Java: recommended solution for deep cloning/copying an instance

Question

I'm wondering if there is a recommended way of doing deep clone/copy of instance in java.

I have 3 solutions in mind, but I can have miss some, and I'd like to have your opinion

edit: include Bohzo propositon and refine question: it's more about deep cloning than shallow cloning.

Do it yourself:

code the clone by hand properties after properties and check that mutable instances are cloned too.
pro:
- control of what will be performed
- quick execution
cons:
- tedious to write and maintain
- bug prone (copy/paste failure, missing property, reassigned mutable property)

Use reflection:

With your own reflection tools or with an external helper (like jakarta common-beans) it is easy to write a generic copy method that will do the job in one line.
pro:
- easy to write
- no maintenance
cons:
- less control of what happens
- bug prone with mutable object if the reflection tool does not clone sub objects too
- slower execution

Use clone framework:

Use a framework that do it for you, like :
commons-lang SerializationUtils
Java Deep Cloning Library
Dozer

pro:
- same as reflection
- more control over what will be exactly be cloned. cons:
- every mutable instance is fully cloned, even at the end of the hierarchy
- could be very slow to execute

Use bytecode instrumentation to write clone at runtime

javassit, BCEL or cglib might be use to generate a dedicated cloner as fast as one hand writed. Someone knows a lib using one of these tools for this purpose ?

What I have missed here ?
Which one would you recommend ?

Thanks.

Answer 1

I'd recommend the DIY way which, combined with a good hashCode() and equals() method should be easy to proof in a unit test.

Answer 2

I'd suggest to override Object.clone(), call super.clone() first and than call ref = ref.clone() on all references that you want to have deep copied. It's more or less Do it yourself approach but needs a bit less coding.

Answer 3

Depends.

For speed, use DIY. For bulletproof, use reflection.

BTW, serialization is not the same as refl, as some objects may provide overridden serialization methods (readObject/writeObject) and they can be buggy

Answer 4

for deep cloning (clones the entire object hierarchy):

• commons-lang SerializationUtils - using serialization - if all classes are in your control and you can force implementing Serializable

• Java Deep Cloning Library - using reflection - in cases when the classes or the objects you want to clone are out of your control (a 3rd party library) and you can't make them implement Serializable, or in cases you don't want to implement Serializable

for shallow cloning (clones only the first level properties):

• commons-beanutils BeanUtils - in most cases

• Spring BeanUtils - if you are already using spring and hence have this utility on the classpath.

I deliberately omitted the "do-it-yourself" option - the API's above provide a good control over what to and what not to clone (for example using transient, or String[] ignoreProperties), so reinventing the wheel isn't preferred.

Answer 5

Joshua Bloch's book has a whole chapter entitled "Item 10: Override Clone Judiciously" in which he goes into why overriding clone for the most part is a bad idea because the Java spec for it creates many problems.

He provides a few alternatives:

• Use a factory pattern in place of a constructor:

       public static Yum newInstance(Yum yum);
  

• Use a copy constructor:

       public Yum(Yum yum);
  

All of the collection classes in Java support the copy constructor (e.g. new ArrayList(l);)

Answer 6

I've made another Generic Deep Copy utility available at http://www.genericdeepcopy.com/.