Copy all values from fields in one class to another through reflection

Question

I have a class which is basically a copy of another class.

public class A {
  int a;
  String b;
}

public class CopyA {
  int a;
  String b;
}

What I am doing is putting values from class A into CopyA before sending CopyA through a webservice call. Now I would like to create a reflection-method that basically copies all fields that are identical (by name and type) from class A to class CopyA.

How can I do this?

This is what I have so far, but it doesn't quite work. I think the problem here is that I am trying to set a field on the field I am looping through.

private <T extends Object, Y extends Object> void copyFields(T from, Y too) {

    Class<? extends Object> fromClass = from.getClass();
    Field[] fromFields = fromClass.getDeclaredFields();

    Class<? extends Object> tooClass = too.getClass();
    Field[] tooFields = tooClass.getDeclaredFields();

    if (fromFields != null && tooFields != null) {
        for (Field tooF : tooFields) {
            logger.debug("toofield name #0 and type #1", tooF.getName(), tooF.getType().toString());
            try {
                // Check if that fields exists in the other method
                Field fromF = fromClass.getDeclaredField(tooF.getName());
                if (fromF.getType().equals(tooF.getType())) {
                    tooF.set(tooF, fromF);
                }
            } catch (SecurityException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (NoSuchFieldException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IllegalArgumentException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

        }
    }

I am sure there must be someone that has already done this somehow

See also http://stackoverflow.com/questions/1432764/any-tool-for-java-object-to-object-mapping — Ruben Bartelink, Nov 03 '09 at 15:05
See also [How do I copy an object in Java?](https://stackoverflow.com/questions/869033/how-do-i-copy-an-object-in-java) — Vadzim, Jul 03 '19 at 17:41

score 117 · Accepted Answer · edited Jul 31 '16 at 16:52

117

If you don't mind using a third party library, BeanUtils from Apache Commons will handle this quite easily, using copyProperties(Object, Object).

edited Jul 31 '16 at 16:52

Colourfit

375
3
19

answered Nov 03 '09 at 15:07

Greg Case

3,200
1
19
17

15

Apparently BeanUtils doesn't work with null Date fields. Use Apache PropertyUtils if this is a problem for you: http://www.mail-archive.com/user@commons.apache.org/msg02246.html – ripper234 Nov 28 '11 at 13:51
11

This apparently does not work for private fields with no getter and setters. Any solution that works on directly with fields, rather than properties? – Andrea Ratto Oct 16 '15 at 17:13
It neither works with plain public fields without getters: https://stackoverflow.com/questions/34263122/copy-object-properties-by-direct-field-access – Vadzim Mar 14 '19 at 22:18

score 20 · Answer 2 · edited Jun 09 '17 at 05:31

20

Why don't you use gson library https://github.com/google/gson

you just convert the Class A to json string. Then convert jsonString to you subClass (CopyA) .using below code:

Gson gson= new Gson();
String tmp = gson.toJson(a);
CopyA myObject = gson.fromJson(tmp,CopyA.class);

edited Jun 09 '17 at 05:31

Giru Bhai

14,370
5
46
74

answered Nov 25 '16 at 06:30

Eric Ho

395
2
4

1

Why generate another String which could also be large? There are better alternatives described as answers here. At least we (the industry) progressed from XML to json for string representations, but we still don't want everything passed to that string representation at any given chance ... – arntg Jan 29 '19 at 02:00
1

Please noted that the string is a side product when using reflection . Even through you did not save it !! This is a answer for java beginners, and aim to be in short and clean way. @arntg – Eric Ho Feb 03 '19 at 06:17
1

You'd still need reflection on both the source and destination objects, but now you're also introducing binary/text/binary formatting and parsing overhead. – Evvo Oct 09 '19 at 17:08
1

Pay attention if you use Proguard to obfuscate the code. If you use it, this code will not work. – SebaReal Jun 03 '20 at 14:21
this is useful in my case thanks – Mugeesh Husain Nov 25 '22 at 01:27

score 11 · Answer 3 · edited Dec 12 '11 at 09:29

BeanUtils will only copy public fields and is a bit slow. Instead go with getter and setter methods.

