copyProperties
copyProperties
copyProperties 是 Spring BeanUtils 工具箱中提供的用于 Bean 之间属性复制的方法。
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
}
上面可以看到经由BeanUtils.copyProperties()复制后m2的属性值确实与m1相同，里面的List物件属性也会被复制，属于深拷贝(deep copy)。下面范例则是把m1复制到另一个类别User，除了articlelist属性名称外几乎相同。
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
}
此时 u1.articlelist 的内容却是 null，由此可知 BeanUtils.copyProperties()只会复制属性名称相同的属性值，若属性名称不同则被忽略。
内部实现
BeanUtils.copyProperties()是利用 Java 的反射 reflection 来达成，以下是原始码。
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);
}
}
}
}
}
}
...
}
BeanUtils.copyProperties()虽然在复制 POJO 物件时非常方便，但在属性的命名上必须统一，所以系统中各属性的命名规则必须严格规范。还有个问题就是 debug 时错误不好找，所以并不推荐使用。大型程序中最好有专人专职负责维护整个系统的命名列表，中英文对照表，名词解释等，让开发人员可以直接查找使用，如果找不到就写信去申请由维护人员建立，否则就常看到同一种东西在程序中出现多种名称，这样 BeanUtils.copyProperties()就派不上用场了，更严重的是维护上常令人困惑。
- 例如顾客编号 customerId，customerID，custId，custID，clientId，clientId；
- 商品数量 quantity, qty；
- 利息收入 interestIncome，intstIncome，intrstIncome，intstRevenue；
- 一次性费用 onetimeExpense，onceExpense，oneExpense 等。