CODEWITHSUNDEEP
javaclassclone
Andrei Savu
Andrei Savu

Reputation: 8685

How do you make a deep copy of an object?

It's a bit difficult to implement a deep object copy function. What steps you take to ensure the original object and the cloned one share no reference?

Upvotes: 373

Views: 481799

Answers (23)

Nick Dong
Nick Dong

Reputation: 3736

  1. A clone() example used in a maven package.

https://github.com/3breadt/dd-plist/blob/v1.28.0/src/main/java/com/dd/plist/NSDictionary.java#L323

public class NSDictionary extends NSObject implements Map<String, NSObject> {

    @Override
    public NSDictionary clone() {
        NSDictionary clone = new NSDictionary();
        for (Entry<String, NSObject> entry : this.dict.entrySet()) {
            clone.dict.put(entry.getKey(), entry.getValue() != null ? entry.getValue().clone() : null);
        }

        return clone;
    }
}

https://github.com/3breadt/dd-plist/blob/v1.28.0/src/main/java/com/dd/plist/NSObject.java#L81

public abstract class NSObject implements Cloneable, Comparable<NSObject> {

    /**
     * Creates and returns a deep copy of this instance.
     * @return A clone of this instance.
     */
    @Override
    public abstract NSObject clone();
}
  1. Others talks in https://www.baeldung.com/java-deep-copy
  • Shallow Copy: only copy values of fields
  • Deep Copy: each mutable object in the object graph is recursively copied
    • Copy Constructor
    • clone(): based on the clone method inherited from Object.
  • External Libraries
    • Apache Commons Lang has SerializationUtils#clone
    • JSON Serialization With Gson
    • JSON Serialization With Jackson
  • Sample code

Upvotes: 0

mipo256
mipo256

Reputation: 3224

I am surprised people are not mentioning this, but sometimes mapstruct can be used for these purposes. For instance, in out code we often have smth like that:

@Mapping(target = "status", constant = "DRAFT")
@Mapping(target = "versionNumber", constant = "1")
Contract create(Contract contract);

And mapstruct is intelligent enough to create a brand new copy with values from the old object, maybe with some modifications like in this case above.

I agree that this is not ideal and there are cases when it does not work. But in an overwhelming amount of cases, it really helps. So check it out.

Upvotes: 0

Fahad Israr
Fahad Israr

Reputation: 1239

A very quick and simple one-liner solution could be to use Jackson.

Have a look at the example snippet :

ObjectMapper objectMapper = new ObjectMapper();

MyClass deepCopyObject = objectMapper
  .readValue(objectMapper.writeValueAsString(originalObject), MyClass.class);

In the above example : "MyClass" refers to the class of the object you want to be copied .

Upvotes: 1

Thargor
Thargor

Reputation: 1872

You can make a deep copy with serialization without creating files.

Your object you wish to deep copy will need to implement serializable. If the class isn't final or can't be modified, extend the class and implement serializable.

Convert your class to a stream of bytes:

ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(bos);
oos.writeObject(object);
oos.flush();
oos.close();
bos.close();
byte[] byteData = bos.toByteArray();

Restore your class from a stream of bytes:

ByteArrayInputStream bais = new ByteArrayInputStream(byteData);
Object object = new ObjectInputStream(bais).readObject();

Upvotes: 63

Adriaan Koster
Adriaan Koster

Reputation: 16209

One way to implement deep copy is to add copy constructors to each associated class. A copy constructor takes an instance of 'this' as its single argument and copies all the values from it. Quite some work, but pretty straightforward and safe.

EDIT: note that you don't need to use accessor methods to read fields. You can access all fields directly because the source instance is always of the same type as the instance with the copy constructor. Obvious but might be overlooked.

Example:

public class Order {

    private long number;

    public Order() {
    }

    /**
     * Copy constructor
     */
    public Order(Order source) {
        number = source.number;
    }
}


public class Customer {

    private String name;
    private List<Order> orders = new ArrayList<Order>();

    public Customer() {
    }

    /**
     * Copy constructor
     */
    public Customer(Customer source) {
        name = source.name;
        for (Order sourceOrder : source.orders) {
            orders.add(new Order(sourceOrder));
        }
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Edit: Note that copy constructors don't take inheritance into account. For example: If you pass an OnlineOrder (a subclass of Order) to a copy constructor a regular Order instance will be created in the copy, unless you solve this explicitly. You could use reflection to look up a copy constructor in the runtime type of the argument. But I would suggest to not go this route and look for another solution if inheritance needs to be covered in a general way.