public Object loadData (RideHotelsService object_a) throws Exception{

        Method[] gettersAndSetters = object_a.getClass().getMethods();

        for (int i = 0; i < gettersAndSetters.length; i++) {
                String methodName = gettersAndSetters[i].getName();
                try{
                  if(methodName.startsWith("get")){
                     this.getClass().getMethod(methodName.replaceFirst("get", "set") , gettersAndSetters[i].getReturnType() ).invoke(this, gettersAndSetters[i].invoke(object_a, null));
                        }else if(methodName.startsWith("is") ){
                            this.getClass().getMethod(methodName.replaceFirst("is", "set") ,  gettersAndSetters[i].getReturnType()  ).invoke(this, gettersAndSetters[i].invoke(object_a, null));
                        }

                }catch (NoSuchMethodException e) {
                    // TODO: handle exception
                }catch (IllegalArgumentException e) {
                    // TODO: handle exception
                }

        }

        return null;
    }

BeanUtils works just fine on private fields, as long as the getters/setters are public. Regarding performance, I haven't done any benchmarking, but I believe it does some internal caching of the bean's it has introspected. — Greg Case, Dec 14 '11 at 21:18
this will only work if the two beans have the same data type of fields. — TimeToCodeTheRoad, Jun 26 '12 at 21:28
@To Kra It would work only if you have getter/setter for that field. — WoLfPwNeR, Jan 09 '17 at 23:58

score 10 · Answer 4 · answered Aug 30 '17 at 08:39

Here is a working and tested solution. You can control the depth of the mapping in the class hierarchy.

public class FieldMapper {

    public static void copy(Object from, Object to) throws Exception {
        FieldMapper.copy(from, to, Object.class);
    }

    public static void copy(Object from, Object to, Class depth) throws Exception {
        Class fromClass = from.getClass();
        Class toClass = to.getClass();
        List<Field> fromFields = collectFields(fromClass, depth);
        List<Field> toFields = collectFields(toClass, depth);
        Field target;
        for (Field source : fromFields) {
            if ((target = findAndRemove(source, toFields)) != null) {
                target.set(to, source.get(from));
            }
        }
    }

    private static List<Field> collectFields(Class c, Class depth) {
        List<Field> accessibleFields = new ArrayList<>();
        do {
            int modifiers;
            for (Field field : c.getDeclaredFields()) {
                modifiers = field.getModifiers();
                if (!Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers)) {
                    accessibleFields.add(field);
                }
            }
            c = c.getSuperclass();
        } while (c != null && c != depth);
        return accessibleFields;
    }

    private static Field findAndRemove(Field field, List<Field> fields) {
        Field actual;
        for (Iterator<Field> i = fields.iterator(); i.hasNext();) {
            actual = i.next();
            if (field.getName().equals(actual.getName())
                && field.getType().equals(actual.getType())) {
                i.remove();
                return actual;
            }
        }
        return null;
    }
}

I have created a similar solution. I cached a class to field names to field map. — Orden, Aug 09 '18 at 20:36
The solution is nice but you're gonna have a problem in this line `i.remove()`. Even if you've created iterator you can't call `remove` on `List`'s `iterator`. It should be `ArrayList` — Farid, Jul 04 '20 at 23:13
Farid, remove can't be a problem because collectFields() creates ArrayList objects. — JHead, Jul 06 '20 at 08:01

score 8 · Answer 5 · answered Mar 28 '17 at 20:46

Spring has a built in BeanUtils.copyProperties method. But it doesn't work with classes without getters/setters. JSON serialization/deserialization can be another option for copying fields. Jackson can be used for this purpose. If you are using Spring In most cases Jackson is already in your dependency list.

