[Java] Annotation Type AutoClone
- groovy.transform.AutoClone
Class annotation used to assist in the creation of 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:
- http://www.codeguru.com/java/tij/tij0128.shtml
- http://www.artima.com/objectsandjava/webuscript/ClonCollInner1.html
- http://courses.dce.harvard.edu/~cscie160/JDCTipsCloning
- http://www.agiledeveloper.com/articles/cloning072002.htm
- See Also:
- AutoCloneStyle
- ExternalizeMethods
- Since:
- 1.8.0
Element Summary
Type | Name and Description |
---|---|
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. |
Inherited Methods Summary
Methods inherited from class | Name |
---|---|
class Object | wait, wait, wait, equals, toString, hashCode, getClass, notify, notifyAll |
Element Detail
public String[] excludes
Comma separated list of property (and/or field) names to exclude from cloning. For convenience, a String with comma separated names can be used in addition to an array (using Groovy's literal list notation) of String values.
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. @default {}
public boolean includeFields
Include fields as well as properties when cloning.
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. @default false
public AutoCloneStyle style
Style to use when cloning. @default AutoCloneStyle.CLONE
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Licensed under the Apache license.
https://docs.groovy-lang.org/2.5.14/html/gapi/groovy/transform/AutoClone.html