Upvotes: 32

Jason Cohen
Jason Cohen

Reputation: 83081

A safe way is to serialize the object, then deserialize. This ensures everything is a brand new reference.

Here's an article about how to do this efficiently.

Caveats: It's possible for classes to override serialization such that new instances are not created, e.g. for singletons. Also this of course doesn't work if your classes aren't Serializable.

Upvotes: 192

Unmitigated
Unmitigated

Reputation: 89472

Here is a generic deep cloning method using object serialization and deserialization with byte array streams (to avoid writing to a file).

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

@SuppressWarnings("unchecked")
public static <T extends Serializable> T deepClone(T t) {
    try (ByteArrayOutputStream baos = new ByteArrayOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(baos);) {
        oos.writeObject(t);
        byte[] bytes = baos.toByteArray();
        try (ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(bytes))) {
            return (T) ois.readObject();
        }
    } catch (IOException | ClassNotFoundException e) {
        throw new RuntimeException(e);
    }
}

Upvotes: 1

Ravi Chinoy
Ravi Chinoy

Reputation: 165

One very easy and simple approach is to use Jackson JSON to serialize complex Java Object to JSON and read it back.

From https://github.com/FasterXML/jackson-databind/#5-minute-tutorial-streaming-parser-generator :

JsonFactory f = mapper.getFactory(); // may alternatively construct directly too

// First: write simple JSON output
File jsonFile = new File("test.json");
JsonGenerator g = f.createGenerator(jsonFile);
// write JSON: { "message" : "Hello world!" }
g.writeStartObject();
g.writeStringField("message", "Hello world!");
g.writeEndObject();
g.close();

// Second: read file back
JsonParser p = f.createParser(jsonFile);

JsonToken t = p.nextToken(); // Should be JsonToken.START_OBJECT
t = p.nextToken(); // JsonToken.FIELD_NAME
if ((t != JsonToken.FIELD_NAME) || !"message".equals(p.getCurrentName())) {
   // handle error
}
t = p.nextToken();
if (t != JsonToken.VALUE_STRING) {
   // similarly
}
String msg = p.getText();
System.out.printf("My message to you is: %s!\n", msg);
p.close();

Upvotes: 10

mastercool
mastercool

Reputation: 523

Here is an easy example on how to deep clone any object: Implement serializable first

public class CSVTable implements Serializable{
    Table<Integer, Integer, String> table; 
    public CSVTable() {
        this.table = HashBasedTable.create();
    }
    
    public CSVTable deepClone() {
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(baos);
            oos.writeObject(this);

            ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
            ObjectInputStream ois = new ObjectInputStream(bais);
            return (CSVTable) ois.readObject();
        } catch (IOException e) {
            return null;
        } catch (ClassNotFoundException e) {
            return null;
        }
    }

}

And then

CSVTable table = new CSVTable();
CSVTable tempTable = table.deepClone();

is how you get the clone.

Upvotes: 0

Karthik Rao
Karthik Rao

Reputation: 219

Using Jackson to serialize and deserialize the object. This implementation does not require the object to implement the Serializable class.

  <T> T clone(T object, Class<T> clazzType) throws IOException {

    final ObjectMapper objMapper = new ObjectMapper();
    String jsonStr= objMapper.writeValueAsString(object);

    return objMapper.readValue(jsonStr, clazzType);

  }

Upvotes: 6

zeroboo
zeroboo

Reputation: 8993

For complicated objects and when performance is not significant i use a json library, like gson to serialize the object to json text, then deserialize the text to get new object.

gson which based on reflection will works in most cases, except that transient fields will not be copied and objects with circular reference with cause StackOverflowError.

public static <T> T copy(T anObject, Class<T> classInfo) {
    Gson gson = new GsonBuilder().create();
    String text = gson.toJson(anObject);
    T newObject = gson.fromJson(text, classInfo);
    return newObject;
}
public static void main(String[] args) {
    String originalObject = "hello";
    String copiedObject = copy(originalObject, String.class);
}

Upvotes: 13

user8690
user8690

Reputation: 491

You can do a serialization-based deep clone using org.apache.commons.lang3.SerializationUtils.clone(T) in Apache Commons Lang, but be careful—the performance is abysmal.