ObjectMapper mapper     = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
Clazz        copyObject = mapper.readValue(mapper.writeValueAsString(sourceObject), Clazz.class);

score 6 · Answer 6 · edited Jun 20 '16 at 14:10

My solution:

public static <T > void copyAllFields(T to, T from) {
        Class<T> clazz = (Class<T>) from.getClass();
        // OR:
        // Class<T> clazz = (Class<T>) to.getClass();
        List<Field> fields = getAllModelFields(clazz);

        if (fields != null) {
            for (Field field : fields) {
                try {
                    field.setAccessible(true);
                    field.set(to,field.get(from));
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
        }
    }

public static List<Field> getAllModelFields(Class aClass) {
    List<Field> fields = new ArrayList<>();
    do {
        Collections.addAll(fields, aClass.getDeclaredFields());
        aClass = aClass.getSuperclass();
    } while (aClass != null);
    return fields;
}

I don't think this doesn't work for custom objects. Only if you have a class with no parent and only primitive fields — Shervin Asgari, Feb 01 '16 at 09:01
For covering super class fields I am using 'getAllModelFields' custom method — Nik Kashi, Feb 01 '16 at 10:45

score 6 · Answer 7 · answered Apr 06 '18 at 14:04

This is a late post, but can still be effective for people in future.

Spring provides a utility BeanUtils.copyProperties(srcObj, tarObj) which copies values from source object to target object when the names of the member variables of both classes are the same.

If there is a date conversion, (eg, String to Date) 'null' would be copied to the target object. We can then, explicitly set the values of the date as required.

The BeanUtils from Apache Common throws an error when there is a mismatch of data-types (esp. conversion to and from Date)

Hope this helps!

This doesn't provide any additional information than accepted https://stackoverflow.com/a/1667911/4589003 answer — Sudip Bhandari, May 31 '18 at 11:05

score 5 · Answer 8 · answered Nov 03 '09 at 15:14

The first argument to tooF.set() should be the target object (too), not the field, and the second argument should be the value, not the field the value comes from. (To get the value, you need to call fromF.get() -- again passing in a target object, in this case from.)

Most of the reflection API works this way. You get Field objects, Method objects, and so on from the class, not from an instance, so to use them (except for statics) you generally need to pass them an instance.

Ruben Bartelink · Answer 9 · 2012-11-19T11:44:29.857

4

Dozer

UPDATE Nov 19 2012: There's now a new ModelMapper project too.

edited Nov 19 '12 at 11:44

answered Nov 03 '09 at 15:04

Ruben Bartelink

59,778
26
187
249

score 3 · Answer 10 · answered May 17 '12 at 10:40

I think you can try dozer. It has good support for bean to bean conversion. Its also easy to use. You can either inject it into your spring application or add the jar in class path and its done.

For an example of your case :

 DozerMapper mapper = new DozerMapper();
A a= new A();
CopyA copyA = new CopyA();
a.set... // set fields of a.
mapper.map(a,copyOfA); // will copy all fields from a to copyA

score 3 · Answer 11 · edited Sep 19 '17 at 13:36

3

Without using BeanUtils or Apache Commons

public static <T1 extends Object, T2 extends Object>  void copy(T1     
origEntity, T2 destEntity) throws IllegalAccessException, NoSuchFieldException {
    Field[] fields = origEntity.getClass().getDeclaredFields();
    for (Field field : fields){
        origFields.set(destEntity, field.get(origEntity));
     }
}

edited Sep 19 '17 at 13:36

Jing Li

14,547
7
57
69

answered Apr 16 '15 at 09:15

Darkhan Iskakov

47
1

This is not a working solution but a good starting point. The fields need to be filtered to handle only the non-static and public fields that are present in both of the classes. – JHead Aug 30 '17 at 07:25
Wouldn't this ignore fields in parent classes? – Evvo Jan 02 '20 at 19:38

Nagappan · Answer 12 · 2015-07-30T03:12:55.860

2

Orika's is simple faster bean mapping framework because it does through byte code generation. It does nested mappings and mappings with different names. For more details, please check here Sample mapping may look complex, but for complex scenarios it would be simple.

