" + m1); // m1:name:Eddy, articleList:[id:1, content:hello world]
System.out.println("u1:" + u1); // u1:name:Eddy, articlelist:null
}
}
class Member {
private String name;
private List<Article> articleList; // <-- small camel case
public Member() {
}
public Member(String name, List<Article> articleList) {
this.name = name;
this.articleList = articleList;
}
@Override
public String toString() {
return "name:" + name + ", " + "articleList:" + articleList;
}
// getters and setters
}
class Article {
private Long id;
private String content;
public Article(Long id, String content) {
this.id = id;
this.content = content;
}
@Override
public String toString() {
return "id:" + id + ", content:" + content;
}
// getters and setters
}
class User {
private String name;
private String articlelist; // <-- all lowercase
public String getName() {
return name;
}
@Override
public String toString() {
return "name:" + name + ", " + "articleList:" + articlelist;
}
// getters and setters
}
此時u1.articlelist的內容卻是null，由此可知BeanUtils.copyProperties()只會複製屬性名稱相同的屬性值，若屬性名稱不同則被忽略。
BeanUtils.copyProperties()是利用Java的反射reflection來達成，以下是原始碼。
BeanUtils
public abstract class BeanUtils {
...
private static void copyProperties(Object source, Object target, @Nullable Class<?> editable,
@Nullable String... ignoreProperties) throws BeansException {
Assert.notNull(source, "Source must not be null");
Assert.notNull(target, "Target must not be null");
Class<?> actualEditable = target.getClass();
if (editable != null) {
if (!editable.isInstance(target)) {
throw new IllegalArgumentException("Target class [" + target.getClass().getName() +
"] not assignable to Editable class [" + editable.getName() + "]");
}
actualEditable = editable;
}
PropertyDescriptor[] targetPds = getPropertyDescriptors(actualEditable);
List<String> ignoreList = (ignoreProperties != null ? Arrays.asList(ignoreProperties) : null);
for (PropertyDescriptor targetPd : targetPds) {
Method writeMethod = targetPd.getWriteMethod();
if (writeMethod != null && (ignoreList == null || !ignoreList.contains(targetPd.getName()))) {
PropertyDescriptor sourcePd = getPropertyDescriptor(source.getClass(), targetPd.getName()); // 以目的對象(target)的屬性名稱來取得來源對象(source)的屬性
if (sourcePd != null) {
Method readMethod = sourcePd.getReadMethod();
if (readMethod != null &&
ClassUtils.isAssignable(writeMethod.getParameterTypes()[0], readMethod.getReturnType())) {
try {
if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers())) {
readMethod.setAccessible(true);
}
Object value = readMethod.invoke(source); // 取得來源對象的屬性值
if (!Modifier.isPublic(writeMethod.getDeclaringClass().getModifiers())) {
writeMethod.setAccessible(true);
}
writeMethod.invoke(target, value); // 把來源對象的屬性值寫入目的對象的同名屬性
}
catch (Throwable ex) {
throw new FatalBeanException(
"Could not copy property '" + targetPd.getName() + "' from source to target", ex);
}
}
}
}
}
}
...
}
使用注意事項
- 由於是利用反射來達到複製的效果，因此效能上會比手動寫set方法來得差。
- 屬性的命名上必須統一，所以系統中各屬性的命名規則必須嚴格規範。
- 不好除錯，例如屬性中有Enum型態，但資料庫為字串等型態不一致，或是Entity類與DTO類命名不一致等也會造成屬性結果為null的情況。
專案中最好有專人專職負責維護整個系統的命名列表，中英文對照表，名詞解釋等，讓開發人員可以直接查找使用，如果找不到就寫信去申請由維護人員建立，否則就常看到同一種東西在程式中出現多種名稱，這樣BeanUtils.copyProperties()就派不上用場了，更嚴重的是維護上常令人困惑。
例如顧客編號customerId，customerID，custId，custID，clientId，clientId；
商品數量quantity, qty；
利息收入interestIncome，intstIncome，intrstIncome，intstRevenue；
一次性費用onetimeExpense，onceExpense，oneExpense等。
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