In general, it is best practice to write your own clone methods for each class of an object in the object graph needing cloning.

Upvotes: 49

Ihor Rybak
Ihor Rybak

Reputation: 3299

For Spring Framework users. Using class org.springframework.util.SerializationUtils:

@SuppressWarnings("unchecked")
public static <T extends Serializable> T clone(T object) {
     return (T) SerializationUtils.deserialize(SerializationUtils.serialize(object));
}

Upvotes: 24

CorayThan
CorayThan

Reputation: 17845

You can use a library that has a simple API, and performs relatively fast cloning with reflection (should be faster than serialization methods).

Cloner cloner = new Cloner();

MyClass clone = cloner.deepClone(o);
// clone is a deep-clone of o

Upvotes: 25

Julien Chastang
Julien Chastang

Reputation: 17784

A few people have mentioned using or overriding Object.clone(). Don't do it. Object.clone() has some major problems, and its use is discouraged in most cases. Please see Item 11, from "Effective Java" by Joshua Bloch for a complete answer. I believe you can safely use Object.clone() on primitive type arrays, but apart from that you need to be judicious about properly using and overriding clone.

The schemes that rely on serialization (XML or otherwise) are kludgy.

There is no easy answer here. If you want to deep copy an object you will have to traverse the object graph and copy each child object explicitly via the object's copy constructor or a static factory method that in turn deep copies the child object. Immutables (e.g. Strings) do not need to be copied. As an aside, you should favor immutability for this reason.

Upvotes: 81

Arun
Arun

Reputation: 1095

1)

public static Object deepClone(Object object) {
   try {
     ByteArrayOutputStream baos = new ByteArrayOutputStream();
     ObjectOutputStream oos = new ObjectOutputStream(baos);
     oos.writeObject(object);
     ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
     ObjectInputStream ois = new ObjectInputStream(bais);
     return ois.readObject();
   }
   catch (Exception e) {
     e.printStackTrace();
     return null;
   }
 }

2)

    // (1) create a MyPerson object named Al
    MyAddress address = new MyAddress("Vishrantwadi ", "Pune", "India");
    MyPerson al = new MyPerson("Al", "Arun", address);

    // (2) make a deep clone of Al
    MyPerson neighbor = (MyPerson)deepClone(al);

Here your MyPerson and MyAddress class must implement serilazable interface

Upvotes: 1

Adisesha
Adisesha

Reputation: 5258

Use XStream(http://x-stream.github.io/). You can even control which properties you can ignore through annotations or explicitly specifying the property name to XStream class. Moreover you do not need to implement clonable interface.

Upvotes: 8

Alfergon
Alfergon

Reputation: 5929

BeanUtils does a really good job deep cloning beans.

BeanUtils.cloneBean(obj);

Upvotes: 2

user946968
user946968

Reputation: 

import com.thoughtworks.xstream.XStream;

public class deepCopy {
    private static  XStream xstream = new XStream();

    //serialize with Xstream them deserialize ...
    public static Object deepCopy(Object obj){
        return xstream.fromXML(xstream.toXML(obj));
    }
}

Upvotes: 6

TheByeByeMan
TheByeByeMan

Reputation: 1424

Apache commons offers a fast way to deep clone an object.

My_Object object2= org.apache.commons.lang.SerializationUtils.clone(object1);

Upvotes: 20

supernova
supernova

Reputation: 3271

I used Dozer for cloning java objects and it's great at that , Kryo library is another great alternative.

Upvotes: 4

sankara
sankara

Reputation: 359

XStream is really useful in such instances. Here is a simple code to do cloning

private static final XStream XSTREAM = new XStream();
...

Object newObject = XSTREAM.fromXML(XSTREAM.toXML(obj));

Upvotes: 10

Orion Adrian
Orion Adrian

Reputation: 19573

Deep copying can only be done with each class's consent. If you have control over the class hierarchy then you can implement the clonable interface and implement the Clone method. Otherwise doing a deep copy is impossible to do safely because the object may also be sharing non-data resources (e.g. database connections). In general however deep copying is considered bad practice in the Java environment and should be avoided via the appropriate design practices.

Upvotes: 6

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