MapperFactory factory = new DefaultMapperFactory.Builder().build();
mapperFactory.registerClassMap(mapperFactory.classMap(Book.class,BookDto.class).byDefault().toClassMap());
MapperFacade mapper = factory.getMapperFacade();
BookDto bookDto = mapperFacade.map(book, BookDto.class);

edited Jul 30 '15 at 03:12

answered Jan 07 '15 at 14:19

Nagappan

346
2
8

This does not do what the question is asking for. `SerializationUtils.clone()` is going to give a new object of the same class. Additionally it only works on serializable classes. – Kirby Jul 29 '15 at 23:24

db80 · Answer 13 · 2023-02-02T08:34:24.737

2

If you have spring in the list of dependencies you can use org.springframework.beans.BeanUtils.

https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/beans/BeanUtils.html

edited Feb 02 '23 at 08:34

answered Nov 10 '15 at 11:15

db80

4,157
1
38
38

score 1 · Answer 14 · answered Sep 09 '19 at 16:34

I solved the above problem in Kotlin that works fine for me for my Android Apps Development:

 object FieldMapper {

fun <T:Any> copy(to: T, from: T) {
    try {
        val fromClass = from.javaClass

        val fromFields = getAllFields(fromClass)

        fromFields?.let {
            for (field in fromFields) {
                try {
                    field.isAccessible = true
                    field.set(to, field.get(from))
                } catch (e: IllegalAccessException) {
                    e.printStackTrace()
                }

            }
        }
    } catch (e: Exception) {
        e.printStackTrace()
    }

}

private fun getAllFields(paramClass: Class<*>): List<Field> {

    var theClass:Class<*>? = paramClass
    val fields = ArrayList<Field>()
    try {
        while (theClass != null) {
            Collections.addAll(fields, *theClass?.declaredFields)
            theClass = theClass?.superclass
        }
    }catch (e:Exception){
        e.printStackTrace()
    }

    return fields
}

}

Zedong · Answer 15 · 2020-03-19T07:21:23.253

public static <T> void copyAvalableFields(@NotNull T source, @NotNull T target) throws IllegalAccessException {
    Field[] fields = source.getClass().getDeclaredFields();
    for (Field field : fields) {
        if (!Modifier.isStatic(field.getModifiers())
                && !Modifier.isFinal(field.getModifiers())) {
            field.set(target, field.get(source));
        }
    }
}

We read all the fields of the class. Filter non-static and non-final fields from the result. But there may be an error accessing non-public fields. For example, if this function is in the same class, and the class being copied does not contain public fields, an access error will occur. The solution may be to place this function in the same package or change access to public or in this code inside the loop call field.setAccessible (true); what will make the fields available

While this code may provide a solution to the question, it's better to add context as to why/how it works. This can help future users learn, and apply that knowledge to their own code. You are also likely to have positive feedback from users in the form of upvotes, when the code is explained. — borchvm, Mar 19 '20 at 07:00

score 1 · Answer 16 · edited Mar 24 '22 at 09:52

Here is my solution, will cover the child class case:

/**
 * This methods transfer the attributes from one class to another class if it
 * has null values.
 * 
 * @param fromClass from class
 * @param toClass   to class
 */
private void loadProperties(Object fromClass, Object toClass) {
    if (Objects.isNull(fromClass) || Objects.isNull(toClass))
        return;
    
    Field[] fields = toClass.getClass().getDeclaredFields();
    Field[] fieldsSuperClass = toClass.getClass().getSuperclass().getDeclaredFields();
    Field[] fieldsFinal = new Field[fields.length + fieldsSuperClass.length];

    Arrays.setAll(fieldsFinal, i -> (i < fields.length ? fields[i] : fieldsSuperClass[i - fields.length]));

    for (Field field : fieldsFinal) {
        field.setAccessible(true);
        try {
            String propertyKey = field.getName();
            if (field.get(toClass) == null) {
                Field defaultPropertyField = fromClass.getClass().getDeclaredField(propertyKey);
                defaultPropertyField.setAccessible(true);
                Object propertyValue = defaultPropertyField.get(fromClass);
                if (propertyValue != null)
                    field.set(toClass, propertyValue);
            }
        } catch (IllegalAccessException e) {
            logger.error(() -> "Error while loading properties from " + fromClass.getClass() +" and to " +toClass.getClass(), e);
        } catch (NoSuchFieldException e) {
            logger.error(() -> "Exception occurred while loading properties from " + fromClass.getClass()+" and to " +toClass.getClass(), e);
        }
    }
}

score 0 · Answer 17 · answered Feb 04 '15 at 18:28

I didn't want to add dependency to another JAR file because of this, so wrote something which would suit my needs. I follow the convention of fjorm https://code.google.com/p/fjorm/ which means that my generally accessible fields are public and that I don't bother to write setters and getters. (in my opinion code is easier to manage and more readable actually)

