How do you make a deep copy of an object

Question

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

User · Answer

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 = (Object) new ObjectInputStream(bais).readObject();

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Using Jackson to serialize and deserialize the object  This implementation does not require the object to implement the Serializable class      lt T gt  T clone T object  Class lt T gt  clazzType  throws IOException        final ObjectMapper objMapper   new ObjectMapper        String jsonStr  objMapper writeValueAsString object        return objMapper readValue jsonStr  clazzType

User · Answer

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

User · Answer

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

User · Answer

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  quot unchecked quot   public static  lt T extends Serializable gt  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

User · Answer

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

User · Answer

BeanUtils does a really good job deep cloning beans   BeanUtils cloneBean obj

User · Answer

For Spring Framework users  Using class org springframework util SerializationUtils    SuppressWarnings  unchecked   public static  lt T extends Serializable gt  T clone T object         return  T  SerializationUtils deserialize SerializationUtils serialize object

User · Answer

For Spring Framework users  Using class org springframework util SerializationUtils    SuppressWarnings  unchecked   public static  lt T extends Serializable gt  T clone T object         return  T  SerializationUtils deserialize SerializationUtils serialize object

User · Answer

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

User · Answer

Here is an easy example on how to deep clone any object  Implement serializable first public class CSVTable implements Serializable      Table lt Integer  Integer  String gt  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

User · Answer

Apache commons offers a fast way to deep clone an object   My Object object2  org apache commons lang SerializationUtils clone object1

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Here is an easy example on how to deep clone any object  Implement serializable first public class CSVTable implements Serializable      Table lt Integer  Integer  String gt  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

User · Answer

Apache commons offers a fast way to deep clone an object   My Object object2  org apache commons lang SerializationUtils clone object1

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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  quot test json quot    JsonGenerator g   f createGenerator jsonFile      write JSON     quot message quot     quot Hello world  quot    g writeStartObject    g writeStringField  quot message quot    quot Hello world  quot    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       quot message quot  equals p getCurrentName             handle error   t   p nextToken    if  t    JsonToken VALUE STRING          similarly   String msg   p getText    System out printf  quot My message to you is   s  n quot   msg   p close

User · Answer

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

User · Answer

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  quot test json quot    JsonGenerator g   f createGenerator jsonFile      write JSON     quot message quot     quot Hello world  quot    g writeStartObject    g writeStringField  quot message quot    quot Hello world  quot    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       quot message quot  equals p getCurrentName             handle error   t   p nextToken    if  t    JsonToken VALUE STRING          similarly   String msg   p getText    System out printf  quot My message to you is   s  n quot   msg   p close

User · Answer

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  quot unchecked quot   public static  lt T extends Serializable gt  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

User · Answer

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

User · Answer

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

User · Answer

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 = (Object) new ObjectInputStream(bais).readObject();

User · Answer

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

User · Answer

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  lt T gt  T copy T anObject  Class lt T gt  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

User · Answer

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

User · Answer

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  lt T gt  T copy T anObject  Class lt T gt  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

User · Answer

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

User · Answer

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

User · Answer

BeanUtils does a really good job deep cloning beans   BeanUtils cloneBean obj

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt Order gt  orders   new ArrayList lt Order gt          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 when using copy constructors you need to know the runtime type of the object you are copying  With the above approach you cannot easily copy a mixed list  you might be able to do it with some reflection code

User · Answer

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

User · Answer

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

User · Answer

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 lt Order gt  orders   new ArrayList lt Order gt          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 when using copy constructors you need to know the runtime type of the object you are copying  With the above approach you cannot easily copy a mixed list  you might be able to do it with some reflection code

User · Answer

Using Jackson to serialize and deserialize the object  This implementation does not require the object to implement the Serializable class      lt T gt  T clone T object  Class lt T gt  clazzType  throws IOException        final ObjectMapper objMapper   new ObjectMapper        String jsonStr  objMapper writeValueAsString object        return objMapper readValue jsonStr  clazzType

User · Answer

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

User · Answer

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

[java] How do you make a deep copy of an object?

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