groovy.lang
Annotation Type Immutable

@Retention(value=RUNTIME)
@Target(value=TYPE)
public @interface Immutable

Class annotation used for making a class immutable.

It allows you to write code snippets like this:

 @Immutable final class Customer {
     String first, last
     int age
     Date since
     Collection favItems
 }
 def d = new Date()
 def c1 = new Customer(first:'Tom', last:'Jones', age:21, since:d, favItems:['Books', 'Games'])
 def c2 = new Customer('Tom', 'Jones', 21, d, ['Books', 'Games'])
 assert c1 == c2
 

A class created in this way has the following characteristics:

• Properties automatically have private, final backing fields with getters. Attempts to update the property will result in a ReadOnlyPropertyException.
• A map-based constructor is provided which allows you to set properties by name.
• A tuple-style constructor is provided which allows you to set properties in the same order as they are defined.
• Default equals, hashCode and toString methods are provided based on the property values.
• Dates, Cloneables and arrays are defensively copied on the way in (constructor) and out (getters). Arrays and cloneable objects use the clone method. For your own classes, it is up to you to define this method and use deep cloning if appropriate.
• Collections and Maps are wrapped by immutable wrapper classes (but not deeply cloned!). Attempts to update them will result in an UnsupportedOperationException.
• Fields that are enums or other @Immutable classes are allowed but for an otherwise possible mutable property type, an error is thrown.
• You don't have to follow Groovy's normal property conventions, e.g. you can create an explicit private field and then you can write explicit get and set methods. Such an approach, isn't currently prohibited (to give you some wiggle room to get around these conventions) but any fields created in this way are deemed not to be part of the significant state of the object and aren't factored into the equals or hashCode methods. Use at your own risk!

Such classes are particularly useful for functional and concurrent styles of programming and for use as key values within maps.

Author:

Paul King