So I wrote something (it's not actually much difficult) which suits my needs (assumes that the class has public constructor without args) and it could be extracted into utility class

  public Effect copyUsingReflection() {
    Constructor constructorToUse = null;
    for (Constructor constructor : this.getClass().getConstructors()) {
      if (constructor.getParameterTypes().length == 0) {
        constructorToUse = constructor;
        constructorToUse.setAccessible(true);
      }
    }
    if (constructorToUse != null) {
      try {
        Effect copyOfEffect = (Effect) constructorToUse.newInstance();
        for (Field field : this.getClass().getFields()) {
          try {
            Object valueToCopy = field.get(this);
            //if it has field of the same type (Effect in this case), call the method to copy it recursively
            if (valueToCopy instanceof Effect) {
              valueToCopy = ((Effect) valueToCopy).copyUsingReflection();
            }
            //TODO add here other special types of fields, like Maps, Lists, etc.
            field.set(copyOfEffect, valueToCopy);
          } catch (IllegalArgumentException | IllegalAccessException ex) {
            Logger.getLogger(Effect.class.getName()).log(Level.SEVERE, null, ex);
          }
        }
        return copyOfEffect;
      } catch (InstantiationException | IllegalAccessException | IllegalArgumentException | InvocationTargetException ex) {
        Logger.getLogger(Effect.class.getName()).log(Level.SEVERE, null, ex);
      }
    }
    return null;
  }

Antipattern: Reinvent the wheel – Pwnstar May 30 '18 at 09:07 — Pwnstar, May 30 '18 at 09:07

score 0 · Answer 18 · answered Sep 19 '19 at 21:35

Mladen's basic idea worked (thanks), but needed a few changes to be robust, so I contributed them here.

The only place where this type of solution should be used is if you want to clone the object, but can't because it is a managed object. If you are lucky enough to have objects that all have 100% side-effect free setters for all fields, you should definitely use the BeanUtils option instead.

Here, I use lang3's utility methods to simplify the code, so if you paste it, you must first import Apache's lang3 library.

Copy code

static public <X extends Object> X copy(X object, String... skipFields) {
        Constructor constructorToUse = null;
        for (Constructor constructor : object.getClass().getConstructors()) {
            if (constructor.getParameterTypes().length == 0) {
                constructorToUse = constructor;
                constructorToUse.setAccessible(true);
                break;
            }
        }
        if (constructorToUse == null) {
            throw new IllegalStateException(object + " must have a zero arg constructor in order to be copied");
        }
        X copy;
        try {
            copy = (X) constructorToUse.newInstance();

            for (Field field : FieldUtils.getAllFields(object.getClass())) {
                if (Modifier.isStatic(field.getModifiers())) {
                    continue;
                }

                //Avoid the fields that you don't want to copy. Note, if you pass in "id", it will skip any field with "id" in it. So be careful.
                if (StringUtils.containsAny(field.getName(), skipFields)) {
                    continue;
                }

                field.setAccessible(true);

                Object valueToCopy = field.get(object);
                //TODO add here other special types of fields, like Maps, Lists, etc.
                field.set(copy, valueToCopy);

            }

        } catch (InstantiationException | IllegalAccessException | IllegalArgumentException
                | InvocationTargetException e) {
            throw new IllegalStateException("Could not copy " + object, e);
        }
        return copy;
}

score 0 · Answer 19 · answered Oct 10 '19 at 06:42

0

    public <T1 extends Object, T2 extends Object> void copy(T1 origEntity, T2 destEntity) {
        DozerBeanMapper mapper = new DozerBeanMapper();
        mapper.map(origEntity,destEntity);
    }

 <dependency>
            <groupId>net.sf.dozer</groupId>
            <artifactId>dozer</artifactId>
            <version>5.4.0</version>
        </dependency>

answered Oct 10 '19 at 06:42

Ran Adler

3,587
30
27

This mapper.map has issues, in an entity it is not copying the Primary keys – Mohammed Rafeeq Jan 30 '20 at 15:00

Copy all values from fields in one class to another through reflection

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