author ： Mingmingruyue senior \
source ：blog.csdn.net/w605283073/article/details/107371462
I mentioned in the column before “ Attribute copying tools are not recommended ”, It is recommended to directly define transformation classes and methods IDEA Plug in auto fill get / set function .
The main reason for not recommending is ：
Some attribute copying tools have poor performance Some attribute copying tools have “BUG” Using the attribute copy tool is prone to some hidden dangers （ Later examples will talk about ）
2 Example
First of all, the company has encountered commons Bag BeanUtils A real case of poor property copy performance , Then the colleague changed to Spring Of BeanUtils The performance is much better , If you are interested, you can use the performance test framework or benchmark framework to compare , There is no comparison here .
Now let's see Spring Of BeanUtils What are the problems with copying the properties of ：
import lombok.Data;
import java.util.List;
@Data
public class A {
private String name;
private List<Integer> ids;
}
@Data
public class B {
private String name;
private List<String> ids;
}
import org.springframework.beans.BeanUtils;
import java.util.Arrays;
public class BeanUtilDemo {
public static void main(String[] args) {
A first = new A();
first.setName("demo");
first.setIds(Arrays.asList(1, 2, 3));
B second = new B();
BeanUtils.copyProperties(first, second);
for (String each : second.getIds()) {// Type conversion exception
System.out.println(each);
}
}
}When you run the above example , A type conversion exception occurs .
At the break point, you can see , After the attribute is copied B Type of second In the object ids Still for Integer type ：
If not converted to a string , Print directly , No errors are reported .
Use CGlib Not defined in Converter Similar problems will be encountered in the case of ：
import org.easymock.cglib.beans.BeanCopier;
import java.util.Arrays;
public class BeanUtilDemo {
public static void main(String[] args) {
A first = new A();
first.setName("demo");
first.setIds(Arrays.asList(1, 2, 3));
B second = new B();
final BeanCopier beanCopier = BeanCopier.create(A.class, B.class, false);
beanCopier.copy(first,second,null);
for (String each : second.getIds()) {// Type conversion exception
System.out.println(each);
}
}
}Again , The problem is exposed at runtime .
Let's take a look mapstruct：
import org.mapstruct.Mapper;
import org.mapstruct.factory.Mappers;
@Mapper
public interface Converter {
Converter INSTANCE = Mappers.getMapper(Converter.class);
B aToB(A car);
}
import java.util.Arrays;
public class BeanUtilDemo {
public static void main(String[] args) {
A first = new A();
first.setName("demo");
first.setIds(Arrays.asList(1, 2, 3));
B second = Converter.INSTANCE.aToB(first);
for (String each : second.getIds()) {// normal
System.out.println(each);
}
}
} Can succeed in A in List<Integer> To B Medium List<String> type . Let's look at the compiled Converter Implementation class ：
import java.util.ArrayList;
import java.util.List;
import javax.annotation.Generated;
import org.springframework.stereotype.Component;
@Generated(
value = "org.mapstruct.ap.MappingProcessor",
comments = "version: 1.3.1.Final, compiler: javac, environment: Java 1.8.0_202 (Oracle Corporation)"
)
@Component
public class ConverterImpl implements Converter {
@Override
public B aToB(A car) {
if ( car == null ) {
return null;
}
B b = new B();
b.setName( car.getName() );
b.setIds( integerListToStringList( car.getIds() ) );
return b;
}
protected List<String> integerListToStringList(List<Integer> list) {
if ( list == null ) {
return null;
}
List<String> list1 = new ArrayList<String>( list.size() );
for ( Integer integer : list ) {
list1.add( String.valueOf( integer ) );
}
return list1;
}
}Automatically helped us with the conversion , We may not realize that the types are inconsistent .
If we were A Add one to the class String number attribute , stay B Add one to the class Long number attribute , Use mapstruect When number When it is set to non numeric type, it will report .NumberFormatException.
@Override
public B aToB(A car) {
if ( car == null ) {
return null;
}
B b = new B();
b.setName( car.getName() );
if ( car.getNumber() != null ) { // Here's the problem
b.setNumber( Long.parseLong( car.getNumber() ) );
}
b.setIds( integerListToStringList( car.getIds() ) );
return b;
}Use cglib By default, there is no mapping number attribute ,B Medium number by null. If you define the converter manually , Use IDEA plug-in unit ( Such as generateO2O) Automatic conversion ：
public final class A2BConverter {
public static B from(A first) {
B b = new B();
b.setName(first.getName());
b.setIds(first.getIds());
return b;
}
}This problem can be found very clearly in the coding phase ：
3 Conclusion
because Java Generics are actually compile time checks , Post compilation generic erasure , Causes the runtime to List<Integer> and List<String> All are List type , It can be assigned normally . This leads to when using many attribute mapping tools , It is not easy to make obvious errors at compile time .
mapstruct Custom annotation processor , The generic types on both sides of the map can be read at compile time , Then map . But this mapping is also terrible , Sometimes we define the wrong type due to carelessness and other reasons , Automatically helped us with the conversion , It will bring a lot of side effects .
The performance of various attribute mapping tools has been briefly compared before , give the result as follows ：
Therefore, use attribute conversion tools with caution , If possible, it is recommended to customize the conversion class , Use IDEA Plug in auto fill , Very efficient , A or B Any property type in does not match , Even delete an attribute , Errors can be reported at the compilation stage , And directly call get set The efficiency is also very high .
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