@Documented
@Retention(value=RUNTIME)
@Target(value=TYPE)
public @interface AutoClone
Cloneable classes.
The @AutoClone annotation instructs the compiler to execute an
AST transformation which adds a public clone() method and adds
Cloneable to the list of interfaces which the class implements.
Because the JVM doesn't have a one-size fits all cloning strategy, several
customizations exist for the cloning implementation. By default, the clone()
method will call super.clone() before calling clone() on each
Cloneable property of the class.
Example usage:
import groovy.transform.AutoClone
@AutoClone
class Person {
String first, last
List favItems
Date since
}
Which will create a class equivalent to the following:
class Person implements Cloneable {
...
public Person clone() throws CloneNotSupportedException {
Person result = (Person) super.clone()
result.favItems = favItems instanceof Cloneable ? (List) favItems.clone() : favItems
result.since = (Date) since.clone()
return result
}
...
}
Which can be used as follows:
def p = new Person(first:'John', last:'Smith', favItems:['ipod', 'shiraz'], since:new Date()) def p2 = p.clone() assert p instanceof Cloneable assert p.favItems instanceof Cloneable assert p.since instanceof Cloneable assert !(p.first instanceof Cloneable) assert !p.is(p2) assert !p.favItems.is(p2.favItems) assert !p.since.is(p2.since) assert p.first.is(p2.first)In the above example,
super.clone() is called which in this case
calls clone() from java.lang.Object. This does a bit-wise
copy of all the properties (references and primitive values). Properties
like first has type String which is not Cloneable
so it is left as the bit-wise copy. Both Date and ArrayList
are Cloneable so the clone() method on each of those properties
will be called. For the list, a shallow copy is made during its clone() method.
If your classes require deep cloning, it is up to you to provide the appropriate
deep cloning logic in the respective clone() method for your class.
If one of your properties contains an object that doesn't support cloning
or attempts deep copying of a data structure containing an object that
doesn't support cloning, then a CloneNotSupportedException may occur
at runtime.
Another popular cloning strategy is known as the copy constructor pattern.
If any of your fields are final and Cloneable you should set
style=COPY_CONSTRUCTOR which will then use the copy constructor pattern.
Here is an example making use of the copy constructor pattern:
import groovy.transform.AutoClone import static groovy.transform.AutoCloneStyle.*Which will create classes equivalent to the following:@AutoClone(style=COPY_CONSTRUCTOR)class Person { final String first, last final Date birthday }@AutoClone(style=COPY_CONSTRUCTOR)class Customer extends Person { final int numPurchases final List favItems }
class Person implements Cloneable {
...
protected Person(Person other) throws CloneNotSupportedException {
first = other.first
last = other.last
birthday = (Date) other.birthday.clone()
}
public Person clone() throws CloneNotSupportedException {
return new Person(this)
}
...
}
class Customer extends Person {
...
protected Customer(Customer other) throws CloneNotSupportedException {
super(other)
numPurchases = other.numPurchases
favItems = other.favItems instanceof Cloneable ? (List) other.favItems.clone() : other.favItems
}
public Customer clone() throws CloneNotSupportedException {
return new Customer(this)
}
...
}
If you use this style on a child class, the parent class must
also have a copy constructor (created using this annotation or by hand).
This approach can be slightly slower than the traditional cloning approach
but the Cloneable fields of your class can be final. When using the copy constructor style,
you can provide your own custom constructor by hand if you wish. If you do so, it is up to you to
correctly copy, clone or deep clone the properties of your class.
As a variation of the last two styles, if you set style=SIMPLE
then the no-arg constructor will be called followed by setting the
individual properties (and/or fields) calling clone() if the
property/field implements Cloneable. Here is an example:
import groovy.transform.AutoClone import static groovy.transform.AutoCloneStyle.*Which will create classes equivalent to the following:@AutoClone(style=SIMPLE)class Person { final String first, last final Date birthday }@AutoClone(style=SIMPLE)class Customer { final List favItems }
class Person implements Cloneable {
...
public Person clone() throws CloneNotSupportedException {
def result = new Person()
copyOrCloneMembers(result)
return result
}
protected void copyOrCloneMembers(Person other) {
other.first = first
other.last = last
other.birthday = (Date) birthday.clone()
}
...
}
class Customer extends Person {
...
public Customer clone() throws CloneNotSupportedException {
def result = new Customer()
copyOrCloneMembers(result)
return result
}
protected void copyOrCloneMembers(Customer other) {
super.copyOrCloneMembers(other)
other.favItems = favItems instanceof Cloneable ? (List) favItems.clone() : favItems
}
...
}
You would typically use this style only for base classes where you didn't
want the normal Object clone() method to be called and
you would typically need to use the SIMPLE style for any child classes.
As a final example, if your class already implements the Serializable
or Externalizable interface, you can choose the following cloning style:
@AutoClone(style=SERIALIZATION)
class Person implements Serializable {
String first, last
Date birthday
}
which outputs a class with the following form:
class Person implements Cloneable, Serializable {
...
Person clone() throws CloneNotSupportedException {
def baos = new ByteArrayOutputStream()
baos.withObjectOutputStream{ it.writeObject(this) }
def bais = new ByteArrayInputStream(baos.toByteArray())
bais.withObjectInputStream(getClass().classLoader){ (Person) it.readObject() }
}
...
}
This will output an error if your class doesn't implement one of
Serializable or Externalizable, will typically be
significantly slower than the other approaches, also doesn't
allow fields to be final, will take up more memory as even immutable classes
like String will be cloned but does have the advantage that it performs
deep cloning automatically.
Further references on cloning:
AutoCloneStyle,
ExternalizeMethods| Modifier and Type | Optional Element and Description |
|---|---|
java.lang.String[] |
excludes
Comma separated list of property (and/or field) names to exclude from cloning.
|
boolean |
includeFields
Include fields as well as properties when cloning.
|
AutoCloneStyle |
style
Style to use when cloning.
|
public abstract java.lang.String[] excludes
NOTE: When using the CLONE style, property (and/or field) copying might occur as part of
calling super.clone() which will ignore this list. You can then use this list to
streamline the provided clone() implementation by selecting which Cloneable properties
(and/or fields) will have a subsequent call to their clone() method. If you have
immutable properties (and/or fields) this can be useful as the extra clone() will
not be necessary and cloning will be more efficient.
NOTE: This doesn't affect property (and/or field) copying that might occur as part
of serialization when using the SERIALIZATION style, i.e. this flag is ignored;
instead adjust your serialization code to include or exclude the desired
properties (and/or fields) which should carry over during cloning.
public abstract boolean includeFields
NOTE: When using the CLONE style, field copying might occur as part of
calling super.clone() and might be all you require; if you turn on
this flag, the provided clone() implementation will also
subsequently call clone() for each Cloneable field which can be
useful if you have mutable fields.
NOTE: This doesn't affect field copying that might occur as part of
serialization when using the SERIALIZATION style, i.e. this flag is ignored;
instead adjust your serialization code to include or exclude your fields.
public abstract AutoCloneStyle style